Characterization and facies analysis of the hydrovolcanic deposits of Montaña Pelada tuff ring: Tenerife, Canary Islands

Journal of African Earth Sciences 59 (2011) 41–50

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Characterization and facies analysis of the hydrovolcanic deposits of Montaña Pelada tuff ring: Tenerife, Canary Islands J. Carmona a,⇑, C. Romero b, J. Dóniz c, A. García a a

Dpto. Volcanología Museo Nacional de Ciencias Naturales, Madrid, Spain Dpto. Geografía, Universidad de La Laguna, Tenerife, Spain c Escuela Universitaria de Turismo Iriarte, Universidad de la Laguna, Puerto de la Cruz, Tenerife, Spain b

a r t i c l e

i n f o

Article history: Received 7 October 2009 Received in revised form 23 July 2010 Accepted 30 July 2010 Available online 10 August 2010 Keywords: Hydrovolcanism Phreatomagmatic Base surges Water/magma ratio Facies analysis Montaña Pelada Tenerife

a b s t r a c t Montaña Pelada is a basaltic Pleistocene tuff ring located in the SE of Tenerife and it is composed of two edifices each with its distinct internal depositional distribution. A detailed stratigraphic analysis was carried out and ten facies were recognized. Deposits interpretation has revealed that water/magma ratio changes controlled the eruptive evolution, distinguishing three main stages of the eruption. Pyroclastic density currents were formed during the initial phreatomagmatic stages depositing the proximal facies, and transformed into turbulent dry surges during the second stage, indicating a reduction in the water/ magma ratio. After deposition of these surges, the opening of an N–S fracture drove the eruption northwards creating a new edifice. The new hydrological conditions allowed the input of phreatic water, which resulted in high proportion of accidental lithics within characteristic of the deposits, increasing the water/magma ratio and reducing the fragmentation degree as can be recognized in the third stage. The evolution of the second tuff was similar, starting with radial-diluted pyroclastic surges and finishing with base surges deposits, suggesting lower water/magma ratio and higher fragmentation degree. Whereas the south cone originates dry pyroclastic surges and many tuff facies, northern one does not go beyond the deposition of a laminated tuff. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Hydrovolcanism is the result of efficient interaction of magma with variable amount of water triggering an explosive eruption when it takes place at shallow marine depths or subaerial environments. Characteristic volcanic edifices and deposits are generated, classified as tuff cones, tuff rings or maars (Wood, 1980; Wohletz and Sheridan, 1983; White, 1991). Explosive hydrovolcanic eruptions generally form small monogenetic volcanoes, whose deposits are mainly controlled by water/ magma (W/M) contact ratio at the time of eruption. Relative low (W/M) ratio generates tuff rings (Wohletz and McQueen, 1984) formed mainly of base-surge deposits resulting in low angle volcano morphology. The morphological parameters of these edifices are: crater diameters ranging from 0.2 to 3.0 km and heights commonly <100 m, showing therefore a low aspect ratio ranging from 0.05 to 0.13 (Wohletz and Sheridan, 1983; Sohn, 1996; Vespermann and Schmincke, 2000). During the course of an eruption, water/magma ratio can change since their evolution is principally controlled by the hydro-

⇑ Corresponding author. Fax: +34 915644740. E-mail address: [email protected] (J. Carmona).

geologic factors around the vent such as the substrate nature and the physical conditions (Dobran and Papale, 1993; Houghton et al., 1999; Kokelaar, 1986; Lorenz, 1986; Sheridan and Wohletz, 1981; Sohn, 1996; White, 1996; Wohletz, 1986). In this sense, pure magmatic phases would be generated when the W/M ratio is very low or non-existent, pyroclasts and lavas being the only volcanic products and hydromagmatic phases would result when the ratio increases. When magma and water interaction occurs, the eruptive mechanism changes to magmatic quenching and chilling, producing explosions, steam flows and eruptive columns formation and collapses. When column collapse occurs, the subsequent base surges and their resultant deposits are characterized by the presence of sand-wave bedforms and tractional structures (Wohletz and Sheridan, 1979). Water–magma interaction may occur during all eruptive period or at any time during the eruption, therefore final structure of the edifice would be highly dependent on the time when the ratio increases, recognized by the presence of gradual or sharp gradations to magmatic or hydromagmatic phases. Furthermore, the post eruptive morphological evolution will depend of the internal distribution of the magmatic and hydromagmatic sequences. In general, relatively little was known about hydrovolcanism at the Canaries. However, detailer studies in other islands such as Lanzarote have shown that this type of volcanism is widespread

