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Eruptive history of the Colima volcanic complex (Mexico)
Journal of Volcanology and Geothermal Research, 31 (1987) 99-113 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
99
ERUPTIVE HISTORY OF THE COLIMA VOLCANIC COMPLEX (MEXICO) CLAUDE ROBIN, PHILIPPE MOSSAND, GUY CAMUS, JEAN-MARIE CANTAGREL, ALAIN GOURGAUD and PIERRE M. VINCENT Centre de Recherches Volcanologiques, Universit~ Clermont H et CNRS UAIO, 5, rue Kessler, 63038 Clermont-Ferrand, France (Received September 27, 1985; revised and accepted June 11, 1986)
Abstract Robin, C., Mossand, P., Camus, G., Cantagrel, J.-M., Gourgaud, A. and Vincent, P.M., 1987. Eruptive history of the Colima volcanic complex (Mexico). J. Volcanol. Geotherm. Res., 31: 99-113. The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nudes ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego. The oldest caldera, C 1 ( 7-8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3-3.5 km wide), ended the evolution of Nevado I. Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4-5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters. The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls. The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II-III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.
100
Introduction The andesitic volcanic complex of Colima is located in the southern part of the N - S Colima graben, in the western part of the Trans-Mexican Volcanic Belt. Two main eruptive centres form this complex: the Nevado de Colima (19033'48" N, 103°36'30" W; 4260 m a.s.l.), a composite volcano without historic activity, and the Volcan de Fuego, or Volcan de Colima (19 ° 30'40" N, 103 ° 37' W; about 3850 m a.s.l. ), located in the caldera of a former edifice built on the southern side of the Nevado. The two summits, only 5 k m from each other, tower above an andesitic shield, slightly elongated N-S, of 25 by 30 kin, whose base is around 1000 m a.s.l.(Fig. 1 ).
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Fuego has been the most active of the Mexican volcanoes during the historic period. Its activityhas been described by Ordonez (1897), Arreola (1903), Waltz (1920, 1936), and most recently by Luhr and Carmichael (1980) and Medina (1983). The descriptions available for the last four centuries (Luhr, 1981 ) enable us to distinguish a succession of long constructive stages of the cone followed by short destructive episodes of its summit by violent outbursts. During the last decades, the cone produced block-lava flows in 1961-1962, 1975-1976 and 1981-1982. Cantaro, the volcano thought to be the oldest in this area, is next to the northern flanks of the Colima group (Fig. i). Some of the products of this volcano interfingerwith those from
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Fig. 1. Location of the Colima volcanic complex in the Trans-Mexican Volcanic Belt (1A) and in the Colima graben (1B). 1 = Volcan Sanganguey; 2 = Ceboruco; 3 = Volcan de Tequila; 4 = Colima volcanic complex; 5 = La Primavera calderic structure; 6=V. Tancitaro; 7=V. Paricutin; 8 = JoruUo; 9=Nevado de Toluca; 10= Ajusco; 11 = Iztaccihuatl; 12= Popocatepetl; 13 = Malinche; 14 = V. Pico de Orizaba; 15 = Los Humeros caldera; 16 = V. Cofre de Perote; 17= PMma Sola volcanic complex; 18 = V. San Martin
101 the Nevado along the road joining Ciudad Guzman to Venustiano Carranza. Basaltic cones, the most famous of which is Volcan Apaxtepec, are on the northern edge of the andesitic complex. Most of them, studied by Luhr and Carmichael (1981), belong to the high-K alkaline series; only three are calc-alkalic. The studies by Luhr and Carmichael (1980, 1982 ) only deal with the recent stage of the southern structure (Volcan de Fuego). Previous reconstructions of the two eruptive centres were attempted by Mooser (1961) and Demant (1981). However, the detailed history of the whole volcanic complex, the definition of the successive stages of its evolution and their connection with the magmatic and eruptive behaviours had not yet been studied. The ancient Nevado
Structure and extent The primitive volcano - Nevado I - represents the main part of Nevado de Colima (Fig. 2). The wall of the oldest caldera C1, though eroded, can be detected in the morphology of the western, northern and southwestern slopes of this edifice. On the basis of this structure, two periods of construction can be distinguished: IA and IB. This elliptical caldera reaches 8 km in length, and the primitive composite volcano is about 25 km in diameter. The latter consists mainly of massive lava flows and domes located near the caldera rim or scattered on its lower slopes. On the outskirts, the lava flows grade into a gently dipping conglomeratic formation, well developed towards the west and the southeast, the Atenquique formation (Mooser, 1961). The evolution of Nevado I ended with the collapse of a second and smaller caldera, C2, the wall of which is clearly exposed on the western side, especially at Cerro el Aguila (Fig. 2).
Nature of the lavas and pyroclastic products The Nevado IA (ante C 1 ) is characterized by plagioclase-phyric andesites (57-59% SiO2),
with ortho- and clinopyroxenes, and without amphibole. More siliceous but less porphyric lavas (61-62% Si02) with rare amphiboles are also present. The Nevado IB lavas are siliceous andesites and very porphyric amphibole-rich two-pyroxene dacites. One can relate the C1 caldera to a major part of thick pyroclastic units making up a formation called informally the "yellow ashes" (Robin et al., 1984). Indeed, the Cantaro-Nevado complex is associated with thick ash deposits of regional extent which cover at least 600 km 2 towards the north and the west. In numerous places the upper part of the deposits has been reworked. In such formations, exact estimates of what belongs to each centre are not easy to make. It is obvious that in the northwestern area, around Venustiano Carranza, yellow ashes must be related to the Cantaro, which is also topped by a caldera and domes. However, on the northwestern slopes of the Nevado and towards the west, between Telcruz and Zapotitlan, the "yellow ashes" are associated with ash flows running down the Nevado (Fig. 2). These "yellow ashes" represent an u n c o m m o n pyroclastic facies, made up of the following: (a) Ash-flow deposits: the most characteristic outcrops can be observed on the northwestern slopes, channeled into andesites of the stage IA. Above Telcruz, the ash-flow series are 50-70 m thick and slightly indurated. On the lower slopes, these products are channeled into deep canyons cut into the Atenquique series. Similar pyroclastic flows have been observed on the slopes of Cantaro. (b) Ash and pumice air-fall deposits: these can be spotted everywhere, even as far as 30 km from the summits. They are 10-15 m thick at a distance of 15 km to the northwest of the New ado. These air-fall deposits include several units that reach thicknesses of meters. At the boundary between the two edifices it was not possible to distinguish between the ashes of Nevado and those of Cantaro. In some places, for example 5 km west of Los Depositos, basic ashes, or even
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Fig. 2. Sketch map ahowing theegtent and structure of the Colima volcanic complex prior to the two Mount St. Helens-type events. Legend: 1 = volcanic series from the basement; 2 = limits of the effusive primitive edifice, Nevado IA; 3 = extent of the conglomeratic dvposits (Atanquique series ); 4 = directions followed by the main ash-flow deposits from the end of period IA (related to C1); 5=extent of the "yellow ashes" related to the Nevado I and to V. Cantaro: ash-flow depositsj air-fall tephra and reworked materials; 6 = L o s Depositos pyroclastic flows related to terminal stages of Nevado I (IB period); 7 = dacitic domes and a~ociated pelean n u ( ~ ardantes; 8 = directions followed by the Los Garcia and the Los Mazos pyroclestic flows of St. Vincent-type ( h e t e ~ mrs), possibly related to C2; 9=laves of Nevado II; 10=pyroclastic flows with hetervgeneous bombs and scoria related to Nevado II; 11 =inferred extent of the Paleofuego; 12=C] and C2 calderic structures; 13 = scoria cone CO 6.
103 strombolian scoriae, emitted by nearby basaltic cinder cones, are interbedded with the "yellow ashes" sequences (location CO6, Fig. 2).
Age of the Nevado I The age of a lava-flow from the middle part of the northwestern flank is 0.53+0.1 m.y. Another flow was dated at 0.35 + 0.05 m.y. These results agree well enough with those obtained by Allan and Luhr (1982), who suggest ages going back to about 1.0 m.y. in the Cantaro basement, and with other unpublished K-At data from this volcano (Cantagrel et al., in prep. ).
The Atenquique conglomeratic series This series is a detrital formation whose elements are exlusively volcanic. Its volume was estimated to be more than 300 km 3 (Luhr and Carmichael, 1980). These sequences are an accumulation of gently dipping strata ( < 5 ° ), up to several metres thick, within a total thickness that in many places exceeds 200 m. The irregular grain-size layers consist of rounded, blunted or angular blocks. The fine-textured strata are composed of beige, pink, and grey ash. An andesitic block from this series was dated at 0.38 + 0.10 m.y. The pyroclastic flows from Nevado I follow deep barrancas cut into these conglomerates. This age indicates the early deposition of most of the Atenquique series, although another block yielding an age of 0.26+0.06 m.y., shows that some of these deposits were contemporaneous with Nevado II. On the other hand, the relations between some parts of this formation and recent clastic deposits (of debris-flow or pyroclastic-flow type; see below) seem to indicate that the deposition of the Atenquique conglomeratic series continued at a declining rate until more recent times.
Other pyroclastic formations related to the Nevado I Although these formations require more work to determine their relationships, we can attribute them to the ancient Nevado, and quite probably to the IB period: (1) Ash-flow deposits with vesiculated dacitic blocks and rare bombs partly cover the northern slopes near Los Depositos. They have an average thickness of 10 m. These amphibole-rich pyroclastic units (Table 1, CO 67) probably originated from the destruction of domes that overflowed the C1 caldera on this side. (2) Back-falling nudes deposits exposed at Los Mazos and near E1 Jasmin (in the eastern and northwestern areas, Fig. 2) show a large proportion of dark-grey bombs (up to 30%) with a glassy crust and xenoliths scattered in an ashy matrix. These pyroclastic flow deposits resemble those of Saint-Vincent (Anderson and Flett, 1903; Shepard et al., 1979), and in the terminal volcanic cones of other great Mexican volcanoes (Robin, 1981; Robin and Cantagrel, 1982; Robin and Boudal, 1987). The structure and the mineralogical composition of the bombs show a more or less heterogeneous magma, one component of which is very basic with unstable olivine and clinopyroxene. With an average SiO2 content of 59% (Table 1, CO 66), these pyroclastic-flows mark a break in the petrological evolution of the differentiated suite. The C2 caldera, about 3 km in diameter, is not clearly related to a well defined pyroclastic unit. Such a structure may be a large crater resulting from strong eruptions of the Saint-Vincent-type (related to the Los Mazos and E1 Jasmin deposits?), as well as a collapse caldera associated with ignimbrites. (3) Pelean nudes ardentes deposits of dacitic composition (64% SiO2 on average) also lie on the northern side. Their location and stratigraphic position allow us to relate their emplacement to a series of neighbouring domes (Fig. 2).
