the role of collapsing and cone rafting on eruption style changes and final cone morphology: los...

Cent. Eur. J. Geosci. • 3(2) • 2011 • 102-118DOI: 10.2478/s13533-011-0008-4

Central European Journal of Geosciences

The role of collapsing and cone rafting on eruptionstyle changes and final cone morphology:Los Morados scoria cone, Mendoza, Argentina

Research Article

Karoly Németh1, Corina Risso2, Francisco Nullo3, Gabor Kereszturi1,4

1 Volcanic Risk Solutions, Massey University,Private Bag 11 222, Palmerston North, New Zealand

2 Departamento de Geología , Area Riesgo Volcánico,FCEyN-Universidad de Buenos Aires, Argentina

3 CONICET-SEGEMAR, Buenos Aires, Argentina

4 Geological Institute of Hungary,Stefánia út 14, Budapest, 1143, Hungary

Received 30 November 2010; accepted 31 January 2011

Abstract: Payún Matru Volcanic Field is a Quaternary monogenetic volcanic field that hosts scoria cones with perfect tobreached morphologies. Los Morados complex is a group of at least four closely spaced scoria cones (LosMorados main cone and the older Cones A, B, and C). Los Morados main cone was formed by a long lived eruptionof months to years. After an initial Hawaiian-style stage, the eruption changed to a normal Strombolian, cone-building style, forming a cone over 150 metres high on a northward dipping (∼4˚) surface. An initial cone graduallygrew until a lava flow breached the cone’s base and rafted an estimated 10% of the total volume. A suddensector collapse initiated a dramatic decompression in the upper part of the feeding conduit and triggered violenta Strombolian style eruptive stage. Subsequently, the eruption became more stable, and changed to a regularStrombolian style that partially rebuilt the cone. A likely increase in magma flux coupled with the gradual growthof a new cone caused another lava flow outbreak at the structurally weakened earlier breach site. For a secondtime, the unstable flank of the cone was rafted, triggering a second violent Strombolian eruptive stage which wasfollowed by a Hawaiian style lava fountain stage. The lava fountaining was accompanied by a steady outpour ofvoluminous lava emission accompanied by constant rafting of the cone flank, preventing the healing of the cone.Santa Maria is another scoria cone built on a nearly flat pre-eruption surface. Despite this it went through similarstages as Los Morados main cone, but probably not in as dramatic a manner as Los Morados. In contrast tothese examples of large breached cones, volumetrically smaller cones, associated to less extensive lava flows,were able to heal raft/collapse events, due to the smaller magma output and flux rates. Our evidence shows thatscoria cone growth is a complex process, and is a consequence of the magma internal parameters (e.g. volatilecontent, magma flux, recharge, output volume) and external conditions such as inclination of the pre-eruptivesurface where they grew and thus gravitational instability.

Keywords: scoria cone • pahoehoe • aa lava • Strombolian, Hawaiian • lava spatter • clastogenic lava flow • debris avalanche• agglutinate • breached cone© Versita Sp. z o.o.

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1. Introduction

Scoria cones are the most common manifestation ofsubaerial small-volume, short-lived volcanism on Earth.These are generally considered to be a result of a mild ex-plosive eruption of mafic to intermediate magmas in a shortperiod of time (days to weeks) [6]. However, long-livedscoria cone eruptions are also known, such as Paricutinin Mexico that was active for 9 years [7]. The main vol-caniclastic deposits that are produced by a scoria coneeruption are the cone-building coarse-grained ash, lapilliand aggolomerate units and the medial to distal coarseash to fine lapilli sized pyroclastic blanket. These pyro-clastic deposits are formed when magmatic gas bubblescoalesce and explosively burst in a relatively establishedvolcanic conduit, high in the growing cone [8–14]. De-tailed analyses of deposits preserved on scoria cones andtheir surroundings led to the clarification of the role ofthe shallow seated magmatic system in the control of theexplosive eruptions of such volcanoes [1, 15–19]. Amongthe identified parameters, the variations in degassing pat-terns, magma ascent rates and degrees of interaction withexternal water are thought to be responsible for suddenchanges in the eruption styles [1, 17, 20].Small-volume mafic cone building eruption styles canbe grouped into four end member types, which primar-ily depend on the magmatic mass flux and the degree offragmentation: 1) Hawaiian, 2) Strombolian, 3) violentStrombolian, and 4) weak ash emission [1, 21]. Hawai-ian style eruptions produce coarse-grained, moderatelyvesicular pyroclastic deposits (e.g. low level of fragmen-tation) through an eruption with high magma flux (i.e. 50– 1000 m3/s) [1, 22, 23]. In contrast, Strombolian activ-ity typically produces pyroclasts that are relatively coarsegrained and vesicular, but finer grained than those pro-duced by Hawaiian style eruptions, reflecting more effec-tive magma fragmentation [6, 24]. Strombolian style erup-tions are associated with a significantly lower magma fluxrate (i.e. 10−3 – 100 m3 per explosions over several explo-sions an hour) [1, 6]. Weak ash emission eruptions producefine ash, with a negligible gas thrust and therefore verylimited (usually in the crater basin) distribution of thedeposit. Violent Strombolian style eruptions are thosethat produce significant volumes of fine ash, as a conse-quence of effective magma fragmentation accompanied byhigh magma flux and high explosive event frequency. Sucheruptions can spread ash over relatively large areas anddisperse fine pyroclasts to over 10 km distance from theirsource [1, 25–30].The closely packed, slightly oriented texture of mod-erately vesicular coarse ash and lapilli accumulationsare a typical result of Hawaiian style lava fountain-

