Los Morados Scoria Cone, Mendoza, Argentina

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

Central European Journal of Geosciences

The role of collapsing and cone rafting on eruption style 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 to breached morphologies. Los Morados complex is a group of at least four closely spaced scoria cones (Los Morados main cone and the older Cones A, B, and C). Los Morados main cone was formed by a long lived eruption of 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 gradually grew until a lava flow breached the cone’s base and rafted an estimated 10% of the total volume. A sudden sector collapse initiated a dramatic decompression in the upper part of the feeding conduit and triggered violent a Strombolian style eruptive stage. Subsequently, the eruption became more stable, and changed to a regular Strombolian style that partially rebuilt the cone. A likely increase in magma flux coupled with the gradual growth of a new cone caused another lava flow outbreak at the structurally weakened earlier breach site. For a second time, the unstable flank of the cone was rafted, triggering a second violent Strombolian eruptive stage which was followed by a Hawaiian style lava fountain stage. The lava fountaining was accompanied by a steady outpour of voluminous 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 similar stages as Los Morados main cone, but probably not in as dramatic a manner as Los Morados. In contrast to these 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 that scoria cone growth is a complex process, and is a consequence of the magma internal parameters (e.g. volatile content, magma flux, recharge, output volume) and external conditions such as inclination of the pre-eruptive surface 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 ing [22, 29, 46, 47]. The common intercalation of scoria beds in scoria cones with welded fall deposits and/or Scoria cones are the most common manifestation of clastogenic lava flows [48] indicate a sudden and frequent subaerial small-volume, short-lived volcanism on Earth. change in eruption style from Strombolian to Hawai- These are generally considered to be a result of a mild ex- ian and vice versa [1], reflecting variable volcanic con- plosive eruption of mafic to intermediate magmas in a short duit dynamics during the eruption. Lava spatter near the period of time (days to weeks) [6]. However, long-lived vent, found typically along the crater rim of a cone, can scoria cone eruptions are also known, such as Paricutin form strongly welded, red, slightly bedded sequences with in Mexico that was active for 9 years [7]. The main vol- large spindle or highly vesiculated fluidal bombs [49, 50]. caniclastic deposits that are produced by a scoria cone These deposits usually reflect strong reworking of the py- eruption are the cone-building coarse-grained ash, lapilli roclasts as they roll back to the vent and are cleared and aggolomerate units and the medial to distal coarse again, resulting in complex amalgamated lapilli textures. ash to fine lapilli sized pyroclastic blanket. These pyro- The inner part of scoria cones suffers strong agglutination clastic deposits are formed when magmatic gas bubbles due to the persistent heat of the magma filled conduit [6]. coalesce and explosively burst in a relatively established Such welded parts of the inner cone, or the collar-like volcanic conduit, high in the growing cone [8–14]. De- welded ring on the crater rim, are more erosion resistant tailed analyses of deposits preserved on scoria cones and and can be preserved for a long time; however, they are their surroundings led to the clarification of the role of also prone to gravitational collapse, especially if there is the shallow seated magmatic system in the control of the mass deficit at the base of the cone. Lava emission points explosive eruptions of such volcanoes [1, 15–19]. Among or fractures at the base of the cone may cause gradual the identified parameters, the variations in degassing pat- rafting of large portions of the cone basal flank that can terns, magma ascent rates and degrees of interaction with lead to gravitational sector collapse of the cone [1, 51]. external water are thought to be responsible for sudden Here we explore the morphological results of lava flow changes in the eruption styles [1, 17, 20]. rafting, and resulting cone sector collapses, as a prime controlling parameter in triggering significant eruptive Small-volume mafic cone building eruption styles can style changes in the course of scoria cone growth, and how be grouped into four end member types, which primar- such processes may be reflected by 1) the accumulated ily depend on the magmatic mass flux and the degree of intra- and inter-cone pyroclastic successions, 2) textural fragmentation: 1) Hawaiian, 2) Strombolian, 3) violent cyclicity of the pyroclastic deposits, and 3) final morphol- Strombolian, and 4) weak ash emission [1, 21]. Hawai- ogy of the scoria cone. ian style eruptions produce coarse-grained, moderately vesicular pyroclastic deposits (e.g. low level of fragmentation) through an eruption with high magma flux (i.e. 50 1.1. Payún Matru Volcanic Field (PMVF) – 1000 m3/s) [1, 22, 23]. In contrast, Strombolian activity typically produces pyroclasts that are relatively coarse The Payún Matru Volcanic Field (PMVF) is located in the grained and vesicular, but finer grained than those pro- southeastern region of the Mendoza province, Argentina, duced by Hawaiian style eruptions, reflecting more effec- between latitudes 36˚ and 36˚ 35’ S and 68˚ 30’and tive magma fragmentation [6, 24]. Strombolian style erup- 69˚30’ W, approximately 200 km east of the trench in the tions are associated with a significantly lower magma flux Southern Volcanic Zone of the Andes (inset in Fig. 1A). rate (i.e. 10−3 – 100 m3 per explosions over several explo- The PMVF covers 5,200 km2 [52, 53] and is a broad back- sions an hour) [1, 6]. Weak ash emission eruptions produce arc lava plateau with hundreds of monogenetic pyroclastic fine ash, with a negligible gas thrust and therefore very cones. limited (usually in the crater basin) distribution of the Two roughly contemporaneous basaltic volcanic fields are deposit. Violent Strombolian style eruptions are those E and W Payún Matru (Fig. 1A). These monogenetic fields that produce significant volumes of fine ash, as a conse- are referred to as the Payén east and Payén west lava quence of effective magma fragmentation accompanied by shields respectively. The eastern Payén is related to an high magma flux and high explosive event frequency. Such east-west trending fault system. This fissure-controlled eruptions can spread ash over relatively large areas and volcanism produced several extremely long lava flows that disperse fine pyroclasts to over 10 km distance from their reach the Salado river valley in La Pampa province to the source [1, 25–30]. east. One of the lava flows has an individual tongue-like The closely packed, slightly oriented texture of mod- shape 181 km long, and is the longest known individual erately vesicular coarse ash and lapilli accumulations Quaternary lava flow on Earth [67]. are a typical result of Hawaiian style lava fountain- Volcanism on the western slope of Payún Matru volcano

