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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, conebuilding 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
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1.
Introduction
Scoria cones are the most common manifestation of
subaerial small-volume, short-lived volcanism on Earth.
These are generally considered to be a result of a mild explosive eruption of mafic to intermediate magmas in a short
period of time (days to weeks) [6]. However, long-lived
scoria cone eruptions are also known, such as Paricutin
in Mexico that was active for 9 years [7]. The main volcaniclastic deposits that are produced by a scoria cone
eruption are the cone-building coarse-grained ash, lapilli
and aggolomerate units and the medial to distal coarse
ash to fine lapilli sized pyroclastic blanket. These pyroclastic deposits are formed when magmatic gas bubbles
coalesce and explosively burst in a relatively established
volcanic conduit, high in the growing cone [8–14]. Detailed analyses of deposits preserved on scoria cones and
their surroundings led to the clarification of the role of
the shallow seated magmatic system in the control of the
explosive eruptions of such volcanoes [1, 15–19]. Among
the identified parameters, the variations in degassing patterns, magma ascent rates and degrees of interaction with
external water are thought to be responsible for sudden
changes in the eruption styles [1, 17, 20].
Small-volume mafic cone building eruption styles can
be grouped into four end member types, which primarily depend on the magmatic mass flux and the degree of
fragmentation: 1) Hawaiian, 2) Strombolian, 3) violent
Strombolian, and 4) weak ash emission [1, 21]. Hawaiian 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
– 1000 m3 /s) [1, 22, 23]. In contrast, Strombolian activity typically produces pyroclasts that are relatively coarse
grained and vesicular, but finer grained than those produced by Hawaiian style eruptions, reflecting more effective magma fragmentation [6, 24]. Strombolian style eruptions are associated with a significantly lower magma flux
rate (i.e. 10−3 – 100 m3 per explosions over several explosions an hour) [1, 6]. Weak ash emission eruptions produce
fine ash, with a negligible gas thrust and therefore very
limited (usually in the crater basin) distribution of the
deposit. Violent Strombolian style eruptions are those
that produce significant volumes of fine ash, as a consequence of effective magma fragmentation accompanied by
high magma flux and high explosive event frequency. Such
eruptions can spread ash over relatively large areas and
disperse fine pyroclasts to over 10 km distance from their
source [1, 25–30].
The closely packed, slightly oriented texture of moderately vesicular coarse ash and lapilli accumulations
are a typical result of Hawaiian style lava fountain-
ing [22, 29, 46, 47]. The common intercalation of scoria beds in scoria cones with welded fall deposits and/or
clastogenic lava flows [48] indicate a sudden and frequent
change in eruption style from Strombolian to Hawaiian and vice versa [1], reflecting variable volcanic conduit dynamics during the eruption. Lava spatter near the
vent, found typically along the crater rim of a cone, can
form strongly welded, red, slightly bedded sequences with
large spindle or highly vesiculated fluidal bombs [49, 50].
These deposits usually reflect strong reworking of the pyroclasts as they roll back to the vent and are cleared
again, resulting in complex amalgamated lapilli textures.
The inner part of scoria cones suffers strong agglutination
due to the persistent heat of the magma filled conduit [6].
Such welded parts of the inner cone, or the collar-like
welded ring on the crater rim, are more erosion resistant
and can be preserved for a long time; however, they are
also prone to gravitational collapse, especially if there is
mass deficit at the base of the cone. Lava emission points
or fractures at the base of the cone may cause gradual
rafting of large portions of the cone basal flank that can
lead to gravitational sector collapse of the cone [1, 51].
Here we explore the morphological results of lava flow
rafting, and resulting cone sector collapses, as a prime
controlling parameter in triggering significant eruptive
style changes in the course of scoria cone growth, and how
such processes may be reflected by 1) the accumulated
intra- and inter-cone pyroclastic successions, 2) textural
cyclicity of the pyroclastic deposits, and 3) final morphology of the scoria cone.
1.1.
Payún Matru Volcanic Field (PMVF)
The Payún Matru Volcanic Field (PMVF) is located in the
southeastern region of the Mendoza province, Argentina,
between latitudes 36˚ and 36˚ 35’ S and 68˚ 30’and
69˚30’ W, approximately 200 km east of the trench in the
Southern Volcanic Zone of the Andes (inset in Fig. 1A).
