Size, distribution, and magma output rate for shield volcanoes of the Michoacán-Guanajuato volcanic field, Central Mexico

Jom'n~ofvoicanololff ano geothermal research ELSEVIER

Journal of Volcanology and Geothermal Research 63 (1994) 13-31

Size, distribution, and magma output rate for shield volcanoes of the Michoac in-Guanajuato volcanic field, Central Mexico T. H a s e n a k a Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Aoba, Sendal Miyagi 980, Japan Received 22 July 1993; revised version accepted 1 March 1994

Abs~a~ The Michoac~in-Guanajuato Volcanic Field (MGVF), in the west-central Mexican Volcanic Belt (MVB), contains nearly 400 medium-sized volcanoes in addition to 1000 small monogenetic cones. The area is distinct from other parts of the MVB, where large composite volcanoes predominate. Shield volcanoes are dominant among medium-sized volcanoes, which also include minor lava domes and composite volcanoes. The location, height, basal diameter, and crater diameter (when applicable) of 378 medium-sized volcanoes were catalogued, and the slope angles and volumes were calculated from these data. The median shield volcano has a height of 340 m, a basal diameter of 4100 m, an average slope angle of 9.4 °, and a volume of 1.7 km 3. The shield volcanoes in the MGVF are similar in size to Icelandic-type shield volcanoes, but the former have much higher slope angles and smaller basal diameters than the latter. Within the MGVF, these medium-sized volcanoes are located between 190 km and 430 km from the Middle America Trench; the distribution area is similar to that of small cones, but clusters and alignments are not as obvious for the medium-sized cones. The shield density is highest between 270 and 280 km from the Middle America Trench, 20 km farther than the density maximum for small cones. Ten medium-sized volcanoes were considered younger than 40,000 yr B.P. from lava flow morphology. The slope angle and a ratio of height to basal diameter are not useful age indicators, because they seem to reflect difference in original shield shape. K-Ar ages of shield volcanoes reveal that the volcanoes located north of latitude 19 °55'N were active between 1 Ma and 3 Ma, whereas those south of latitude 19 °55'N were active since 1 Ma. The average volcanic output rate estimated for the last 1 Ma is 0.7 kin3/1000 yr., whereas that for the period of 13 Ma is 0.2 km3/1000 yr. An increase in magma production occurred around 1 Ma in response to migration of volcanism and probably to a change in tectonics and the thermal structure of the magma source region.

1. Introduction T h e states o f Michoac~tn a n d G u a n a j u a t o , in central Mexico, contain o v e r 1000 small volcanic centers in an area o f 40,000 k m 2, which f o r m s the west-central part o f the M e x i c a n Volcanic Belt ( M V B ) (Fig. 1). Scoria cones, lava domes, thick lava flows, and m a a r s are the m a i n

volcanic features o f this field, but active large c o m p o s i t e volcanoes as seen in other parts o f the M V B are absent ( H a s e n a k a a n d Carmichael, 1985a,b). T h e area, however, also contains close to 400 m e d i u m - s i z e d volcanoes ( H a s e n a k a a n d Carmichael, 1986). T h e y are smaller t h a n the c o m p o s i t e volcanoes in the MVB, and thus are i n t e r m e d i a t e in size between cinder cones and

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72.Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

typical composite volcanoes. Most of them are shield volcanoes that are mainly composed of lava flows as opposed to the mostly pyroclastic small cones. Each of these medium-sized volcanoes represents a greater volume of magma output than a small volcanic center. Therefore, these volcanoes represent an important part of the overall magma supply to the volcanic field. Geomorphologically, most of the lava flows from these medium-sized volcanoes appear older than those from cinder cones and other small volcanic centers. They are possible precursors to the small volcanoes. Information on the distribution, age, and magma output rate for these medium-sized volcanoes is indispensable for understanding the currently active monogenetic volcanism in the MGVF. In contrast to the cinder cones, which have been quantitatively described in terms of size, distribution, and magma output rate (Hasenaka and Carmichael, 1985a), medium-sized volcan-

oes in the MGVF have been poorly described for those parameters. Studies related to the medium-sized volcanoes are as follows. Williams (1950) described the geology, petrography, and chemistry of volcanoes in the Paricutin region and included several medium-sized volcanoes. Hasenaka and Carmichael (1985b) cataloged over 1000 volcanic centers of the volcanic field, in which they included several medium-sized volcanoes with summit cones. Connor (1987) compiled over 2000 volcanoes from the area including the MGVF, however, he did not include information on ages and types of volcanoes. Roggensack ( 1988 ) made cluster analyses of the shield volcanoes and found six overlapping clusters with different lava chemistries. Ban et al. (1992) dated lavas from nine medium-sized volcanoes of the MGVF, which revealed that the volcanism migrated toward the south around 1 Ma. The purpose of this paper is to present the resuits of a compilation for medium-sized volcan-

Fig. 1. Index map of the Miehoac~ln-Guanajuato volcanic field and tectonic boundaries. Small dots are catalogued, small, monogenetic volcanic centers after Hasenaka and Carmichael ( 1985b). Filled triangles indicate active composite volcanoes or silicic volcanic centers. Stars indicate clusters of small monogenetic volcanoes. Boundaries between Rivera, Cocos, and North American plates are simplified after D r u m m o n d ( 1981 ). See DeMets and Stein (1990) for details.

T. Hasenaka / Journal of Volcanologyand GeothermalResearch 63 (1994) 13-31

oes according to volcano-type, size, and distribution, and to use Ban et al.'s ( 1992 ) K-Ar ages to make an estimate of the magma output rate. These data will be useful in :interpreting the overall tectonics and volcanism of this part of the MVB during the period from late Pliocene to Recent. The compiled data of location, size, and lava morphology of medium-sized volcanoes will be published elsewhere.