103 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

Figure 1. A) Simplified geological map of the central sector of the Payen Volcanic Complex modified after Pasquaré et al. 2008 [41]. The youngest basaltic eruptive products are shown in black without distinction of individual vents. The youngest lava flows are associated with the Santa 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 Shield Volcano. Diagonal line pattern represents parts of the Payen Eastern Shield Volcano covered by younger eruptive products of trachyte and trachyandesite lavas derived from Payún Matru caldera or Payún Liso stratovolcano (“vvv” pattern). Major lava fields such as Los Carrizales 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 Ramos and Folguera 2010 [40] and Germa et al 2010 [39], in lava flows exposed along the Rio Grande valley; these lavas pre-date the youngest lava flows and scoria associated to Los Morados. Inset shows the general tectonic setting of the study area (black star) and the location of 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 lava pulses can be recognized. The older formed pahoehoe flows with tumuli, pressure ridges, skylights, ropy lavas and 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 rubble levees (Tromen Group) [52]. Some of the scoria cones are very well preserved, with small craters and slope values close to 33˚. The younger eruptive period includes the Holocene age volcanoes of the Carbonilla region, east of Payún Matru, and Santa María to the west (Fig. 1A). The post-caldera and fissure phase volcanic activity at the Payen west shield generated more than 100 monogenetic scoria cones and lava flows that reach 30 km from their individual point sources in the Los Volcanes area (Fig. 1B). These lava flows overlap each other, creating a fan-like lava field on the eastern margin of the Río Grande river (Fig. 1B) [71]. A large number of scoria cones have associated 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, and they have crater diameters between 140 and 750 m. They consist mainly of agglomerates and volcanic breccias with variable degrees of welding. Eruptive mechanisms were primarily of Strombolian and Hawaiian type, with occa- Figure 2. A) Google Earth image of Los Volcanes area. Dispersal areas of black ash fall issued by Los Morados volcanic sional violent Strombolian eruptions. The average cone complex are outlined with white lines; I - first major pyro- density for the area is 33 cones per 100 km2. The high clastic fall, II-A - second major pyroclastic fall, II-B - probably part of the second major fall event. cone density is probably a result of a strong magmatic B) An interpretative geological map based on mapping in pulse initiated the volcanic field formation [72]. volcanic units of the same area shown on 2A. Pampas Negras (“black flat surface”) defines an area among young scoria cones of Los Volcanes that is covered by black scoriaceous ash and lapilli deposits (Fig. 2) that ucts of the Los Morados main cone, named as Cone A partially or totally cover the slopes of older scoria cones. (Figs 2A, B & 3A, B), as well as another older cone to It is inferred to be the product of a far less explosive scoria the north of Los Morados main cone referred here as cone building period than those earlier more silicic erup- Cone B, and an eroded older cone west of Cone A, named tions associated to the Payún Matru composite volcano. here as Cone C (Figs 2A, B & 3A). The radiometric age Some scoria cones produced ash and lapilli deposits that of Los Morados scoria cone complex is unknown, but it are restricted chiefly to their cone edifice and within a few rests atop other lavas erupted from the Payén west shield kilometres from their source vents. Today ash and lapilli volcano (Fig. 1A) [62]. The youngest age of the lava has been re-worked by wind, forming elongated and par- flows of the shield volcano in the Rio Grande basin is allel dunes perpendicular to the prevailing westerly wind 0.016±0.001 Ma, putting a limit to the age of Los Mora- direction. dos volcanic complex [62]. These stratigraphic and radiometric age data coincide well with the general morphological characteristics of both the Los Morados scoria cone 2. Los Morados scoria cone complex and associated lava flows. The outer slope of the Los Morados main cone is steep (up to 30 degrees) Los Morados is a young scoria cone complex with closely and youthful in appearance with a fresh, reddish to black spaced edifices in the western edge of the PMVF in Men- lapilli blanket with no significant gully network. However, doza (Fig. 1). Here we concentrate on the morphologi- dominantly dry climatic conditions with annual precipita- cal evolution of the youngest of the cones, named as Los tion of a few hundred mm favours the preservation of the Morados main cone, and an older cone west of the Los volcanic edifices over a relatively long period of time. Morados main cone, partially covered by eruptive prod- Los Morados main cone is part of an alignment of vents,

105 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

elongated deep crater. To the north of Los Morados main cone, about 1.7 km from its crater, an older scoria cone (Cone B, Figs 3A & B) formed a barrier that diverted the lava ﬂows issued from the base of Los Morados main cone (Figs 3A & B). This older cone (Cone B) has a smooth surface with a light vegetation cover and it has a less well preserved crater than Los Morados main cone, indicating that it is part of an older eruption in the area (Fig. 3B). Los Morados scoria cone complex was probably active over a prolonged time period. Los Morados main cone is inferred to be the youngest volcano in the Los Morados scoria cone complex, and potentially is the source of a major ash fall event that issued black ash and lapilli throughout a fairly large area, commonly referred to as the “ash and lapilli plain” or “Pampas Negras” (Figs 1 & 2).

