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

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Journal of African Earth Sciences

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Characterization and facies analysis of the hydrovolcanic deposits of MontañaPelada 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, Spainb Dpto. Geografía, Universidad de La Laguna, Tenerife, Spainc Escuela Universitaria de Turismo Iriarte, Universidad de la Laguna, Puerto de la Cruz, Tenerife, Spain

a r t i c l e i n f o

Article history:Received 7 October 2009Received in revised form 23 July 2010Accepted 30 July 2010Available online 10 August 2010

Keywords:HydrovolcanismPhreatomagmaticBase surgesWater/magma ratioFacies analysisMontaña PeladaTenerife

1464-343X/$ - see front matter Crown Copyright � 2doi:10.1016/j.jafrearsci.2010.07.003

⇑ Corresponding author. Fax: +34 915644740.E-mail address: jcarmona@mncn.csic.es (J. Carmon

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 twoedifices each with its distinct internal depositional distribution. A detailed stratigraphic analysis was car-ried out and ten facies were recognized. Deposits interpretation has revealed that water/magma ratiochanges controlled the eruptive evolution, distinguishing three main stages of the eruption. Pyroclasticdensity 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 north-wards creating a new edifice. The new hydrological conditions allowed the input of phreatic water, whichresulted in high proportion of accidental lithics within characteristic of the deposits, increasing thewater/magma ratio and reducing the fragmentation degree as can be recognized in the third stage. Theevolution of the second tuff was similar, starting with radial-diluted pyroclastic surges and finishing withbase surges deposits, suggesting lower water/magma ratio and higher fragmentation degree. Whereasthe south cone originates dry pyroclastic surges and many tuff facies, northern one does not go beyondthe deposition of a laminated tuff.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Hydrovolcanism is the result of efficient interaction of magmawith variable amount of water triggering an explosive eruptionwhen it takes place at shallow marine depths or subaerial environ-ments. Characteristic volcanic edifices and deposits are generated,classified as tuff cones, tuff rings or maars (Wood, 1980; Wohletzand Sheridan, 1983; White, 1991).

Explosive hydrovolcanic eruptions generally form small mono-genetic 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 vol-cano morphology. The morphological parameters of these edificesare: crater diameters ranging from 0.2 to 3.0 km and heightscommonly <100 m, showing therefore a low aspect ratio rangingfrom 0.05 to 0.13 (Wohletz and Sheridan, 1983; Sohn, 1996;Vespermann and Schmincke, 2000).

During the course of an eruption, water/magma ratio canchange since their evolution is principally controlled by the hydro-

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a).

geologic factors around the vent such as the substrate nature andthe physical conditions (Dobran and Papale, 1993; Houghtonet al., 1999; Kokelaar, 1986; Lorenz, 1986; Sheridan and Wohletz,1981; Sohn, 1996; White, 1996; Wohletz, 1986). In this sense, puremagmatic phases would be generated when the W/M ratio is verylow or non-existent, pyroclasts and lavas being the only volcanicproducts and hydromagmatic phases would result when the ratioincreases. When magma and water interaction occurs, the eruptivemechanism changes to magmatic quenching and chilling, produc-ing explosions, steam flows and eruptive columns formation andcollapses. When column collapse occurs, the subsequent basesurges and their resultant deposits are characterized by the pres-ence of sand-wave bedforms and tractional structures (Wohletzand Sheridan, 1979).

Water–magma interaction may occur during all eruptive periodor at any time during the eruption, therefore final structure of theedifice would be highly dependent on the time when the ratio in-creases, recognized by the presence of gradual or sharp gradationsto magmatic or hydromagmatic phases. Furthermore, the posteruptive morphological evolution will depend of the internal dis-tribution of the magmatic and hydromagmatic sequences.