104 TABLE 1 Selectedanalyses from the Nevado complex (northern centre) Lava flow
Lava flows IA period
Pyr. flow
Pelean Pyr. Flows
IB
Depositos
Ash and scoriae pyr. flows from Nevado I (Los Mazos) heterogeneous m-omas C066
Los
CO 70
CO 40
CO3
CO 17
CO 39
CO 17
CO 117
CO 67
SiO~ Al2Oa Fe~03 FeO MgO CaO Na~O K20 Ti02 MnO H20 + H20-
57.50 17.70 5.63 1.01 4.30 7.40 3,90 1.40 0.55 0.11 0.10
61.00 17.80 4.42 1.43 2.10 5.40 4.90 1.35 0.90 0.08 0.04 -
61,30 18.20 4.31 1.16 2.00 5.00 4.85 1.20 0.60 0.09 0.19 0.27
61.10 17.50 3.89 2.17 3.40 5.70 4,05 1.10 0.85 0.12 0.47 0.10
63.20 17.00 4.82 0.43 1.80 4.80 4.55 1.45 0.80 0.08 0.14 -
63.50 17.00 3.26 1.16 2.00 4.80 4.50 1.70 0.50 0.09 1.03 0.14
63.90 17.30 2.70 1.44 1.60 4.00 4.40 1.75 0.55 0.10 0.31 0.17
62.70 16.90 3.60 1.44 1.90 4.70 4.45 1.60 0.55 0.09 1.00 0.09
59,00 16.60 4.58 1.73 4.50 6.90 4,15 1.45 0.60 0.09 0.43 0.22
59,70 17.30 2.91 2.74 3.50 6.00 4.25 1.50 0.55 0.08 0.32 0.20
Total
99.60
99.42
99.17
100.45
99.07
99.98
99.12
99.02
100.25
99:05
Conglomeratic block CO
Nevado II lava flows
Nevado III lava flow
Ash and scoriae pyr.flows from Nevado HI (Fuego
Ash and scoriae flow (heterogeneous magmas) -~0.08 m.y.
llB CO 46
CO 55
Fe203 FeO MgO CaO Na20 K20 TiO2 MnO H20 + H20-
64.20 16.80 3.52 0.43 1.55 4.55 4.30 2.10 0.60 0.07 1.63 0,.14
58.90 17.20 4.94 1.59 4.90 6.50 3.85 1.45 1.05 0.10 0.13 0.11
60.40 17.50 3.51 2.60 4.10 5.80 4.10 1.30 0.80 0.11 0.07
Total
99.89
100.72
100.29
SiO~
track; Los Garcia)
CO 57 CO 45
AI203
CO 68
CO 56
C09
CO10
CO 24
CO 25
59.50 18.10 5.11 1.16 3.60 6.10 4.10 1.50 0.85 0.10 0.04
59.00 17.10 3.88 2.45 3.80 5.90 4.25 1.45 0.60 0,09 0.27 0.25
59.90 17.30 3.54 " 2.31 3.40 6.00 4.25 1.45 0,50 0.10 0.30 0.01
60.80 17.60 3.19 2.17 3.10 5.70 4.55 1.45 0.70 0.11 0.72 0.02
59.40 16.50 5.40 1.44 4.10 5.50 4.00 1.90 0.80 0.10 0.14 0.14
58.90 17.15 3.86 2.02 3.50 6:20 4.25 1.45 0.60 0.09 0.98 0.05
60.50 17.00 4.34 1.59 3.15 5.70 4.40 1.35 0.90 0.09 1.23 -
100.16
99,04
99.06
100.11
99.42
99.05
100.25
CO 70 to CO 39: two-pyroxane andesite and siliceous andesites from the primitive volcano; CO 17 and CO 117: amphibole dacites from pelaan pyroclastiC flow dIR~its; CO 67: siliceous andesite from the Los Depositos pyroclastic flows;CO 66 and CO 68: juvenile heterogeneous ~ ~ o f ash and scoriae p ~ i c flows (outcrops of Los Mazos a n d ~ l Jasmin); CO lIB: Amphibole dacite block from the Atenquiqus series; CO 45 to CO 56: andesitic lava flows from Nevado II;CO 9 and CO 10: dark and light bombs from h e t e ~ ash and p y r i t i c flow in Barmnca Platanos; CO 57: summit lava flow from Nevado III; CO 24 and CO 25: dark and light bombs from the Los Garcia pyroclastic flows (related to Nevado III).
105
The modern Nevado
Lava-flows and pyroclastic products from Nevado H (or Intermediate Nevado) Mooser (1961) was the first to point out the possible existence of two calderas at the top of the Nevado. The composite structure corresponds to the superposition of the C2 and C3 calderas. C2 ended the evolution of the primitive volcano. A new effusive cone, here called the intermediate Nevado, filled C2. Its products overflowed the C2 rim at least towards the southwest, north and northeast. Towards the northwest, the flows from this volcano were stopped by the C2 wall at Cerro el Aguila (Fig. 2). The Nevado II was not very extensive and only a small part of it remains: this edifice was destroyed by a Mount St. Helens-type event whose consequence was the formation of the summit amphitheatre (or avalanche caldera C3 ), 4-5 km wide, opening to the east. Two KAr ages obtained on lava-flows which overflowed C2 towards the southwest and on a lava flow from the C3 wall, show that the age of this volcano is in order of 150,000-200,000 y. Compared to the andesites and dacites of the older edifice, the lavas of Nevado II have less silica, like the back-falling nu6es deposits mentioned above. They are relatively basic (58-59% SiO2) with two pyroxenes (cpx/opx = 5) and rare amphibole microcrysts. The groundmass is locally heterogeneous. These characteristics clearly distinguish the series of Nevado II from the older andesitic to dacitic suite of Nevado I (Table 1A). A second group of back-falling ash-flow deposits with heterogeneous bombs can be distinguished from the older ones: (1) Several flow-deposits of this type outcrop in the upper part of the Barranca Platanos (location, Fig. 2 ). Their stratigraphic position suggests a relatively ancient age. (2) In the Atenquique river, a second outcrop of St. Vincent-type deposits yielded an age of 0.08 + 0.03 m.y.
(3) At Puerto La Calle, products of this type were stopped by the C2 wall (Fig. 3 ). In the Barranca Platanos, the avalanche deposits related to the C3 caldera formation of debris-flow type, see below - are younger than these amphibole-rich pyroclastic units of andesitic composition (CO 9). This highly explosive volcanism can thus be related to the Nevado II. Pyroclastic deposits of pelean type and autobrecciated lava-flows are present on the northern slope between 2700 and 3000 m a.s.1. They too belong to Nevado II. As in the case of the lava-flows or domes exposed in the C3 wall on this side, they are andesitic with opx, cpx, amphibole and unstable zoned plagioclases. Other deposits, one quite visible along the Fuego track (alt. 1600 m a.s.1.), and two ash and pumice flows with scoria bombs located near Los Garcia (Fig. 3), could also be related to this explosive activity of Nevado II. Nevertheless, these two units are covered only by slight aerial deposits and thus are more likely to be associated with Nevado III.
The Mount St.-Helens-type eruption of Nevado The C3 caldera is related to a huge avalanche resulting from an eruption similar to that of Mount St. Helens (MSH) in May 1980 (Also called a Bezymianny-type event; Siebert, 1984). The debris-flow (DF) followed channels towards the east, then turned towards the southeast about 20 km from the summit (Fig. 3). Near the avalanche caldera, these DF deposits are made up of piles of large fractured and tilted slabs, each of which includes several flows whose disposition was not disturbed during the slide and still preserves their initial stratigraphy. On its slopes, for example in the upper part of the Atenquique Barranca, the pile of slabs reaches a thickness of 500 m. In the downhill part, the deposits are more finely crushed: the strong fracturing yields blocks up to a centimeter to decimeter in size, in a pulverized dusty matrix. In the distal part, the
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landslide partly covers the slopes of Paleofuego, the basal edifice of the southern eruptive centre (see below). Between San Marcos and Atenquique much of this debris flow turned into lahars. A gradual transformation can be seen between these deposits and the conglomerates making up the Atenquique series. The MSH-type eruption was followed by intense explosive activity that deposited air-fall layers - 25 m thick on the whole - over the entire summit area of Nevado. Ash, scoriae, pumice and pumiceous or dense andesitic lapilli, are piled up in layers from several centimeters to more than a meter thick. Numerous paleosoils are found interbedded in these layers. On the southern slopes of Nevado, at the boundary between Nevado II and Paleofuego, these plinian sequences are overlaid by another series of air falls, emitted by the southern eruptive centre, which has already been studied by Luhr and Carmichael (1982). The lower part of the Nevado air-fall deposits is older than Fuego's plinian series. However, the upper layers of Nevado's air falls interfinger with those of Fuego. The terminal cone of Nevado, or Nevado III (Figs. 3-4) This small cone, about 3 km in diameter and 800 m high, is composed of thick, viscous and steeply dipping lava flows ( 30 ° to 45 ° ). On the
outerflanks of the cone, some of these lava flows can be observed overlying the plinian sequences, close to the opening of the C3 caldera. The top of this volcano is near the southwestern wall of C3. Some andesitic flows reached the northern caldera wall and were slightly deflected towards the East. (Fig. 4). The lavas are poorly crystallized andesites (about 59% SiO2; Table 1, CO 56) with unstable sodic plagioclase, cpx, opx, and relic crystals of amphibole. The present summit of Nevado preserves the remains of a crater; it corresponds to the summit of the last edifice which, contrary to what was generally assumed, has been only slightly eroded. Nevado III was probably short-lived, in comparison to Nevado I and Nevado II. It was probably active only a few thousand years. In addition to the post-C3 plinian air falls starting with the new constructional stage, and the St. Vincent-type pyroclastic flows outcropping on the trail to Fuego and near Los Garcia, two other units can be related to the late stages of Nevado: white ash flows with decimetricsized dacitic pumice overlie the debris flow in the southwest, in the barrancas Platanos and Atenquique (symbol No 13, Fig. 3 ). Twelve km from the summit, their thickness ranges from 10 to 25 m. It is likely that these dacitic products were emitted prior to construction of the cone by andesitic lava flows, probably a short time after the MSH-type event, as was the case at Popocatepetl (Robin and Boudal, 1987 ), and at Mount St. Helens.