1464-343X/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2010.07.003

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and is relatively abundant, showing characteristic structures and textures (Martí and Colombo, 1990; Aparicio et al., 1994; De la Nuez et al., 1997; García-Cacho and Romero, 2000; Romero et al., 2007). Only the general features of the hydrovolcanism at Tenerife has been characterized (Alonso and Rodríguez, 1992; De la Nuez et al., 1993), and no detailed studies about the deposits that configure these edifices conducted as focus of previous studies mainly was on the activity of the Teide-Cañadas edifice. However, there are some specific studies which focused on the petrologic, stratigraphic, geologic and geomorphologic analysis of these types of volcanoes (Araña et al., 1986; Alonso et al., 1992; Notario del Pino et al., 1996; Clarke et al., 2005). 2. Regional setting Canary Islands are an archipelago composed of seven islands, located on the north-western African continental margin. Tenerife constitutes the largest (2034 km2) and highest (3718 m a.s.l.) of the Canary Islands (Fig. 1). It was formed as a result of the accumulation of varied volcanic materials (mafic, felsic and intermediate) through a long period of geologic time (see Ancochea et al., 1990). The oldest subaerial volcanic rocks (Old Basaltic Series) are found in the Massifs of Anaga (NE), Teno (NW) and Roque del Conde (S), with ages ranging from 12 Ma in Roque del Conde to 7 Ma at the bottom of Anaga Massif (Ancochea et al., 1990). Around 3 Ma ago volcanic activity shifted to the central part of the island (Cañadas Series, Cañadas Edifice). The Central Complex has an elongated morphology and a complex structure resulting from the superposition of different volcanic edifices. Most of the eruptions that gave rise to the Cañadas edifice were explosive producing a large variety of phonolitic pyroclastic deposits mostly exposed along the southern slopes of Tenerife (Marti et al., 1994; Bryan et al., 1998). Coeval with the construction of the Cañadas edifice, shield basaltic volcanism has continued till the present along rift zones oriented NW–SE and NE–SW, and covered a large area at the south (Ancochea et al., 1990; Dóniz, 2004; Galindo et al., 2005). This basaltic volcanism is responsible for the formation of hundreds of monogenetic volcanoes, characterized by effusive and explosive Strombolian activity (Dóniz, 2005; Dóniz et al., 2008), although only four of them correspond to historical times (the last 500 years) (Romero, 1991). Hydrovolcanism is relatively common at the Canary Archipelago and it is present in all islands except in La Gomera and Fuerteventura (De la Nuez et al., 1997). Most of these volcanoes are the result of the interaction of basaltic magma and sea water, while

phreatomagmatic events originated from ground water involvement are scarce, which explains why almost all hydrovolcanic edifices are located on the coast or next to, always less than 3 km from the shore line (Romero, 2003). Usually these edifices begin with hydromagmatic phases evolving towards dryer and magmatic phases as water availability decreases (e.g. Montaña Erales, Montaña Mosta, Montaña Cavera, and others). Hydrovolcanic edifices in Tenerife are less common than in the rest of the islands, at least four monogenetic volcanoes have been recognized: Caldera del Rey, Montaña de los Erales, Montaña Pelada and Montaña Amarilla (Araña et al., 1986; De la Nuez et al., 1997). All of these edifices are located in the south (Fig. 1), with distances less than 3 km from a very populated coast with important touristic resources. A proper understanding of hydrovolcanic evolution, their distribution and eruptive mechanisms is essential in order to evaluate the possible impact of these eruptions on a territory that currently has a population of 2 million people, and which is visited each year by approximately 10 million tourists. This work presents for first time a detailed stratigraphic study and facies analysis of the deposits of the Montaña Pelada (also locally known as Motaña Escachada) hydrovolcanic monogenic edifice of Tenerife. 3. Montaña Pelada tuff ring (MPTR) Although hydrovolcanism is not very common in Tenerife, almost all edifices are located along a narrow band in the south of the island. The Montaña Pelada tuff ring (MPTR) has been selected since it is a well-known tuff ring that has not been described before. MPTR is overlain by a basaltic scoria field which age date is not determined, although according to Hernández-Pacheco and Fernández (1978) MPTR would have formed during the Lower Pleistocene. The edifice configuration is a tuff ring of 105 m high and displays 852 m maximum rim diameter (Fig. 2). The aspect ratio (height/rim diameter) of MPTR is 0.13 which is similar to many others tuff rings worldwide (Sohn, 1996). Deposits distribution, dipping directions and morphological features suggest the presence of another hydrovolcanic edifice to the south of Montaña Pelada. This edifice has been completely eroded by the oceanic action and rim collapses. An unconformity can be recognized within the beds, the southern crater exhibit beds dipping to the north overlaid by beds dipping to the south from the northern edifice. MPTR hydrovolcanic deposit components are grouped as follows: (1) vitric yellow pyroclasts (sideromelane) and dark basaltic dark fragments (tachylite), both appearing in all granulometric