The PMVF covers 5,200 km2 [52, 53] and is a broad backarc lava plateau with hundreds of monogenetic pyroclastic
cones.
Two roughly contemporaneous basaltic volcanic fields are
E and W Payún Matru (Fig. 1A). These monogenetic fields
are referred to as the Payén east and Payén west lava
shields respectively. The eastern Payén is related to an
east-west trending fault system. This fissure-controlled
volcanism produced several extremely long lava flows that
reach the Salado river valley in La Pampa province to the
east. One of the lava flows has an individual tongue-like
shape 181 km long, and is the longest known individual
Quaternary 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 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 Formation (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 occasional violent Strombolian eruptions. The average cone
density for the area is 33 cones per 100 km2 . The high
cone density is probably a result of a strong magmatic
pulse initiated the volcanic field formation [72].
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
partially or totally cover the slopes of older scoria cones.
It is inferred to be the product of a far less explosive scoria
cone building period than those earlier more silicic eruptions associated to the Payún Matru composite volcano.
Some scoria cones produced ash and lapilli deposits that
are restricted chiefly to their cone edifice and within a few
kilometres from their source vents. Today ash and lapilli
has been re-worked by wind, forming elongated and parallel dunes perpendicular to the prevailing westerly wind
direction.
2.
Los Morados scoria cone
Los Morados is a young scoria cone complex with closely
spaced edifices in the western edge of the PMVF in Mendoza (Fig. 1). Here we concentrate on the morphological evolution of the youngest of the cones, named as Los
Morados main cone, and an older cone west of the Los
Morados main cone, partially covered by eruptive prod-
Figure 2.
A) Google Earth image of Los Volcanes area. Dispersal
areas of black ash fall issued by Los Morados volcanic
complex are outlined with white lines; I - first major pyroclastic fall, II-A - second major pyroclastic fall, II-B - probably part of the second major fall event.
B) An interpretative geological map based on mapping in
volcanic 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 to
the north of Los Morados main cone referred here as
Cone B, and an eroded older cone west of Cone A, named
here as Cone C (Figs 2A, B & 3A). The radiometric age
of Los Morados scoria cone complex is unknown, but it
rests atop other lavas erupted from the Payén west shield
volcano (Fig. 1A) [62]. The youngest age of the lava
flows of the shield volcano in the Rio Grande basin is
0.016±0.001 Ma, putting a limit to the age of Los Morados volcanic complex [62]. These stratigraphic and radiometric age data coincide well with the general morphological characteristics of both the Los Morados scoria cone
complex and associated lava flows. The outer slope of
the Los Morados main cone is steep (up to 30 degrees)
and youthful in appearance with a fresh, reddish to black
lapilli blanket with no significant gully network. However,
dominantly dry climatic conditions with annual precipitation of a few hundred mm favours the preservation of the
volcanic edifices over a relatively long period of time.
Los Morados main cone is part of an alignment of vents,
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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.
Figure 3.
A) Close up Google Earth image of the Los Morados scoria cone complex. Thick yellow line outlines the younger
lava flow with rafted cone material of Los Morados main
cone. Thick white line bounds to the zone from where the
cone flank was rafted away. Dashed white line marks Los
Morados main cone older lava flow and hummocky area
in the northern side of Los Morados main cone. Yellow arrow points to the ash-covered hummocky surfaces north of
Los Morados main cone. Dashed yellow line corresponds
to the base of an older scoria cone (Cone A) west of Los
Morados main cone. Red star shows the location of an
older, small-volume, lava outbreak at the base of Cone
A. The red star is also shown air photo on 3B. Note the
fractures marked by white arrow parallel to the cone flank
breach. White dot marks the point photo 4B was taken.