In general, relatively little was known about hydrovolcanism atthe Canaries. However, detailer studies in other islands such asLanzarote have shown that this type of volcanism is widespread

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42 J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50

and is relatively abundant, showing characteristic structures andtextures (Martí and Colombo, 1990; Aparicio et al., 1994; De laNuez et al., 1997; García-Cacho and Romero, 2000; Romero et al.,2007). Only the general features of the hydrovolcanism at Tenerifehas been characterized (Alonso and Rodríguez, 1992; De la Nuezet al., 1993), and no detailed studies about the deposits that config-ure these edifices conducted as focus of previous studies mainlywas on the activity of the Teide-Cañadas edifice. However, thereare some specific studies which focused on the petrologic, strati-graphic, geologic and geomorphologic analysis of these types ofvolcanoes (Araña et al., 1986; Alonso et al., 1992; Notario del Pinoet 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. Tenerifeconstitutes the largest (2034 km2) and highest (3718 m a.s.l.) of theCanary Islands (Fig. 1). It was formed as a result of the accumula-tion 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 delConde (S), with ages ranging from 12 Ma in Roque del Conde to7 Ma at the bottom of Anaga Massif (Ancochea et al., 1990).

Around 3 Ma ago volcanic activity shifted to the central part ofthe island (Cañadas Series, Cañadas Edifice). The Central Complexhas an elongated morphology and a complex structure resultingfrom the superposition of different volcanic edifices. Most of theeruptions that gave rise to the Cañadas edifice were explosive pro-ducing a large variety of phonolitic pyroclastic deposits mostly ex-posed along the southern slopes of Tenerife (Marti et al., 1994;Bryan et al., 1998).

Coeval with the construction of the Cañadas edifice, shieldbasaltic volcanism has continued till the present along rift zonesoriented NW–SE and NE–SW, and covered a large area at the south(Ancochea et al., 1990; Dóniz, 2004; Galindo et al., 2005). Thisbasaltic volcanism is responsible for the formation of hundredsof monogenetic volcanoes, characterized by effusive and explosiveStrombolian activity (Dóniz, 2005; Dóniz et al., 2008), althoughonly four of them correspond to historical times (the last500 years) (Romero, 1991).

Hydrovolcanism is relatively common at the Canary Archipel-ago and it is present in all islands except in La Gomera and Fuert-eventura (De la Nuez et al., 1997). Most of these volcanoes are theresult of the interaction of basaltic magma and sea water, while

Fig. 1. Location map of Tenerife in the Canary Islands inset (left); and the

phreatomagmatic events originated from ground water involve-ment are scarce, which explains why almost all hydrovolcanic edi-fices are located on the coast or next to, always less than 3 km fromthe shore line (Romero, 2003). Usually these edifices begin withhydromagmatic phases evolving towards dryer and magmaticphases as water availability decreases (e.g. Montaña Erales, Mont-aña Mosta, Montaña Cavera, and others).

Hydrovolcanic edifices in Tenerife are less common than in therest of the islands, at least four monogenetic volcanoes have beenrecognized: Caldera del Rey, Montaña de los Erales, Montaña Pela-da and Montaña Amarilla (Araña et al., 1986; De la Nuez et al.,1997). All of these edifices are located in the south (Fig. 1), withdistances less than 3 km from a very populated coast with impor-tant touristic resources. A proper understanding of hydrovolcanicevolution, their distribution and eruptive mechanisms is essentialin order to evaluate the possible impact of these eruptions on a ter-ritory that currently has a population of 2 million people, andwhich is visited each year by approximately 10 million tourists.This work presents for first time a detailed stratigraphic studyand facies analysis of the deposits of the Montaña Pelada (alsolocally known as Motaña Escachada) hydrovolcanic monogenicedifice of Tenerife.

3. Montaña Pelada tuff ring (MPTR)

Although hydrovolcanism is not very common in Tenerife, al-most all edifices are located along a narrow band in the south ofthe island. The Montaña Pelada tuff ring (MPTR) has been selectedsince it is a well-known tuff ring that has not been describedbefore.