Fig. 1. Location map of Tenerife in the Canary Islands inset (left); and the hydromagmatic edifices distribution in the south of the island (right).

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Fig. 2. Geological setting of Montaña Pelada tuff ring.

range; (2) sedimentary or volcanosedimentary fragments which normally show coarse lapilli and fine block sizes corresponding to limestone, sandstone, pumice fragments and breccia tuffs; and (3) trachybasaltic and dioritic fragments, showing coarse lapilli and block sizes. Juvenile fragments correspond to basaltic bombs and the vitric and basaltic dark pyroclasts; cognate lithics are non-vesiculated juvenile basaltic and trachybasaltic fragments, very common at the sequence; and accessory lithics correspond to the sedimentary, pumitic and plutonic nature. Basaltic fine and medium lapilli and ash are the dominant grain size components in the cone, although coarse lapilli and block size fragments can be frequent in some sections of the volcano. Ballistic ejecta, usually fine blocks (<256 mm), with impact-sags structures are relatively common, although the impact clasts are not always preserved.

3.1. Facies analysis Detailed facies analysis has been performed to characterize the processes that formed the MPTR deposits and evolution therein. Five sections across the MPTR volcano have been studied for cm-scale stratigraphic detail, although incomplete exposure of sections in some cases hamper a more detailed characterization of the facies. Two of these sections (Sections 1 and 2) belong to the south edifice (hereafter SMPTR) and the other three (Sections 3–5) to the north edifice (NMPTR). Sections from NMPTR are very similar in grain-size distribution and in stratification pattern to SMPTR. With the exception of its upper part, the SMPTR volcanic sequence has been recognized also in the NMPTR, the implications of which will be discussed in the next sections. The entire sequence of MPTR is divided into 10 facies based on grain-size distribution,

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stratigraphic features and the presence of a particular sedimentary structure. The classification of Ingram (1954) is used to classify the beds according to thickness as follows: lamina <1 cm; very thin bed, 1–3 cm; thin bed, 3–10 cm; medium bed, 10–30 cm; thick bed, 30–100 cm; and very thick bed, >100 cm. The granulometric classification followed in this work is as follows: fine ash, <1/ 16 mm; medium ash, <1/2 mm; coarse ash, <2 mm; fine lapillus, <4 mm; medium lapillus, <16 mm; coarse lapillus, <64 mm; fine block, <256 mm; and coarse block, P256 mm. 3.1.1. Facies LT1: diffuse-bedding to massive stratified lapilli-tuff The facies consists of fine to coarse lapilli and medium to coarse ash matrix, in which only medium lapilli occasionally are normally graded (represented in all sections, Fig. 3). Juvenile basaltic lapilli dominate, although variations within the sequence are frequent which are more pronounced in the uppermost parts of the layers. Non-vesiculated fine basaltic and sedimentary blocks are frequent and exceptionally coarse basaltic blocks show impact sag structures. The diffuse bedding is formed by laterally consistent layers marked by either changes in granulometry or by grain-supported lapilli trains presence. Thickness ranges from thin to thick beds and normally grain-supported beds appear on the top of the sequence (Fig. 4A and B and Table 1). A dark biconvex lenticular layer of grain-supported, medium to coarse, basaltic lapilli lenses in a matrix of medium to coarse ash is encountered embedded in some parts of this massive stratified facies. Juvenile lapilli appear occasionally vesiculated in the same size range. The lenses do not exhibit erosional lower contact in the main bedding sequence, and occur as isolated lenses in the uppermost part of the facies. The lenses range in thickness from few cms to a maximum of 30 cm and with a maximum length of 50 cm. Interpretation. LT1 deposits correspond to the proximal deposits of the near-vent areas formed by the rapid emplacement of radialdiluted pyroclastic density currents, whose basal parts were highconcentration or granular fluid-based pyroclastic density currents (Branney and Kokelaar, 2002). Grain size variations and the presence of lenses suggest changes in depositional environment indicating the existence of turbulent condition during the emplacement. Furthermore, lapilli lenses inside LT1 could suggest grain transportation from previous fallout deposits (Sohn and Chough, 1992, 1993) and are indicative of a dryer evolution of the main sequence since the diffuse bedding and massive stratified