B) Aerial view of Los Morados scoria cone complex, as
seen from the north. White dot marks the point photo 4B
was taken.
on an E-W trending fissure system (Carbonilla Fault System) (Fig. 1A). The westernmost vent at Los Morados scoria cone complex is an older scoria cone, with an eroded
morphology (Cone C) (Figs 2A, B & 3A, B). The easternmost part of Los Morados scoria cone complex forms a row
of fissure vents which has at least 5 individual vent sites
spaced a few tens of metres apart (Fig. 2A). The central
part of the Los Morados scoria cone complex is composed
of two overlapping scoria cones. The older one, named as
Cone A has a near-circular well-preserved crater, while
the Los Morados main cone partially covered Cone A.The
Los Morados main cone has an oval shaped, east–west
Morphology and Erupted Volume
Precise delineation of the dimensions of Los Morados main
cone is not simple due to the thick tephra cover that obscures 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
covered surface of the outer flank of Los Morados main
cone (Fig. 3B). The base of Cone A is about 2100 metres
above sea level, which allows us to estimate a value of
2150 metres for the basement of Los Morados main cone
based on spot elevation readings on GoogleEarth images
as well as direct hand-held GPS measurements (Fig. 3A).
This value is a good average for the western side of the
Los Morados main cone; however, the eastern basement
lies on higher ground, but probably not more than 100 metres higher than the western side. This inferred elevation
difference indicates that the Los Morados main cone itself
erupted on a north-westward dipping topography with an
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
elevation values. These elevation points were digitalized
from areas where no tephra blanket exists and effusive
products (e.g. lava flows) of Los Morados cannot be found
(e.g. areas that are interpreted as the syn-eruptive surface). This calculation supports that the Los Morados
scoria cone complex erupted on a surface gently inclined
(∼4˚) towards the northwest. The Los Morados main cone
however, was likely formed on a very irregular topography
determined by the steep slopes of Cone A (Fig. 3B).
The highest point on the Los Morados main cone crater
rim today lies in the south-east and is about 2425 metres 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 (similar to the older cone to the west, Cone A), a 2150 metres
above sea level horizon in the west (the same level as
the base of Cone A) and a value of 2300 metres for the
basement level in the east (Fig. 3). This together provides
rough estimates of the Los Morados main cone height with
a minimum 125 and maximum of 275 m. The older Cone
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 (Hco ) to basal diameter (Wco ) ratio
of 0.167. Our observations indicate that if Los Morados
main cone had grown on a flat surface it could have similar dimensions to Cone A. However, Los Morados is sitting
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,
depending on which reference elevation of the cone base
is used.
To estimate the cumulative volume of pyroclastic deposits
associated with Los Morados main cone, measurements
of the total thickness of pyroclastic deposits associated
with the main cone were taken in the Pampas Negras
area (Figs 2B & 5A). The measurements included the estimated deposit thicknesses on the Los Morados main cone
and the primary (ie: not wind re-worked) distal inter-cone
field tephra layer thicknesses. We also included the windredeposited ash and lapilli that form thick accumulations
of tephra on the west facing side of older cones and lava
flows east of Los Morados main cone (Fig. 5A and Table 1).
The total volume of the pyroclastic deposits, including Los
Morados main cone, is about 0.3 km3 . This value is likely
an underestimate due to the uncertainties in the delineation of the cone base and the thickness of the tephra
in proximal regions. However, it appears to be a reasonable estimate with Los Morados emitting approximately
0.3 km3 of tephra that accumulated in the cone and in the
vicinity. This could be recalculated to a dense rock equiv-
Figure 4.
A) Panoramic view of Los Morados main cone from the
east.
B) Breached cone flank of Los Morados main cone from
the eastern side of the younger lava flow initiated from the
cone.
C) Inner crater wall of Los Morados main cone exposes
welded lava spatters steeply dipping inward.
D) Lava spine in the proximal area of Los Morados main
cone younger lava flow.
alent (DRE), using an average 50% vesicularity of typical
scoriaceous pyroclasts, to give a value of about 0.15 km3
magma involved in the formation of the pyroclastic units
of Los Morados main cone.
Table 1.
Calculated eruptive volumes associated with measured sectors on Fig. 5A.
Measured regions
[on Fig. 5]
1
2
3
4
5
5a
6
7
8
9
10
11
12
13
14
18
Area
Thickness
covered in m2 in metres
148831.70
384564.60
761612.54
6060053.10
2895932.15
1711069.76
734716.58
1282405.62
4122036.43
5822792.84
2273830.63
99731.43
112160.12
158664.13
71610.12
1262616.12
150
80
60
20
10
10
0.5
4
0.5
0.5
4
4
0.5
4
4
0.5
In m3
In km3
22324755.000
30765168.000
45696752.400
121201062.000
28959321.500
17110697.600
367358.290
5129622.480
2061018.215
2911396.420
9095322.520
398925.720
56080.060
634656.520
286440.480
631308.060
0.022325
0.030765
0.045697
0.121201
0.028959
0.017111
0.000367
0.00513
0.002061
0.002911
0.009095
0.000399
5.61E-05
0.000635
0.000286
0.000631
Total 0.28763
We also estimated the lava flow volume, which was emitted
in 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-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).