MPTR is overlain by a basaltic scoria field which age date is notdetermined, although according to Hernández-Pacheco and Ferná-ndez (1978) MPTR would have formed during the Lower Pleisto-cene. The edifice configuration is a tuff ring of 105 m high anddisplays 852 m maximum rim diameter (Fig. 2). The aspect ratio(height/rim diameter) of MPTR is 0.13 which is similar to manyothers tuff rings worldwide (Sohn, 1996). Deposits distribution,dipping directions and morphological features suggest the pres-ence of another hydrovolcanic edifice to the south of MontañaPelada. This edifice has been completely eroded by the oceanic ac-tion and rim collapses. An unconformity can be recognized withinthe beds, the southern crater exhibit beds dipping to the northoverlaid by beds dipping to the south from the northern edifice.

MPTR hydrovolcanic deposit components are grouped as fol-lows: (1) vitric yellow pyroclasts (sideromelane) and dark basalticdark fragments (tachylite), both appearing in all granulometric

hydromagmatic edifices distribution in the south of the island (right).

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

J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50 43

range; (2) sedimentary or volcanosedimentary fragments whichnormally show coarse lapilli and fine block sizes correspondingto limestone, sandstone, pumice fragments and breccia tuffs; and(3) trachybasaltic and dioritic fragments, showing coarse lapilliand block sizes. Juvenile fragments correspond to basaltic bombsand the vitric and basaltic dark pyroclasts; cognate lithics arenon-vesiculated juvenile basaltic and trachybasaltic fragments,very common at the sequence; and accessory lithics correspondto the sedimentary, pumitic and plutonic nature. Basaltic fineand medium lapilli and ash are the dominant grain size compo-nents in the cone, although coarse lapilli and block size fragmentscan be frequent in some sections of the volcano. Ballistic ejecta,usually fine blocks (<256 mm), with impact-sags structures are rel-atively common, although the impact clasts are not alwayspreserved.

3.1. Facies analysis

Detailed facies analysis has been performed to characterize theprocesses that formed the MPTR deposits and evolution therein.

Five sections across the MPTR volcano have been studied forcm-scale stratigraphic detail, although incomplete exposure of sec-tions in some cases hamper a more detailed characterization of thefacies. Two of these sections (Sections 1 and 2) belong to the southedifice (hereafter SMPTR) and the other three (Sections 3–5) to thenorth edifice (NMPTR). Sections from NMPTR are very similar ingrain-size distribution and in stratification pattern to SMPTR. Withthe exception of its upper part, the SMPTR volcanic sequence hasbeen recognized also in the NMPTR, the implications of whichwill be discussed in the next sections. The entire sequence ofMPTR is divided into 10 facies based on grain-size distribution,

44 J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50

stratigraphic features and the presence of a particular sedimentarystructure. The classification of Ingram (1954) is used to classify thebeds according to thickness as follows: lamina <1 cm; very thinbed, 1–3 cm; thin bed, 3–10 cm; medium bed, 10–30 cm; thickbed, 30–100 cm; and very thick bed, >100 cm. The granulometricclassification 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; fineblock, <256 mm; and coarse block, P256 mm.

3.1.1. Facies LT1: diffuse-bedding to massive stratified lapilli-tuffThe facies consists of fine to coarse lapilli and medium to coarse

ash matrix, in which only medium lapilli occasionally are normallygraded (represented in all sections, Fig. 3). Juvenile basaltic lapillidominate, although variations within the sequence are frequentwhich are more pronounced in the uppermost parts of the layers.Non-vesiculated fine basaltic and sedimentary blocks are frequentand exceptionally coarse basaltic blocks show impact sag struc-tures. The diffuse bedding is formed by laterally consistent layersmarked by either changes in granulometry or by grain-supportedlapilli trains presence. Thickness ranges from thin to thick bedsand normally grain-supported beds appear on the top of the se-quence (Fig. 4A and B and Table 1).

A dark biconvex lenticular layer of grain-supported, medium tocoarse, basaltic lapilli lenses in a matrix of medium to coarse ash isencountered embedded in some parts of this massive stratified fa-cies. Juvenile lapilli appear occasionally vesiculated in the samesize range. The lenses do not exhibit erosional lower contact inthe main bedding sequence, and occur as isolated lenses in theuppermost part of the facies. The lenses range in thickness fromfew cms to a maximum of 30 cm and with a maximum length of50 cm.