deposits could separate out and evolve into clusters of lapilli lenses. According to Sumita et al. (2004) other possibility could be that lapilli lenses are relatively dry tephra jets that may evolve into a debris flow if the flow enters wet ground conditions or if the flow has higher water content. The presence of Mud crack-like structures on the top of some strata suggests a temporal gap of surges formation. 3.1.2. Facies T1: diffuse-bedding tuff The diffuse-bedding tuff include fine to coarse ash with occasionally fine basaltic and sandstone blocks, always in the lowermost part of the sequence (Sections 1 and 4 in Figs. 3 and 5). Bedding ranges from thin to medium thickness. Some diffuse planar bedding displaying a tiny wavy pattern can be distinguished (Section 1) although in general this facies exhibits a diffuse-bedding aspect. Interpretation. The diffuse bedding and the finer granulometry of this facies is interpreted as medial to distal deposits from turbulent hydromagmatic surges and concurrent simultaneous fallout. 3.1.3. Facies LT2: thinly stratified lapilli-tuff Thinly stratified lapilli-tuff consists of a well-defined alternation of fine to coarse grain-supported lapilli and fine to coarse ash rich layer (Sections 4 and 5 in Fig. 3). The thickness of the lapilli layers range from 10 to 30 cm, whereas the tuff layers range in thickness from 5 to 10 cm (Fig. 4C and Table 1). Inversely graded units which are characteristics of this facies; begins with fine normally graded layers and evolve to grain-supported layers reaching to a maximum thickness of 80 cm. These units are repeated several times within the facies. Interpretation. Thin stratification, alternation of grain-supported layers and inversely-graded layers is interpreted as being constituted of proximal-to-medial deposits from pyroclastic surges and fallout. 3.1.4. Facies TB: Tuff lapilli breccia The tuff lapilli breccia is composed of more than 30% of compositionally heterogeneous angular fine blocks and non-vesiculated basaltic lapilli (Section 1, Fig. 3). Blocky size fragments of trachybasalt, sandstone and limestone embedded in a white coarse ash matrix are frequent. The apparent maximum thickness of the exposed breccia is about 50 cm, although since the outcrop is partially covered by slope deposits, the real thickness cannot be determined.

Fig. 3. Stratigraphic profiles and facies recognized at Montaña Pelada tuff ring. Doted areas in Sections 2 and 3 are fossilized dune and climbing dune deposits, respectively. Fragments composition: black, basaltic; white, vitric; doted, sandstones; bricked, limestones.

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Fig. 4. Field photograph of the outcrop aspect of facies from MPTR: (A) diffuse-bedding to massive stratified lapilli-tuff (LT1), where the main bedding changes are recognized by changes in the granulometry and lapilli composition; (B) massive stratified aspect of LT1 lapilli-tuff where bedding can be seen though the general aspect is massive; (C) thinly stratified lapilli-tuff (LT2) composed of the alternation of lapilli and ash beds, in this picture some erosive surfaces can be recognized; (D) aspect of the disorganized lapilli-tuff (LT3) where almost all granulometric range is present, note the apparent massive to crude stratification pattern.