The older lava flow and rafted collapsed cone flank cover
an area of approximately 1.3 km2 (Fig. 5B). The flow volume was estimated based on field observations on lava
flow thicknesses. The average lava flow thickness estimates of 10 m and 30 m were obtained at the proximal
flow margins. Thus the erupted volume of the older lava
flow ranges between 0.01 and 0.04 km3 (Fig. 5B).
The younger and more extensive lava flow covers an area
of approximately 10.57 km2 . Using an estimated minimum
of 3 metres and maximum of 10 metres average thickness
from field observations, a minimum of 0.03 and a maximum
of 0.11 km3 magma erupted through this lava flow stage
(Fig. 5B).
The total estimated volume of magma involved in the eruption of Los Morados main cone therefore ranges between
0.19 and 0.3 km3 in dense rock equivalent.
2.2.
Volcanic Facies
Several volcanic facies were identified in and around the
volcanic construct of Los Morados main cone. The scoriaceous deposits were divided according to their colour,
referring to red (Sr) and black (Sb) scoria beds ranging
from coarse ash to coarse lapilli grain sizes.
Agglutinate facies (A) are moderately to strongly welded,
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 recognized in proximal volcanic units. Densely welded pyroclasts commonly form coherent interbeds of dark colour,
lava-like rocks where original clasts can barely be recognized, and only some irregularities and the occasional,
commonly random clast outlines indicate the pyroclastic origin of the rock. These beds are clastogenic lava
flows that formed as a result of rheomorphic processes
that nearly completely destroyed the original fabric. Such
clastogenic lava flows probably represent fast accumulation of lava spatter from low eruption columns.
Lava facies (L) can be distinguished on the basis of the
lack of any indicators of clast outlines. In addition, they
show typical surface features, such as spines, ropy surfaces, 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), rugged
surface morphology with m-sized spines and vesicular
zones in the lava plates. The lava facies is covered by rubble derived from fragmented lava blocks and clinker that is
mixed with m-sized of agglutinate blocks that still retain
original bedding in a contorted fashion (Fig. 4D). The lava
field is considerably wider from its point source located
at the northern breach of the scoria cone, and reaches
a width of about 300 metres. There are at least two distinguishable lava flows issued from the breach of the scoria
cone (Fig. 2B). An older lava flow extends toward the north
and is blocked by an older scoria cone, reaching a length
of about 1 kilometre before being diverted toward the west
(Fig. 2). This older flow can be traced at least another
1.3 km further down beneath a more extensive and younger
lava flow. The older lava flow is covered extensively by
clinker and rubble derived from the margin of the younger
lava flow. In addition, the older lava flow is partially covered by a red and black scoria blanket which is missing
on top of the younger lava flow. The young lava flow, after
reaching the older scoria cone barrier (Cone B; Fig. 3A),
was diverted to the west, and travelled at least another
7.5 km before stopping. About 1 km and 2.3 km from the
first diversion point, the younger lava flow branches to
two separate narrower arms that were emplaced slightly
toward the northwest. These lava flow arms have typical aa lava surface morphologies, with a central channel
and a rampart of broken lava fragments on both sides the
main flow channel (Fig. 2A). The widest arm also displays
in its final 2.5 km multiple channelized flow structures and
pressure ridges. However, occasional agglutinate metresized blocks are still randomly distributed on the top of
the younger lava flow, especially in the deeper parts of
the rugged surface.
3.