Interpretation. LT1 deposits correspond to the proximal depositsof the near-vent areas formed by the rapid emplacement of radial-diluted pyroclastic density currents, whose basal parts were high-concentration or granular fluid-based pyroclastic density currents(Branney and Kokelaar, 2002). Grain size variations and thepresence of lenses suggest changes in depositional environmentindicating the existence of turbulent condition during theemplacement. Furthermore, lapilli lenses inside LT1 could suggestgrain transportation from previous fallout deposits (Sohn andChough, 1992, 1993) and are indicative of a dryer evolution ofthe main sequence since the diffuse bedding and massive stratified

Fig. 3. Stratigraphic profiles and facies recognized at Montaña Pelada tuff ring. Doted areFragments composition: black, basaltic; white, vitric; doted, sandstones; bricked, limest

deposits could separate out and evolve into clusters of lapillilenses. According to Sumita et al. (2004) other possibility couldbe that lapilli lenses are relatively dry tephra jets that may evolveinto a debris flow if the flow enters wet ground conditions or if theflow has higher water content. The presence of Mud crack-likestructures on the top of some strata suggests a temporal gap ofsurges formation.

3.1.2. Facies T1: diffuse-bedding tuffThe diffuse-bedding tuff include fine to coarse ash with occa-

sionally fine basaltic and sandstone blocks, always in the lower-most part of the sequence (Sections 1 and 4 in Figs. 3 and 5).Bedding ranges from thin to medium thickness. Some diffuse pla-nar bedding displaying a tiny wavy pattern can be distinguished(Section 1) although in general this facies exhibits a diffuse-bed-ding aspect.

Interpretation. The diffuse bedding and the finer granulometryof this facies is interpreted as medial to distal deposits from turbu-lent hydromagmatic surges and concurrent simultaneous fallout.

3.1.3. Facies LT2: thinly stratified lapilli-tuffThinly stratified lapilli-tuff consists of a well-defined alterna-

tion of fine to coarse grain-supported lapilli and fine to coarseash rich layer (Sections 4 and 5 in Fig. 3). The thickness of the lapillilayers range from 10 to 30 cm, whereas the tuff layers range inthickness from 5 to 10 cm (Fig. 4C and Table 1). Inversely gradedunits which are characteristics of this facies; begins with fine nor-mally graded layers and evolve to grain-supported layers reachingto a maximum thickness of 80 cm. These units are repeated severaltimes within the facies.

Interpretation. Thin stratification, alternation of grain-supportedlayers and inversely-graded layers is interpreted as being consti-tuted of proximal-to-medial deposits from pyroclastic surges andfallout.

3.1.4. Facies TB: Tuff lapilli brecciaThe tuff lapilli breccia is composed of more than 30% of compo-

sitionally heterogeneous angular fine blocks and non-vesiculatedbasaltic lapilli (Section 1, Fig. 3). Blocky size fragments of trachyba-salt, sandstone and limestone embedded in a white coarse ash ma-trix are frequent. The apparent maximum thickness of the exposedbreccia is about 50 cm, although since the outcrop is partially cov-ered by slope deposits, the real thickness cannot be determined.

as in Sections 2 and 3 are fossilized dune and climbing dune deposits, respectively.ones.

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 recognizedby 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 disorganizedlapilli-tuff (LT3) where almost all granulometric range is present, note the apparent massive to crude stratification pattern.

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

Facies Sample r Md

LT1 M1-01 1.810 2.297LT1 M1-02 1.850 2.297LT1 M3-01 2.397 1.557LT1 M3-02 2.575 1.557LT2 M2-01 2.359 2.500LT2 M2-02 2.389 2.432LT3 M4-01 1.762 2.297LT3 M4-02 1.749 2.344Ta 2A-01 1.620 1.950Ta 2A-02 1.615 1.950Tb 2A2-01 2.090 1.019Tb 2A2-02 2.090 1.019

J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50 45

Interpretation. The matrix supported and unsorted texture, to-gether with the presence of fragments of country rock in the pyro-clastic beds of this facies, suggests an over-excavation offragmentation levels.