ing [22, 29, 46, 47]. The common intercalation of sco-ria beds in scoria cones with welded fall deposits and/orclastogenic lava flows [48] indicate a sudden and frequentchange in eruption style from Strombolian to Hawai-ian and vice versa [1], reflecting variable volcanic con-duit dynamics during the eruption. Lava spatter near thevent, found typically along the crater rim of a cone, canform strongly welded, red, slightly bedded sequences withlarge spindle or highly vesiculated fluidal bombs [49, 50].These deposits usually reflect strong reworking of the py-roclasts as they roll back to the vent and are clearedagain, resulting in complex amalgamated lapilli textures.The inner part of scoria cones suffers strong agglutinationdue to the persistent heat of the magma filled conduit [6].Such welded parts of the inner cone, or the collar-likewelded ring on the crater rim, are more erosion resistantand can be preserved for a long time; however, they arealso prone to gravitational collapse, especially if there ismass deficit at the base of the cone. Lava emission pointsor fractures at the base of the cone may cause gradualrafting of large portions of the cone basal flank that canlead to gravitational sector collapse of the cone [1, 51].Here we explore the morphological results of lava flowrafting, and resulting cone sector collapses, as a primecontrolling parameter in triggering significant eruptivestyle changes in the course of scoria cone growth, and howsuch processes may be reflected by 1) the accumulatedintra- and inter-cone pyroclastic successions, 2) texturalcyclicity of the pyroclastic deposits, and 3) final morphol-ogy of the scoria cone.

1.1. Payún Matru Volcanic Field (PMVF)

The Payún Matru Volcanic Field (PMVF) is located in thesoutheastern region of the Mendoza province, Argentina,between latitudes 36˚ and 36˚ 35’ S and 68˚ 30’and69˚30’ W, approximately 200 km east of the trench in theSouthern Volcanic Zone of the Andes (inset in Fig. 1A).The PMVF covers 5,200 km2 [52, 53] and is a broad back-arc lava plateau with hundreds of monogenetic pyroclasticcones.Two roughly contemporaneous basaltic volcanic fields areE and W Payún Matru (Fig. 1A). These monogenetic fieldsare referred to as the Payén east and Payén west lavashields respectively. The eastern Payén is related to aneast-west trending fault system. This fissure-controlledvolcanism produced several extremely long lava flows thatreach the Salado river valley in La Pampa province to theeast. One of the lava flows has an individual tongue-likeshape 181 km long, and is the longest known individualQuaternary lava flow on Earth [67].Volcanism on the western slope of Payún Matru volcano

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Figure 1. A) Simplified geological map of the central sector of the Payen Volcanic Complex modified after Pasquaré et al. 2008 [41]. The youngestbasaltic eruptive products are shown in black without distinction of individual vents. The youngest lava flows are associated with theSanta Maria (SM) scoria cone NW from the Payún Matru caldera (PM) and to La Carbonilla lava flow just east of Payún Matru (PM)caldera along the Carbonilla Fault system (marked by a thick red line). The light grey area corresponds to the Payen Eastern ShieldVolcano. Diagonal line pattern represents parts of the Payen Eastern Shield Volcano covered by younger eruptive products of trachyteand trachyandesite lavas derived from Payún Matru caldera or Payún Liso stratovolcano (“vvv” pattern). Major lava fields such as LosCarrizales and Pampas Onduladas fields are also labelled east of the Payen Eastern Shield Volcano. Yellow stars mark the Los Morados(LM) and Santa Maria (SM) scoria cones. The yellow rectangle corresponds to Fig. 1B. Red stars are K-Ar ages obtained by Ramosand Folguera 2010 [40] and Germa et al 2010 [39], in lava flows exposed along the Rio Grande valley; these lavas pre-date the youngestlava flows and scoria associated to Los Morados. Inset shows the general tectonic setting of the study area (black star) and the locationof the Southern Volcanic Zone.B) Major volcanic units in the Los Volcanes area around Los Morados scoria cone complex modified after Risso et al. 2009 [43].

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is trachyandesitic and basaltic, and two different lavapulses can be recognized. The older formed pahoehoeflows with tumuli, pressure ridges, skylights, ropy lavasand occasional lava tubes belonging to the Puente For-mation (Fig. 1B) [68], while the younger. are aa-type,blocky lava flows with conspicuous ridge crests and rub-ble levees (Tromen Group) [52]. Some of the scoria conesare very well preserved, with small craters and slope val-ues close to 33˚. The younger eruptive period includesthe Holocene age volcanoes of the Carbonilla region, eastof Payún Matru, and Santa María to the west (Fig. 1A).The post-caldera and fissure phase volcanic activity at thePayen west shield generated more than 100 monogeneticscoria cones and lava flows that reach 30 km from their in-dividual point sources in the Los Volcanes area (Fig. 1B).These lava flows overlap each other, creating a fan-likelava field on the eastern margin of the Río Grande river(Fig. 1B) [71]. A large number of scoria cones have associ-ated dark aa and pahoehoe lava flows belonging to ”older”and “younger” lavas of the Tromen Group (Fig. 1B) [52].The scoria cones range from 75 to 225 m in height, andthey have crater diameters between 140 and 750 m. Theyconsist mainly of agglomerates and volcanic breccias withvariable degrees of welding. Eruptive mechanisms wereprimarily of Strombolian and Hawaiian type, with occa-sional violent Strombolian eruptions. The average conedensity for the area is 33 cones per 100 km2. The highcone density is probably a result of a strong magmaticpulse initiated the volcanic field formation [72].Pampas Negras (“black flat surface”) defines an areaamong young scoria cones of Los Volcanes that is coveredby black scoriaceous ash and lapilli deposits (Fig. 2) thatpartially or totally cover the slopes of older scoria cones.It is inferred to be the product of a far less explosive scoriacone building period than those earlier more silicic erup-tions associated to the Payún Matru composite volcano.Some scoria cones produced ash and lapilli deposits thatare restricted chiefly to their cone edifice and within a fewkilometres from their source vents. Today ash and lapillihas been re-worked by wind, forming elongated and par-allel dunes perpendicular to the prevailing westerly winddirection.