Fig. 3. Generalized map of the Colima volcanic complex (see also Fig. 2 for major volcanic events prior to the Mount St. Helens-type eruptions). 1 = andesites from the basement; 2 = extent of the lava-flows of Nevado I (major pyroclastic events related to Nevado I are shown in Fig. 2); 3 = conglomerates; 4 = Nevado II (andesites); see also Fig. 2 for the distribution of pyroclastic flows related to this edifice; 5 =andesite lava flows of V. Paleofuego; 6 =debris-flow deposits resulting from the MSH events of Nevado II and Paleofuego (largely laharic in their distal parts) ; 7 = outcrops of pyroclastic deposits (surge type) related to the Paleofuego's MSH event; 8=Nevado III lava-flows (andesites); 9--Volcan de Fuego (see Fig. 4 for details); 10 = ash and scoria pyroclastic flow deposits (heterogeneous magmas) related to Fuego; 11 = cinder cones, 12 ---dacitic domes and related pelean pyroclastic flow deposits; 13 =emplacement of ash and pumice deposits related to Nevado III; 14 = scattered outcrops of St. Vincent-type ash-flow deposits; a = related to Nevado II (La Calle, B. Platanos and Atenquique); b = related to the Nevado I I I ( Los Garcia, Fuego track); 15 = approximate extent of the plinian series; a ="yellow ashes" related to C 1; b = summit air-fall deposits from the Nevado (transition Nevado I-Nevado II); c = plinian series from the Fuego; 16 = collapse caldera (C 1 ); large crater or collapse caldera (C2); avalanche calderas (C3 and C4).
108
Fig. 4. Generalizedmap of the slimmlt area of the Colima volcanic complex./=primitive volcano (Nevado IA and IB ); IIA = V. Nevado II ( mainly effusive); l i b = Nevado's debris-flow; IIIA and IIIB=s,mmit cone of the Nevado, probably two phases of building (separated by one of the majorpyroclasticevents?); IVA = Paleof~Jego'slava-shield; IVB=Paleofuego's debris-flow; V=Volcan de Fuego; C1= collapse caldera; C2 =large crater or collapse caldera; C3 and C4 = avalanche calderas related to the MSH-type events.
T h e southern eruptive centre Paleofuegeo (or volcan de Colima )
Like the intermediate cone of Nevado, Paleofuego is an effusive central-vent volcano whose history ended with a gigantic M S H - t y p e eruption. This event was followed by construction
of a new cone, the modern volcan de Fuego. T h e lava flows of Paleofuego extend towards the south (Figs. 3, 4). This volcano, of which only remains its northern side, grew like an adventive, or neighbouring center of Nevado. Although the age of its f~rst outbursts is yet to be defined, it was at least contemporaneous with the upper formations of Nevado It. The destruction of its upper part by a M S H eruption about 10,000 y. ago was responsible for the C4 avalanche caldera. Thus, it was also partly contemporaneous with Nevado III. It was a larger volcano t h a n the intermediate Nevado II, as some of its laves flowed as far as 13 km from the present-day summit. On the edge of C4, the dip of the flows seems to indicate t h a t its summit was somewhat higher t h a n the present cone and probably higher than that of Nevado. It is composed of two-pyroxene andesites, and amphibole appears in the upper lavas. One andesite from the lower part is notable for its basicity (Table 2, CO 35). T h e most differentiated rocks are siliceous andesites exposed in the wall of the C4 caldera. After the M S H event, the whole summit area disappeared, as clearly indicated by the horseshoe-shaped avalanche catdera, 4-5 km wide, opening towards the south. The floor of this amphitheatre dips to the south, from 3150 to 2700 m a.s.1. In the north, the C4 wall is still 300 m high, despite the filling by the lavas from Fuego; in the east, the caldera wall was partly covered by the recent growth of Volcancito and several historic flows (Fig. 4). Judging from its volume - which is more than 10 km 3 and could be as m u c h as 20 k m 3 (Vincent et al., 1983 ) - and its extent of more than 500 km ~, the debris flow is one of the most important known today. Its deposits spread over a fan-shaped area with a radius of 25-35 km and an angle of up to 100 ° . T h e morphology of the debris-flow is distinctive. It has m a n y hummocks including some more t h a n 200 m high. A few pyroclastic sequences, described elsewhere (Vincent et al., 1983 ), were observed at three places where they rest on the debris flow (B1
109 TABLE 2 Selected analyses from the southern centre Fuego lava-flows
Paleofuego lava-flows CO 35
F 19
F6
F 15
F 13
F7
CO 72
F 20
CO 149
Si02 A1203 Fe20~ FeO MgO CaO Na20 K20 TiO2 MnO H20 + HeO
55.90 18.30 5.14 1.59 4.00 7.70 4.40 1.10 1.00 0.11 0.11 0.01
57.20 18.00 3.47 3.18 4.00 6.90 4.25 1.20 0.70 0.12
62.40 16.80 3.30 1.44 2.90 5.80 4.20 1.35 0.75 0.09 0.04 -
64.90 16.40 2.58 1.73 2.50 5.20 4.50 1.25 0.55 0.07 0.07
57.00 16.90 2.59 3.61 6.30 7.20 3.85 1.00 0.85 0.12 0.12 0.08
57.80 17.10 3.78 2.45 4.80 7.30 4.10 1.00 0.75 0.10 -
59.80 17.60 3.74 2.31 3.50 5.50 4.70 1.40 0.70 0.11
62.40 17.10 3.76 1.30 2.90 5.40 4.70 1.45 0.80 0.11
0.01
61.20 17.30 3.30 1.44 3.05 5.70 4.20 1.55 0.75 0.08 0.33 0.74
Total
99.36
99.03
99.64
99.07
99.75
99.62
99.18
99.42 Crater dome 1982
Ash and Scoriae pyr. flows from V. de Fuego 1913 pyr. flows
0.06
B. Montegrande
B. Beltran
F 23
CO 77
CO 32
CO 146
99.92 Scoriaecone
CO 6 F2
F 26
CO 79
Si02 A1203 Fe203 FeO MgO CaO Na20 K20 Ti02 MnO H20* H20-
60.70 17.50 3.29 2.17 3.20 6.00 4.20 1.25 0.65 0.10 0.36 0.13
61.00 17.70 4.30 1.44 3.10 5.80 4.40 1.45 0.60 0.10 0.16
57.30 17.70 3.76 2.74 4.40 6.35 4.40 1.30 0.85 0.11 0.07 0.12
58.50 17.00 3.47 3.18 4.60 6.40 4.00 1.20 0.85 0.12 0.20 -
57.00 15.70 3.54 3.75 7.70 7.30 3.60 0.95 0.85 0.13 -
58.10 16.80 4.86 2.02 4.60 6.70 4.25 1.35 0.70 0.11 0.26 0.09
60.70 16.50 3.19 2.17 4.60 6.00 4.50 1.30 0.70 0.12 -
53.20 14.50 3.95 3.47 8.50 8.00 3.50 2.35 1.10 0.12 0.42 0.32
Total
99.55
100.05
99.10
99.52
100.52
99.84
99.75
99.43
CO 35 to F 13: Paleofuego's magmatic suite; basic andesites to dacites; F 7 to CO 149: andesites from the terminal cone; F 7 and F 20: northern side; CO 72 and CO 149 respectively from Barranca de Montegrande and from the southern slope at 3050 m; F 2 to F 23: ash and scoriae pyroclastic flow deposits from the 1913 eruption; F 2 and F 26: juvenile bombs; F 23 from a co-eruptive aerial pumice layer; CO 77 and CO 79: bombs from the historic St. Vincent-type deposits in B. Montegrande; CO 32: juvenile magma of other historic pyroclastic flow of the same eruptive type in B. Beltran; CO 146: crater dome 1982; CO 6: cinder cone from the northern flank, near Los Depositos. Analyses presented here have been selected from 85 new analyses of the whole complex, available on request.
110
to B3, Fig. 3). Although these deposits have certain characteristics different from those laid down just after the avalanche of Mount St. Helens in 1980 (Voight et al., 1981, 1983), the field relations between these pyroclastic products and the landslide deposits show that they were emplaced at the same time or immediately after the avalanche. According to a C14 age measured for wood from a sedimentary series overlying the debris flow, Luhr and Prestegaard (1985) suggest that the caldera and related debris flow are about 4300 y. old. We obtained a similar age (4730_+ 100 y. B.P.) from similar epiclastic deposits from the right bank of the Arroyo La Lumbra. However, fragments of charcoal collected in a typical pyroclastic layer, at the bottom of the B1 outcrop overlying the debris-flow, are 9370_+400 y. old (C14 analysis No 6139, C.F,R. Gif sur Yvette, France).