2. Volcanoes of the MGVF

The tectonic setting of the western MVB is given in Fig. 1. The tectonics and volcanism of the western MVB are strongly influenced by the combined subduction of the Cocos and Rivera plates under the North American plate (e.g., Burbach et al., 1984) and three evolving rift systems (Luhr et al., 1985). Subduction of these oceanic plates is not well defined; the relatively young Rivera plate subducts aseismically, and where the Cocos plate descends beneath the North American plate earthquakes are found only at depths less than 100 kin. Three rift systems are present at the western end of the MVB and intersect west of Chapala lake (Fig. I ): the NWtrending Tepic-Zacoalco rift zone; the N-Strending Colima rift zone; and the E-W-trending Chapala rift zone. The first two are known to be recently active (Allan et al., 1991; NietoObregon et al., 1993), but no geological or geophysical evidence has been found for Recent activity in the Chapala graben, a proposed aulacogen of the rift system. The eastern extension of the Chapala graben goes through the northern part of the MGVF, where E-W-trending subparallel normal faults and depressions like Lake Cuitzeo and Lake Yuriria (Fig. 1 ) are found. Some of the medium-sized volcanoes are offset by these E-W-trending normal faults. Maximum vertical offset of 500 m is observed east of Lake Chapala. Delgado ( 1992 ) studied the geology of the Lake Chapala area and concluded that the rifting initiated in the Pliocene. Another important aspect of the MVB is that it is not parallel to the Middle America Trench, but makes an oblique angle to it; thus, volcanoes

15

in the east are progressively farther from the trench. Because of this characteristic, Mooser (1969) and Shurbet and Cebull (1984), for example, proposed that the volcanism of the MVB is related to major crustal fractures and not to subduction of oceanic lithosphere. The extensive monogenetic volcanism in the MGVF favors such a fracturing hypothesis, although traceelement geochemistry indicates that these magmas have the signature of a subduction zone (Hasenaka and Carmichael, 1987; Hasenaka et al., 1994). Johnson and Harrison (1990) studied the lineaments of the LANDSAT TM images of the MVB, and found that three major fracture zones surround the MGVF, thus forming the "Michoac~in triangle", where a tensional stress field is created, and causing the extensive monogenetic volcanism. The area included in the present study lies between 100 ° 30' W and 102 o40' W, and between 19 ° 00' N and 21 ° 00' N, which is a little larger than the Michoac~ln-Guanajuato Volcanic Field as defined by Hasenaka and Carmichael (1985a). In the northwestern part of the volcanic field, the cinder cones become sparse, whereas medium-sized volcanoes are continuous westward to the Chapala Lake area. Medium-sized volcanoes have mainly erupted calc-alkaline andesites with the majority of lavas falling in the SiO2 range between 55% and 61% (Hasenaka et al., 1994). They present an interesting contrast with small volcanic centers, which have discharged all types of lavas from calc-alkaline to alkaline, with a wide variation of silica content from 47% to 65%, among which calc-alkaline olivine basalt or olivine basaltic andesite is dominant (Hasenaka and Carmichael, 1987 ). Thus, the voluminous lavas from medium-sized volcanoes represent more evolved magmas than those from small monogenetic volcanic centers. 3. Classification of medium-sized volcanoes 3.1. Character&tics o f the M G V F shieM volcanoes

Most medium-sized volcanoes in the MGVF are 2-12 km in basal diameter, 100-1000 m in

16

T. Hasenaka /Journal of Volcanology and Geothermal Research 63 (1994) 13-31

(b) B-type shield volcanoes

(a) A-type shield volcanoes

Cerro Culiacan

Cerro Grande

NW -

W Cerro El Metate

J

~

~

~

SE

Cerro El Tule

.............................................

NW

~

~

N

SE

s

Cerro Pscsrscua Cerro Camataran

SN ~

w

E

w

Cerro Buenavista Tometlan SW

~

NE

w .~--~-'-~-~.~_ ~ - - - - . ~ - - - - ~

NE

NW ~

~

......

E

~

SE

Cerro Cspaxtiro WNW

E

Cerro El Picacho

Cerro Fresno NW

~

Cerro Brlnco del DIsblo

Cerros Custes Sw

NE

Cerro Yahuarsto

SE . . . . .

[lokm

0

5 km

ESE

..........

1 km o 5 km

0

(d) MGVF Composite volcanoes

(c) Domes and lava flows Cerro Tancl'tsro

Cerro El Burro SW

NE

NW

.

SE

~

Cerro La Cantera sw

~

Volc~n Grande

NE

Cerro Los Lobos NNW ~

~

w

~

E

SSE 1 km

0

5 km

0 5 km

0

(d) Composite volcano Volcrtn de Colima

w

1 km . . . . . 0

E

I0 5 km

Fig. 2. Profiles of medium-sized volcanoes in the MGVF. (a) A-type shield volcanoes (gentle-sloped); (b) B-type shield volcanoes (steep-sloped); (c) domes and lava flows; and (d) composite volcanoes. Vertical:horizontal scale is 1:1. A dotted line indicates a base line for volume calculation. Profiles of Volc~in de Colima are added in (d) for comparison.

T. Hasenaka / Journal of Volcanologyand GeothermalResearch 63 (1994) 13-31

height, and 0.5-10 km 3 in volume. Shield volcanoes are abundant in this size range, but several tens of lava domes or thick lava flows and a few composite volcanoes are also found. The lava domes and flows are among the smallest, whereas the composite volcanoes are among the largest of the size distribution. The shield volcanoes and composite volcanoes as described in this paper follow the definitions of Williams and McBirney (1979). The former are volcanoes produced by accumulation of fluid lavas, and the latter are cones built partly of lava flows and partly of pyroclastic layers. Profiles of representative medium-sized volcanoes in the MGVF are shown in Fig. 2, their plan views with contour lines in Fig. 3, and surface morphology of Metate and neighboring volcanoes in Fig. 4. Shield volcanoes in the MGVF have slopes forming nearly straight lines or curves slightly convex upward, and show profiles of typical shields (Fig. 2 ). The MGVF shields most closely resemble Icelandic shields in size, shape, and eruption mode, among three representative terrestrial shield volcanoes, namely Hawaiian-type, Icelandic-type, and Galapagos-type. In contrast to these three shield types, which have a caldera or crater depressions at the summit (Williams and McBirney, 1979; Cas and Wright, 1987), those from the MGVF do not show such depressions. On the contrary, some of them have a small summit cone (Cerro Grande, Cerro E1 Metate, Cerro Buenavista Tomatlan, Cerros Cuates, Cerro Capaxtiro, Cerro E1 Picacho in Figs. 2 and 3 ), which is either a lava dome or a scoria cone with agglutinated bombs and scoriae. A few of them, such as Cerros Cuates (Figs. 2 and 3 ), have two or more summit cones, probably reflecting a shift of the vent position during eruption. Others, like Cerro Grande, have aligned parasitic cones suggesting the existence of a fissure dike. In the MGVF, small shields with summit cones are similar to cinder cones with associated lava flows, except that they have much greater volumetric proportions of lava flows to cone than the latter, thus forming an elevated shield base below a cone. Field observations indicate that shield volcanoes in the MGVF consist mainly of lava flows.