Volcanic Facies Distribution
The eruptive products of Los Morados main cone can be
grouped into 3 major volcanic facies. The cone itself is
predominantly composed of Group I facies formed by red
scoria beds, agglutinate and minor clastogenic lava flow
inter-beds. These lithologies are exposed on the steep
inner crater wall. The outer upper flank of the cone is
also composed of red scoria; however there is no exposed
agglutinate collar on the crater rim (Fig. 3B & 4A). The
Group II facies is exposed in the region of the lower cone
flank and the inter-cone field, which is composed by black
scoria of coarse ash and lapilli (Fig. 2). The black scoria
beds can be traced in three distinct directions and are inferred to be sourced from Los Morados main cone as this
is the only vent located in the right position in respect to
the bed thickness variations (Fig. 5A). The larger black
scoria dispersion pattern represents a significant eruption
event, as tephra reached areas at least 10 km from the vent
across an east-west axis (Fig. 2A). The estimated area
covered by black scoria is about 100 km2 . Another, not
so clearly traceable black scoria dispersal pattern, with
a 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 about
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 subtracting 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
a sub-Plinian eruption [74, 75]. This type of activity is
commonly referred in a basaltic monogenetic context as
a violent Strombolian type of eruption [1, 34, 76–79]. It
is also likely that the smaller black NE-trending dispersal pattern was also of the violent end-member of normal
Strombolian type eruptions based on the similar dispersion values as the E-trending ash and lapilli field.
Agglutinate is primarily exposed in the inner crater wall
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
in situ agglutinate forms dm-thick beds of strongly welded,
slightly flattened, moderately vesicular scoriaceous lapilli
hosted in red scoriaceous coarse ash. In situ agglutinate
units commonly form coarse to fine bed couplets with thin,
few cm-thick fine ash beds. The fine ash beds are also
commonly welded and the original pyroclast morphology
is difficult to recognize. Thin (dm thick) clastogenic lava
flows form discontinuous horizons exposed on the crater
wall. The rocks exposed in the crater wall are sufficiently
welded to support the agglutinate beds that dip steeply
inward.
Agglutinate facies are exposed north of the crater wall
breach as individual blocks with random bed dip directions (Fig. 6A). The abundance of well-defined zones of
agglutinate-dominated rocks on the rough lava flow surface decreases with distance from the Los Morados main
cone. Near to the lava flow point source at the cone
breach, large blocks of contorted, truncated and randomly
tilted agglutinate blocks are exposed, commonly stacked
over each other. Bed dip directions in this area do not
resemble any systematic distribution, thus we interpret
these blocks as fragments derived from the breached portion 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 rafted
and/or collapsed cone flank blocks in the breach area.
Dashed line marks the contact between the older and
younger lava flows.
B) Overview of the tephra-covered hummocky surface
north of Los Morados main cone. Arrows point to flattened
lava spatters.
C) Flattened, moderately vesicular, stratified lava spatters
in a rafted cone block on the surface of the young lava
flow of Los Morados main cone.
D) Spill over of the younger lava flow, interpreted as the
result of a local collapse of a large rafted block. Dashed
arrow show the path of the spill.
Red scoria beds are common in the upper and outer cone
flank. In the upper exposed section of the crater wall there
are individual well-sorted beds of coarse ash to fine lapilli
red scoria. A similar red scoria blanket seems to follow
the east – west oriented fissure network. In situ agglutinate outcrops on the upper crater wall and red scoria
beds 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 in
the displaced agglutinate blocks. The reddish scoria blanket clearly marks a coloured zone in the entire length of
the nearly 10 km long younger lava flow.
In the northern side of Los Morados main scoria cone, just
in front of older Cone B, a hummocky surface is exposed
(Fig. 6B). Hummocks are up to 10 meters tall and randomly distributed and covered by a red and black scoria lapilli blanket (Fig. 6B). The larger hummocks are
formed agglutinate and show very diverse dip and bedding characteristics indicating that the individual blocks
were displaced and rotated. The hummocky surface forms
a broader zone than the younger lava flow (Fig. 3). It
seems that the original width of the cone breach was
slightly 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 of
the older lava flow region partially covered by either the
younger lava flow or the black scoria blanket (Fig. 6D).
4. Los Morados Main Scoria Cone
Evolution
4.1.