2.1. Morphology and Erupted Volume

Precise delineation of the dimensions of Los Morados main cone is not simple due to the thick tephra cover that ob- scures parts of the cone (Figs 3A & B). The main cone of Los Morados scoria cone complex is located next to another older cone (Cone A, Fig. 3) that is inferred to be older than Los Morados; the thin vegetation cover of the older Cone A contrasts with the barren, red and black ash Figure 3. A) Close up Google Earth image of the Los Morados sco- covered surface of the outer flank of Los Morados main ria cone complex. Thick yellow line outlines the younger cone (Fig. 3B). The base of Cone A is about 2100 metres lava flow with rafted cone material of Los Morados main cone. Thick white line bounds to the zone from where the above sea level, which allows us to estimate a value of cone flank was rafted away. Dashed white line marks Los 2150 metres for the basement of Los Morados main cone Morados main cone older lava flow and hummocky area in the northern side of Los Morados main cone. Yellow ar- based on spot elevation readings on GoogleEarth images row points to the ash-covered hummocky surfaces north of as well as direct hand-held GPS measurements (Fig. 3A). Los Morados main cone. Dashed yellow line corresponds to the base of an older scoria cone (Cone A) west of Los This value is a good average for the western side of the Morados main cone. Red star shows the location of an Los Morados main cone; however, the eastern basement older, small-volume, lava outbreak at the base of Cone lies on higher ground, but probably not more than 100 me- A. The red star is also shown air photo on 3B. Note the fractures marked by white arrow parallel to the cone flank tres higher than the western side. This inferred elevation breach. White dot marks the point photo 4B was taken. difference indicates that the Los Morados main cone itself B) Aerial view of Los Morados scoria cone complex, as seen from the north. White dot marks the point photo 4B erupted on a north-westward dipping topography with an was taken. inclination angle of 4˚ (Fig. 3B). The general slope angle values were approximated by using planes as a “trend surface” fitted to digitalized spot heights with individual on an E-W trending fissure system (Carbonilla Fault Sys- elevation values. These elevation points were digitalized tem) (Fig. 1A). The westernmost vent at Los Morados sco- from areas where no tephra blanket exists and effusive ria cone complex is an older scoria cone, with an eroded products (e.g. lava flows) of Los Morados cannot be found morphology (Cone C) (Figs 2A, B & 3A, B). The eastern- (e.g. areas that are interpreted as the syn-eruptive sur- most part of Los Morados scoria cone complex forms a row face). This calculation supports that the Los Morados of fissure vents which has at least 5 individual vent sites scoria cone complex erupted on a surface gently inclined spaced a few tens of metres apart (Fig. 2A). The central (∼4˚) towards the northwest. The Los Morados main cone part of the Los Morados scoria cone complex is composed however, was likely formed on a very irregular topography of two overlapping scoria cones. The older one, named as determined by the steep slopes of Cone A (Fig. 3B). Cone A has a near-circular well-preserved crater, while The highest point on the Los Morados main cone crater the Los Morados main cone partially covered Cone A.The rim today lies in the south-east and is about 2425 me- Los Morados main cone has an oval shaped, east–west tres above sea level (Figs 3A, B & 4A). From this highest

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point, the crater rim has a crescent shape that gradually decreases its elevation to 2275 m to the north and to 2250 m to the northwest side. The crescent shaped edifice encircles a U-shaped scar on the northern side of the volcano (Fig. 3B). The U-shaped scar is the source of an extensive lava flow and associated rafted cone material (Figs 3B& 4B). The scar is about 400 metres across where it meets the crater rim top, and at its base is about 200 metres wide (Figs 3A & B). The crater rim has an elliptical shape with an E-W, 680 m long axis, and a 500 m long N-S axis (Figs 3A & B). The base of Los Morados main cone is obscured by a pyroclastic blanket, thus its basal diameter cannot be determined easily, but our best estimates based on direct GPS-measurements, spot elevation data from GoogleEarth images and field observations give a maximum base diameter of about 1200 metres (sim- Figure 4. A) Panoramic view of Los Morados main cone from the ilar to the older cone to the west, Cone A), a 2150 metres east. above sea level horizon in the west (the same level as B) Breached cone flank of Los Morados main cone from the eastern side of the younger lava flow initiated from the the base of Cone A) and a value of 2300 metres for the cone. basement level in the east (Fig. 3). This together provides C) Inner crater wall of Los Morados main cone exposes rough estimates of the Los Morados main cone height with welded lava spatters steeply dipping inward. D) Lava spine in the proximal area of Los Morados main a minimum 125 and maximum of 275 m. The older Cone cone younger lava flow. A in the west has an average cone base diameter of 1200 metres and cone height of about 200 metres, providing an estimated cone height (H ) to basal diameter (W ) ratio co co alent (DRE), using an average 50% vesicularity of typical of 0.167. Our observations indicate that if Los Morados scoriaceous pyroclasts, to give a value of about 0.15 km3 main cone had grown on a flat surface it could have simi- magma involved in the formation of the pyroclastic units lar dimensions to Cone A. However, Los Morados is sitting of Los Morados main cone. on a north-westward gently inclined depositional surface with a potential slope angle of at least ∼4˚. The Hco/Wco ratio of Los Morados is calculated between 0.1 to 0.23, Table 1. Calculated eruptive volumes associated with measured sec- depending on which reference elevation of the cone base tors on Fig. 5A. is used. Measured regions Area Thickness In m3 In km3 [on Fig. 5] covered in m2 in metres To estimate the cumulative volume of pyroclastic deposits 1 148831:70 150 22324755:000 0:022325 associated with Los Morados main cone, measurements 2 384564:60 80 30765168:000 0:030765 of the total thickness of pyroclastic deposits associated 3 761612.54 60 45696752.400 0:045697 with the main cone were taken in the Pampas Negras 4 6060053.10 20 121201062.000 0:121201 area (Figs 2B& 5A). The measurements included the es- 5 2895932.15 10 28959321.500 0.028959 timated deposit thicknesses on the Los Morados main cone 5a 1711069.76 10 17110697.600 0.017111 and the primary (ie: not wind re-worked) distal inter-cone 6 734716.58 0.5 367358.290 0.000367 field tephra layer thicknesses. We also included the wind- 7 1282405.62 4 5129622.480 0.00513 8 4122036.43 0.5 2061018.215 0.002061 redeposited ash and lapilli that form thick accumulations 9 5822792.84 0.5 2911396.420 0.002911 of tephra on the west facing side of older cones and lava 10 2273830.63 4 9095322.520 0.009095 flows east of Los Morados main cone (Fig. 5A and Table 1). 11 99731.43 4 398925.720 0.000399 The total volume of the pyroclastic deposits, including Los 12 112160.12 0.5 56080.060 5.61E-05 Morados main cone, is about 0.3 km3. This value is likely 13 158664.13 4 634656.520 0.000635 an underestimate due to the uncertainties in the delin- 14 71610.12 4 286440.480 0.000286 eation of the cone base and the thickness of the tephra 18 1262616.12 0.5 631308.060 0.000631 Total 0.28763 in proximal regions. However, it appears to be a reason- able estimate with Los Morados emitting approximately 0.3 km3 of tephra that accumulated in the cone and in the We also estimated the lava flow volume, which was emitted vicinity. This could be recalculated to a dense rock equiv- in two stages through the northern breach of Los Morados