17

Pyroclastic materials, when observed, are limited to the summit area. Typical basaltic shield volcanoes are almost entirely composed of lava flows with pyroclastic deposits forming only a minor (<1%) proportion (Cas and Wright, 1987 ). The MGVF shields, in comparison, may have a larger proportion of pyroclastic deposits, considering their andesitic compositions. Although basaltic compositions dominate for the fluidal lava flows of typical terrestrial shields (Williams and McBirney, 1979; Cas and Wright, 1987), the lava flows from the MGVF shield volcanoes are mostly andesitic. Reflecting this compositional difference, the MGVF shields issue either aa or block lava flows, in contrast to the numerous thin pahoehoe flows of Icelandic shields. Individual flow units are hard to discern due to erosion, except for a few youthful shields including 4700 year-old Metate (Fig. 4). Late Metate lava flows, such as those that flowed to west and north, have a thickness of up to 120 m, indicating high aspect ratios concordant with high SiO2 contents of 62 wt.% (Hasenaka and Carmichael, 1986). Most shields whose lava flow boundaries are ambiguous, have gullies radiating from the center as shown in Fig. 4. Shield volcanoes like Metate and Cerro Capaxtiro, which erupted on a tilted base, are elongated in the down-slope direction (Fig. 2a). 3.2. A-type and B-type shield volcanoes

The MGVF shield volcanoes have a wide range of slope angles in contrast to typical basaltic shield volcanoes, which have gentle slopes of < 10 ° (Williams and McBirney, 1979; Cas and Wright, 1987 ). They are subdivided into A-type, which have gentle slope angles (around 5 °, Figs. 2a and 3a) and B-type, which have steeper slope angles (around 10 °, Figs. 2b and 3b). The former are similar to Skjaldbreidur from Iceland, which shows a perfectly symmetrical shape with widely spaced concentric contours in the topographic map (Compare Fig. 3a with p. 368 of Cas and Wright, 1987). In contrast, B-type shields are represented by narrowly spaced concentric contours like Cerro Culiacan, or a concentric pattern obstructed by neighboring volcanoes as in

18

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

the case of Cerro E1 Picacho (Fig. 3b ). The cause of this high slope is discussed later in section 7 (Slope angle of volcanoes). Because no clear distinction in slope angle is found between the shield types, they are treated together in the discussion of volcano dimensions. Shield volcanoes, mostly B-type, are also found

in other parts of the MVB including the Lake Chapala area, immediately west of the MGVF (Delgado, 1992), and the Sierra Chichinautzin area (Martin del Pozzo, 1983), 500 km east of the MGVF. On both the northern and southern sides of Lake Chapala are distributed about forty shield volcanoes and large lava domes that form

(b) B-type shield volcano

(a) A-type shield volcano

--

Cerro Camataran

100
j~~2~30"N

i

~

~

Cerros Cuates o

20°17'30"N~_ 101o00'00"W

_L_19o52.30,,N

..

Cerro El Picacho

0 Jr-19°45'00"N 102°00'00"W

1

2

3

4

5 km

-t- 19~50'00"N~ 102°00'00"W ml~.~ ()

1

2

:3

4

5 km

Cerro La Cantera Cerro Los Lobos

(c) Domes and lava flows Cerro El Burro

0"W :'2T30"N

101o32,30 x~J "~..W~ ~ 2 - J

0~

1'

2

3

'~

5 km

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

19

(d) Composite volcano

(d) Composite volcano Volc6n

Gra~

101 °00'00"W . . . . . . . . . .

102o17,30.W

~ (~

1j

2

3

4

5 km

Fig. 3. Plan view of medium-sized volcanoes. (a) A-type shield volcanoes (gentle-sloped); (b) B-type shield volcanoes (steepsloped); (c) domes and lava flows; and (d) composite volcanoes. Contour interval is 100 m.

a continuation of the medium-sized volcanoes from the western part of the MGVF. Most of them are offset by E - W normal faults that are related to the formation of the Chapala graben. In this region, however, small youthful monogenetic volcanoes as observed in the southern part of the M G V F are not found. In contrast, Sierra Chichinautzin is similar to the M G V F in that both small monogenetic volcanoes and mediumsized shield volcanoes coexist (Martin del Pozzo, 1983).

chael, 1985b). A few domes sometimes form a cluster, as in the case of Cerro E1 Burro (Figs. 2c and 3c). A lava dome like Cerro Los Lobos issues thick lava flows. An extreme case occurs when the lava flows form a large portion of the dome: these are called thick lava flows. Because domes and lava flows without cones are continuous in size from small-size to medium-size, and show similar forms in both size range, they are better grouped together.