Original Size and Shape of the Cone
It is difficult to establish the Los Morados main cone original size as it was built on a highly irregular pre-eruptive
volcanic surface. It can be accepted that the cone itself is
at least 125 m high and probably as high as 275 m. The
young age of the scoria cone and abundance of inward
dipping welded lava spatter preserved in its crater support that the morphology of Los Morados main cone has
not been significantly modified since its formation. This is
also suggested by the lack of gullies, erosional scars, or
other landforms indicative of cone degradation. The minimum total volume of the cone prior to its breaching can be
estimated in the range of 0.25-0.35 km3 . The width of the
breach on the northern side of Los Morados main cone is
nearly as large as the crater of the preserved cone, indicating significant mass wasting. Visual estimation of the
volume of the obscured part of the Los Morados main cone
suggests that about 10% of the cone flank is disturbed or
missing. This value indicates a significant mass wasting
on the northern flank of the Los Morados main cone.
4.2.
Inferred Eruption History
The earliest eruptive stage on Los Morados main cone
that can be documented with present day exposures at
the bottom of the crater wall was likely a Hawaiian style
eruption, as evidenced by the abundance of agglutinate
and minor clastogenic lava flows (Figs 7A & B). This early
Hawaiian style eruption changed to a normal Strombolian
scoria cone forming-eruption. This eruptive style permitted the establishment of a volcanic conduit with a semisealed 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 that
were favourable to generate gas pockets that caused periodic outbursts and normal Strombolian style explosions.
The magma rise speed and flux likely dropped at this stage
in comparison with the earlier Hawaiian lava fountaining
stage. Volcanic activity at this point was dominated by
the formation of red scoria and minor agglutinate, as evidenced by the increased volume of red scoria beds in
the 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 potentially
a trigger of some dramatic processes that modified the
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 inevitable due to the generally inclined pre-eruptive landscape and the irregular morphology of the area of Los
Morados main cone basement. Magmatic pressure combined with the gravitationally unstable geometry (having
a dense magma filled conduit well above the pre-eruptive
surface in the growing scoria cone) provided significant
pressure on the northern, unsupported flank of the cone,
which eventually collapsed and issued a lava flow through
the lower part of the cone (Fig. 7A). The first lava outbreak
created the older lava flow, which is now partially exposed
on the northern side of the breached Los Morados main
cone, rafting large parts of the initial transient cone flank.
The lava flow probably accelerated the gravitational cone
collapse process, opening up a wide breach in the inclined
unstable northern side of the initial cone. As a result,
a minor volcanic debris avalanche probably was created
that was carried and/or covered by the still moving older
lava flow (Fig. 7A). The lava flow was diverted by the old
scoria cone, Cone B. The rafted cone blocks and collapsed
sectors of the initial Los Morados main scoria cone piled
up against the southern flank of older Cone B, which acted
as barrier, and may have resulted in the formation of the
hummocky surface (Figs 2B & 7A).
Sudden decompression of the shallow magma column
caused a short lived, relatively violent Strombolian eruption. This eruption produced a tall eruption column that
drifted slightly towards the NE and deposited the first
black scoria blanket (Fig. 2A). Present day winds are from
NW to SE which is inconsistent with NE-distributed ash
fall pattern suggesting a) the eruption took place in an unusual wind pattern or b) the ash cover accumulated from
directed blasts.
This first violent Strombolian stage probably caused a minor shift in the vent location in the crater of Los Morados, moving about 100 metres toward the north in the
area of today’s crater wall breach. This is supported by
slightly offset bedding dip directions of the agglutinate
facies mantling the inner crater wall (Fig. 3). The scoria
cone was likely healed through resumed Hawaiian style
eruptions, providing agglutinate accumulation and associated red scoria emission through the subsequent moderate
Strombolian style eruption (Fig. 7A). The red scoria of this
stage covered the original scars, the hummocky surface
and, partially, the older lava flow. The rebuilding phase of
the scoria cone followed a similar evolutionary trend, and
probably culminated in a stage when the scoria cone once
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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.
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again became gravitationally unstable leading to a new
outbreak of lava in its northern foothill (Fig. 7B). This
younger lava flow subsequently rafted away large blocks
of the cone flank. The rafted cone blocks likely reached
sizes of tens of metres across. Many large blocks gradually disaggregated and caused small scale collapses on
the lava surface, generating some ash tongues outbreaks
in the main lava flow channel. In similar fashion to the
first collapse, the removal of a large portion of the cone
caused a pressure drop, and facilitated the production of
a fountain-like magma discharge and tall eruption column
during a second violent Strombolian eruption. This eruption produced the second, more extensive, black scoria
deposit that followed an approximate west to east dispersal direction (Fig. 7B). The fact that the younger lava flow
is not covered by black scoria suggests that the lava flow
emission and cone wall rafting were still proceeding during this violent Strombolian eruption stage. The volume of
the younger lava flow indicates that the second collapse of
the eruption was likely triggered by the arrival of a new,
fast rising magma batch, which was able to sustain lava
flow effusion and lava fountaining after the collapse and
rafting of the rebuilt cone (Fig. 7B).