107 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

Figure 5. A) Black scoria fall thickness map calculated from field measurements of tephra layers and corrected by the estimated pre-eruptive surface morphology. B) Simplified outline map of the two lava flows produced by Los Morados main cone. The black area corresponds to the younger Los Morados lava flow. Calculated eruptive volume estimates listed in the figure using 3 m (min.) or 10 m (max.) average lava flow thicknesses 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). at the northern breach of the scoria cone, and reaches The older lava flow and rafted collapsed cone flank cover a width of about 300 metres. There are at least two dis- an area of approximately 1.3 km2(Fig.5B). The flow vol- tinguishable lava flows issued from the breach of the scoria ume was estimated based on field observations on lava cone (Fig. 2B). An older lava flow extends toward the north flow thicknesses. The average lava flow thickness esti- and is blocked by an older scoria cone, reaching a length mates of 10 m and 30 m were obtained at the proximal of about 1 kilometre before being diverted toward the west flow margins. Thus the erupted volume of the older lava (Fig. 2). This older flow can be traced at least another flow ranges between 0.01 and 0.04 km3 (Fig.5B). 1.3 km further down beneath a more extensive and younger The younger and more extensive lava flow covers an area lava flow. The older lava flow is covered extensively by of approximately 10.57 km2. Using an estimated minimum clinker and rubble derived from the margin of the younger of 3 metres and maximum of 10 metres average thickness lava flow. In addition, the older lava flow is partially cov- from field observations, a minimum of 0.03 and a maximum ered by a red and black scoria blanket which is missing of 0.11 km3 magma erupted through this lava flow stage on top of the younger lava flow. The young lava flow, after (Fig.5B). reaching the older scoria cone barrier (Cone B; Fig. 3A), was diverted to the west, and travelled at least another The total estimated volume of magma involved in the erup- 7.5 km before stopping. About 1 km and 2.3 km from the tion of Los Morados main cone therefore ranges between first diversion point, the younger lava flow branches to 0.19 and 0.3 km3 in dense rock equivalent. two separate narrower arms that were emplaced slightly toward the northwest. These lava flow arms have typi- 2.2. Volcanic Facies cal aa lava surface morphologies, with a central channel and a rampart of broken lava fragments on both sides the Several volcanic facies were identified in and around the main flow channel (Fig. 2A). The widest arm also displays volcanic construct of Los Morados main cone. The sco- in its final 2.5 km multiple channelized flow structures and riaceous deposits were divided according to their colour, pressure ridges. However, occasional agglutinate metre- referring to red (Sr) and black (Sb) scoria beds ranging sized blocks are still randomly distributed on the top of from coarse ash to coarse lapilli grain sizes. the younger lava flow, especially in the deeper parts of Agglutinate facies (A) are moderately to strongly welded, the rugged surface. coarse grained pyroclastic rocks that are commonly associated with red scoria interbeds (Fig.4C). The agglutinate facies also includes welded lava spatter and welded scoria, where original clast outlines can be confidently rec- 3. Volcanic Facies Distribution ognized in proximal volcanic units. Densely welded pyroclasts commonly form coherent interbeds of dark colour, The eruptive products of Los Morados main cone can be lava-like rocks where original clasts can barely be recog- grouped into 3 major volcanic facies. The cone itself is nized, and only some irregularities and the occasional, predominantly composed of Group I facies formed by red commonly random clast outlines indicate the pyroclas- scoria beds, agglutinate and minor clastogenic lava flow tic origin of the rock. These beds are clastogenic lava inter-beds. These lithologies are exposed on the steep flows that formed as a result of rheomorphic processes inner crater wall. The outer upper flank of the cone is that nearly completely destroyed the original fabric. Such also composed of red scoria; however there is no exposed clastogenic lava flows probably represent fast accumula- agglutinate collar on the crater rim (Fig. 3B& 4A). The tion of lava spatter from low eruption columns. Group II facies is exposed in the region of the lower cone Lava facies (L) can be distinguished on the basis of the flank and the inter-cone field, which is composed by black lack of any indicators of clast outlines. In addition, they scoria of coarse ash and lapilli (Fig. 2). The black scoria show typical surface features, such as spines, ropy sur- beds can be traced in three distinct directions and are in- faces, vesicle pipes and/or associated flow front features. ferred to be sourced from Los Morados main cone as this The lava facies show characteristic features of an aa lava, is the only vent located in the right position in respect to with pressure ridges across the lava field (Fig. 2A), rugged the bed thickness variations (Fig. 5A). The larger black surface morphology with m-sized spines and vesicular scoria dispersion pattern represents a significant eruption zones in the lava plates. The lava facies is covered by rub- event, as tephra reached areas at least 10 km from the vent ble derived from fragmented lava blocks and clinker that is across an east-west axis (Fig. 2A). The estimated area mixed with m-sized of agglutinate blocks that still retain covered by black scoria is about 100 km2. Another, not original bedding in a contorted fashion (Fig. 4D). The lava so clearly traceable black scoria dispersal pattern, with field is considerably wider from its point source located a 6 km long axis toward the NE can also be recognized