2. Los Morados scoria coneLos Morados is a young scoria cone complex with closelyspaced edifices in the western edge of the PMVF in Men-doza (Fig. 1). Here we concentrate on the morphologi-cal evolution of the youngest of the cones, named as LosMorados main cone, and an older cone west of the LosMorados main cone, partially covered by eruptive prod-

Figure 2. A) Google Earth image of Los Volcanes area. Dispersalareas of black ash fall issued by Los Morados volcaniccomplex are outlined with white lines; I - first major pyro-clastic fall, II-A - second major pyroclastic fall, II-B - prob-ably part of the second major fall event.B) An interpretative geological map based on mapping involcanic units of the same area shown on 2A.

ucts of the Los Morados main cone, named as Cone A(Figs 2A, B & 3A, B), as well as another older cone tothe north of Los Morados main cone referred here asCone B, and an eroded older cone west of Cone A, namedhere as Cone C (Figs 2A, B & 3A). The radiometric ageof Los Morados scoria cone complex is unknown, but itrests atop other lavas erupted from the Payén west shieldvolcano (Fig. 1A) [62]. The youngest age of the lavaflows of the shield volcano in the Rio Grande basin is0.016±0.001 Ma, putting a limit to the age of Los Mora-dos volcanic complex [62]. These stratigraphic and radio-metric age data coincide well with the general morpholog-ical characteristics of both the Los Morados scoria conecomplex and associated lava flows. The outer slope ofthe Los Morados main cone is steep (up to 30 degrees)and youthful in appearance with a fresh, reddish to blacklapilli blanket with no significant gully network. However,dominantly dry climatic conditions with annual precipita-tion of a few hundred mm favours the preservation of thevolcanic edifices over a relatively long period of time.Los Morados main cone is part of an alignment of vents,

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Figure 3. A) Close up Google Earth image of the Los Morados sco-ria cone complex. Thick yellow line outlines the youngerlava flow with rafted cone material of Los Morados maincone. Thick white line bounds to the zone from where thecone flank was rafted away. Dashed white line marks LosMorados main cone older lava flow and hummocky areain the northern side of Los Morados main cone. Yellow ar-row points to the ash-covered hummocky surfaces north ofLos Morados main cone. Dashed yellow line correspondsto the base of an older scoria cone (Cone A) west of LosMorados main cone. Red star shows the location of anolder, small-volume, lava outbreak at the base of ConeA. The red star is also shown air photo on 3B. Note thefractures marked by white arrow parallel to the cone flankbreach. White dot marks the point photo 4B was taken.B) Aerial view of Los Morados scoria cone complex, asseen from the north. White dot marks the point photo 4Bwas taken.

on an E-W trending fissure system (Carbonilla Fault Sys-tem) (Fig. 1A). The westernmost vent at Los Morados sco-ria cone complex is an older scoria cone, with an erodedmorphology (Cone C) (Figs 2A, B & 3A, B). The eastern-most part of Los Morados scoria cone complex forms a rowof fissure vents which has at least 5 individual vent sitesspaced a few tens of metres apart (Fig. 2A). The centralpart of the Los Morados scoria cone complex is composedof two overlapping scoria cones. The older one, named asCone A has a near-circular well-preserved crater, whilethe Los Morados main cone partially covered Cone A.TheLos Morados main cone has an oval shaped, east–west

elongated deep crater. To the north of Los Morados maincone, about 1.7 km from its crater, an older scoria cone(Cone B, Figs 3A & B) formed a barrier that diverted thelava flows issued from the base of Los Morados main cone(Figs 3A & B). This older cone (Cone B) has a smooth sur-face with a light vegetation cover and it has a less wellpreserved crater than Los Morados main cone, indicatingthat it is part of an older eruption in the area (Fig. 3B).Los Morados scoria cone complex was probably active overa prolonged time period. Los Morados main cone is in-ferred to be the youngest volcano in the Los Morados sco-ria cone complex, and potentially is the source of a majorash fall event that issued black ash and lapilli throughouta fairly large area, commonly referred to as the “ash andlapilli plain” or “Pampas Negras” (Figs 1 & 2).

2.1. Morphology and Erupted Volume

Precise delineation of the dimensions of Los Morados maincone is not simple due to the thick tephra cover that ob-scures parts of the cone (Figs 3A & B). The main cone ofLos Morados scoria cone complex is located next to an-other older cone (Cone A, Fig. 3) that is inferred to beolder than Los Morados; the thin vegetation cover of theolder Cone A contrasts with the barren, red and black ashcovered surface of the outer flank of Los Morados maincone (Fig. 3B). The base of Cone A is about 2100 metresabove sea level, which allows us to estimate a value of2150 metres for the basement of Los Morados main conebased on spot elevation readings on GoogleEarth imagesas well as direct hand-held GPS measurements (Fig. 3A).This value is a good average for the western side of theLos Morados main cone; however, the eastern basementlies on higher ground, but probably not more than 100 me-tres higher than the western side. This inferred elevationdifference indicates that the Los Morados main cone itselferupted on a north-westward dipping topography with aninclination angle of 4˚ (Fig. 3B). The general slope an-gle values were approximated by using planes as a “trendsurface” fitted to digitalized spot heights with individualelevation values. These elevation points were digitalizedfrom areas where no tephra blanket exists and effusiveproducts (e.g. lava flows) of Los Morados cannot be found(e.g. areas that are interpreted as the syn-eruptive sur-face). This calculation supports that the Los Moradosscoria cone complex erupted on a surface gently inclined(∼4˚) towards the northwest. The Los Morados main conehowever, was likely formed on a very irregular topographydetermined by the steep slopes of Cone A (Fig. 3B).The highest point on the Los Morados main cone craterrim today lies in the south-east and is about 2425 me-tres above sea level (Figs 3A, B & 4A). From this highest

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point, the crater rim has a crescent shape that gradu-ally decreases its elevation to 2275 m to the north andto 2250 m to the northwest side. The crescent shapededifice encircles a U-shaped scar on the northern side ofthe volcano (Fig. 3B). The U-shaped scar is the source ofan extensive lava flow and associated rafted cone mate-rial (Figs 3B & 4B). The scar is about 400 metres acrosswhere it meets the crater rim top, and at its base is about200 metres wide (Figs 3A & B). The crater rim has an el-liptical shape with an E-W, 680 m long axis, and a 500 mlong N-S axis (Figs 3A & B). The base of Los Moradosmain cone is obscured by a pyroclastic blanket, thus itsbasal diameter cannot be determined easily, but our bestestimates based on direct GPS-measurements, spot eleva-tion data from GoogleEarth images and field observationsgive a maximum base diameter of about 1200 metres (sim-ilar to the older cone to the west, Cone A), a 2150 metresabove sea level horizon in the west (the same level asthe base of Cone A) and a value of 2300 metres for thebasement level in the east (Fig. 3). This together providesrough estimates of the Los Morados main cone height witha minimum 125 and maximum of 275 m. The older ConeA in the west has an average cone base diameter of 1200metres and cone height of about 200 metres, providing anestimated cone height (Hco) to basal diameter (Wco) ratioof 0.167. Our observations indicate that if Los Moradosmain cone had grown on a flat surface it could have simi-lar dimensions to Cone A. However, Los Morados is sittingon a north-westward gently inclined depositional surfacewith a potential slope angle of at least ∼4˚. The Hco/Wco

ratio of Los Morados is calculated between 0.1 to 0.23,depending on which reference elevation of the cone baseis used.