The Volcan de Fuego In the case of Nevado, the period following the M S H event was characterized by intense plinian activity.In the southern eruptive structure, the ash and lapiUi air falls cover all the summit area except the active cone. This series, about 40 m thick,had been attributed to an earlier volcano (Paleofuego or V. de Colima) by Demant (1981). Luhr and Carmichael (1982) showed that at least one part of these air falls, the upper 7-8 m, are related to the recent cone of Fuego. However, according to these authors, the cone would be only 4300 y. old.The greatest age we obtained for the plinian deposits is 9300+400 y., approximately the age of the caldera. The air-fall sequences, under the dated layer, are about 15 m thick at a distance of 2 or 3 km from the edge of C4. They show no discontinuity nor paleosoil, suggesting that the greater part, if not the whole of the aerial deposits, is subsequent to the MSH-type event, as in the case of Nevado. Fuego is centered in C4. It is a very asymmetric cone: its rough northern side reaches
about 900 m, whereas its smoother southern slopes (35-40 ° ) are 2000 m in height. It consists of a pile of block lavas, some of which, including those of 1975-1976, ran beyond the bottom of the volcano. These effusive stages alternate with explosive periods. The pyroclastic activity of Fuego is represented both by the products found in the plinian sequences mentioned above and by episodes which produced block and ash flows of the St. Vincent type. These latter, known from the 1913 eruption, are quite numerous. They filled most of the earlier hummocky topography on the southern area, near the foot of the volcano. A 20-30-m thick series of three ash-flow units was observed in the Barranca de Montegrande (Fig. 3); other deposits of similar type originating in historic events had been recognized in the Barranca Beltran. These products can be used as markers to define the eruptive cycles of the Fuego. The two-pyroxene and amphibole andesites represent the more widespread facies of this cone, but there are also opx- and cpx-rich andesites without amphibole and basic andesites containing unstable olivine. Bombs resulting from collapse of the eruptive column commonly have a heterogeneous composition and unstable mineral assemblage. In the field, these products can be related to similar heterogeneous airfall pumices, but the latter were not always emitted during the St. Vincent-type eruptions; an example of this association is given by the 1909 air-fall deposits and the 1913 block-andash flows. Discussion and conclusions Although quite different in the distribution and volume of their products, the two main centres comprising the Colima volcanic complex have had similar evolutions. At an advanced stage of their development each was marked by a huge eruption of MSH type followed by radical changes in their eruptive behavior. The history, prior to the MSH events, appears to be even more complex in Nevado
111
than in Fuego, because of its longer stages of growth over several hundreds thousands years ( Nevado IA, IB and II ). The first period of construction, mainly effusive, led to a great andesitic primitive volcano with a volume of 500-700 km 3. Comparable ancient edifices are known in other major Mexican volcanic complexes (Robin, 1981,1984; Cantagrel et al., 1981; Robin and Cantagrel, 1982). At Nevado de Colima, the initial effusive stage (IA) was related to the evolution and emptying of a shallow magmatic reservoir. It ended with an outburst of dacitic ash flows and important associated air falls. A large collapse caldera resulted from this sudden eruption. Later activity was mainly explosive, because of the growth of domes in the summit area and on the slopes, producing abundant pyroclastic products during the IB period (Los Depositos pyroclastic flows, pelean nudes ardentes from the northern side...). The ancient Nevado ended about 0.2-0.3 m.y. ago with formation of the C2 caldera whose products cannot be clearly identified but were probably related to St. Vincent-type deposits or other pyroclastic flows. One can note a break in the late magmatic evolution of the ancient volcano of Nevado, owing to the emission of heterogeneous lavas and to the occurrence of new andesitic magmas. The intermediate Nevado grew and overflowed C2. In the same way, Paleofuego began to grow, about the time of Nevado II, but whereas the latter was affected by pyroclastic events producing heterogeneous magmas about 0.08 m.y. ago, no phenomenon of this type is seen in the development of Paleofuego. Was Paleofuego formed as the result of one of these reinjections? The basic lavas of this volcano seem to support this hypothesis. Thus, the question is whether there was a connection between the two modern volcanoes, and whether their evolution was related to a single magma chamber or to two different ones. The two modern volcanoes, less voluminous than the common ancient edifice, had comparable, but not synchronous histories: they were
marked by eruptions of the MSH type leading to gigantic landslides and to destruction of their summit areas. Two avalanche calderas, 4-5 km wide, opening towards the East and the South are related to two debris flows, of which Paleofuego's debris flow is among the most voluminous known. These destructive events occured no more than some tens of thousands of years ago in the Nevado, and only 9370_+ 400 years ago in the case of Paleofuego. Although the age of the MSH type eruption is not yet defined at Nevado, it appears to be relatively recent; it rejuvenated the terminal cone of Nevado, which is usually considered to be an old volcano dissected by the erosion. Both centers had intense pyroclastic activity that left numerous plinian air-fall layers. At Nevado, dacitic ash-flows erupted during this stage. The age of the debris-flow deposits from the southern centre (9000-10,000 y.) shows that the greater part, if not all of the summit plinian series surrounding the Fuego, follows the MSH event and thus is reasonably to be attributed to the active cone. As a consequence, the position and the role of the MSH event with respect to the tephra, as they were defined by Luhr and Carmichael (1982), must be reconsidered. A similar evolution appears in the upper Nevado. Moreover, the relations between the two series of air falls corroborate the belief that the transition periods between the Nevado II and Nevado III on the one hand, and between Paleofuego and Fuego on the other, were very close in terms of the overall evolution of the volcanic complex. The terminal cones are characterized by the alternation of episodes of growth and destruction. For Nevado III, the role of the St. Vincenttype eruptions, leading to pyroclastic flows with heterogeneous magmas, remains to be clarified by further investigations and chronological data. At Fuego, these eruptions dominated the explosive periods and played a determining role in the cycles of activity. According to Luhr and Carmichael (1980), the recent activity was characterized by cycles of one hundred years or
112
more. Each had an effusive dynamism with emissions of very homogeneous andesitic flows (60-61% SiO2), followed by a short explosive episode with more basic magma (57-59% SiO2 ) that ended the cycle. For the historic period, which includes three of that cycles, the following factor should be considered: (1) The cataclysmic episodes, in particular those characterized by deposits of the St. Vincent-type, were probably at the beginning of cycles that commonly ended in effusive or extrusive activity, as it did in the case of the flows of 1975-1976 and the present dome. Basic juvenile material released into the shallow chamber could have led to renewed explosive activity,as proposed by Sparks et al. (1977). (2 ) It is not obvious that a cycle necessarily starts with such a sequence. An important plinian eruption giving heterogeneous pumice-rich products could have the same significance. A slighthomogenization isalready apparent in the 1913 St. Vincent-type deposits ifthey are compared to previous products emitted between 1903 and 1909. At the end of each cycle, the magma may have been totallyhomogenized. At Fuego, the frequency of these phases and their duration is an important element in evaluating potential hazards: this is the purpose of our work in progress. Acknowledgments
Fieldwork (1982-1984) was supported by the Programme Interdisciplinaire de Recherches pour la PrSvision et la Surveillance des Eruptions Volcaniques (C.N.R.S.-I.N.A.G.). W e thank Dr. Joss Guerrero, U.N.A. Mexico and Mission Arch~ologique Fran~aise in Mexico for their assistance in the form of 4 W D cars. The proposed chronology is based on K-At ages (L.A. 10 C.N.R.S., Clermont-Ferrand) and C14 ages (C.F.R., Gif sur Yvette). Chemical analyses were made by Mrs. F. Canta$1~ and S. Couturid,D ~ m e n t de Gdologie et MinSralogie,Clermont-Ferrand. W e thank Prof. A.R. McBirney and an unknown reviewer for con-
structive and helpful comments for the elaboration of the final manuscript. References Allan, J.F. and Luhr, J.F., 1982. K-At ages of late Cenozoic alkaline and calk-alkaline lavas of the Colima graben area, 8W Mexico (abstr.). EOS, Trans. Am: Geophys. Union, 63: 1154. Anderson, T. and Flett, J.S., 1903. Report on the eruptions of the Soufri~re in St. Vincent in 1902, and on a visit to Montagne Pelde in Martinique. Philos. Trans. R. Soc. London, 200: 353-553. Arreola, J.M., 1903. The recent eruptions of Colima. J. Geol., 11: 749-761. Cantagrel, J.M., Robin, C. and Vincent, P.M., 1981. Les grandes Stapes d'Svolution d'un volcan anddsitique: l'example du Nevado Toluca (Mexique). Bull. Volcanol., 44: 177-188. Demant, A., 1981. L'axe n~o-volcanique trans-mexicain Etude volcanologique et pStrographique - Significatio gSodynamique. Doctorat d'Etat Thesis, Aix-Marseille Univ,, 259 pp. Luhr, J.F., 1981. Colima: History and cyclicity of eruptions. Volcano News, 7: 1-3. Luhr, J.F. and Carmichael, I.S.E., 1980. The Colima volcanic complex, Mexico - part 1. Post-caldera andesites from volcan Colima. Contrib. Mineral Petrol., 71: 343-372. Luhr, J.F. and Carmichael, I.S.E., 1981. The Colima volcanic complex, Mexico - part 2. Late Quaternary cinder cones. Contrib, Mineral. PetroL, 76: 127-147. Luhr, J.F. and Carmichael, I.S.E., 1982. The Colima volcanic complex, Mexico - Part 3, Ash and scoria fall deposits from the upper slopes of volcan Colima. Contrib. Mineral. Petrol., 80: 262-275. Luhr, J.F. and Prsstegaard, K., 1985. Caldera formation at Volcan ColimA~Mexico: A lage Mount St. Helens-Type Avalanche event 4,300 years ago (Abstr.). EOS, Trans. Am. Geophys. Union, 66: 411. Medina Martinez, F., 1983. Analysis of the eruptive history of the Volcan de Colima, Mexico (1560-1980). Geofis. Int., 22: 157-178. Mooser, F., 1961. Los volcanes de Colima. Univ. Nac. Mexico, Geol. Bol., 61: 49-71. Ordonez, E., 1897, Les volcans de Colima et Ceboruco. Mere. Soc. Cient. Antonio Alzate, tomo II, Mexico, pp. 325-329. Robin, C., 1981. Relations volcanologie-magnmtologiegc~dynamique: application au passage entre volcanismes alcalin et and~sitique dans le Sud mexicain (Axe trans-mexicain et Province alcaline orientale). Doctorat d'Etat Thesis, Publ. Ann. Univ, Clermont-Ferrand, 2nd Ser.,31, fasc.70, 503 pp.
113
Robin, C., 1984. Le Volcan Popocateptl (Mexique): structure, ~volution p6trologique et risques. Bull. Volcanol., 47: 1-23. Robin, C. and Boudal, C., 1984. Une 6ruption remarquable par son volume: l'~v~nement de type Saint-Helens du Popocatepetl (Mexique). C. R. Acad. Sci. Paris, 299: 881-886. Robin, C. and Boudal, C., 1987. A gigantic Bezimianny type event at the beginning of modern V. Popocatepetl. J. Volcanol. Geotherm. Res., 31: 113-128. Robin, C. and Cantagrel, J.M., 1982. Le Pico de Orizaba (Mexique): structure et ~volution d'un grand volcan and~sitique complexe. Bull. Volcanol., 45 (4): 299-316. Robin, C., Camus, G., Cantagrel, J.M., Gourgaud, A., Mossand, Ph. and Vincent, P.M., 1983. The Colima volcanic complex (Mexico): Main eruptive units (abstr.). 2nd E.U.G. Meeting, Strasbourg, Terra Cognita, 3: 153. Robin, C., Camus, G., Cantagrel, J.M., Gourgaud, A., Mossand, Ph., Vincent, P.M., Aubert, M., Dorel, J. and Murray, J.B., 1984. Les volcans de Colima (Mexique). Bull. P.I.R.P.S.E.V.-C.N.R.S.-I.N.A.G., 87:98 pp. Shepard, J.B., Aspinall, W.P., Rowley, K.C., Pereira, J., Sigurdsson, H., Fiske, R.S. and Tomblin, J.F., 1979. The eruption of Soufri~re volcano, St. Vincent April-June 1979. Nature, 282: 24-28. Siebert, L., 1984. Large volcanic debris avalanches: char-
acteristics of source areas, deposits, and associated eruptions. J. Volcanol. Geotherm. Res., 22: 163-197. Sparks, R.S.J., Sigurdsson, H. and Wilson, L., 1977. Magma mixing: a mechanism for trigering acid erplosive eruptions. Nature, 267: 315-318. Vincent, P.M., Camus, G., Cantagrel, J.M., Mossand, Ph., Robin, C. and Gourgaud, A., 1983. Gigantic debris flow and blast deposits from Colima Volcano (Mexico) (abstr.). 2nd E.U.G. Meeting, Strasbourg, Terra Cognita, 3: 154. Voight, B., Glicken, H., Janda, R.J. and Douglass, P.M., 1981. Catastrophic rockslide avalanche of May 18. In: P.W. Lipman and D.R. Mullineaux (Editors), The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geol. Surv., Prof. Pap., 1250: 347-378. Voight, B., Janda, R.J., Glicken, H. and Douglass, P.M., 1983. Nature and mechanics of the Mount St. Helens rockslide-avalanche of 18 May 1980. Geotechnique, 33: 243-273. Waitz, P., 1920. "Nubes ardientes" observadas en las erupciones del Jorullo (1859), Ceboruco (1870), y del volcan de Colima {1913). Mem. Soc. Cient. Antonio Alzate, Tomo 37, Mexico, pp. 267-277. Waitz, P., 1936. Datos historicos y bibliograficos acerca del volcan de Colima. Mem. Soc. Cient. Antonio Alzate, Mex., 53: 349-384.