109 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

(Fig. 2A). This black scoria blanket covers an area of about agglutinate beds exposed in the crater wall. 60 km2. The thickness of the black scoria units is variable as shown in Figure 5A. Thickness is in the metre scale in the flank of the Los Morados main cone. Between 5 to 10 kilometres from the source the thickness of the scoria blanket is still in the cm-scale (Fig. 5A). Determination of the precise thickness of the scoria blanket in many places is hindered by the lack of natural outcrops, and the difficulty in digging as the surface is covered by desert pavement. The total volume of the black ash and lapilli blanket can be estimated from field measurements (Table 1) by subtract- ing the near vent pyroclasts included previously in the total pyroclast volume measurements (Table 1 and Fig. 5A). In this way, an estimated volume of 0.068 km3 tephra is obtained for the likely products of two major, and one likely minor, individual eruptions (Fig. 2A). This value indicates that the major black scoria blanket must have been produced by an eruptive stage in the lower end member of Figure 6. A) Field relationships between lava flows and rafted a sub-Plinian eruption [74, 75]. This type of activity is and/or collapsed cone flank blocks in the breach area. commonly referred in a basaltic monogenetic context as Dashed line marks the contact between the older and younger lava flows. a violent Strombolian type of eruption [1, 34, 76–79]. It B) Overview of the tephra-covered hummocky surface is also likely that the smaller black NE-trending disper- north of Los Morados main cone. Arrows point to flattened sal pattern was also of the violent end-member of normal lava spatters. C) Flattened, moderately vesicular, stratified lava spatters Strombolian type eruptions based on the similar disper- in a rafted cone block on the surface of the young lava sion values as the E-trending ash and lapilli field. flow of Los Morados main cone. D) Spill over of the younger lava flow, interpreted as the Agglutinate is primarily exposed in the inner crater wall result of a local collapse of a large rafted block. Dashed arrow show the path of the spill. either as plastered and steeply inward inclined stacks of beds or as outward dipping layers which are exposed by rock falls in the crater wall interior (Fig. 4C). The exposed Red scoria beds are common in the upper and outer cone in situ agglutinate forms dm-thick beds of strongly welded, flank. In the upper exposed section of the crater wall there slightly flattened, moderately vesicular scoriaceous lapilli are individual well-sorted beds of coarse ash to fine lapilli hosted in red scoriaceous coarse ash. In situ agglutinate red scoria. A similar red scoria blanket seems to follow units commonly form coarse to fine bed couplets with thin, the east – west oriented fissure network. In situ agglu- few cm-thick fine ash beds. The fine ash beds are also tinate outcrops on the upper crater wall and red scoria commonly welded and the original pyroclast morphology beds are more common than at the base of the crater. is difficult to recognize. Thin (dm thick) clastogenic lava Similarly loose red scoria deposits tend to fill some small flows form discontinuous horizons exposed on the crater (metres scale) depressions on top of the lava flow and in wall. The rocks exposed in the crater wall are sufficiently the displaced agglutinate blocks. The reddish scoria blan- welded to support the agglutinate beds that dip steeply ket clearly marks a coloured zone in the entire length of inward. the nearly 10 km long younger lava flow. Agglutinate facies are exposed north of the crater wall In the northern side of Los Morados main scoria cone, just breach as individual blocks with random bed dip direc- in front of older Cone B, a hummocky surface is exposed tions (Fig. 6A). The abundance of well-defined zones of (Fig. 6B). Hummocks are up to 10 meters tall and ran- agglutinate-dominated rocks on the rough lava flow sur- domly distributed and covered by a red and black sco- face decreases with distance from the Los Morados main ria lapilli blanket (Fig. 6B). The larger hummocks are cone. Near to the lava flow point source at the cone formed agglutinate and show very diverse dip and bed- breach, large blocks of contorted, truncated and randomly ding characteristics indicating that the individual blocks tilted agglutinate blocks are exposed, commonly stacked were displaced and rotated. The hummocky surface forms over each other. Bed dip directions in this area do not a broader zone than the younger lava flow (Fig. 3). It resemble any systematic distribution, thus we interpret seems that the original width of the cone breach was these blocks as fragments derived from the breached por- slightly wider than the breach width evident today. Also, tion of the cone. These blocks are similar to the in situ the younger lava flow of Los Morados main cone over-