To estimate the cumulative volume of pyroclastic depositsassociated with Los Morados main cone, measurementsof the total thickness of pyroclastic deposits associatedwith the main cone were taken in the Pampas Negrasarea (Figs 2B & 5A). The measurements included the es-timated deposit thicknesses on the Los Morados main coneand the primary (ie: not wind re-worked) distal inter-conefield tephra layer thicknesses. We also included the wind-redeposited ash and lapilli that form thick accumulationsof tephra on the west facing side of older cones and lavaflows east of Los Morados main cone (Fig. 5A and Table 1).The total volume of the pyroclastic deposits, including LosMorados main cone, is about 0.3 km3. This value is likelyan underestimate due to the uncertainties in the delin-eation of the cone base and the thickness of the tephrain proximal regions. However, it appears to be a reason-able estimate with Los Morados emitting approximately0.3 km3 of tephra that accumulated in the cone and in thevicinity. This could be recalculated to a dense rock equiv-

Figure 4. A) Panoramic view of Los Morados main cone from theeast.B) Breached cone flank of Los Morados main cone fromthe eastern side of the younger lava flow initiated from thecone.C) Inner crater wall of Los Morados main cone exposeswelded lava spatters steeply dipping inward.D) Lava spine in the proximal area of Los Morados maincone younger lava flow.

alent (DRE), using an average 50% vesicularity of typicalscoriaceous pyroclasts, to give a value of about 0.15 km3

magma involved in the formation of the pyroclastic unitsof Los Morados main cone.

Table 1. Calculated eruptive volumes associated with measured sec-tors on Fig. 5A.

Measured regions Area Thickness In m3 In km3

[on Fig. 5] covered in m2 in metres

1 148831.70 150 22324755.000 0.0223252 384564.60 80 30765168.000 0.0307653 761612.54 60 45696752.400 0.0456974 6060053.10 20 121201062.000 0.1212015 2895932.15 10 28959321.500 0.0289595a 1711069.76 10 17110697.600 0.0171116 734716.58 0.5 367358.290 0.0003677 1282405.62 4 5129622.480 0.005138 4122036.43 0.5 2061018.215 0.0020619 5822792.84 0.5 2911396.420 0.00291110 2273830.63 4 9095322.520 0.00909511 99731.43 4 398925.720 0.00039912 112160.12 0.5 56080.060 5.61E-0513 158664.13 4 634656.520 0.00063514 71610.12 4 286440.480 0.00028618 1262616.12 0.5 631308.060 0.000631

Total 0.28763

We also estimated the lava flow volume, which was emittedin two stages through the northern breach of Los Morados

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Figure 5. A) Black scoria fall thickness map calculated from field measurements of tephra layers and corrected by the estimated pre-eruptivesurface morphology.B) Simplified outline map of the two lava flows produced by Los Morados main cone. The black area corresponds to the youngerLos Morados lava flow. Calculated eruptive volume estimates listed in the figure using 3 m (min.) or 10 m (max.) average lava flowthicknesses for the young lava flows while 10 m (min.) or 30 m (max.) average lava flow thicknesses based on field observations.

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main cone (Fig. 5B).The older lava flow and rafted collapsed cone flank coveran area of approximately 1.3 km2(Fig. 5B). The flow vol-ume was estimated based on field observations on lavaflow thicknesses. The average lava flow thickness esti-mates of 10 m and 30 m were obtained at the proximalflow margins. Thus the erupted volume of the older lavaflow ranges between 0.01 and 0.04 km3 (Fig. 5B).The younger and more extensive lava flow covers an areaof approximately 10.57 km2. Using an estimated minimumof 3 metres and maximum of 10 metres average thicknessfrom field observations, a minimum of 0.03 and a maximumof 0.11 km3 magma erupted through this lava flow stage(Fig. 5B).The total estimated volume of magma involved in the erup-tion of Los Morados main cone therefore ranges between0.19 and 0.3 km3 in dense rock equivalent.

2.2. Volcanic Facies

Several volcanic facies were identified in and around thevolcanic construct of Los Morados main cone. The sco-riaceous deposits were divided according to their colour,referring to red (Sr) and black (Sb) scoria beds rangingfrom coarse ash to coarse lapilli grain sizes.Agglutinate facies (A) are moderately to strongly welded,coarse grained pyroclastic rocks that are commonly asso-ciated with red scoria interbeds (Fig. 4C). The agglutinatefacies also includes welded lava spatter and welded sco-ria, where original clast outlines can be confidently rec-ognized in proximal volcanic units. Densely welded py-roclasts commonly form coherent interbeds of dark colour,lava-like rocks where original clasts can barely be recog-nized, and only some irregularities and the occasional,commonly random clast outlines indicate the pyroclas-tic origin of the rock. These beds are clastogenic lavaflows that formed as a result of rheomorphic processesthat nearly completely destroyed the original fabric. Suchclastogenic lava flows probably represent fast accumula-tion of lava spatter from low eruption columns.Lava facies (L) can be distinguished on the basis of thelack of any indicators of clast outlines. In addition, theyshow typical surface features, such as spines, ropy sur-faces, vesicle pipes and/or associated flow front features.The lava facies show characteristic features of an aa lava,with pressure ridges across the lava field (Fig. 2A), ruggedsurface morphology with m-sized spines and vesicularzones in the lava plates. The lava facies is covered by rub-ble derived from fragmented lava blocks and clinker that ismixed with m-sized of agglutinate blocks that still retainoriginal bedding in a contorted fashion (Fig. 4D). The lavafield is considerably wider from its point source located