3.1.5. Facies Ta: wavy bedded tuffThe wavy bedded tuff is composed of fine to coarse ash and fine

to medium lapilli (Section 1, Figs. 3 and 5). This facies is character-ized by normal grading, very thin and thick continuous wavy bed-ding, 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 sidesand smoothly rounded wave crest. Cross-laminated lenses whichare always located under the waveform crests, appear interbeddedwithin 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 maximumlength of 50 cm. Impact sag structures are also common preservingin some cases the original block (Table 1).

Interpretation. The fine-grained and wavy bedding are indicativeof pyroclastic density currents deposition.

3.1.6. Facies Tb: dune bedded tuffThis facies is composed of fine to coarse ash and fine to medium

basaltic lapilli (Section 1, Figs. 3 and 5). It is characterized by thepresence of lower flow regime structures such as dunes and rip-ples. Dunes are dominant in the whole sequence, showing a slopeangle of 5–10� on the lee side. Their wavelengths could not bedetermined due to bad outcrop exposure. Cross-laminated lensesunderlying the dunes are present. Dunes are distinguished fromthe cross-laminated lenses by the larger amplitude and the lowerdipping angle, usually less than 10�. The whole sequence show amaximum thickness of 70 cm between the two crests of the under-lying facies (Table 1).

Interpretation. Cross-lamination and dune bedding suggestdeposition from turbulent pyroclastic density currents with lowparticle concentration (Sohn and Chough, 1989, 1992) proximal-to-medial from dry pyroclastic surges.

3.1.7. Facies Tc: erosive-wavy bedded tuffWith respect to the granulometry and bedding features, this

facies is very similar to Ta but differs in the erosive base of thesequence and in having beds with lower dipping angle (Section 1,Figs. 3 and 5). This facies starts with an erosive contact wheresymmetrical lower flow regime structures, such as ripples develop.

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.

46 J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50

The wave lengths of these structures range from 60 to 100 cm andamplitude from 10 to 20 cm and with no internal structure hasbeen preserved. At the bottom of this facies ash proportion domi-nates whereas upward in the sequence lapilli appears more fre-quently compared to Ta facies, where lapilli concentrationdecreases upwards.

Interpretation. The fine-grained, laminated and wavy texturebeds with erosive basal contact are indicative of deposition fromhorizontally mobile pyroclastic density currents, such as basesurges capable of reworking the underlying deposits.

3.1.8. Facies Td: blocky tuffThis 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 faciesthickness, it shows frequent impact sag structures that sometimesare deep enough to penetrate in the underlying facies.

Interpretation. Coarse granulometry embedded into a finer ashmatrix suggest rapid ash fallout from dense suspension with syn-chronous emplacement of ballistic block.

3.1.9. Facies Te: thinly bedded tuffThis facies is characterized by a well-defined alternation of

coarse and fine lapilli grained thin beds (Section 1 and 5, Figs. 3and 5). Bedding is dominantly horizontal, only impact-sags struc-tures break the layer and the horizontal continuity.

Interpretation. The thinly horizontal bedding and grain sizealternation suggest deposition of base surges.

3.1.10. Facies LT3: disorganized lapilli-tuffThis facies comprises a thick sequence of fine to coarse lapilli

embedded into a medium to coarse ash matrix with frequent fineand coarse basaltic and sedimentary blocks (Section 3, Fig. 3). Largeblocks corresponding to non-vesiculated basaltic compositions are

J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50 47

common, whereas accessory lithics such as fine blocks appearing inminor proportions are exposed (Fig. 4D).

Interpretation. Thick stratification and poorly sorted depositsindicate deposition from pyroclastic surges lacking tractionalprocesses.

4. Volcanic evolution

Facies distribution suggests that the first hydrovolcanic edificewas SMPTR, and NMPTR was built at the final stages or completelyafter SMPTR was formed.