99
ERUPTIVE HISTORY OF THE COLIMA VOLCANIC COMPLEX (MEXICO) CLAUDE ROBIN, PHILIPPE MOSSAND, GUY CAMUS, JEAN-MARIE CANTAGREL, ALAIN GOURGAUD and PIERRE M. VINCENT Centre de Recherches Volcanologiques, Universit~ Clermont H et CNRS UAIO, 5, rue Kessler, 63038 Clermont-Ferrand, France (Received September 27, 1985; revised and accepted June 11, 1986)
Abstract Robin, C., Mossand, P., Camus, G., Cantagrel, J.-M., Gourgaud, A. and Vincent, P.M., 1987. Eruptive history of the Colima volcanic complex (Mexico). J. Volcanol. Geotherm. Res., 31: 99-113. The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nudes ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego. The oldest caldera, C 1 ( 7-8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3-3.5 km wide), ended the evolution of Nevado I. Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4-5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters. The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls. The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II-III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.
100
Introduction The andesitic volcanic complex of Colima is located in the southern part of the N - S Colima graben, in the western part of the Trans-Mexican Volcanic Belt. Two main eruptive centres form this complex: the Nevado de Colima (19033'48" N, 103°36'30" W; 4260 m a.s.l.), a composite volcano without historic activity, and the Volcan de Fuego, or Volcan de Colima (19 ° 30'40" N, 103 ° 37' W; about 3850 m a.s.l. ), located in the caldera of a former edifice built on the southern side of the Nevado. The two summits, only 5 k m from each other, tower above an andesitic shield, slightly elongated N-S, of 25 by 30 kin, whose base is around 1000 m a.s.l.(Fig. 1 ).
%.
/O~',,Vr V.!
Fuego has been the most active of the Mexican volcanoes during the historic period. Its activityhas been described by Ordonez (1897), Arreola (1903), Waltz (1920, 1936), and most recently by Luhr and Carmichael (1980) and Medina (1983). The descriptions available for the last four centuries (Luhr, 1981 ) enable us to distinguish a succession of long constructive stages of the cone followed by short destructive episodes of its summit by violent outbursts. During the last decades, the cone produced block-lava flows in 1961-1962, 1975-1976 and 1981-1982. Cantaro, the volcano thought to be the oldest in this area, is next to the northern flanks of the Colima group (Fig. i). Some of the products of this volcano interfingerwith those from
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,% %
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Fig. 1. Location of the Colima volcanic complex in the Trans-Mexican Volcanic Belt (1A) and in the Colima graben (1B). 1 = Volcan Sanganguey; 2 = Ceboruco; 3 = Volcan de Tequila; 4 = Colima volcanic complex; 5 = La Primavera calderic structure; 6=V. Tancitaro; 7=V. Paricutin; 8 = JoruUo; 9=Nevado de Toluca; 10= Ajusco; 11 = Iztaccihuatl; 12= Popocatepetl; 13 = Malinche; 14 = V. Pico de Orizaba; 15 = Los Humeros caldera; 16 = V. Cofre de Perote; 17= PMma Sola volcanic complex; 18 = V. San Martin
101 the Nevado along the road joining Ciudad Guzman to Venustiano Carranza. Basaltic cones, the most famous of which is Volcan Apaxtepec, are on the northern edge of the andesitic complex. Most of them, studied by Luhr and Carmichael (1981), belong to the high-K alkaline series; only three are calc-alkalic. The studies by Luhr and Carmichael (1980, 1982 ) only deal with the recent stage of the southern structure (Volcan de Fuego). Previous reconstructions of the two eruptive centres were attempted by Mooser (1961) and Demant (1981). However, the detailed history of the whole volcanic complex, the definition of the successive stages of its evolution and their connection with the magmatic and eruptive behaviours had not yet been studied. The ancient Nevado
Structure and extent The primitive volcano - Nevado I - represents the main part of Nevado de Colima (Fig. 2). The wall of the oldest caldera C1, though eroded, can be detected in the morphology of the western, northern and southwestern slopes of this edifice. On the basis of this structure, two periods of construction can be distinguished: IA and IB. This elliptical caldera reaches 8 km in length, and the primitive composite volcano is about 25 km in diameter. The latter consists mainly of massive lava flows and domes located near the caldera rim or scattered on its lower slopes. On the outskirts, the lava flows grade into a gently dipping conglomeratic formation, well developed towards the west and the southeast, the Atenquique formation (Mooser, 1961). The evolution of Nevado I ended with the collapse of a second and smaller caldera, C2, the wall of which is clearly exposed on the western side, especially at Cerro el Aguila (Fig. 2).
Nature of the lavas and pyroclastic products The Nevado IA (ante C 1 ) is characterized by plagioclase-phyric andesites (57-59% SiO2),
with ortho- and clinopyroxenes, and without amphibole. More siliceous but less porphyric lavas (61-62% Si02) with rare amphiboles are also present. The Nevado IB lavas are siliceous andesites and very porphyric amphibole-rich two-pyroxene dacites. One can relate the C1 caldera to a major part of thick pyroclastic units making up a formation called informally the "yellow ashes" (Robin et al., 1984). Indeed, the Cantaro-Nevado complex is associated with thick ash deposits of regional extent which cover at least 600 km 2 towards the north and the west. In numerous places the upper part of the deposits has been reworked. In such formations, exact estimates of what belongs to each centre are not easy to make. It is obvious that in the northwestern area, around Venustiano Carranza, yellow ashes must be related to the Cantaro, which is also topped by a caldera and domes. However, on the northwestern slopes of the Nevado and towards the west, between Telcruz and Zapotitlan, the "yellow ashes" are associated with ash flows running down the Nevado (Fig. 2). These "yellow ashes" represent an u n c o m m o n pyroclastic facies, made up of the following: (a) Ash-flow deposits: the most characteristic outcrops can be observed on the northwestern slopes, channeled into andesites of the stage IA. Above Telcruz, the ash-flow series are 50-70 m thick and slightly indurated. On the lower slopes, these products are channeled into deep canyons cut into the Atenquique series. Similar pyroclastic flows have been observed on the slopes of Cantaro. (b) Ash and pumice air-fall deposits: these can be spotted everywhere, even as far as 30 km from the summits. They are 10-15 m thick at a distance of 15 km to the northwest of the New ado. These air-fall deposits include several units that reach thicknesses of meters. At the boundary between the two edifices it was not possible to distinguish between the ashes of Nevado and those of Cantaro. In some places, for example 5 km west of Los Depositos, basic ashes, or even
102
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Fig. 2. Sketch map ahowing theegtent and structure of the Colima volcanic complex prior to the two Mount St. Helens-type events. Legend: 1 = volcanic series from the basement; 2 = limits of the effusive primitive edifice, Nevado IA; 3 = extent of the conglomeratic dvposits (Atanquique series ); 4 = directions followed by the main ash-flow deposits from the end of period IA (related to C1); 5=extent of the "yellow ashes" related to the Nevado I and to V. Cantaro: ash-flow depositsj air-fall tephra and reworked materials; 6 = L o s Depositos pyroclastic flows related to terminal stages of Nevado I (IB period); 7 = dacitic domes and a~ociated pelean n u ( ~ ardantes; 8 = directions followed by the Los Garcia and the Los Mazos pyroclestic flows of St. Vincent-type ( h e t e ~ mrs), possibly related to C2; 9=laves of Nevado II; 10=pyroclastic flows with hetervgeneous bombs and scoria related to Nevado II; 11 =inferred extent of the Paleofuego; 12=C] and C2 calderic structures; 13 = scoria cone CO 6.
103 strombolian scoriae, emitted by nearby basaltic cinder cones, are interbedded with the "yellow ashes" sequences (location CO6, Fig. 2).
Age of the Nevado I The age of a lava-flow from the middle part of the northwestern flank is 0.53+0.1 m.y. Another flow was dated at 0.35 + 0.05 m.y. These results agree well enough with those obtained by Allan and Luhr (1982), who suggest ages going back to about 1.0 m.y. in the Cantaro basement, and with other unpublished K-At data from this volcano (Cantagrel et al., in prep. ).
The Atenquique conglomeratic series This series is a detrital formation whose elements are exlusively volcanic. Its volume was estimated to be more than 300 km 3 (Luhr and Carmichael, 1980). These sequences are an accumulation of gently dipping strata ( < 5 ° ), up to several metres thick, within a total thickness that in many places exceeds 200 m. The irregular grain-size layers consist of rounded, blunted or angular blocks. The fine-textured strata are composed of beige, pink, and grey ash. An andesitic block from this series was dated at 0.38 + 0.10 m.y. The pyroclastic flows from Nevado I follow deep barrancas cut into these conglomerates. This age indicates the early deposition of most of the Atenquique series, although another block yielding an age of 0.26+0.06 m.y., shows that some of these deposits were contemporaneous with Nevado II. On the other hand, the relations between some parts of this formation and recent clastic deposits (of debris-flow or pyroclastic-flow type; see below) seem to indicate that the deposition of the Atenquique conglomeratic series continued at a declining rate until more recent times.
Other pyroclastic formations related to the Nevado I Although these formations require more work to determine their relationships, we can attribute them to the ancient Nevado, and quite probably to the IB period: (1) Ash-flow deposits with vesiculated dacitic blocks and rare bombs partly cover the northern slopes near Los Depositos. They have an average thickness of 10 m. These amphibole-rich pyroclastic units (Table 1, CO 67) probably originated from the destruction of domes that overflowed the C1 caldera on this side. (2) Back-falling nudes deposits exposed at Los Mazos and near E1 Jasmin (in the eastern and northwestern areas, Fig. 2) show a large proportion of dark-grey bombs (up to 30%) with a glassy crust and xenoliths scattered in an ashy matrix. These pyroclastic flow deposits resemble those of Saint-Vincent (Anderson and Flett, 1903; Shepard et al., 1979), and in the terminal volcanic cones of other great Mexican volcanoes (Robin, 1981; Robin and Cantagrel, 1982; Robin and Boudal, 1987). The structure and the mineralogical composition of the bombs show a more or less heterogeneous magma, one component of which is very basic with unstable olivine and clinopyroxene. With an average SiO2 content of 59% (Table 1, CO 66), these pyroclastic-flows mark a break in the petrological evolution of the differentiated suite. The C2 caldera, about 3 km in diameter, is not clearly related to a well defined pyroclastic unit. Such a structure may be a large crater resulting from strong eruptions of the Saint-Vincent-type (related to the Los Mazos and E1 Jasmin deposits?), as well as a collapse caldera associated with ignimbrites. (3) Pelean nudes ardentes deposits of dacitic composition (64% SiO2 on average) also lie on the northern side. Their location and stratigraphic position allow us to relate their emplacement to a series of neighbouring domes (Fig. 2).