3.3. Lava domes and flows

3.4. Composite volcanoes

Lava domes having higher slope angles ( > 15 ° ) than B-type shield volcanoes are included in this compilation. They are few in number, and smaller in size than most M G V F shields, but larger than lava domes included in the compilation of small cones (Hasenaka and Carmi-

Volcanoes classified as composite volcanoes include Volc~in Grande and Cerro Tancitaro, both of which display unique profiles among medium-sized volcanoes in the M G V F (Figs. 2d and 3d). Williams (1950) described Cerro Tancitaro as a shield volcano from its shape as shown

20

T. Hasenaka /Journal of Volcanologyand Geothermal Research 63 (I 994) 13-3 1

19”35’N

19”30’N

---y,.~

Railroad

Fig. 4. Map showing the volcano morphology of Cerro El Metate and neighboring volcanoes northeast of Uruapan. M=Cerro El Metate; P= Cerro Paracho; L= Cerro La Cruz; C= Cerro Colorado. Individual lava flow margins, pressure ridges, gullies on the slope of shield volcanoes as observed in air photographs (published by DETENAL, Mexico City. scale l:SO,OOO) are drawn. Lava flow margins of Metate is enhanced by horizontal lines. Note a deep gully of Cerro Paracho opening to north.

in Fig. 2d, although he observed interbedded breccia and agglomerate. Such interbeds of pyroelastic rocks and lavas were not found from the limited field work of the present study. However, the highly dissected nature of Tancitaro for its relatively young K-Ar ages of 0.53 Ma (Ban et al., 1992) with deep gullies and valleys is noteworthy. It implies the presence of easily eroded pyroclastic material. In contrast, the shield volcano Cerro Culiacan, which is little dissected (Figs. 2b and 3b), has a K-Ar age of 2.10 Ma (Ban et al., 1992). Volcdn Grande, on the other hand, has a wide, flat summit crater area and a concave-upward slope, and resembles a typical composite volcano whose slope is mainly made of scoriae and ash (Fig. 2d).

4. Method of measurement Volcanic landforms were identified from topographic maps, geological maps, air photographs, and field observations. Maps and photographs ( 1:50,000) published by DETENAL (Mexico, D.F. ) were used in this study. Contour interval is either 10 m or 20 m, depending on the area of the maps. Topographically high erosional remanents were carefully excluded by observations in the field and from air photographs. A volcano with more than two craters is regarded as a single volcano, as long as it maintains a more or less conical shape with continuous slopes. The position of the volcano is taken as either the highest point or the center of the summit crater.

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (I 994) 13-31

The diameter and the basal height of a volcano are taken as the average of maximum and minim u m values. The base of a volcano is determined where the generally equidistant contour lines become somewhat wider. In cases where the bases of two or more volcanoes overlap, the maximum basal height is taken at the rim of the overlap. The minimum diameter is measured from where two volcanoes overlap (e.g., Cerro E1 Picacho in Fig. 3b). In this way the underestimation of volume is less than the case if the maximum basal height is taken at the plunge of the overlap. The volume of a volcano is calculated generally by approximating its shape as a cone. For fiat-topped volcanoes, a truncated cone shape was used for calculation. The volume of the summit cone is added if it is significantly large. In many cases, though, it makes a very small (a few % or less) portion of the entire volcano. Slope angle is calculated as the a r c - tangent of the height to basal radius ratio ( t a n - ~ ( 2 H / D) ). This is a very rough estimate because it neglects any irregularity in shape; however it is useful in showing the general tendency of steepness of the volcanoes. Because conical shape is assumed in calculating volcano volume, the deviation of the actual shape from conical will cause the greatest error. Such deviations include elongated shape, concave or convex profiles, and disintegrated shape due to erosion. Model volume calculations as compared with actual volcano profiles and a simple cone shape showed differences less than 20%.

21 °N

a i IIA iA

I

IA

I

i

21

[ ~A

A

Ai

I •

i "

.5

20 °N

~9°N

21°N

,b, i

102°W

101°W

i

i

i

20°N

19ON 102°W

101°W

Fig. 5. (a) Distribution of medium-sized volcanoes in the MGVF. Only those with volumes greater than 1 km 3 are shown. Those with volumes greater than 10 km 3 are shown in filled triangles. (b) Distribution of small volcanic centers in the MGVF. Triangles are much larger than the actual volcano size, which enhances the locations of volcano clusters. Data from Hasenaka and Carmichael (1985b) and this study.

5. Distribution of medium-sized volcanoes

The location of shield volcanoes in the MGVF is shown in Fig. 5a; only volcanoes with volumes greater than 1 km 3 are included. Though Roggensack ( 1988 ) reported clustering of shield volcanoes in the MGVF, such clusters are not obvious on the first look at Fig. 5a, in which medium-sized volcanoes are evenly distributed and show no clustering or regional alignments. In contrast, several clusters with high cone density are clearly shown in the distribution of small

monogenetic volcanoes (Fig. 5b), as described by Hasenaka and Carmichael (1985a) and Connor (1987, 1990). Medium-sized volcanoes are sparse to absent in the north-central portion of the field (around 102 ° W and above 20 ° N). A large N-S graben structure is found in this area (Fig. 1 ), which disrupts a generally E-W-trending normal fault system in the northern part of the MGVF. The comparison of Fig. 5a and b shows the different distribution patterns of volcanoes be-

72 Hasenaka /Journal of Volcanology and Geothermal Research 63 (1994) 13-31

22 3O 25f

•

Medium-sized volcanoes ]

15

~

lO

CO

5

~

o

,* 1~o

100

50

0 200

250 300 350 400 Distance from the trench (km)

450

Fig. 6. Number of volcanoes plotted against their distance from the Middle America Trench. (top) medium-sized volcanoes, (bottom) small monogenetic cones.

tween these two size groups. Proportionally more medium-sized volcanoes are found in the northern part of the volcanic field or farther from the trench compared to the small cones. This becomes more obvious when the numbers of volcanoes are plotted against the distance from the Middle America Trench (Fig. 6). The frequency peak for the medium-sized volcanoes is located 20-30 km farther from the trench than that of the small monogenetic volcanoes. Locally, in the southern part, the small volcanic centers are mainly distributed at the feet of these mediumsized volcanoes, and the latter appear older than the former.