Table 1 Table of sorting (rphi) and median size (Mdphi) values (Inman, 1952). Facies

Sample

r

Md

LT1 LT1 LT1 LT1 LT2 LT2 LT3 LT3 Ta Ta Tb Tb

M1-01 M1-02 M3-01 M3-02 M2-01 M2-02 M4-01 M4-02 2A-01 2A-02 2A2-01 2A2-02

1.810 1.850 2.397 2.575 2.359 2.389 1.762 1.749 1.620 1.615 2.090 2.090

2.297 2.297 1.557 1.557 2.500 2.432 2.297 2.344 1.950 1.950 1.019 1.019

Interpretation. The matrix supported and unsorted texture, together with the presence of fragments of country rock in the pyroclastic beds of this facies, suggests an over-excavation of fragmentation levels. 3.1.5. Facies Ta: wavy bedded tuff The wavy bedded tuff is composed of fine to coarse ash and fine to medium lapilli (Section 1, Figs. 3 and 5). This facies is characterized by normal grading, very thin and thick continuous wavy bedding, and the presence of sinuous and symmetrical waveforms. Waveforms show large wavelength ranging between 2 and 2.5 m, and with 10–50 cm in amplitude gently dipping stoss and lee sides and smoothly rounded wave crest. Cross-laminated lenses which are always located under the waveform crests, appear interbedded within these layers. Very thin to thin laminae are normally graded

and the foreset laminate shows 15°NW of maximum dipping. These lenses have a maximum thickness of 30 cm and maximum length of 50 cm. Impact sag structures are also common preserving in some cases the original block (Table 1). Interpretation. The fine-grained and wavy bedding are indicative of pyroclastic density currents deposition. 3.1.6. Facies Tb: dune bedded tuff This facies is composed of fine to coarse ash and fine to medium basaltic lapilli (Section 1, Figs. 3 and 5). It is characterized by the presence of lower flow regime structures such as dunes and ripples. Dunes are dominant in the whole sequence, showing a slope angle of 5–10° on the lee side. Their wavelengths could not be determined due to bad outcrop exposure. Cross-laminated lenses underlying the dunes are present. Dunes are distinguished from the cross-laminated lenses by the larger amplitude and the lower dipping angle, usually less than 10°. The whole sequence show a maximum thickness of 70 cm between the two crests of the underlying facies (Table 1). Interpretation. Cross-lamination and dune bedding suggest deposition from turbulent pyroclastic density currents with low particle concentration (Sohn and Chough, 1989, 1992) proximalto-medial from dry pyroclastic surges. 3.1.7. Facies Tc: erosive-wavy bedded tuff With respect to the granulometry and bedding features, this facies is very similar to Ta but differs in the erosive base of the sequence and in having beds with lower dipping angle (Section 1, Figs. 3 and 5). This facies starts with an erosive contact where symmetrical lower flow regime structures, such as ripples develop.

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Fig. 5. Field photograph showing outcropping aspect of tuff facies from upper part of SMPTR where crudely stratified tuff (T1), undulatory bedded tuff (Ta), dune bedded tuff (Tb), erosive-undulatory bedded tuff (Tc), blocky tuff (Td) and thinly bedded tuff (Te) develop within just 8 m thickness.

The wave lengths of these structures range from 60 to 100 cm and amplitude from 10 to 20 cm and with no internal structure has been preserved. At the bottom of this facies ash proportion dominates whereas upward in the sequence lapilli appears more frequently compared to Ta facies, where lapilli concentration decreases upwards. Interpretation. The fine-grained, laminated and wavy texture beds with erosive basal contact are indicative of deposition from horizontally mobile pyroclastic density currents, such as base surges capable of reworking the underlying deposits.

3.1.8. Facies Td: blocky tuff This facies is composed of coarse basaltic lapilli and basaltic and sedimentary fine blocks embedded in a fine to coarse ash matrix (Section 1, Figs. 3 and 5). With a maximum of one meter facies thickness, it shows frequent impact sag structures that sometimes are deep enough to penetrate in the underlying facies.

Interpretation. Coarse granulometry embedded into a finer ash matrix suggest rapid ash fallout from dense suspension with synchronous emplacement of ballistic block. 3.1.9. Facies Te: thinly bedded tuff This facies is characterized by a well-defined alternation of coarse and fine lapilli grained thin beds (Section 1 and 5, Figs. 3 and 5). Bedding is dominantly horizontal, only impact-sags structures break the layer and the horizontal continuity. Interpretation. The thinly horizontal bedding and grain size alternation suggest deposition of base surges. 3.1.10. Facies LT3: disorganized lapilli-tuff This facies comprises a thick sequence of fine to coarse lapilli embedded into a medium to coarse ash matrix with frequent fine and coarse basaltic and sedimentary blocks (Section 3, Fig. 3). Large blocks corresponding to non-vesiculated basaltic compositions are