104 TABLE 1 Selectedanalyses from the Nevado complex (northern centre) Lava flow
Lava flows IA period
Pyr. flow
Pelean Pyr. Flows
IB
Depositos
Ash and scoriae pyr. flows from Nevado I (Los Mazos) heterogeneous m-omas C066
Los
CO 70
CO 40
CO3
CO 17
CO 39
CO 17
CO 117
CO 67
SiO~ Al2Oa Fe~03 FeO MgO CaO Na~O K20 Ti02 MnO H20 + H20-
57.50 17.70 5.63 1.01 4.30 7.40 3,90 1.40 0.55 0.11 0.10
61.00 17.80 4.42 1.43 2.10 5.40 4.90 1.35 0.90 0.08 0.04 -
61,30 18.20 4.31 1.16 2.00 5.00 4.85 1.20 0.60 0.09 0.19 0.27
61.10 17.50 3.89 2.17 3.40 5.70 4,05 1.10 0.85 0.12 0.47 0.10
63.20 17.00 4.82 0.43 1.80 4.80 4.55 1.45 0.80 0.08 0.14 -
63.50 17.00 3.26 1.16 2.00 4.80 4.50 1.70 0.50 0.09 1.03 0.14
63.90 17.30 2.70 1.44 1.60 4.00 4.40 1.75 0.55 0.10 0.31 0.17
62.70 16.90 3.60 1.44 1.90 4.70 4.45 1.60 0.55 0.09 1.00 0.09
59,00 16.60 4.58 1.73 4.50 6.90 4,15 1.45 0.60 0.09 0.43 0.22
59,70 17.30 2.91 2.74 3.50 6.00 4.25 1.50 0.55 0.08 0.32 0.20
Total
99.60
99.42
99.17
100.45
99.07
99.98
99.12
99.02
100.25
99:05
Conglomeratic block CO
Nevado II lava flows
Nevado III lava flow
Ash and scoriae pyr.flows from Nevado HI (Fuego
Ash and scoriae flow (heterogeneous magmas) -~0.08 m.y.
llB CO 46
CO 55
Fe203 FeO MgO CaO Na20 K20 TiO2 MnO H20 + H20-
64.20 16.80 3.52 0.43 1.55 4.55 4.30 2.10 0.60 0.07 1.63 0,.14
58.90 17.20 4.94 1.59 4.90 6.50 3.85 1.45 1.05 0.10 0.13 0.11
60.40 17.50 3.51 2.60 4.10 5.80 4.10 1.30 0.80 0.11 0.07
Total
99.89
100.72
100.29
SiO~
track; Los Garcia)
CO 57 CO 45
AI203
CO 68
CO 56
C09
CO10
CO 24
CO 25
59.50 18.10 5.11 1.16 3.60 6.10 4.10 1.50 0.85 0.10 0.04
59.00 17.10 3.88 2.45 3.80 5.90 4.25 1.45 0.60 0,09 0.27 0.25
59.90 17.30 3.54 " 2.31 3.40 6.00 4.25 1.45 0,50 0.10 0.30 0.01
60.80 17.60 3.19 2.17 3.10 5.70 4.55 1.45 0.70 0.11 0.72 0.02
59.40 16.50 5.40 1.44 4.10 5.50 4.00 1.90 0.80 0.10 0.14 0.14
58.90 17.15 3.86 2.02 3.50 6:20 4.25 1.45 0.60 0.09 0.98 0.05
60.50 17.00 4.34 1.59 3.15 5.70 4.40 1.35 0.90 0.09 1.23 -
100.16
99,04
99.06
100.11
99.42
99.05
100.25
CO 70 to CO 39: two-pyroxane andesite and siliceous andesites from the primitive volcano; CO 17 and CO 117: amphibole dacites from pelaan pyroclastiC flow dIR~its; CO 67: siliceous andesite from the Los Depositos pyroclastic flows;CO 66 and CO 68: juvenile heterogeneous ~ ~ o f ash and scoriae p ~ i c flows (outcrops of Los Mazos a n d ~ l Jasmin); CO lIB: Amphibole dacite block from the Atenquiqus series; CO 45 to CO 56: andesitic lava flows from Nevado II;CO 9 and CO 10: dark and light bombs from h e t e ~ ash and p y r i t i c flow in Barmnca Platanos; CO 57: summit lava flow from Nevado III; CO 24 and CO 25: dark and light bombs from the Los Garcia pyroclastic flows (related to Nevado III).
105
The modern Nevado
Lava-flows and pyroclastic products from Nevado H (or Intermediate Nevado) Mooser (1961) was the first to point out the possible existence of two calderas at the top of the Nevado. The composite structure corresponds to the superposition of the C2 and C3 calderas. C2 ended the evolution of the primitive volcano. A new effusive cone, here called the intermediate Nevado, filled C2. Its products overflowed the C2 rim at least towards the southwest, north and northeast. Towards the northwest, the flows from this volcano were stopped by the C2 wall at Cerro el Aguila (Fig. 2). The Nevado II was not very extensive and only a small part of it remains: this edifice was destroyed by a Mount St. Helens-type event whose consequence was the formation of the summit amphitheatre (or avalanche caldera C3 ), 4-5 km wide, opening to the east. Two KAr ages obtained on lava-flows which overflowed C2 towards the southwest and on a lava flow from the C3 wall, show that the age of this volcano is in order of 150,000-200,000 y. Compared to the andesites and dacites of the older edifice, the lavas of Nevado II have less silica, like the back-falling nu6es deposits mentioned above. They are relatively basic (58-59% SiO2) with two pyroxenes (cpx/opx = 5) and rare amphibole microcrysts. The groundmass is locally heterogeneous. These characteristics clearly distinguish the series of Nevado II from the older andesitic to dacitic suite of Nevado I (Table 1A). A second group of back-falling ash-flow deposits with heterogeneous bombs can be distinguished from the older ones: (1) Several flow-deposits of this type outcrop in the upper part of the Barranca Platanos (location, Fig. 2 ). Their stratigraphic position suggests a relatively ancient age. (2) In the Atenquique river, a second outcrop of St. Vincent-type deposits yielded an age of 0.08 + 0.03 m.y.
(3) At Puerto La Calle, products of this type were stopped by the C2 wall (Fig. 3 ). In the Barranca Platanos, the avalanche deposits related to the C3 caldera formation of debris-flow type, see below - are younger than these amphibole-rich pyroclastic units of andesitic composition (CO 9). This highly explosive volcanism can thus be related to the Nevado II. Pyroclastic deposits of pelean type and autobrecciated lava-flows are present on the northern slope between 2700 and 3000 m a.s.1. They too belong to Nevado II. As in the case of the lava-flows or domes exposed in the C3 wall on this side, they are andesitic with opx, cpx, amphibole and unstable zoned plagioclases. Other deposits, one quite visible along the Fuego track (alt. 1600 m a.s.1.), and two ash and pumice flows with scoria bombs located near Los Garcia (Fig. 3), could also be related to this explosive activity of Nevado II. Nevertheless, these two units are covered only by slight aerial deposits and thus are more likely to be associated with Nevado III.
The Mount St.-Helens-type eruption of Nevado The C3 caldera is related to a huge avalanche resulting from an eruption similar to that of Mount St. Helens (MSH) in May 1980 (Also called a Bezymianny-type event; Siebert, 1984). The debris-flow (DF) followed channels towards the east, then turned towards the southeast about 20 km from the summit (Fig. 3). Near the avalanche caldera, these DF deposits are made up of piles of large fractured and tilted slabs, each of which includes several flows whose disposition was not disturbed during the slide and still preserves their initial stratigraphy. On its slopes, for example in the upper part of the Atenquique Barranca, the pile of slabs reaches a thickness of 500 m. In the downhill part, the deposits are more finely crushed: the strong fracturing yields blocks up to a centimeter to decimeter in size, in a pulverized dusty matrix. In the distal part, the
106
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landslide partly covers the slopes of Paleofuego, the basal edifice of the southern eruptive centre (see below). Between San Marcos and Atenquique much of this debris flow turned into lahars. A gradual transformation can be seen between these deposits and the conglomerates making up the Atenquique series. The MSH-type eruption was followed by intense explosive activity that deposited air-fall layers - 25 m thick on the whole - over the entire summit area of Nevado. Ash, scoriae, pumice and pumiceous or dense andesitic lapilli, are piled up in layers from several centimeters to more than a meter thick. Numerous paleosoils are found interbedded in these layers. On the southern slopes of Nevado, at the boundary between Nevado II and Paleofuego, these plinian sequences are overlaid by another series of air falls, emitted by the southern eruptive centre, which has already been studied by Luhr and Carmichael (1982). The lower part of the Nevado air-fall deposits is older than Fuego's plinian series. However, the upper layers of Nevado's air falls interfinger with those of Fuego. The terminal cone of Nevado, or Nevado III (Figs. 3-4) This small cone, about 3 km in diameter and 800 m high, is composed of thick, viscous and steeply dipping lava flows ( 30 ° to 45 ° ). On the
outerflanks of the cone, some of these lava flows can be observed overlying the plinian sequences, close to the opening of the C3 caldera. The top of this volcano is near the southwestern wall of C3. Some andesitic flows reached the northern caldera wall and were slightly deflected towards the East. (Fig. 4). The lavas are poorly crystallized andesites (about 59% SiO2; Table 1, CO 56) with unstable sodic plagioclase, cpx, opx, and relic crystals of amphibole. The present summit of Nevado preserves the remains of a crater; it corresponds to the summit of the last edifice which, contrary to what was generally assumed, has been only slightly eroded. Nevado III was probably short-lived, in comparison to Nevado I and Nevado II. It was probably active only a few thousand years. In addition to the post-C3 plinian air falls starting with the new constructional stage, and the St. Vincent-type pyroclastic flows outcropping on the trail to Fuego and near Los Garcia, two other units can be related to the late stages of Nevado: white ash flows with decimetricsized dacitic pumice overlie the debris flow in the southwest, in the barrancas Platanos and Atenquique (symbol No 13, Fig. 3 ). Twelve km from the summit, their thickness ranges from 10 to 25 m. It is likely that these dacitic products were emitted prior to construction of the cone by andesitic lava flows, probably a short time after the MSH-type event, as was the case at Popocatepetl (Robin and Boudal, 1987 ), and at Mount St. Helens.