Grande, an A-type shield volcano with an uncorrected volume of 54 km 3, average basal diameter of 17 km, and a height of 680 m from the base. The second is Cerro Tancitaro (49 km 3, 11 km, and 950 m, respectively). The size-frequency diagrams of Fig. 7, if combined with those of small cinder cones (Hasenaka and Carmichael, 1985a, fig. 5 ) show gaps in size distribution. Shield volcanoes (mediumsized volcanoes ) and cinder cones (small cones ) are not continuous in terms of volcano height, basal diameter, or volume. This implies that magma supply, storage, and output systems are different between these two groups of volcanoes. Height and basal diameter, plotted on the diagram of Whitford-Stark (1975), show a clear distinction between the M G V F shields and other terrestrial shields (Fig. 8). Most medium-sized volcanoes from the MGVF fall in the field of Icelandic-type shields and part of Scutulum-types, but in the region with greater height and smaller basal diameter. The comparison of shield volcanoes from the M G V F and Iceland shows an overlap at similar height range between 100 m and 1000 m, though Icelandic shields plot in the larger diameter range. The M G V F shields, especially B-types, seem to represent a different class of shield volcano from Icelandic shields. Data points plotted in the Scutulum-type field represent the relatively small MGVF volcanoes. They are either domes or small shields. The former are transitional to thick lava flows or small domes, and the latter include a cinder cone (or lava cone) associated with abundant lava flows that form a shield at the foot of a cone.

6. Size and volume of volcanoes Frequency histograms for height, basal diameter, slope angle, and volume of medium-sized volcanoes are shown in Fig. 7. No distinctions are made between different types of volcanoes, because dimensional differences are gradational to each other. Median values are 340 m for the height, 4100 m for the basal diameter, 9.4 ° for slope angle, and 1.7 km 3 for volume. Mean values are 390 m, 4500 m, 10.2 °, and 3.3 km 3, respectively. Among 378 medium-sized volcanoes that have been compiled, the largest is Cerro

7. Slope angle of volcanoes The slope angles of other terrestrial shields were calculated in the same way as the Mexican shields for the purpose of comparison (Fig. 9). The data for height, crater diameter, and basal diameter are taken from Pike (1978 ), as is the classification of shield volcanoes. KT- and KMtypes represent a greater size range of shield volcanoes with summit calderas, mostly from Hawaii and Galapagos. SI-type or shield volcanoes

T. Hasenaka /Journal of Volcanology and Geothermal Research 63 (I 994) 13-31

100 ,

~ 8 ~

,

100 B1.36

8O

ao

60

8o

40

40

2

20

~=

o o

200

,oo

600

600

23

looo 12oo

lUll..

.....

~'~';'~'~'1~'1~'1:~'1~'~;~'2~'2~

Height (m)

Volume (kin3)

100 o c u

lu >

80

60

'3

40

.1~

20

E z

O 0

,ill,,,. 2

4

6

8

10

12

40 30 20 10

o 14 16

Basaldiameter( k i n )

iii

18

5

10

15

20

25

Slope angle (degree)

Fig. 7. Frequency distributions of height (H), basal diameter (D), slope angle, and vo]ume(V) for 378 medium-sized volcanoes in the MGVF. Dark bars correspond to volcanoes with volumes > l km 3, whereas light-colored bars correspond to those with volumes < 1 km 3. Slope angle was calculated as tan- ~(2H/D). Volume was calculated as a symmetrical cone shape or truncated cone shape, and summit cone volume was added when it was not negligible.

10000 .#

• •

•

.

#

""

•

10000

MGVF

~lan-type

1000

1000

, •I - 1 0 0

"1" 1011

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/

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/

Iceland o" Galapagos Is. Hawaiian Is.

4-

i

I

10

100

Basal diameter (km)

1000

10

100

Basal diameter (km)

Fig. 8. Height-basal diameter relation for terrestrial shields. Division, boundary lines, and data of shield volcanoes from the Faeroes, Iceland, Galapagos, and Hawaii are taken from Whitford-Stark ( 1975 ). All data for medium-sized volcanoes from the MGVF are shown on the right-hand plot, and is shown on the left for comparison. As discussed in text, the MGVF data also include lava domes, which have small heights and basal diameters.

T. Hasenaka / Journal of I'blcanology and Geothermal Research 63 (1994) 13-31

24

8

KT

~6 ° 4 82

'~

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2

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6

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8

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0 0

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6

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8

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4

6

Slope angle (degree) Fig. 9. Frequency of slope angle for flanks of terrestrial shield volcanoes. Slope angle was calculated in the same manner as for Fig. 7 from data and classifications of Pike ( 1978 ). The division of shield volcanoes is as follows. KT= shield volcano with summit caldera: predominantly tholeiitic lava. KM=shield volcano with summit caldera: predominantly non-tholeiitic lava. S l = s h i e l d volcano with summit crater: Icelandic type. SS= shield volcano with summit crater: small, steep type. SL = shield volcano with summit crater: low-sloping type. The MGVF data for comparison are shown in Fig. 7.

with summit craters having a similar size range to the MGVF are from Iceland and the western U.S.A. SS-types are smaller shield volcanoes from Iceland and Hawaii. SL is a low-sloping type mostly from the Snake River Plain, western U.S.A. These compiled terrestrial shield volcanoes have slope angles less than 10 ° regardless of size, with SL-types showing distinctly low slope angle. In contrast, the medium-sized volcanoes from the MGVF show a wide range of slope angles from 3 ° to 25 °, and nearly half have slope

angles greater than 10 ° (Fig. 7 ). Some of the high slope angles were measured for lava domes, but shield volcanoes also show slope angles greater than 10 °. The reasons for this relatively higher slope angle for the MGVF shields than other terrestrial shields are probably explained by: (1) higher viscosities of lavas; (2) smaller effusion rates; and (3) sedimentation of pyroclastic materials near the summit crater, or a combination of these. The viscosity of silicate melt is strongly dependent on composition, especially on concentration of silicon, the most abundant network forming atom (Bottinga and Weill, 1972). The calculated viscosity of Mount Hood andesite (SIO2=60.71 wt.%), for example, is one order of magnitude larger than that of Columbia River basalt (SIO2=50.71 wt.%) at the same temperature (Murase and McBirney, 1973 ). Even when non-Newtonian behavior of lava flows is considered, and they are treated as Bingham fluids (Show et al., 1968), their yield strength increases with SiO2 content (Hulme, 1974). As stated earlier, most MGVF shield lavas show SiO2 ranges between 55% and 61% in contrast to other terrestrial shield lavas, which are mostly basaltic. Therefore, MGVF lavas have effective viscosities at least one order of magnitude larger than basaltic lavas with 50% SiO2. When difference in magmatic temperature and crystal content are considered, this viscosity contrast is even larger. Higher viscosity would certainly contribute to the greater slope angles of MGVF shield volcanoes. Walker ( 1973 ) showed that the length of lava flows is related to magma effusion rate. Small effusion rates produce short lava flows that pile one after another, resulting in cones with steep slope angles. This mechanism, although possible, is hard to confirm, because eruptions of shield volcanoes have never been observed in the MGVF. The third possibility is based on the observation of pyroclastic summit cones at some of the MGVF shield volcanoes. It is quite possible that ash- and scoria-fall deposits would raise the base level of lava flows especially near the summit crater. This would imply that some of the shield