at the northern breach of the scoria cone, and reachesa width of about 300 metres. There are at least two dis-tinguishable lava flows issued from the breach of the scoriacone (Fig. 2B). An older lava flow extends toward the northand is blocked by an older scoria cone, reaching a lengthof about 1 kilometre before being diverted toward the west(Fig. 2). This older flow can be traced at least another1.3 km further down beneath a more extensive and youngerlava flow. The older lava flow is covered extensively byclinker and rubble derived from the margin of the youngerlava flow. In addition, the older lava flow is partially cov-ered by a red and black scoria blanket which is missingon top of the younger lava flow. The young lava flow, afterreaching the older scoria cone barrier (Cone B; Fig. 3A),was diverted to the west, and travelled at least another7.5 km before stopping. About 1 km and 2.3 km from thefirst diversion point, the younger lava flow branches totwo separate narrower arms that were emplaced slightlytoward the northwest. These lava flow arms have typi-cal aa lava surface morphologies, with a central channeland a rampart of broken lava fragments on both sides themain flow channel (Fig. 2A). The widest arm also displaysin its final 2.5 km multiple channelized flow structures andpressure ridges. However, occasional agglutinate metre-sized blocks are still randomly distributed on the top ofthe younger lava flow, especially in the deeper parts ofthe rugged surface.

3. Volcanic Facies Distribution

The eruptive products of Los Morados main cone can begrouped into 3 major volcanic facies. The cone itself ispredominantly composed of Group I facies formed by redscoria beds, agglutinate and minor clastogenic lava flowinter-beds. These lithologies are exposed on the steepinner crater wall. The outer upper flank of the cone isalso composed of red scoria; however there is no exposedagglutinate collar on the crater rim (Fig. 3B & 4A). TheGroup II facies is exposed in the region of the lower coneflank and the inter-cone field, which is composed by blackscoria of coarse ash and lapilli (Fig. 2). The black scoriabeds can be traced in three distinct directions and are in-ferred to be sourced from Los Morados main cone as thisis the only vent located in the right position in respect tothe bed thickness variations (Fig. 5A). The larger blackscoria dispersion pattern represents a significant eruptionevent, as tephra reached areas at least 10 km from the ventacross an east-west axis (Fig. 2A). The estimated areacovered by black scoria is about 100 km2. Another, notso clearly traceable black scoria dispersal pattern, witha 6 km long axis toward the NE can also be recognized

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(Fig. 2A). This black scoria blanket covers an area of about60 km2. The thickness of the black scoria units is variableas shown in Figure 5A. Thickness is in the metre scale inthe flank of the Los Morados main cone. Between 5 to 10kilometres from the source the thickness of the scoria blan-ket is still in the cm-scale (Fig. 5A). Determination of theprecise thickness of the scoria blanket in many places ishindered by the lack of natural outcrops, and the difficultyin digging as the surface is covered by desert pavement.The total volume of the black ash and lapilli blanket can beestimated from field measurements (Table 1) by subtract-ing the near vent pyroclasts included previously in the to-tal pyroclast volume measurements (Table 1 and Fig. 5A).In this way, an estimated volume of 0.068 km3 tephra is ob-tained for the likely products of two major, and one likelyminor, individual eruptions (Fig. 2A). This value indicatesthat the major black scoria blanket must have been pro-duced by an eruptive stage in the lower end member ofa sub-Plinian eruption [74, 75]. This type of activity iscommonly referred in a basaltic monogenetic context asa violent Strombolian type of eruption [1, 34, 76–79]. Itis also likely that the smaller black NE-trending disper-sal pattern was also of the violent end-member of normalStrombolian type eruptions based on the similar disper-sion values as the E-trending ash and lapilli field.

Agglutinate is primarily exposed in the inner crater walleither as plastered and steeply inward inclined stacks ofbeds or as outward dipping layers which are exposed byrock falls in the crater wall interior (Fig. 4C). The exposedin situ agglutinate forms dm-thick beds of strongly welded,slightly flattened, moderately vesicular scoriaceous lapillihosted in red scoriaceous coarse ash. In situ agglutinateunits commonly form coarse to fine bed couplets with thin,few cm-thick fine ash beds. The fine ash beds are alsocommonly welded and the original pyroclast morphologyis difficult to recognize. Thin (dm thick) clastogenic lavaflows form discontinuous horizons exposed on the craterwall. The rocks exposed in the crater wall are sufficientlywelded to support the agglutinate beds that dip steeplyinward.

Agglutinate facies are exposed north of the crater wallbreach as individual blocks with random bed dip direc-tions (Fig. 6A). The abundance of well-defined zones ofagglutinate-dominated rocks on the rough lava flow sur-face decreases with distance from the Los Morados maincone. Near to the lava flow point source at the conebreach, large blocks of contorted, truncated and randomlytilted agglutinate blocks are exposed, commonly stackedover each other. Bed dip directions in this area do notresemble any systematic distribution, thus we interpretthese blocks as fragments derived from the breached por-tion of the cone. These blocks are similar to the in situ

agglutinate beds exposed in the crater wall.

Figure 6. A) Field relationships between lava flows and raftedand/or collapsed cone flank blocks in the breach area.Dashed line marks the contact between the older andyounger lava flows.B) Overview of the tephra-covered hummocky surfacenorth of Los Morados main cone. Arrows point to flattenedlava spatters.C) Flattened, moderately vesicular, stratified lava spattersin a rafted cone block on the surface of the young lavaflow of Los Morados main cone.D) Spill over of the younger lava flow, interpreted as theresult of a local collapse of a large rafted block. Dashedarrow show the path of the spill.