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runs the older lava flow with highly irregular morphology cone building phase, a fairly large scoria cone formed. (Fig.6C). The hummocky surface is likely to be part of Lava draining back into the upper conduit was potentially the older lava flow region partially covered by either the a trigger of some dramatic processes that modified the younger lava flow or the black scoria blanket (Fig. 6D). original cone morphology and allowed lava to drain near the base of the cone. The breach of the cone with a lava flow outbreak was in- 4. Los Morados Main Scoria Cone evitable due to the generally inclined pre-eruptive land- Evolution scape and the irregular morphology of the area of Los Morados main cone basement. Magmatic pressure com- 4.1. Original Size and Shape of the Cone bined with the gravitationally unstable geometry (having a dense magma filled conduit well above the pre-eruptive It is difficult to establish the Los Morados main cone orig- surface in the growing scoria cone) provided significant inal size as it was built on a highly irregular pre-eruptive pressure on the northern, unsupported flank of the cone, volcanic surface. It can be accepted that the cone itself is which eventually collapsed and issued a lava flow through at least 125 m high and probably as high as 275 m. The the lower part of the cone (Fig. 7A). The first lava outbreak young age of the scoria cone and abundance of inward created the older lava flow, which is now partially exposed dipping welded lava spatter preserved in its crater sup- on the northern side of the breached Los Morados main port that the morphology of Los Morados main cone has cone, rafting large parts of the initial transient cone flank. not been significantly modified since its formation. This is The lava flow probably accelerated the gravitational cone also suggested by the lack of gullies, erosional scars, or collapse process, opening up a wide breach in the inclined other landforms indicative of cone degradation. The mini- unstable northern side of the initial cone. As a result, mum total volume of the cone prior to its breaching can be a minor volcanic debris avalanche probably was created estimated in the range of 0.25-0.35 km3. The width of the that was carried and/or covered by the still moving older breach on the northern side of Los Morados main cone is lava flow (Fig. 7A). The lava flow was diverted by the old nearly as large as the crater of the preserved cone, indi- scoria cone, Cone B. The rafted cone blocks and collapsed cating significant mass wasting. Visual estimation of the sectors of the initial Los Morados main scoria cone piled volume of the obscured part of the Los Morados main cone up against the southern flank of older Cone B, which acted suggests that about 10% of the cone flank is disturbed or as barrier, and may have resulted in the formation of the missing. This value indicates a significant mass wasting hummocky surface (Figs 2B& 7A). on the northern flank of the Los Morados main cone. Sudden decompression of the shallow magma column caused a short lived, relatively violent Strombolian erup- 4.2. Inferred Eruption History tion. This eruption produced a tall eruption column that drifted slightly towards the NE and deposited the first The earliest eruptive stage on Los Morados main cone black scoria blanket (Fig. 2A). Present day winds are from that can be documented with present day exposures at NW to SE which is inconsistent with NE-distributed ash the bottom of the crater wall was likely a Hawaiian style fall pattern suggesting a) the eruption took place in an un- eruption, as evidenced by the abundance of agglutinate usual wind pattern or b) the ash cover accumulated from and minor clastogenic lava flows (Figs 7A & B). This early directed blasts. Hawaiian style eruption changed to a normal Strombolian This first violent Strombolian stage probably caused a mi- scoria cone forming-eruption. This eruptive style permit- nor shift in the vent location in the crater of Los Mora- ted the establishment of a volcanic conduit with a semi- dos, moving about 100 metres toward the north in the sealed wall that allowed a stable magma column to form; area of today’s crater wall breach. This is supported by this column hosted larger bubbles that tended to coalesce slightly offset bedding dip directions of the agglutinate (Fig. 7A). The bubble coalescence created conditions that facies mantling the inner crater wall (Fig. 3). The scoria were favourable to generate gas pockets that caused pe- cone was likely healed through resumed Hawaiian style riodic outbursts and normal Strombolian style explosions. eruptions, providing agglutinate accumulation and associ- The magma rise speed and flux likely dropped at this stage ated red scoria emission through the subsequent moderate in comparison with the earlier Hawaiian lava fountaining Strombolian style eruption (Fig. 7A). The red scoria of this stage. Volcanic activity at this point was dominated by stage covered the original scars, the hummocky surface the formation of red scoria and minor agglutinate, as ev- and, partially, the older lava flow. The rebuilding phase of idenced by the increased volume of red scoria beds in the scoria cone followed a similar evolutionary trend, and the upper and outer cone flank (Fig. 7A). During this first probably culminated in a stage when the scoria cone once

111 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

Figure 7. Cartoons show the inferred evolutionary steps of Los Morados main cone. North – south cross sectional views represent the evolutionary steps of the eruptions. Small rectangular figures provide graphic demonstrations of the inferred processes in the upper conduit of each stage. Left hand side the magma is shown with various bubbles (oval features). Black fields represent the chilled magma in the conduit wall. Right hand side white basal zone represent the conduit cut into the pre-eruptive country rocks. The dark grey field in the left hand side represents the scoria cone edifice. Vertical black arrows indicate magma discharge rate; thicker the arrow the higher the magma discharge rate. Evolutionary steps of the first rafting and healing events: 1) initial lava fountaining, fragmentation of magma took place in an open and wide conduit (small diagram) below the cone construct; 2) pyroclastic cone growth and the conduit became more localized. The conduit wall became more defined (small diagram) with stable wall (black field on small diagram) high in the edifice. Fragmentation became typical Strombolian style with bubble coalescent and rhythmic outbursts; 3) lava flow movement initiated at the northward inclined 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 this stage 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 the previous 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 the cone leading to a second decompression of the conduit and triggering a second violent Strombolian eruption which produced dispersion pattern 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 removal of portions of the cone flank and preventing cone healing.