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

volcanoes in the MGVF could actually be composite volcanoes. Delgado (pers. commun., 1993 ) found outcrops of interbedded scoriae and lavas in a deep gully of Cerro Paracho (Fig. 4, calculated average slope angle of 19 ° ) and suggested that it is a composite volcano. As only a few shield volcanoes have been dissected by deep gullies, the importance of pyroclastic material is difficult to evaluate. This paper maintains the view that most medium-sized volcanoes in the MGVF are shield volcanoes, as long as lava flows are exposed on the surface of their slopes, and they have a shield shape.

8. Morphological indicators of shield age Hasenaka and Carmichael (1985a) showed that geomorphological parameters such as height to basal diameter ratio (H/D), slope angle, and degree of preservation of surface features on lava flows were useful indicators of cinder cone age especially for those younger than 40,000 yr B.P. Most shield volcanoes compiled in this study do not preserve primary lava surface features such as pressure ridges, individual flows, or flow edges. Thus according to the criteria of Hasenaka and Carmichael (1985a), they are older than 40,000 yr B.P. and are classified as Pleistocene (Plv2_3, Plv2, and Plv~ ) or Tertiary lavas. It is possible, however, that the preservation of andesitic lava flows from shield volcanoes may be different from that of mostly basaltic lava flows associated with cinder cones. Like cinder cones, shield volcanoes show various HID ratios and slope angles. However, in contrast to cinder cones, which are built of loosely piled up scoriae and bombs, shield volcanoes are mainly built of lava flows that appear to be more resistant to erosion. Their erosional mechanism is probably different from the degradation process of cinder cones. The decrease of slope angle and H/D ratio with increasing degree of degradation as observed among cinder cones are probably not applicable to the MGVF shield volcanoes that are younger than 3 Ma. Variations in slope angles and HID ratios of shield volcanoes are likely to reflect their original morphology, and

25

not degree of erosion. The main difference between shield volcanoes older than 1 Ma and those younger than 1 Ma is the depth of gullies that cut the lava flows on their slopes. Older shields in the northern part of the volcanic field are obviously more deeply eroded by gullies.

9. Estimation of magma eruption rate

Ban et al. (1992) reported l0 K-Ar ages and compiled other published age data from the MGVF. The results of their study show that: ( l ) the focus of volcanism migrated from the north ( > 19°55'N) to the south ( < 19°55'N) around 1 Ma; and (2) the volcanism continued at a nearly constant rate for the last 3 m.y (Fig. 10). The K-Ar ages were obtained mainly from medium-sized volcanoes, but some were obtained from small monogenetic volcanoes that erupted during the same period. The dated samples were collected from volcanoes with various erosional stages, and sampling bias is not apparent in Fig. 10. Magma output rate was estimated separately for two different periods corresponding to the northern and the southern parts of the volcanic field. In calculating the magmatic volumes of shield volcanoes, the volume of associated tephra was neglected, because it must be minor for the Hawaiian-type eruptions of shield volcanoes. A correction factor of 0.7 was applied for calculating the dense rock equivalent volume of lava I

E 3 <

o

I

•

Medium-sized, r o l c a n o e s

[]

Small c o n e s

I

I

I

I North

•

•

m m ln South

o E 0

,

i

0

2

K-Ar Age (Ma) Fig. 10. Histogram of K-Ar age samples. Ages obtained from the volcanoes in the north ( > 19 ° 55 'N) are shown in the upper half, and those from the volcanoes in the south ( < 19 o55'N) are shown in the lower half. Data are taken from Ban et al. (1992).

26

T. Hasenaka /Journal of Volcanology and Geothermal Research 63 (1994) 13-31

Table 1 Magma output rate estimation for the MGVF Period

Volcano type

Number of volcanoes

Total Volume*

Magma output rate

3 M a - 1 Ma

Shield volcanoes

186

390 km 3

0.2 km3/1000 yr

1 Ma-Present

Shield volcanoes Small cones

192 832

470 km 3 230 km 3

0.7 km3/1000 yr (for both)

0.04 Ma-Present

Shield volcanoes Small cones

10 66

30 km 3.* 18 km 3.*

t.2 km3/1000 yr** (for both)

*Dense rock equivalent volume. See explanation in text. **Revised from Hasenaka and Carmichael ( 1985a ).

flows and shields. In comparison, the magmatic volume estimations for cinder cones include corrections for vesicles and void space between lava flows and the cone, and a large fraction erupted as tephra (Hasenaka and Carmichael, 1985a). Using measurements of Fries ( 1953 ) at Paricutin and assumptions of Crisp ( 1984 ), factors of 0.5 (cone), 0.5 (ash), and 0.7 (lava) were multiplied for dense-rock equivalent conversion, and volume of ash was estimated to be 7.7 times that of a cinder cone. The magma output volume from shield volcanoes in the north during the period between 3 Ma and 1 Ma is 390 km 3. Thus, the average magma output rate is 390 km3/2 m.y. = 0.2 km3/ 1000 yrs (Table 1 ). In contrast, the magma output volume from shield volcanoes in the south during the last 1 Ma is 470 km 3, yielding a much greater magma output rate of 0.47 km3/1000 yrs. However, as explained above, this estimation excludes the contribution from smaller cones. Small cones in the north are highly degraded and volume estimation is difficult. Approximately 200 small cones identified north of latitude 19 ° 55' N indicate a small ratio of magma output to shield volcanoes. The exclusion of their volume would not substantially change the magma output rate estimate (see Table 1 ). In the southern part of the volcanic field where cones are not much degraded, the total magma output for 76 volcanoes during the last 40,000 years is 48 km 3, as revised from Hasenaka and Carmichael (1985a). This estimate includes the volume of

ten shield volcanoes (30 km 3) and that of 66 small cones ( 18 k m 3). The total magma output volume from all 832 small cones erupted south of latitude 19°55'N is 18)<832/66=230 km 3, when the same average volume of cone, lava, and tephra is assumed for both periods. Thus the total erupted volume in the south is estimated as 700 km 3 for the last 1 m.y., or 0.7 km3/1000 years, which is not significantly different from the rate of 1.2 km3/1000 years estimated for the last 40,000 years (Table 1 ).