Red scoria beds are common in the upper and outer coneflank. In the upper exposed section of the crater wall thereare individual well-sorted beds of coarse ash to fine lapillired scoria. A similar red scoria blanket seems to followthe east – west oriented fissure network. In situ agglu-tinate outcrops on the upper crater wall and red scoriabeds are more common than at the base of the crater.Similarly loose red scoria deposits tend to fill some small(metres scale) depressions on top of the lava flow and inthe displaced agglutinate blocks. The reddish scoria blan-ket clearly marks a coloured zone in the entire length ofthe nearly 10 km long younger lava flow.In the northern side of Los Morados main scoria cone, justin front of older Cone B, a hummocky surface is exposed(Fig. 6B). Hummocks are up to 10 meters tall and ran-domly distributed and covered by a red and black sco-ria lapilli blanket (Fig. 6B). The larger hummocks areformed agglutinate and show very diverse dip and bed-ding characteristics indicating that the individual blockswere displaced and rotated. The hummocky surface formsa broader zone than the younger lava flow (Fig. 3). Itseems that the original width of the cone breach wasslightly wider than the breach width evident today. Also,the younger lava flow of Los Morados main cone over-

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runs the older lava flow with highly irregular morphology(Fig. 6C). The hummocky surface is likely to be part ofthe older lava flow region partially covered by either theyounger lava flow or the black scoria blanket (Fig. 6D).

4. Los Morados Main Scoria ConeEvolution

4.1. Original Size and Shape of the Cone

It is difficult to establish the Los Morados main cone orig-inal size as it was built on a highly irregular pre-eruptivevolcanic surface. It can be accepted that the cone itself isat least 125 m high and probably as high as 275 m. Theyoung age of the scoria cone and abundance of inwarddipping welded lava spatter preserved in its crater sup-port that the morphology of Los Morados main cone hasnot been significantly modified since its formation. This isalso suggested by the lack of gullies, erosional scars, orother landforms indicative of cone degradation. The mini-mum total volume of the cone prior to its breaching can beestimated in the range of 0.25-0.35 km3. The width of thebreach on the northern side of Los Morados main cone isnearly as large as the crater of the preserved cone, indi-cating significant mass wasting. Visual estimation of thevolume of the obscured part of the Los Morados main conesuggests that about 10% of the cone flank is disturbed ormissing. This value indicates a significant mass wastingon the northern flank of the Los Morados main cone.

4.2. Inferred Eruption History

The earliest eruptive stage on Los Morados main conethat can be documented with present day exposures atthe bottom of the crater wall was likely a Hawaiian styleeruption, as evidenced by the abundance of agglutinateand minor clastogenic lava flows (Figs 7A & B). This earlyHawaiian style eruption changed to a normal Strombolianscoria cone forming-eruption. This eruptive style permit-ted the establishment of a volcanic conduit with a semi-sealed wall that allowed a stable magma column to form;this column hosted larger bubbles that tended to coalesce(Fig. 7A). The bubble coalescence created conditions thatwere favourable to generate gas pockets that caused pe-riodic outbursts and normal Strombolian style explosions.The magma rise speed and flux likely dropped at this stagein comparison with the earlier Hawaiian lava fountainingstage. Volcanic activity at this point was dominated bythe formation of red scoria and minor agglutinate, as ev-idenced by the increased volume of red scoria beds inthe upper and outer cone flank (Fig. 7A). During this first

cone building phase, a fairly large scoria cone formed.Lava draining back into the upper conduit was potentiallya trigger of some dramatic processes that modified theoriginal cone morphology and allowed lava to drain nearthe base of the cone.The breach of the cone with a lava flow outbreak was in-evitable due to the generally inclined pre-eruptive land-scape and the irregular morphology of the area of LosMorados main cone basement. Magmatic pressure com-bined with the gravitationally unstable geometry (havinga dense magma filled conduit well above the pre-eruptivesurface in the growing scoria cone) provided significantpressure on the northern, unsupported flank of the cone,which eventually collapsed and issued a lava flow throughthe lower part of the cone (Fig. 7A). The first lava outbreakcreated the older lava flow, which is now partially exposedon the northern side of the breached Los Morados maincone, rafting large parts of the initial transient cone flank.The lava flow probably accelerated the gravitational conecollapse process, opening up a wide breach in the inclinedunstable northern side of the initial cone. As a result,a minor volcanic debris avalanche probably was createdthat was carried and/or covered by the still moving olderlava flow (Fig. 7A). The lava flow was diverted by the oldscoria cone, Cone B. The rafted cone blocks and collapsedsectors of the initial Los Morados main scoria cone piledup against the southern flank of older Cone B, which actedas barrier, and may have resulted in the formation of thehummocky surface (Figs 2B & 7A).Sudden decompression of the shallow magma columncaused a short lived, relatively violent Strombolian erup-tion. This eruption produced a tall eruption column thatdrifted slightly towards the NE and deposited the firstblack scoria blanket (Fig. 2A). Present day winds are fromNW to SE which is inconsistent with NE-distributed ashfall pattern suggesting a) the eruption took place in an un-usual wind pattern or b) the ash cover accumulated fromdirected blasts.This first violent Strombolian stage probably caused a mi-nor shift in the vent location in the crater of Los Mora-dos, moving about 100 metres toward the north in thearea of today’s crater wall breach. This is supported byslightly offset bedding dip directions of the agglutinatefacies mantling the inner crater wall (Fig. 3). The scoriacone was likely healed through resumed Hawaiian styleeruptions, providing agglutinate accumulation and associ-ated red scoria emission through the subsequent moderateStrombolian style eruption (Fig. 7A). The red scoria of thisstage covered the original scars, the hummocky surfaceand, partially, the older lava flow. The rebuilding phase ofthe scoria cone followed a similar evolutionary trend, andprobably culminated in a stage when the scoria cone once