4.1. SMPTR

SMPTR can be subdivided in two main parts. The first part ismainly composed of diffuse-bedding to massive stratified lapilli-tuff facies (LT1), whereas the second part is composed of tuff facies(T1, Ta, Tb, Tc, Td and Te), separated by a breccia tuff (TB). Howeverlateral facies variation cannot be observed and no correlation withother 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 itcan be recognized in both sections. It constitutes the bulk of theSMPTR and is located on the near-vent area of the volcano, andcan be correlated to the proximal deposits formed by the emplace-ment of the base surges synchronous to fallout processes. Blockand bombs impact sags indicates ballistically emplaced ejecta fromexplosions simultaneously with surges formation. Mud crak-likestructures on the top of some strata suggests a temporal gap of

Fig. 6. W/M ratio and effective fragmentation evolution of MPTR. On the margins grain-s(Inman, 1952), SMPTR: LT1 histogram representing the grain-size distribution of theeruption; Ta histogram representing the grain-size distribution of undulatory bedded tuNMPTR: LT1 histogram representing the grain-size distribution of thickly stratified lapilltuff.

surges, indicating also water-saturated depositional condition (Ris-so et al., 2008). Lapilli lenses are indicative of a dryer evolution ofthe main sequence since LT1 deposits could separate out andevolve into clusters of lapilli lenses. Therefore deposition of diffusebedding 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 estimatedbecause it is partially eroded.

The heterogeneous composition of TB which is mainly com-posed of sedimentary and trachybasaltic blocks suggest an increaseof explosivity and over-excavation of fragmentation levels, proba-bly by phreatic waters input. During this part of the eruptionwater/magma ratios decreased, increasing the fragmentation de-gree and depositing the medial-to-distal tuff facies from turbulentpyroclastic density currents and fallout.

Deposition of crudely stratified and massive facies suggestshigh W/M ratios. During the eruption water availability decreased,increasing the effective fragmentation and depositing the tuff fa-cies (Fig. 6).

4.2. NMPTR

NMPTR edifice is completely preserved in Sections 3–5 used forthe stratigraphic analysis (Fig. 3). The deposits are similar to thefirst part of SMPTR, but only tuff deposits have been recognizedat distal parts of the volcano.

The first recognized deposits correspond to LT1 representingthe proximal deposits of the near-vent areas formed in the same

ize distribution of main facies and the median sorting (rphi) and size (Mdphi) valuescrudely stratified lapilli-tuff, typical facies of the first stage of the hydrovolcanicff, note that in the SMPTR evolution the median size decreases towards the tuffs;

i-tuff and LT2 histogram representing the grain-size distribution of stratified lapilli-

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 toright spherical particle, indicating ductile fragmentation; highly vesicular juvenile crystal and blocky and equant particle of fine ash; from the bottom left side to rightfeldspar crystal and blocky and equant fragment; (C) aspect of an irregular vitric fragment showing concoidal fragmentation and (D) blocky and equant glass fragment.

48 J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50

way as in SMPTR, followed by a thick medial LT3 facies from pyro-clastic surges with no tractional processes. Impact sags indicateballistically emplacement of bombs at the same time of the surgeemplacement. High concentration of accidental lithics indicateshigh magma ascend rate and/or more energetic fragmentationphase within the eruption.

Facies T1 separates Sections 3 and 4, the smaller grain size andthe laminated pattern could indicate a reduction in W/M ratio.Then surges evolve to relatively slow fall out rate resulting in grainsegregation depositing the proximal-to-medial LT2 facies in dryerconditions. Finally surges return to radial-diluted pyroclastic den-sity currents depositing an important thick LT1 facies.

Distal deposits as observed in Section 5 (Fig. 3) are composed ofan alternation of LT2 and Te facies originated by base surges depo-sition. LT2 exhibits erosive surfaces, produced by aqueous turbulentflows (Fig. 4C) indicating also an interruption of surge emission.

Eruptive evolution of NMPTR follow the same pattern thanSMPTR but the effective fragmentation and W/M ratios were notenough to generate the tuff facies (Fig. 6).

5. Summary and conclusions

The growth of Montaña Pelada tuff ring can be subdivided intothree main stages, based on changes in the dominant depositional

processes. The fist stage corresponds to the formation of SMPTRcomposed of proximal deposits; consisting of diffuse-bedding tomassive stratified lapilli-tuff facies (LT1) deposited by successivediluted pyroclastic density currents synchronous to fallout and bal-listic ejecta generation.