10. Were the activities of cinder cones and shield volcanoes coeval?

Although shield volcanoes appear to be older than neighboring cinder cones, it is difficult to judge from stratigraphic relationships whether the small monogenetic cones and medium-sized volcanoes were both active during the entire period, or whether medium-sized volcanoes always predate the small monogenetic cones. This section tries to find the solution from the eruption rate estimation of each volcano group. Sixty-six small monogenetic cones younger than 40,000 yr B.P. were found in the southern part of the M G V F (Table 1 ). If the constant rate of eruptions of 66 cones/40,000 years ( = 1.7 cones/1000 years) is assumed, the period for the formation of the 832 monogenetic cones found in the south ( < 19°55'N) correspond to a period of 0.5 m.y. This estimate is much less than

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (I 994) 13-31

the period ( 1 m.y.) when medium-sized volcanoes were assumed active in the southern part of the volcanic field (Fig. 10); however, it covers the K-Ar age of Cerro Pelon, one of the oldest cinder cones in the south (0.37 Ma, Hasenaka and Carmichael, 1985a). The number of cinder cones may be underestimated due to erosion or burial by younger lavas or sediments, but the error in estimation should not be as large as a factor of two or three. In the same context, about 200 medium-sized volcanoes that erupted in the south during the last 1 m.y. correspond to a rate of 8 volcanoes/40,000 years. Ten shield volcanoes were included in the estimation of magma eruption rate for the last 40,000 years (Table 1 ) with the youngest one being Cerro E1 Metate (3700 yr B.P. ). They have similar surface morphologies of lava flows to those associated with cinder cones erupted during that period, following the classification of Hasenaka and Carmichael (1985a). The rate of 10 volcanoes/40,000 years is not different from the average rate estimated for the entire period of 1 m.y. If these estimations do not bear large errors, it may be possible to conclude that small monogenetic volcanism in the south was restricted to the last 0.5 Ma, whereas volcanic activity of mediumsized volcanoes in the south was continuous for the last l Ma. Thus, around 0.5 Ma, there possibly was a shift from mostly medium-sized volcanism to coexistence of both volcano types. As discussed in the section on volcano distribution, more medium-sized volcanoes occur farther from trench than small cinder cones. This implies that the focus of volcanism continued to gradually shift toward the trench even after 1 Ma, because those medium-sized volcanoes are probably older than the cinder cones. The above conclusions assume that the conditions of preservation of lava surface morphology are the same for medium-sized shield volcanoes and small cinder cones, especially for the last 40,000 years. The age of 0.06 (_+0.01) Ma for Cerro Paracho (Ban et al., 1992), a shield volcano located north of Metate (Fig. 4 ), is surprisingly young, given its poorly preserved lava flow surfaces, a deeply dissected gully, and a protruding summit cone. As discussed above in section

27

7 (Slope angle of volcanoes), the possible composite origin of this volcano with interbedded layers of pyroclastics and lavas may account for the high erosional character. However, it is also probable that the degree of preservation of lava flow surfaces, one of the age indices applied by Hasenaka and Carmichael (1985a) and by this paper, may be different between lava flows associated with cinder cones and those with shield volcanoes. In this case, the activities of most shield volcanoes would be significantly younger. An alternative model for the volcanic history of the MGVF is that most shield volcanoes in the south have been active since sometime before 0.5 Ma, about the same period as the cinder cones. This alternative implies coeval activities of both cinder cones and shield volcanoes and a period of sparse volcanic activity between 0.6-0.7 Ma and 1 Ma. Two volcanoes older than 0.6 Ma, a 0.75-Ma volcano east of Lake Cuitzeo (Ferrari et al., 1991 ) and 0.87-Ma volcano east of Lake Patzcuaro (Nixon et al., 1987 ), are both located at the eastern edge of the MGVF. All the K-Ar ages obtained for shield volcanoes in the southcentral part of the MGVF are younger than 0.6 Ma (Ban et al., 1992 ). In addition, 20 lava samples from shield volcanoes in the south-central part are all normally magnetized, indicating the Brunhes period, thus younger than 0.78 Ma (Delgado et al., 1993 ). The exact time of initiation of volcanism in the southern part of the MGVF is difficult to specify with the limited age data available at present. It may be 0.7-0.6 Ma, at least in the south-central part where the main phase of volcanism occurred. It is also possible that magma output rate was smaller at least during the initial phase of volcanism when the magma supply to the crust started. At the moment, both models are likely; additional age data are needed in order to evaluate them.