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common, whereas accessory lithics such as fine blocks appearing in minor proportions are exposed (Fig. 4D). Interpretation. Thick stratification and poorly sorted deposits indicate deposition from pyroclastic surges lacking tractional processes. 4. Volcanic evolution Facies distribution suggests that the first hydrovolcanic edifice was SMPTR, and NMPTR was built at the final stages or completely after SMPTR was formed. 4.1. SMPTR SMPTR can be subdivided in two main parts. The first part is mainly composed of diffuse-bedding to massive stratified lapillituff facies (LT1), whereas the second part is composed of tuff facies (T1, Ta, Tb, Tc, Td and Te), separated by a breccia tuff (TB). However lateral facies variation cannot be observed and no correlation with other parts of the volcano can be established since Section 2 (Fig. 3) is partially covered with a fossilized dune. The first part is the thickest of the stratigraphic sequence and it can be recognized in both sections. It constitutes the bulk of the SMPTR and is located on the near-vent area of the volcano, and can be correlated to the proximal deposits formed by the emplacement of the base surges synchronous to fallout processes. Block and bombs impact sags indicates ballistically emplaced ejecta from explosions simultaneously with surges formation. Mud crak-like structures on the top of some strata suggests a temporal gap of

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surges, indicating also water-saturated depositional condition (Risso et al., 2008). Lapilli lenses are indicative of a dryer evolution of the main sequence since LT1 deposits could separate out and evolve into clusters of lapilli lenses. Therefore deposition of diffuse bedding to massive facies suggests high W/M ratios. The second of SMPTR is recognized only at Section 1 (Fig. 3), starting with an explosion breccia (TB) followed by the tuff facies (T1, Ta, Tb, Tc, Td and Te). The real thickness cannot be estimated because it is partially eroded. The heterogeneous composition of TB which is mainly composed of sedimentary and trachybasaltic blocks suggest an increase of explosivity and over-excavation of fragmentation levels, probably by phreatic waters input. During this part of the eruption water/magma ratios decreased, increasing the fragmentation degree and depositing the medial-to-distal tuff facies from turbulent pyroclastic density currents and fallout. Deposition of crudely stratified and massive facies suggests high W/M ratios. During the eruption water availability decreased, increasing the effective fragmentation and depositing the tuff facies (Fig. 6). 4.2. NMPTR NMPTR edifice is completely preserved in Sections 3–5 used for the stratigraphic analysis (Fig. 3). The deposits are similar to the first part of SMPTR, but only tuff deposits have been recognized at distal parts of the volcano. The first recognized deposits correspond to LT1 representing the proximal deposits of the near-vent areas formed in the same

Fig. 6. W/M ratio and effective fragmentation evolution of MPTR. On the margins grain-size distribution of main facies and the median sorting (rphi) and size (Mdphi) values (Inman, 1952), SMPTR: LT1 histogram representing the grain-size distribution of the crudely stratified lapilli-tuff, typical facies of the first stage of the hydrovolcanic eruption; Ta histogram representing the grain-size distribution of undulatory bedded tuff, note that in the SMPTR evolution the median size decreases towards the tuffs; NMPTR: LT1 histogram representing the grain-size distribution of thickly stratified lapilli-tuff and LT2 histogram representing the grain-size distribution of stratified lapillituff.

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Fig. 7. Selected of SEM photographs of the fine ash clasts: (A) general aspect of clasts of different nature, blocky and equant vitric ash and feldspar crystals; (B) from top left to right spherical particle, indicating ductile fragmentation; highly vesicular juvenile crystal and blocky and equant particle of fine ash; from the bottom left side to right feldspar crystal and blocky and equant fragment; (C) aspect of an irregular vitric fragment showing concoidal fragmentation and (D) blocky and equant glass fragment.

way as in SMPTR, followed by a thick medial LT3 facies from pyroclastic surges with no tractional processes. Impact sags indicate ballistically emplacement of bombs at the same time of the surge emplacement. High concentration of accidental lithics indicates high magma ascend rate and/or more energetic fragmentation phase within the eruption. Facies T1 separates Sections 3 and 4, the smaller grain size and the laminated pattern could indicate a reduction in W/M ratio. Then surges evolve to relatively slow fall out rate resulting in grain segregation depositing the proximal-to-medial LT2 facies in dryer conditions. Finally surges return to radial-diluted pyroclastic density currents depositing an important thick LT1 facies. Distal deposits as observed in Section 5 (Fig. 3) are composed of an alternation of LT2 and Te facies originated by base surges deposition. LT2 exhibits erosive surfaces, produced by aqueous turbulent flows (Fig. 4C) indicating also an interruption of surge emission. Eruptive evolution of NMPTR follow the same pattern than SMPTR but the effective fragmentation and W/M ratios were not enough to generate the tuff facies (Fig. 6).