112 K. Németh et al.

again became gravitationally unstable leading to a new a lava flow that travelled about 16 km from its source. outbreak of lava in its northern foothill (Fig. 7B). This The cone itself has a slightly elliptical base and stepped younger lava flow subsequently rafted away large blocks architecture that is intact in its southern side (Fig. 8B). of the cone flank. The rafted cone blocks likely reached The present day cone, the result of its last cone building sizes of tens of metres across. Many large blocks grad- eruption stage, is surrounded by a complex and irregu- ually disaggregated and caused small scale collapses on lar lava field and large rafted agglutinate blocks of a few the lava surface, generating some ash tongues outbreaks tens of metres across. The cone is breached to the NNW, in the main lava flow channel. In similar fashion to the but not as widely as Los Morados main cone. The pre- first collapse, the removal of a large portion of the cone eruptive slope dip was probably less than in Los Mora- caused a pressure drop, and facilitated the production of dos (Fig. 8C). The cone has a slight east-west elongation a fountain-like magma discharge and tall eruption column along which at least 3 individual vents can be identified during a second violent Strombolian eruption. This erup- (Fig. 8A). One is slightly offset to the east, while two vents tion produced the second, more extensive, black scoria are defined by a semicircular array of agglutinate and red deposit that followed an approximate west to east disper- scoria-dominated beds in the west. The northern side of sal direction (Fig. 7B). The fact that the younger lava flow this double vent is breached and a red and black scoria is not covered by black scoria suggests that the lava flow flank indicates some healing of the northern side of the emission and cone wall rafting were still proceeding dur- cone after the initial breach. From the northern breach ing this violent Strombolian eruption stage. The volume of an irregular, rough lava surface can be recognized. This the younger lava flow indicates that the second collapse of aa lava flow is partially covered by randomly oriented ag- the eruption was likely triggered by the arrival of a new, glutinate blocks similar to those described for Los Mora- fast rising magma batch, which was able to sustain lava dos. A discontinuous black scoria blanket covers an oval flow effusion and lava fountaining after the collapse and shaped area, and is visible on satellite images extending rafting of the rebuilt cone (Fig. 7B). up to 4 km NE from the vent (Fig. 8A). This scoria blanket partially covers the eastern side of the cone, but no such cover is recognized on the lava flow outbreak to the north. 5. Discussion on Scoria Cone Raft- These relationships suggest similar cone-breaching process as described for Los Morados main cone. The Santa ing, Eruption Styles and Morphology Maria scoria cone likely collapsed through rafting during its growing stage, followed by some partial healing of the Los Morados main scoria cone erupted on an inclined cone flank, creating a very rugged and complex morphol- pre-eruptive surface with complex local topography. This ogy on its northern side. At some point, a single, more pre-eruptive condition determined the fate of the cone. violent Strombolian style stage (but judging by the in- It seems that gradual changes in eruption style, from ferred volume of the black ash, probably not as violent lava fountain-dominated to typical Strombolian eruptions as at Los Morados main cone) occurred. An increased formed a significant misbalance in the weight on the grad- magma flux was likely responsible for the outpouring of ually established magmatic column which resided in the the extensive lava flow that was subsequently covered by centre of the cone. As a result, the unsupported parts of the rafted portions of the northern part of the cone, attain- the cones were doomed to collapse. Due to the inferred ing its present day morphology. It seems the Santa Maria length of the eruption and the substantial magma supply, scoria cone evolution was not as dramatic as Los Morados the vent hosted sufficient magma to cause multiple raft- main cone, as it was formed on a nearly flat pre-volcanic ing and collapse events. It seems that lava flow-triggered surface. rafting, collapse and fast decompression of the magma- filled upper conduit are key parameters in the evolution- Elsewhere in the PMVF there are evidences of such ob- ary trend of some breached scoria cones. Thus, if the scured and breached scoria cone morphologies. However, cone is built on an inclined pre-eruptive landscape, such there are – especially among the older cones –cones as- processes can trigger multiple eruptive stages, and create sociated with similar rough surfaced lava flows with rafted a complex final morphology that markedly departs from agglutinate blocks next to near perfect, red scoria dom- the ideal symmetrical shape of pyroclastic cones. Here inated cones. It has been suggested for Sunset Crater we explore other cones in the same volcanic field to test in northern Arizona that scoria cone healing could lead this idea. to a complete re-establishment of youthful cone morphol- Another young breached scoria cone, called Santa Maria, ogy after lava flow-triggered cone flank rafting. There is is located on a relatively flat surface about 15 km NE of a good example of this process next to the western side of Los Morados (Fig. 8A); this is also a complex cone with Los Morados main cone (Cone A; Figs 4B& 8D). An un-

113 The role of collapsing and cone rafting on eruption style changes and ﬁnal cone morphology

of material removed from the initial cone and therefore, of the character and intensity of post-raft eruptive activity [80]. It has been proposed that in the case of low-flux, but vigorous eruptions that result in voluminous solid fall backs the cone can be rebuilt; while in the case of higher magma 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 prolonged periods of time, this will also sustain lava flow movement, and therefore constant removal of material of the freshly accumulated agglutinates and clastogenic lava flow pads, keeping the cone flank breached [80]. Los Morados main cone and, to lesser degree, Santa Maria are examples that support these interpretations.. At Los Morados main cone the first collapse probably triggered a tall eruption column for a significant period of Figure 8. A) Google Earth image of the Santa Maria scoria cone and time, leading to partial healing of the cone. The subse- associated long lava flow. Inset shows the irregular morphology in the northern side of the cone, where blocks quent collapse was probably able to provide only a sin- derived from a breached cone can be recognized. gular violent Strombolian eruption due to the sudden de- B) Google Earth image of the area west of Los Morados, where an older, still intact scoria cone stands (Cone A). compression over the magma filled upper conduit. After The old Cone A, however surrounded by lava flow and this second stage the cone was never able to regain its rafted block-rich, rough and irregular surfaced region sug- gestive for an earlier rafting event associated with this original morphology. The situation is inferred to be sim- cone. The still intact shape of the cone suggests that the ilar, but simpler, in the case of Santa Maria. In other cone was able to heal after its initial rafting event. Another, circular cones surrounded by rafted small blocks covered smaller cone (Cone C) nearby demonstrates similar sce- nario as Cone B. by restricted lava fields, the original rafting did not cause C) The steep pre-eruptive surface can be viewed from the dramatic geometrical changes in the upper conduit and north of Los Morados scoria cone complex. Note the significant elevation differences the lava flow mark from its the eruption was likely driven by lower magma flux (and proximal to its distal regions. maybe smaller total magma volume), providing an opportu- D) Santa Maria scoria cone sitting on a relatively flat surface. nity to develop a normal Strombolian style of eruption that was able to rebuild the cone into its original near-perfect circular shape, similar to the Sunset Crater eruption in Arizona [51]. breached scoria cone (Cone A) with the red scoria blanket stands next to a highly irregular shaped lava flow that is partially covered by some agglutinate blocks. The lava flow associated with this cone is not as extensive as those 6. Conclusion associated with Los Morados or Santa Maria scoria cone, suggesting a lower volume of available melt. The young monogenetic volcanic field of PMVF is an ideal The studied scoria cones compare well with those de- area to investigate scoria cone morphologies caused by scribed from the San Francisco Volcanic Field (SFVF) [80]. various degrees of cone collapse. We were able to iden- The Red Mountain, which has a significant breaching scar, tify three end-member styles of scoria cone forming erup- indicating there was a mass wasting of nearly 15% of the tions; Los Morados main cone, Cone A and Santa María. total volume of the cone [80], could be a good analogy Los Morados main cone represents an eruption that was for Los Morados main cone. It seems that such a large probably long lived (months to years) and provided magma amount of mass wasting cannot be healed after resumption to build an initial scoria cone on an inclined and rough of Strombolian activity [80]. Other cones that are nearly pre-eruptive surface. When the cone became gravitational perfect in shape at PMVF (e.g. Cone A. west of Los Mora- unstable, a first lava flow outbreak rafted part of the cone dos main cone) resemble situations described from Sunset flank and initiated a partial sector collapse of the cone. Crater in the SFVF [51, 81, 82], where syn-eruptive mass The cone collapse caused decompression in the upper con- wasting did not exceed 1% of the total cone volume. It has duit that triggered a violent Strombolian style stage. Sub- also been suggested for Red Mountain that the potential sequently, the eruption changed to a more typical Strom- of the rebuilding of a scoria cone after breaching, raft- bolian style that partially rebuilt the cone. Due to a likely ing and partial collapse is a function of the total volume increase of magma flux, and the gradual growth of the