11. Magma supply in the MGVF Multiple vents o f medium-sized volcanoes evenly distributed in the MGVF imply an absence of long-lived shallow magma reservoirs in the same way as those ofsmaU monogenetic cones

28

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

(Hasenaka and Carmichael, 1985a). The cinder cones of Paricutin and Jorullo, the only two historically active volcanoes in the volcanic field, had never been reactivated after the cessation of volcanic activity. Other cinder cones have erupted multiple times, however. Cerro Negro in Nicaragua has erupted at least 10 times since its first activity in 1850 (Viramonte and DiScala, 1970; Walker and Carr, 1986), and a debatable case exists at Lathrop Wells in Nevada, which may show different K-Ar ages and buried soil horizons between tephra sequence from the cone (Wells et al., 1990, 1992; Turrin et al., 1991 ). Therefore, examples of Paricutin and Jorullo do not necessarily imply that all the cinder cones in the MGVF are monogenetic, although they are quite likely so. Whether the MGVF shield volcanoes are monogenetic or not is more difficult to determine, because there have been no historic eruptions. Shield volcanoes in Iceland having similar sizes to the MGVF shields are interpreted as being monogenetic (Williams and McBirney, 1979; Cas and Wright, 1987), and a 0.66-Ma-old shield volcano from Las Laja (north of Los Volcanes in Fig. 1 ), Mexico presents evidence from the dammed lake, into which its lava flowed, that it formed within 20 to 40 years (Righter and Carmichael, 1992). The geomorphology of the MGVF shield volcanoes seems to indicate no resumed volcanic activity after a relatively long period of erosion. Although the monogenetic character is still debatable, it would be safe to state that they formed in a geologically short period, and unlike typical large composite volcanoes, they did not have a long period of erosion between lava flow events. Petrological observations show that shield lavas from the MGVF are more differentiated than cinder cone lavas (Hasenaka et al., 1994), which implies a residence time for the magmas that is sufficiently long to allow crystal fractionation. The greater output volume of a shield volcano suggests a larger magma batch or magma reservoir, which will be thermally more stable in the crust and have a longer residence time. Magma mixing due to replenishment of new magma, however, is rarely observed among these lavas (Hasenaka et al., 1994). Thus shallow magma

reservoirs feeding the shield volcanoes, even if they existed, did not have such a long lifetimes as inferred for large composite volcanoes, which show petrological evidence of magma reservoirs replenished by unfractionated magmas (e.g., Luhr and Carmichael, 1980). The evidence for the MGVF during the period younger than 1 Ma suggests either an earlier dominance of shield volcanoes compared to later coeval shield volcanoes and cinder cones, or an earlier period of sparse volcanic activity followed by coeval shield volcanoes and cinder cones. Because lavas from shield volcanoes are more fractionated, it implies a greater volume of unfractionated magma input from the mantle source to the crust compared with unfractionated cinder cone lavas. An earlier input of magmas in the period of the last 1 m.y might not have produced long-lived magma chambers in the crust as discussed above, but it at least elevated the temperature of the crust and probably facilitated the ascent of relatively unfractionated magmas that later built the cinder cones. No geophysical observations or plate reconstruction models are available now to explain the cause of the trench-ward migration of volcanism in the MGVF around 1 Ma. The migration probably results from the change of stress regime as well as the change of thermal structure of the mantle source region where magmas are generated by partial melting. The different magma output rate estimations before and after 1 Ma are related to different magma compositions between these two periods. Concentrations of Mg, Ni, and Cr in lavas of the same SiO2 content show a general decrease with distance from the trench (Hasenaka and Carmichael, 1987). High-Mg ( > 9%) basalts and andesites are only found between 200 and 270 km from the trench. Thus, older volcanism ( > 1 Ma) in the north is characterized by the eruptions of relatively low-temperature magmas, and younger volcanism ( < 1 Ma) in the south by high-temperature magmas. It seems reasonable to have a greater magma output rate during the period while more primitive, high-temperature magmas erupted. This difference may also reflect the different thermal structures of the magma source region in the upper

T. Hasenaka / Journal of Volcanology and Geothermal Research 63 (1994) 13-31

mantle. A detailed tectonic model of the region as well as a petrological model to constrain the change in physicochemical conditions of the magma source region will be necessary to find the direct relationship between the migration of volcanism on the surface and the magmatic processes deep in the mantle and crust.

12. Summary and conclusions ( 1 ) The Michoac~in-Guanajuato Volcanic Field (MGVF) contains 378 medium-sized volcanoes and over 1,000 small volcanoes within an area of 40,000 km 2. Most of the former are shield volcanoes but they also include lava domes and composite volcanoes. (2) The A-type, or gentle-sloped shield volcanoes in the MGVF are comparable in size and form to Icelandic-type shield volcanoes. The Btype, or steep-sloped shields seem unique among terrestrial shield volcanoes. Both types lack the crater depressions that are common among terrestrial shield volcanoes, and issue mostly andesitic lava flows. (3) Medium-sized volcanoes are distributed over the same area as small cones, but do not show clustering or local alignments, and their highest density is found a little farther from the trench than for small cones. (4) The average (mean) medium-sized volcano in the MGVF has a height of 390 m, a basal diameter of 4500 m, a slope angle of 10.2 °, and a volume of 3.3 km 3. Median values are 340 m, 4100 m, 9.4 °, and 1.7 km 3, respectively. (5) The volcanism migrated from the northern part of the MGVF to south (trench-ward) around 1 Ma. Average magma output rate prior to the migration is 0.2 km3/1000 years, and the post-migration rate is 0.7 km3/1000 years, which is comparable to the revised estimate of 1.2 km3/ 1000 years for the last 40,000 years. (6) The age data and magma output rate estimations present two models of post-migration volcanic history: ( 1 ) earlier shield volcano dominated activities followed by coeval activities of shield volcanoes and small cones after 0.5 Ma, and (2) a period of sparse volcanic activity fol-

29

lowed by the activity of both volcano types after 0.6-0.7 Ma.

Acknowledgements Field work was carried out in December, 1989 and November, 1991 as a joint project with Universidad Nacional Autonoma de Mexico (Japanese leader, Prof. K. Aoki, Mexican leader Prof. J. Urrutia-Fucugauchi), with Dr. M. Ban and Dr. H. Delgado-Granados being principal members to study the shield project. The author wishes to thank Prof. K. Aoki for his encouragement and support throughout this project, and Dr. M. Ban for help in the measurement of volcano dimensions. The work was defrayed by Monbusho Grant #01740468, #03201107 to Hasenaka and #03041014 to Prof. Aoki. Dr Jim Luhr and an anonymous reviewer greatly improved the organization and English of this paper.

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Wood, C.A., 1977. Non-basaltic shield volcanoes. Abstr. Planetary Geology Field Conference on the Snake River Plain, Idaho, pp. 34-39.