Fig. 3. Generalized map of the Colima volcanic complex (see also Fig. 2 for major volcanic events prior to the Mount St. Helens-type eruptions). 1 = andesites from the basement; 2 = extent of the lava-flows of Nevado I (major pyroclastic events related to Nevado I are shown in Fig. 2); 3 = conglomerates; 4 = Nevado II (andesites); see also Fig. 2 for the distribution of pyroclastic flows related to this edifice; 5 =andesite lava flows of V. Paleofuego; 6 =debris-flow deposits resulting from the MSH events of Nevado II and Paleofuego (largely laharic in their distal parts) ; 7 = outcrops of pyroclastic deposits (surge type) related to the Paleofuego's MSH event; 8=Nevado III lava-flows (andesites); 9--Volcan de Fuego (see Fig. 4 for details); 10 = ash and scoria pyroclastic flow deposits (heterogeneous magmas) related to Fuego; 11 = cinder cones, 12 ---dacitic domes and related pelean pyroclastic flow deposits; 13 =emplacement of ash and pumice deposits related to Nevado III; 14 = scattered outcrops of St. Vincent-type ash-flow deposits; a = related to Nevado II (La Calle, B. Platanos and Atenquique); b = related to the Nevado I I I ( Los Garcia, Fuego track); 15 = approximate extent of the plinian series; a ="yellow ashes" related to C 1; b = summit air-fall deposits from the Nevado (transition Nevado I-Nevado II); c = plinian series from the Fuego; 16 = collapse caldera (C 1 ); large crater or collapse caldera (C2); avalanche calderas (C3 and C4).
108
Fig. 4. Generalizedmap of the slimmlt area of the Colima volcanic complex./=primitive volcano (Nevado IA and IB ); IIA = V. Nevado II ( mainly effusive); l i b = Nevado's debris-flow; IIIA and IIIB=s,mmit cone of the Nevado, probably two phases of building (separated by one of the majorpyroclasticevents?); IVA = Paleof~Jego'slava-shield; IVB=Paleofuego's debris-flow; V=Volcan de Fuego; C1= collapse caldera; C2 =large crater or collapse caldera; C3 and C4 = avalanche calderas related to the MSH-type events.
T h e southern eruptive centre Paleofuegeo (or volcan de Colima )
Like the intermediate cone of Nevado, Paleofuego is an effusive central-vent volcano whose history ended with a gigantic M S H - t y p e eruption. This event was followed by construction
of a new cone, the modern volcan de Fuego. T h e lava flows of Paleofuego extend towards the south (Figs. 3, 4). This volcano, of which only remains its northern side, grew like an adventive, or neighbouring center of Nevado. Although the age of its f~rst outbursts is yet to be defined, it was at least contemporaneous with the upper formations of Nevado It. The destruction of its upper part by a M S H eruption about 10,000 y. ago was responsible for the C4 avalanche caldera. Thus, it was also partly contemporaneous with Nevado III. It was a larger volcano t h a n the intermediate Nevado II, as some of its laves flowed as far as 13 km from the present-day summit. On the edge of C4, the dip of the flows seems to indicate t h a t its summit was somewhat higher t h a n the present cone and probably higher than that of Nevado. It is composed of two-pyroxene andesites, and amphibole appears in the upper lavas. One andesite from the lower part is notable for its basicity (Table 2, CO 35). T h e most differentiated rocks are siliceous andesites exposed in the wall of the C4 caldera. After the M S H event, the whole summit area disappeared, as clearly indicated by the horseshoe-shaped avalanche catdera, 4-5 km wide, opening towards the south. The floor of this amphitheatre dips to the south, from 3150 to 2700 m a.s.1. In the north, the C4 wall is still 300 m high, despite the filling by the lavas from Fuego; in the east, the caldera wall was partly covered by the recent growth of Volcancito and several historic flows (Fig. 4). Judging from its volume - which is more than 10 km 3 and could be as m u c h as 20 k m 3 (Vincent et al., 1983 ) - and its extent of more than 500 km ~, the debris flow is one of the most important known today. Its deposits spread over a fan-shaped area with a radius of 25-35 km and an angle of up to 100 ° . T h e morphology of the debris-flow is distinctive. It has m a n y hummocks including some more t h a n 200 m high. A few pyroclastic sequences, described elsewhere (Vincent et al., 1983 ), were observed at three places where they rest on the debris flow (B1
109 TABLE 2 Selected analyses from the southern centre Fuego lava-flows
Paleofuego lava-flows CO 35
F 19
F6
F 15
F 13
F7
CO 72
F 20
CO 149
Si02 A1203 Fe20~ FeO MgO CaO Na20 K20 TiO2 MnO H20 + HeO
55.90 18.30 5.14 1.59 4.00 7.70 4.40 1.10 1.00 0.11 0.11 0.01
57.20 18.00 3.47 3.18 4.00 6.90 4.25 1.20 0.70 0.12
62.40 16.80 3.30 1.44 2.90 5.80 4.20 1.35 0.75 0.09 0.04 -
64.90 16.40 2.58 1.73 2.50 5.20 4.50 1.25 0.55 0.07 0.07
57.00 16.90 2.59 3.61 6.30 7.20 3.85 1.00 0.85 0.12 0.12 0.08
57.80 17.10 3.78 2.45 4.80 7.30 4.10 1.00 0.75 0.10 -
59.80 17.60 3.74 2.31 3.50 5.50 4.70 1.40 0.70 0.11
62.40 17.10 3.76 1.30 2.90 5.40 4.70 1.45 0.80 0.11
0.01
61.20 17.30 3.30 1.44 3.05 5.70 4.20 1.55 0.75 0.08 0.33 0.74
Total
99.36
99.03
99.64
99.07
99.75
99.62
99.18
99.42 Crater dome 1982
Ash and Scoriae pyr. flows from V. de Fuego 1913 pyr. flows
0.06
B. Montegrande
B. Beltran
F 23
CO 77
CO 32
CO 146
99.92 Scoriaecone
CO 6 F2
F 26
CO 79
Si02 A1203 Fe203 FeO MgO CaO Na20 K20 Ti02 MnO H20* H20-
60.70 17.50 3.29 2.17 3.20 6.00 4.20 1.25 0.65 0.10 0.36 0.13
61.00 17.70 4.30 1.44 3.10 5.80 4.40 1.45 0.60 0.10 0.16
57.30 17.70 3.76 2.74 4.40 6.35 4.40 1.30 0.85 0.11 0.07 0.12
58.50 17.00 3.47 3.18 4.60 6.40 4.00 1.20 0.85 0.12 0.20 -
57.00 15.70 3.54 3.75 7.70 7.30 3.60 0.95 0.85 0.13 -
58.10 16.80 4.86 2.02 4.60 6.70 4.25 1.35 0.70 0.11 0.26 0.09
60.70 16.50 3.19 2.17 4.60 6.00 4.50 1.30 0.70 0.12 -
53.20 14.50 3.95 3.47 8.50 8.00 3.50 2.35 1.10 0.12 0.42 0.32
Total
99.55
100.05
99.10
99.52
100.52
99.84
99.75
99.43
CO 35 to F 13: Paleofuego's magmatic suite; basic andesites to dacites; F 7 to CO 149: andesites from the terminal cone; F 7 and F 20: northern side; CO 72 and CO 149 respectively from Barranca de Montegrande and from the southern slope at 3050 m; F 2 to F 23: ash and scoriae pyroclastic flow deposits from the 1913 eruption; F 2 and F 26: juvenile bombs; F 23 from a co-eruptive aerial pumice layer; CO 77 and CO 79: bombs from the historic St. Vincent-type deposits in B. Montegrande; CO 32: juvenile magma of other historic pyroclastic flow of the same eruptive type in B. Beltran; CO 146: crater dome 1982; CO 6: cinder cone from the northern flank, near Los Depositos. Analyses presented here have been selected from 85 new analyses of the whole complex, available on request.
110
to B3, Fig. 3). Although these deposits have certain characteristics different from those laid down just after the avalanche of Mount St. Helens in 1980 (Voight et al., 1981, 1983), the field relations between these pyroclastic products and the landslide deposits show that they were emplaced at the same time or immediately after the avalanche. According to a C14 age measured for wood from a sedimentary series overlying the debris flow, Luhr and Prestegaard (1985) suggest that the caldera and related debris flow are about 4300 y. old. We obtained a similar age (4730_+ 100 y. B.P.) from similar epiclastic deposits from the right bank of the Arroyo La Lumbra. However, fragments of charcoal collected in a typical pyroclastic layer, at the bottom of the B1 outcrop overlying the debris-flow, are 9370_+400 y. old (C14 analysis No 6139, C.F,R. Gif sur Yvette, France).