5. Summary and conclusions The growth of Montaña Pelada tuff ring can be subdivided into three main stages, based on changes in the dominant depositional

processes. The fist stage corresponds to the formation of SMPTR composed of proximal deposits; consisting of diffuse-bedding to massive stratified lapilli-tuff facies (LT1) deposited by successive diluted pyroclastic density currents synchronous to fallout and ballistic ejecta generation. The second stage consists of the deposition of dry base surges. This low-concentration and pulsatory surges should be responsible for the deposition of stratified and laminated tuff facies. The thickness and lateral continuity of each lamina suggest that the turbulent eddies were large-scale structures, coherent for long distances and becoming a well-segregated surges depositing the fine-grained layers indicating lower W/M ratios. At the third stage, NMPTR was formed 750 m northwards of the SMPTR with deposits analogous to the first phase of SMPTR with the only difference of well represented distal deposits. The connection between the SMPTR and NMPTR edifices can be explained by a N–S fracture that drove the magma flow towards the present position of NMPTR where the hydrological conditions allowed new water input. The decrease in particle concentration of surges would be responsible for the distal facies deposits such as the stratified lapilli-tuff and the laminated tuff. This last facies represents the highest fragmentation and magma ratio that could reach to about 0.3 (Sheridan and Wohletz, 1981, 1983; Wohletz, 1986). The study of ash using SEM may elucidate the fragmentation process operating during explosive eruptions (Wohletz, 1986).

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Fig. 8. Outcrop aspect: top picture: contact between previous scorias deposits and hydrovolcanic deposits of MPTR. Bottom: detailed aspect of the contact: (A) trachybasaltic scorias; (B) paleosoil overlying the scorias; (C and D) fine ash and well sorted lapilli-fall deposits, respectively, from SMPTR distal facies; and (E) LT3 facies from NMPTR.

The occurrence of blocky and equant clasts (fragments with equilateral sides crossing at right angles, Fig. 7) is considered indicative of phreatomagmatic fragmentation mechanism and the presence of spherical particles of ductile fragmentation ash is characteristic of fine-grained pyroclasts of preatomagmatic origin. Furthermore, high concentrations of accidental lithics suggest the presence of an aquifer in contact to the conduit. The presence of a basaltic scoria flow underlying the NMPTR hydrovolcanic deposits suggests a previous eruption from other surrounding volcano although studies from Hernández-Pacheco and Fernández (1978) related these flows to the MPTR eruption (Fig. 8). If this was true, the eruption should have started with a magmatic phase that has not been recognized in any section of MPTR. The deposits suggest a large period of time between the scoria flow and the hydrovolcanic eruption since an incipient paleosoil has been developed (Fig. 8). Also the first hydrovolcanic deposits are characterized by fine ash and well sorted lapilli corresponding to the distal facies of SMPTR originated by fall out, suggesting an ash cloud expansion wider than 1 km. Then a coarse facies develops overlying the fine ash and well sorted lapilli deposits with similar characteristic to LT3 facies from the NMPTR.

The volcanic evolution and deposits formation has been mainly controlled by two closely related parameters, the W/M ratio and the effective fragmentation. Proximal deposits suggest high W/M ratios at the time of this eruption, although with relatively low effective fragmentation. This situation changed with the deposition of the turbulent pyroclastic surges that resulted in the formation of the tuff facies. These facies indicate lower W/M ratios than in the initial stages and higher effective fragmentation. After deposition of these surges the opening of a N–S fracture drove the eruption northwards creating a new vent. The new hydrological conditions allowed the input of phreatic waters, increasing the W/M and reducing the fragmentation degree. Dryer facies were deposited indicating lower W/M ratio and higher fragmentation degree. As can be seen the evolution of both edifices is similar (Fig. 6), although whereas dry pyroclastic surges and many tuff facies are developed in the SMPTR, the NMPTR do not evolve beyond the deposition of a laminated tuff. Acknowledgements This project was supported by the Project Reference 200430E438 of the Spanish CSIC (Scientific Research Council).

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