5. Discussion on Scoria Cone Rafting, Eruption Styles and Morphology
Los Morados main scoria cone erupted on an inclined
pre-eruptive surface with complex local topography. This
pre-eruptive condition determined the fate of the cone.
It seems that gradual changes in eruption style, from
lava fountain-dominated to typical Strombolian eruptions
formed a significant misbalance in the weight on the gradually established magmatic column which resided in the
centre of the cone. As a result, the unsupported parts of
the cones were doomed to collapse. Due to the inferred
length of the eruption and the substantial magma supply,
the vent hosted sufficient magma to cause multiple rafting and collapse events. It seems that lava flow-triggered
rafting, collapse and fast decompression of the magmafilled upper conduit are key parameters in the evolutionary trend of some breached scoria cones. Thus, if the
cone is built on an inclined pre-eruptive landscape, such
processes can trigger multiple eruptive stages, and create
a complex final morphology that markedly departs from
the ideal symmetrical shape of pyroclastic cones. Here
we explore other cones in the same volcanic field to test
this idea.
Another young breached scoria cone, called Santa Maria,
is located on a relatively flat surface about 15 km NE of
Los 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 stepped
architecture that is intact in its southern side (Fig. 8B).
The present day cone, the result of its last cone building
eruption stage, is surrounded by a complex and irregular lava field and large rafted agglutinate blocks of a few
tens of metres across. The cone is breached to the NNW,
but not as widely as Los Morados main cone. The preeruptive slope dip was probably less than in Los Morados (Fig. 8C). The cone has a slight east-west elongation
along which at least 3 individual vents can be identified
(Fig. 8A). One is slightly offset to the east, while two vents
are defined by a semicircular array of agglutinate and red
scoria-dominated beds in the west. The northern side of
this double vent is breached and a red and black scoria
flank indicates some healing of the northern side of the
cone after the initial breach. From the northern breach
an irregular, rough lava surface can be recognized. This
aa lava flow is partially covered by randomly oriented agglutinate blocks similar to those described for Los Morados. A discontinuous black scoria blanket covers an oval
shaped area, and is visible on satellite images extending
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.
These relationships suggest similar cone-breaching process as described for Los Morados main cone. The Santa
Maria scoria cone likely collapsed through rafting during
its growing stage, followed by some partial healing of the
cone flank, creating a very rugged and complex morphology on its northern side. At some point, a single, more
violent Strombolian style stage (but judging by the inferred volume of the black ash, probably not as violent
as at Los Morados main cone) occurred. An increased
magma flux was likely responsible for the outpouring of
the extensive lava flow that was subsequently covered by
the rafted portions of the northern part of the cone, attaining its present day morphology. It seems the Santa Maria
scoria cone evolution was not as dramatic as Los Morados
main cone, as it was formed on a nearly flat pre-volcanic
surface.
Elsewhere in the PMVF there are evidences of such obscured and breached scoria cone morphologies. However,
there are – especially among the older cones –cones associated with similar rough surfaced lava flows with rafted
agglutinate blocks next to near perfect, red scoria dominated cones. It has been suggested for Sunset Crater
in northern Arizona that scoria cone healing could lead
to a complete re-establishment of youthful cone morphology after lava flow-triggered cone flank rafting. There is
a good example of this process next to the western side of
Los Morados main cone (Cone A; Figs 4B & 8D). An un-
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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 accumulate [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..
Figure 8.
A) Google Earth image of the Santa Maria scoria cone and
associated long lava flow. Inset shows the irregular morphology in the northern side of the cone, where blocks
derived 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 and
rafted block-rich, rough and irregular surfaced region suggestive for an earlier rafting event associated with this
cone. The still intact shape of the cone suggests that the
cone was able to heal after its initial rafting event. Another,
smaller cone (Cone C) nearby demonstrates similar scenario as Cone B.