The second stage consists of the deposition of dry base surges.This low-concentration and pulsatory surges should be responsiblefor the deposition of stratified and laminated tuff facies. The thick-ness and lateral continuity of each lamina suggest that the turbu-lent eddies were large-scale structures, coherent for longdistances and becoming a well-segregated surges depositing thefine-grained layers indicating lower W/M ratios.

At the third stage, NMPTR was formed 750 m northwards of theSMPTR with deposits analogous to the first phase of SMPTR withthe only difference of well represented distal deposits. The connec-tion between the SMPTR and NMPTR edifices can be explained by aN–S fracture that drove the magma flow towards the present posi-tion of NMPTR where the hydrological conditions allowed newwater input. The decrease in particle concentration of surges wouldbe responsible for the distal facies deposits such as the stratifiedlapilli-tuff and the laminated tuff. This last facies represents thehighest fragmentation and magma ratio that could reach to about0.3 (Sheridan and Wohletz, 1981, 1983; Wohletz, 1986).

The study of ash using SEM may elucidate the fragmentationprocess operating during explosive eruptions (Wohletz, 1986).

Fig. 8. Outcrop aspect: top picture: contact between previous scorias deposits and hydrovolcanic deposits of MPTR. Bottom: detailed aspect of the contact: (A) trachybasalticscorias; (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.

J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50 49

The occurrence of blocky and equant clasts (fragments with equi-lateral sides crossing at right angles, Fig. 7) is considered indicativeof phreatomagmatic fragmentation mechanism and the presenceof spherical particles of ductile fragmentation ash is characteristicof fine-grained pyroclasts of preatomagmatic origin. Furthermore,high concentrations of accidental lithics suggest the presence ofan aquifer in contact to the conduit.

The presence of a basaltic scoria flow underlying the NMPTRhydrovolcanic deposits suggests a previous eruption from othersurrounding volcano although studies from Hernández-Pachecoand Fernández (1978) related these flows to the MPTR eruption(Fig. 8). If this was true, the eruption should have started witha magmatic phase that has not been recognized in any sectionof MPTR. The deposits suggest a large period of time betweenthe scoria flow and the hydrovolcanic eruption since an incipientpaleosoil has been developed (Fig. 8). Also the first hydrovolcanicdeposits are characterized by fine ash and well sorted lapilli cor-responding to the distal facies of SMPTR originated by fall out,suggesting an ash cloud expansion wider than 1 km. Then acoarse facies develops overlying the fine ash and well sorted lap-illi deposits with similar characteristic to LT3 facies from theNMPTR.

The volcanic evolution and deposits formation has been mainlycontrolled by two closely related parameters, the W/M ratio andthe effective fragmentation. Proximal deposits suggest high W/Mratios at the time of this eruption, although with relatively loweffective fragmentation. This situation changed with the depositionof the turbulent pyroclastic surges that resulted in the formation ofthe tuff facies. These facies indicate lower W/M ratios than in theinitial stages and higher effective fragmentation. After depositionof these surges the opening of a N–S fracture drove the eruptionnorthwards creating a new vent. The new hydrological conditionsallowed the input of phreatic waters, increasing the W/M andreducing the fragmentation degree. Dryer facies were depositedindicating lower W/M ratio and higher fragmentation degree. Ascan be seen the evolution of both edifices is similar (Fig. 6),although whereas dry pyroclastic surges and many tuff facies aredeveloped in the SMPTR, the NMPTR do not evolve beyond thedeposition of a laminated tuff.

Acknowledgements

This project was supported by the Project Reference200430E438 of the Spanish CSIC (Scientific Research Council).

50 J. Carmona et al. / Journal of African Earth Sciences 59 (2011) 41–50

The authors would like to acknowledge to ITER (Instituto Tecnológ-ico y de Energías Renovables de Tenerife) for the permission to ac-cess the outcrops near their installations and to Woldai Ghebreadfor his valuable comments and suggestions.

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