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Figure 7. Cartoons show the inferred evolutionary steps of Los Morados main cone. North – south cross sectional views represent the evolutionarysteps of the eruptions. Small rectangular figures provide graphic demonstrations of the inferred processes in the upper conduit of eachstage. Left hand side the magma is shown with various bubbles (oval features). Black fields represent the chilled magma in the conduitwall. Right hand side white basal zone represent the conduit cut into the pre-eruptive country rocks. The dark grey field in the left handside represents the scoria cone edifice. Vertical black arrows indicate magma discharge rate; thicker the arrow the higher the magmadischarge rate.Evolutionary steps of the first rafting and healing events: 1) initial lava fountaining, fragmentation of magma took place in an openand wide conduit (small diagram) below the cone construct; 2) pyroclastic cone growth and the conduit became more localized. Theconduit wall became more defined (small diagram) with stable wall (black field on small diagram) high in the edifice. Fragmentationbecame typical Strombolian style with bubble coalescent and rhythmic outbursts; 3) lava flow movement initiated at the northwardinclined pre-eruptive surface, rafting parts of the cone away (blue arrow) and triggering a small volume volcanic debris avalanche; 4)sudden decompression over vent caused violent lava fountaining, and was by accompanied violent Strombolian activity. During thisstage of Los Morados main cone the eruption produced black ash and lapilli that covers large area (“I” dispersion pattern on Fig. 2A).5) the stage when the older lava flow probably was short lived and the cone activity changed to normal Strombolian type. However theprevious collapse and rafting event and the steeply inclined morphology kept the growing cone in an unstable condition.The instability of the cone increased by the outbreak of the younger lava flow at the base of the cone, rafting the healed part of thecone leading to a second decompression of the conduit and triggering a second violent Strombolian eruption which produced dispersionpattern of black ash and lapilli of “II-A” and “II-B” shown on Fig. 2A. 7) the large mass output rate of magma lead to a steady removalof portions of the cone flank and preventing cone healing.

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again became gravitationally unstable leading to a newoutbreak of lava in its northern foothill (Fig. 7B). Thisyounger lava flow subsequently rafted away large blocksof the cone flank. The rafted cone blocks likely reachedsizes of tens of metres across. Many large blocks grad-ually disaggregated and caused small scale collapses onthe lava surface, generating some ash tongues outbreaksin the main lava flow channel. In similar fashion to thefirst collapse, the removal of a large portion of the conecaused a pressure drop, and facilitated the production ofa fountain-like magma discharge and tall eruption columnduring a second violent Strombolian eruption. This erup-tion produced the second, more extensive, black scoriadeposit that followed an approximate west to east disper-sal direction (Fig. 7B). The fact that the younger lava flowis not covered by black scoria suggests that the lava flowemission and cone wall rafting were still proceeding dur-ing this violent Strombolian eruption stage. The volume ofthe younger lava flow indicates that the second collapse ofthe eruption was likely triggered by the arrival of a new,fast rising magma batch, which was able to sustain lavaflow effusion and lava fountaining after the collapse andrafting of the rebuilt cone (Fig. 7B).

5. Discussion on Scoria Cone Raft-ing, Eruption Styles and MorphologyLos Morados main scoria cone erupted on an inclinedpre-eruptive surface with complex local topography. Thispre-eruptive condition determined the fate of the cone.It seems that gradual changes in eruption style, fromlava fountain-dominated to typical Strombolian eruptionsformed a significant misbalance in the weight on the grad-ually established magmatic column which resided in thecentre of the cone. As a result, the unsupported parts ofthe cones were doomed to collapse. Due to the inferredlength of the eruption and the substantial magma supply,the vent hosted sufficient magma to cause multiple raft-ing and collapse events. It seems that lava flow-triggeredrafting, collapse and fast decompression of the magma-filled upper conduit are key parameters in the evolution-ary trend of some breached scoria cones. Thus, if thecone is built on an inclined pre-eruptive landscape, suchprocesses can trigger multiple eruptive stages, and createa complex final morphology that markedly departs fromthe ideal symmetrical shape of pyroclastic cones. Herewe explore other cones in the same volcanic field to testthis idea.Another young breached scoria cone, called Santa Maria,is located on a relatively flat surface about 15 km NE ofLos Morados (Fig. 8A); this is also a complex cone with

a lava flow that travelled about 16 km from its source.The cone itself has a slightly elliptical base and steppedarchitecture that is intact in its southern side (Fig. 8B).The present day cone, the result of its last cone buildingeruption stage, is surrounded by a complex and irregu-lar lava field and large rafted agglutinate blocks of a fewtens of metres across. The cone is breached to the NNW,but not as widely as Los Morados main cone. The pre-eruptive slope dip was probably less than in Los Mora-dos (Fig. 8C). The cone has a slight east-west elongationalong which at least 3 individual vents can be identified(Fig. 8A). One is slightly offset to the east, while two ventsare defined by a semicircular array of agglutinate and redscoria-dominated beds in the west. The northern side ofthis double vent is breached and a red and black scoriaflank indicates some healing of the northern side of thecone after the initial breach. From the northern breachan irregular, rough lava surface can be recognized. Thisaa lava flow is partially covered by randomly oriented ag-glutinate blocks similar to those described for Los Mora-dos. A discontinuous black scoria blanket covers an ovalshaped area, and is visible on satellite images extendingup to 4 km NE from the vent (Fig. 8A). This scoria blanketpartially covers the eastern side of the cone, but no suchcover is recognized on the lava flow outbreak to the north.These relationships suggest similar cone-breaching pro-cess as described for Los Morados main cone. The SantaMaria scoria cone likely collapsed through rafting duringits growing stage, followed by some partial healing of thecone flank, creating a very rugged and complex morphol-ogy on its northern side. At some point, a single, moreviolent Strombolian style stage (but judging by the in-ferred volume of the black ash, probably not as violentas at Los Morados main cone) occurred. An increasedmagma flux was likely responsible for the outpouring ofthe extensive lava flow that was subsequently covered bythe rafted portions of the northern part of the cone, attain-ing its present day morphology. It seems the Santa Mariascoria cone evolution was not as dramatic as Los Moradosmain cone, as it was formed on a nearly flat pre-volcanicsurface.