C) The steep pre-eruptive surface can be viewed from the
north of Los Morados scoria cone complex. Note the significant elevation differences the lava flow mark from its
proximal to its distal regions.
D) Santa Maria scoria cone sitting on a relatively flat surface.
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
associated with Los Morados or Santa Maria scoria cone,
suggesting a lower volume of available melt.
The studied scoria cones compare well with those described 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 the
total volume of the cone [80], could be a good analogy
for Los Morados main cone. It seems that such a large
amount of mass wasting cannot be healed after resumption
of Strombolian activity [80]. Other cones that are nearly
perfect in shape at PMVF (e.g. Cone A. west of Los Morados main cone) resemble situations described from Sunset
Crater in the SFVF [51, 81, 82], where syn-eruptive mass
wasting did not exceed 1% of the total cone volume. It has
also been suggested for Red Mountain that the potential
of the rebuilding of a scoria cone after breaching, rafting and partial collapse is a function of the total volume
At Los Morados main cone the first collapse probably triggered a tall eruption column for a significant period of
time, leading to partial healing of the cone. The subsequent collapse was probably able to provide only a singular violent Strombolian eruption due to the sudden decompression over the magma filled upper conduit. After
this second stage the cone was never able to regain its
original morphology. The situation is inferred to be similar, but simpler, in the case of Santa Maria. In other
circular cones surrounded by rafted small blocks covered
by restricted lava fields, the original rafting did not cause
dramatic geometrical changes in the upper conduit and
the eruption was likely driven by lower magma flux (and
maybe smaller total magma volume), providing an opportunity 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].
6.
Conclusion
The young monogenetic volcanic field of PMVF is an ideal
area to investigate scoria cone morphologies caused by
various degrees of cone collapse. We were able to identify three end-member styles of scoria cone forming eruptions; Los Morados main cone, Cone A and Santa María.
Los Morados main cone represents an eruption that was
probably long lived (months to years) and provided magma
to build an initial scoria cone on an inclined and rough
pre-eruptive surface. When the cone became gravitational
unstable, a first lava flow outbreak rafted part of the cone
flank and initiated a partial sector collapse of the cone.
The cone collapse caused decompression in the upper conduit that triggered a violent Strombolian style stage. Subsequently, the eruption changed to a more typical Strombolian style that partially rebuilt the cone. Due to a likely
increase of magma flux, and the gradual growth of the
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cone, a second lava flow outbreak removed the northern
side of the cone, triggering a violent Strombolian stage
that was followed by a Hawaiian lava fountain stage that
was accompanied by constant lava flow emission. During this stage constant rafting of the cone flank prevented
cone healing.
Santa Maria, went through a similar phase as the second
stage of the Los Morados main cone, but probably not in
as dramatic a manner as Los Morados.
The Cone A represents a scoria cone that went through
moderate rafting and a complete healing.
We conclude that scoria cone growth is a complex process,
and is a consequence of the interaction of magma internal parameters such as volatile content, flux, recharge,
and overall output volume and external parameters such
as slope angle and surface roughness of the pre-volcanic
surface.
Acknowledgements
This project emerged from the Hungarian – Argentine
Bilateral Science and Technology Cooperation Project
(AR02/3) to KN and the ISAT Argentine – New Zealand
Science and Technology cooperation project to CR and
KN. The research is also part of the NZ Foundation for
Research, Science and Technology International Investment Opportunities Fund Project (MAUX0808) “Facing
the challenges of Auckland’s volcanism” and the Volcanic
Risk Solutions, Massey University PhD Research Fellowship to GK. Logistical help by the Rangers of the Dirección
de Recursos Naturales Renovables of Malargüe, Mendoza
provided excellent field work conditions. A research grant,
UBACyT-X102 and grant PIP 2009-2011: 112-20080102084 to CR provided financial support for conducting
this research. Detailed reading by Kate Arentsen made
the manuscript clear and focused. Suggestions by journal reviewers (Andrea Borgia, Jorge Aranda-Gomez and
an anonymus reviewer), the Managing Editor, Katarzyna
Cyran and the Technical Editor Michel Ciemała greatly
improved the quality of this report.
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