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cone, a second lava flow outbreak removed the northern [3] Walker G.P.L., Basaltic-volcano systems, In: Prichard side of the cone, triggering a violent Strombolian stage H.M., Alabaster T., Harris N.B.W., Nearly C.R. (Eds), that was followed by a Hawaiian lava fountain stage that Magmatic Processes and Plate Tectonics. Geological was accompanied by constant lava flow emission. Dur- Society, London, Special Publications, 76, 3-38, 1993 ing this stage constant rafting of the cone flank prevented [4] Brenna M., Cronin S.J., Smith I.E.M., Sohn Y.K., cone healing. Németh K., Mechanisms driving polymagmatic activ- Santa Maria, went through a similar phase as the second ity at a monogenetic volcano, Udo, Jeju Island, South stage of the Los Morados main cone, but probably not in Korea. Contrib. Miner. Petr., 2010, 160, 931-950, as dramatic a manner as Los Morados. DOI: 10.1007/s00410-010-0515-1 The Cone A represents a scoria cone that went through [5] Kereszturi G., Csillag G., Németh K., Sebe K., Kadosa moderate rafting and a complete healing. B., Jáger V., Volcanic architecture, eruption mech- We conclude that scoria cone growth is a complex process, anism and landform evolution of a Plio/Pleistocene and is a consequence of the interaction of magma inter- intracontinental basaltic polycyclic monogenetic vol- nal parameters such as volatile content, flux, recharge, cano from the Bakony-Balaton Highland Volcanic and overall output volume and external parameters such Field, Hungary. Cent. Eur. J. Geosci., 2010, 2, 362- as slope angle and surface roughness of the pre-volcanic 384, DOI: 10.2478/v10085-010-0019-2 surface. [6] Vespermann D., Schmincke H.-U., Scoria cones and tuff rings. In: Sigurdsson H., Houghton B.F., McNutt S.R., Rymer H., Stix J. (Eds), Encyclopedia of Volca- Acknowledgements noes. Academic Press, 2000, 683-694 [7] Luhr J.F., Simkin T., Paricutin. The volcano born in a Mexican cornfield. Geosciences Press, Phoenix, This project emerged from the Hungarian – Argentine 1993 Bilateral Science and Technology Cooperation Project [8] McGetchin T.R., Settle M., Chouet B.A., Geologic and (AR02/3) to KN and the ISAT Argentine – New Zealand photoballistic studies at Mt Etna and Stromboli. T. Science and Technology cooperation project to CR and Am. Geophys. Un., 1972, 53, 1-533 KN. The research is also part of the NZ Foundation for [9] Chouet B.A., Hamisevi N., McGetchin T.R., Photobal- Research, Science and Technology International Invest- listic analysis of main volcanic jet, Stromboli, Italy. T. ment Opportunities Fund Project (MAUX0808) “Facing Am. Geophys. Un., 1973, 54, 1-510 the challenges of Auckland’s volcanism” and the Volcanic [10] Chouet B., Hamisevi N., McGetchin T.R., Photobal- Risk Solutions, Massey University PhD Research Fellow- listics of volcanic jet activity at Stromboli, Italy. J. ship to GK. Logistical help by the Rangers of the Dirección Geophys. Res., 1974, 79, 4961-4976 de Recursos Naturales Renovables of Malargüe, Mendoza [11] McGetchin T.R., Settle M., Chouet B.A., Cinder cone provided excellent field work conditions. A research grant, growth modeled after Northeast Crater, Mount-Etna, UBACyT-X102 and grant PIP 2009-2011: 112-200801- Sicily. J. Geophys. Res., 1974, 79, 3257-3272 02084 to CR provided financial support for conducting [12] Wilson L., Head J.W., Ascent and eruption of basaltic this research. Detailed reading by Kate Arentsen made magma on Earth and Moon. J. Geophys. Res., 1981, the manuscript clear and focused. Suggestions by jour- 86, 2971-3001 nal reviewers (Andrea Borgia, Jorge Aranda-Gomez and [13] Head J.W., Wilson L., Basaltic pyroclastic eruptions: an anonymus reviewer), the Managing Editor, Katarzyna influence of gas release patterns and volume fluxes on Cyran and the Technical Editor Michel Ciemała greatly fountain structure, and the formation of cinder cones, improved the quality of this report. spatter cones, rootless flows, lava ponds and lava flows. J. Volcanol. Geoth. Res., 1989, 37, 261-271 [14] Riedel C., Ernst G.G.J., Riley M., Controls on the References growth and geometry of pyroclastic constructs. J. Vol- canol. Geoth. Res., 2003, 127, 121-152 [1] Valentine G.A., Gregg T.K.P., Continental basaltic [15] Houghton B.F., Hackett W.R., Strombolian and volcanoes - Processes and problems. J. Volcanol. phreatomagmatic deposits of Ohakune Craters, Ru- Geoth. Res., 2008, 177, 857-873 apehu, New Zealand; a complex interaction between [2] Németh K., Monogenetic volcanic fields: Origin, sed- external water and rising basaltic magma. J. Volcanol. imentary record, and relationship with polygenetic Geoth. Res., 1984, 21, 207-231 volcanism. In: Cañón-Tapia E., Szakács A. (Eds), [16] Houghton B.F., Schmincke H.U., Rothenberg sco- What Is a Volcano? Geol. S. Am. S., 2010, 470, 43-67 ria cone, East Eifel; a complex strombolian and

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