Elsewhere in the PMVF there are evidences of such ob-scured and breached scoria cone morphologies. However,there are – especially among the older cones –cones as-sociated with similar rough surfaced lava flows with raftedagglutinate blocks next to near perfect, red scoria dom-inated cones. It has been suggested for Sunset Craterin northern Arizona that scoria cone healing could leadto a complete re-establishment of youthful cone morphol-ogy after lava flow-triggered cone flank rafting. There isa good example of this process next to the western side ofLos Morados main cone (Cone A; Figs 4B & 8D). An un-

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Figure 8. A) Google Earth image of the Santa Maria scoria cone andassociated long lava flow. Inset shows the irregular mor-phology in the northern side of the cone, where blocksderived from a breached cone can be recognized.B) Google Earth image of the area west of Los Morados,where an older, still intact scoria cone stands (Cone A).The old Cone A, however surrounded by lava flow andrafted block-rich, rough and irregular surfaced region sug-gestive for an earlier rafting event associated with thiscone. The still intact shape of the cone suggests that thecone was able to heal after its initial rafting event. Another,smaller cone (Cone C) nearby demonstrates similar sce-nario as Cone B.C) The steep pre-eruptive surface can be viewed from thenorth of Los Morados scoria cone complex. Note the sig-nificant elevation differences the lava flow mark from itsproximal to its distal regions.D) Santa Maria scoria cone sitting on a relatively flat sur-face.

breached scoria cone (Cone A) with the red scoria blanketstands next to a highly irregular shaped lava flow that ispartially covered by some agglutinate blocks. The lavaflow associated with this cone is not as extensive as thoseassociated with Los Morados or Santa Maria scoria cone,suggesting a lower volume of available melt.The studied scoria cones compare well with those de-scribed from the San Francisco Volcanic Field (SFVF) [80].The Red Mountain, which has a significant breaching scar,indicating there was a mass wasting of nearly 15% of thetotal volume of the cone [80], could be a good analogyfor Los Morados main cone. It seems that such a largeamount of mass wasting cannot be healed after resumptionof Strombolian activity [80]. Other cones that are nearlyperfect in shape at PMVF (e.g. Cone A. west of Los Mora-dos main cone) resemble situations described from SunsetCrater in the SFVF [51, 81, 82], where syn-eruptive masswasting did not exceed 1% of the total cone volume. It hasalso been suggested for Red Mountain that the potentialof the rebuilding of a scoria cone after breaching, raft-ing and partial collapse is a function of the total volume

of material removed from the initial cone and therefore,of the character and intensity of post-raft eruptive activ-ity [80]. It has been proposed that in the case of low-flux,but vigorous eruptions that result in voluminous solid fallbacks the cone can be rebuilt; while in the case of highermagma flux, and conversely low lava fountaining events,that agglutinate or even clastogenic lava flows will ac-cumulate [80]. If the eruptions can go on over prolongedperiods of time, this will also sustain lava flow movement,and therefore constant removal of material of the freshlyaccumulated agglutinates and clastogenic lava flow pads,keeping the cone flank breached [80]. Los Morados maincone and, to lesser degree, Santa Maria are examples thatsupport these interpretations..At Los Morados main cone the first collapse probably trig-gered a tall eruption column for a significant period oftime, leading to partial healing of the cone. The subse-quent collapse was probably able to provide only a sin-gular violent Strombolian eruption due to the sudden de-compression over the magma filled upper conduit. Afterthis second stage the cone was never able to regain itsoriginal morphology. The situation is inferred to be sim-ilar, but simpler, in the case of Santa Maria. In othercircular cones surrounded by rafted small blocks coveredby restricted lava fields, the original rafting did not causedramatic geometrical changes in the upper conduit andthe eruption was likely driven by lower magma flux (andmaybe smaller total magma volume), providing an opportu-nity to develop a normal Strombolian style of eruption thatwas able to rebuild the cone into its original near-perfectcircular shape, similar to the Sunset Crater eruption inArizona [51].

6. Conclusion

The young monogenetic volcanic field of PMVF is an idealarea to investigate scoria cone morphologies caused byvarious degrees of cone collapse. We were able to iden-tify three end-member styles of scoria cone forming erup-tions; Los Morados main cone, Cone A and Santa María.Los Morados main cone represents an eruption that wasprobably long lived (months to years) and provided magmato build an initial scoria cone on an inclined and roughpre-eruptive surface. When the cone became gravitationalunstable, a first lava flow outbreak rafted part of the coneflank and initiated a partial sector collapse of the cone.The cone collapse caused decompression in the upper con-duit that triggered a violent Strombolian style stage. Sub-sequently, the eruption changed to a more typical Strom-bolian style that partially rebuilt the cone. Due to a likelyincrease of magma flux, and the gradual growth of the

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cone, a second lava flow outbreak removed the northernside of the cone, triggering a violent Strombolian stagethat was followed by a Hawaiian lava fountain stage thatwas accompanied by constant lava flow emission. Dur-ing this stage constant rafting of the cone flank preventedcone healing.Santa Maria, went through a similar phase as the secondstage of the Los Morados main cone, but probably not inas dramatic a manner as Los Morados.The Cone A represents a scoria cone that went throughmoderate rafting and a complete healing.We conclude that scoria cone growth is a complex process,and is a consequence of the interaction of magma inter-nal parameters such as volatile content, flux, recharge,and overall output volume and external parameters suchas slope angle and surface roughness of the pre-volcanicsurface.

AcknowledgementsThis project emerged from the Hungarian – ArgentineBilateral Science and Technology Cooperation Project(AR02/3) to KN and the ISAT Argentine – New ZealandScience and Technology cooperation project to CR andKN. The research is also part of the NZ Foundation forResearch, Science and Technology International Invest-ment Opportunities Fund Project (MAUX0808) “Facingthe challenges of Auckland’s volcanism” and the VolcanicRisk Solutions, Massey University PhD Research Fellow-ship to GK. Logistical help by the Rangers of the Direcciónde Recursos Naturales Renovables of Malargüe, Mendozaprovided excellent field work conditions. A research grant,UBACyT-X102 and grant PIP 2009-2011: 112-200801-02084 to CR provided financial support for conductingthis research. Detailed reading by Kate Arentsen madethe manuscript clear and focused. Suggestions by jour-nal reviewers (Andrea Borgia, Jorge Aranda-Gomez andan anonymus reviewer), the Managing Editor, KatarzynaCyran and the Technical Editor Michel Ciemała greatlyimproved the quality of this report.

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