The Volcan de Fuego In the case of Nevado, the period following the M S H event was characterized by intense plinian activity.In the southern eruptive structure, the ash and lapiUi air falls cover all the summit area except the active cone. This series, about 40 m thick,had been attributed to an earlier volcano (Paleofuego or V. de Colima) by Demant (1981). Luhr and Carmichael (1982) showed that at least one part of these air falls, the upper 7-8 m, are related to the recent cone of Fuego. However, according to these authors, the cone would be only 4300 y. old.The greatest age we obtained for the plinian deposits is 9300+400 y., approximately the age of the caldera. The air-fall sequences, under the dated layer, are about 15 m thick at a distance of 2 or 3 km from the edge of C4. They show no discontinuity nor paleosoil, suggesting that the greater part, if not the whole of the aerial deposits, is subsequent to the MSH-type event, as in the case of Nevado. Fuego is centered in C4. It is a very asymmetric cone: its rough northern side reaches
about 900 m, whereas its smoother southern slopes (35-40 ° ) are 2000 m in height. It consists of a pile of block lavas, some of which, including those of 1975-1976, ran beyond the bottom of the volcano. These effusive stages alternate with explosive periods. The pyroclastic activity of Fuego is represented both by the products found in the plinian sequences mentioned above and by episodes which produced block and ash flows of the St. Vincent type. These latter, known from the 1913 eruption, are quite numerous. They filled most of the earlier hummocky topography on the southern area, near the foot of the volcano. A 20-30-m thick series of three ash-flow units was observed in the Barranca de Montegrande (Fig. 3); other deposits of similar type originating in historic events had been recognized in the Barranca Beltran. These products can be used as markers to define the eruptive cycles of the Fuego. The two-pyroxene and amphibole andesites represent the more widespread facies of this cone, but there are also opx- and cpx-rich andesites without amphibole and basic andesites containing unstable olivine. Bombs resulting from collapse of the eruptive column commonly have a heterogeneous composition and unstable mineral assemblage. In the field, these products can be related to similar heterogeneous airfall pumices, but the latter were not always emitted during the St. Vincent-type eruptions; an example of this association is given by the 1909 air-fall deposits and the 1913 block-andash flows. Discussion and conclusions Although quite different in the distribution and volume of their products, the two main centres comprising the Colima volcanic complex have had similar evolutions. At an advanced stage of their development each was marked by a huge eruption of MSH type followed by radical changes in their eruptive behavior. The history, prior to the MSH events, appears to be even more complex in Nevado
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than in Fuego, because of its longer stages of growth over several hundreds thousands years ( Nevado IA, IB and II ). The first period of construction, mainly effusive, led to a great andesitic primitive volcano with a volume of 500-700 km 3. Comparable ancient edifices are known in other major Mexican volcanic complexes (Robin, 1981,1984; Cantagrel et al., 1981; Robin and Cantagrel, 1982). At Nevado de Colima, the initial effusive stage (IA) was related to the evolution and emptying of a shallow magmatic reservoir. It ended with an outburst of dacitic ash flows and important associated air falls. A large collapse caldera resulted from this sudden eruption. Later activity was mainly explosive, because of the growth of domes in the summit area and on the slopes, producing abundant pyroclastic products during the IB period (Los Depositos pyroclastic flows, pelean nudes ardentes from the northern side...). The ancient Nevado ended about 0.2-0.3 m.y. ago with formation of the C2 caldera whose products cannot be clearly identified but were probably related to St. Vincent-type deposits or other pyroclastic flows. One can note a break in the late magmatic evolution of the ancient volcano of Nevado, owing to the emission of heterogeneous lavas and to the occurrence of new andesitic magmas. The intermediate Nevado grew and overflowed C2. In the same way, Paleofuego began to grow, about the time of Nevado II, but whereas the latter was affected by pyroclastic events producing heterogeneous magmas about 0.08 m.y. ago, no phenomenon of this type is seen in the development of Paleofuego. Was Paleofuego formed as the result of one of these reinjections? The basic lavas of this volcano seem to support this hypothesis. Thus, the question is whether there was a connection between the two modern volcanoes, and whether their evolution was related to a single magma chamber or to two different ones. The two modern volcanoes, less voluminous than the common ancient edifice, had comparable, but not synchronous histories: they were
marked by eruptions of the MSH type leading to gigantic landslides and to destruction of their summit areas. Two avalanche calderas, 4-5 km wide, opening towards the East and the South are related to two debris flows, of which Paleofuego's debris flow is among the most voluminous known. These destructive events occured no more than some tens of thousands of years ago in the Nevado, and only 9370_+ 400 years ago in the case of Paleofuego. Although the age of the MSH type eruption is not yet defined at Nevado, it appears to be relatively recent; it rejuvenated the terminal cone of Nevado, which is usually considered to be an old volcano dissected by the erosion. Both centers had intense pyroclastic activity that left numerous plinian air-fall layers. At Nevado, dacitic ash-flows erupted during this stage. The age of the debris-flow deposits from the southern centre (9000-10,000 y.) shows that the greater part, if not all of the summit plinian series surrounding the Fuego, follows the MSH event and thus is reasonably to be attributed to the active cone. As a consequence, the position and the role of the MSH event with respect to the tephra, as they were defined by Luhr and Carmichael (1982), must be reconsidered. A similar evolution appears in the upper Nevado. Moreover, the relations between the two series of air falls corroborate the belief that the transition periods between the Nevado II and Nevado III on the one hand, and between Paleofuego and Fuego on the other, were very close in terms of the overall evolution of the volcanic complex. The terminal cones are characterized by the alternation of episodes of growth and destruction. For Nevado III, the role of the St. Vincenttype eruptions, leading to pyroclastic flows with heterogeneous magmas, remains to be clarified by further investigations and chronological data. At Fuego, these eruptions dominated the explosive periods and played a determining role in the cycles of activity. According to Luhr and Carmichael (1980), the recent activity was characterized by cycles of one hundred years or
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more. Each had an effusive dynamism with emissions of very homogeneous andesitic flows (60-61% SiO2), followed by a short explosive episode with more basic magma (57-59% SiO2 ) that ended the cycle. For the historic period, which includes three of that cycles, the following factor should be considered: (1) The cataclysmic episodes, in particular those characterized by deposits of the St. Vincent-type, were probably at the beginning of cycles that commonly ended in effusive or extrusive activity, as it did in the case of the flows of 1975-1976 and the present dome. Basic juvenile material released into the shallow chamber could have led to renewed explosive activity,as proposed by Sparks et al. (1977). (2 ) It is not obvious that a cycle necessarily starts with such a sequence. An important plinian eruption giving heterogeneous pumice-rich products could have the same significance. A slighthomogenization isalready apparent in the 1913 St. Vincent-type deposits ifthey are compared to previous products emitted between 1903 and 1909. At the end of each cycle, the magma may have been totallyhomogenized. At Fuego, the frequency of these phases and their duration is an important element in evaluating potential hazards: this is the purpose of our work in progress. Acknowledgments
Fieldwork (1982-1984) was supported by the Programme Interdisciplinaire de Recherches pour la PrSvision et la Surveillance des Eruptions Volcaniques (C.N.R.S.-I.N.A.G.). W e thank Dr. Joss Guerrero, U.N.A. Mexico and Mission Arch~ologique Fran~aise in Mexico for their assistance in the form of 4 W D cars. The proposed chronology is based on K-At ages (L.A. 10 C.N.R.S., Clermont-Ferrand) and C14 ages (C.F.R., Gif sur Yvette). Chemical analyses were made by Mrs. F. Canta$1~ and S. Couturid,D ~ m e n t de Gdologie et MinSralogie,Clermont-Ferrand. W e thank Prof. A.R. McBirney and an unknown reviewer for con-
structive and helpful comments for the elaboration of the final manuscript. References Allan, J.F. and Luhr, J.F., 1982. K-At ages of late Cenozoic alkaline and calk-alkaline lavas of the Colima graben area, 8W Mexico (abstr.). EOS, Trans. Am: Geophys. Union, 63: 1154. Anderson, T. and Flett, J.S., 1903. Report on the eruptions of the Soufri~re in St. Vincent in 1902, and on a visit to Montagne Pelde in Martinique. Philos. Trans. R. Soc. London, 200: 353-553. Arreola, J.M., 1903. The recent eruptions of Colima. J. Geol., 11: 749-761. Cantagrel, J.M., Robin, C. and Vincent, P.M., 1981. Les grandes Stapes d'Svolution d'un volcan anddsitique: l'example du Nevado Toluca (Mexique). Bull. Volcanol., 44: 177-188. Demant, A., 1981. L'axe n~o-volcanique trans-mexicain Etude volcanologique et pStrographique - Significatio gSodynamique. Doctorat d'Etat Thesis, Aix-Marseille Univ,, 259 pp. Luhr, J.F., 1981. Colima: History and cyclicity of eruptions. Volcano News, 7: 1-3. Luhr, J.F. and Carmichael, I.S.E., 1980. The Colima volcanic complex, Mexico - part 1. Post-caldera andesites from volcan Colima. Contrib. Mineral Petrol., 71: 343-372. Luhr, J.F. and Carmichael, I.S.E., 1981. The Colima volcanic complex, Mexico - part 2. Late Quaternary cinder cones. Contrib, Mineral. PetroL, 76: 127-147. Luhr, J.F. and Carmichael, I.S.E., 1982. The Colima volcanic complex, Mexico - Part 3, Ash and scoria fall deposits from the upper slopes of volcan Colima. Contrib. Mineral. Petrol., 80: 262-275. Luhr, J.F. and Prsstegaard, K., 1985. Caldera formation at Volcan ColimA~Mexico: A lage Mount St. Helens-Type Avalanche event 4,300 years ago (Abstr.). EOS, Trans. Am. Geophys. Union, 66: 411. Medina Martinez, F., 1983. Analysis of the eruptive history of the Volcan de Colima, Mexico (1560-1980). Geofis. Int., 22: 157-178. Mooser, F., 1961. Los volcanes de Colima. Univ. Nac. Mexico, Geol. Bol., 61: 49-71. Ordonez, E., 1897, Les volcans de Colima et Ceboruco. Mere. Soc. Cient. Antonio Alzate, tomo II, Mexico, pp. 325-329. Robin, C., 1981. Relations volcanologie-magnmtologiegc~dynamique: application au passage entre volcanismes alcalin et and~sitique dans le Sud mexicain (Axe trans-mexicain et Province alcaline orientale). Doctorat d'Etat Thesis, Publ. Ann. Univ, Clermont-Ferrand, 2nd Ser.,31, fasc.70, 503 pp.
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Robin, C., 1984. Le Volcan Popocateptl (Mexique): structure, ~volution p6trologique et risques. Bull. Volcanol., 47: 1-23. Robin, C. and Boudal, C., 1984. Une 6ruption remarquable par son volume: l'~v~nement de type Saint-Helens du Popocatepetl (Mexique). C. R. Acad. Sci. Paris, 299: 881-886. Robin, C. and Boudal, C., 1987. A gigantic Bezimianny type event at the beginning of modern V. Popocatepetl. J. Volcanol. Geotherm. Res., 31: 113-128. Robin, C. and Cantagrel, J.M., 1982. Le Pico de Orizaba (Mexique): structure et ~volution d'un grand volcan and~sitique complexe. Bull. Volcanol., 45 (4): 299-316. Robin, C., Camus, G., Cantagrel, J.M., Gourgaud, A., Mossand, Ph. and Vincent, P.M., 1983. The Colima volcanic complex (Mexico): Main eruptive units (abstr.). 2nd E.U.G. Meeting, Strasbourg, Terra Cognita, 3: 153. Robin, C., Camus, G., Cantagrel, J.M., Gourgaud, A., Mossand, Ph., Vincent, P.M., Aubert, M., Dorel, J. and Murray, J.B., 1984. Les volcans de Colima (Mexique). Bull. P.I.R.P.S.E.V.-C.N.R.S.-I.N.A.G., 87:98 pp. Shepard, J.B., Aspinall, W.P., Rowley, K.C., Pereira, J., Sigurdsson, H., Fiske, R.S. and Tomblin, J.F., 1979. The eruption of Soufri~re volcano, St. Vincent April-June 1979. Nature, 282: 24-28. Siebert, L., 1984. Large volcanic debris avalanches: char-
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