Some common conditions favoring sector collapse of volcanoes
Spetrum of volcaniclastic deposits and relation with
their origin.
The distinction between cohesive or clay-rich lahars and
non-cohesive lahars is related to the amount of clay
in the matrix. Kevin Scott et al. (1992)
proposed empirically that about 3-5 % is the
boundary based on his study in Mount Rainier.
You can remember that Lee Siebert proposed these different ways to
form lahars associated with edifice failure. However,
only the first one directly result from
a flank or sector collapse event that can produce
cohesive lahars.
The Mexican Volcanic Belt (MVB) is an irregular province, about
1,000 km long and 20-150 km broad, that lies oblique
to the Middle American Trench, and extends east-west
between Veracruz (Gulf of Mexico) and Puerto Vallarta (Pacific Ocean). A characteristic feature of the MVB is the occurrence of high-relief,
nearly north-south trending volcanic ranges formed by
large stratovolcanoes, separated by wide intramontane
lacustrine/playa basins.
The Citlaltépetl-Cofre de Perote volcanic range (CCPVR) forms an
important physiographic barrier that separates the
Central Altiplano (2,500 masl) from the Gulf Coastal
Plain (GCP)(1,300 masl).
The CCPVR consists of a
wide variety of volcanic centers forming the 70-km long, nearly N‑S trending chain that includes several large
stratovolcanoes, minor cinder cones, and a few silicic
domes. The varied morphologic characteristics of the volcanic structures show different degrees of erosion and indicate a
relative southward younger age of the volcanism. There
are two main alignment directions: a NE-SW trend containing
the northernmost Las Lajas-Cofre de Perote-northern La Gloria volcanoes, and a N-S trend with the La Gloria-Las Cumbres-Citlaltépetl
volcanoes.
Distribution of main deposits derived from multiple collapsing
events at the Citlaltépetl-Cofre de Perote volcanic
range. All directed to the east (Gulf od Mexico). The
abrupt eastward drop in relief between the Altiplano (west) and the Gulf of Mexico (east) provinces
gives rise to unstable conditions and consequent gravitational
collapse of large volcanic edifices built at the edge of the Altiplano like Citlaltépetl, Las Cumbres, and Cofre de Perote. There have also
been several small-scale landslides and debris flows
in Holocene times, some of which are not related to the
activity of the large volcanoes (e.g. the 1920 seismogenic event). There are
also a few isolated exposures of other volcaniclastic
deposits, but their sources remain unknown. Some of the resulting avalanches and
transformed flows have exceptionally long runouts and
reach the Gulf of Mexico after traveling more than 120 km from their source, mainly as hyperconcentrated flows (not shown in this
figure).
Teteltzingo, Metlac and Jamapa are
related to Citlaltépetl volcano and will be described
as follows:
Two major voluminous deposits (Jamapa avalanche and Teteltzingo
lahar) are related to collapses of ancestral volcanoes
( Torrecillas, the oldest and Espolón de Oro, the intermediate
volcano) above of which the present Citlaltépetl cone grew. A comparatively small-scale debris avalanche deposit located about 20
km southeast of the volcano crops out along the Metlac
river-valley, which is behind the Citlaltépetl edifice.
Scar remnants of Espolón de
Oro cone. It is assumed that a horsehoe-like scarp was produced but was later destroyed and/or covered by subsequent lavas
of the present cone.
Photograph
on the left hand side show alternance of altered pyroclastic and brecciated deposits with platy-jointed andesitic lava flows of the interior of
the Espolón de Oro former edifice.
The Teteltzingo avalanche-lahar appears to be a single massive,
unbedded, poorly sorted mixture of heterolithologic
pebbles, cobbles, and boulders supported within a characteristic
yellow-brown, clayey, silty sand matrix that contains small vesicles suggestive of air bubbles trapped in a water-saturated matrix.
Hydrothermal alteration on the matrix is extensive and
dominant in the whole deposit.
The deposit's
features suggest that it had an origin as a sector collapse of weakened, water-saturated hydrothermally altered rock that transformed from a
debris avalanche to a cohesive lahar very close to its
source, similar to the Osceola lahar (Vallance and Scott,
1997).
In this picture you can see very large
boulders that were transported to this location at 65
km from the source area.
Cross sections
showing relations of stratigraphic units:
1.9 ky
pyroclastic flow;
2. 13 ky banded
ignimbrite
3. Tetelzingo
deposit- foming flat terraces with veneer deposits up to 60 m above the thick fill deposits (section B-B’), and showing small hummocks,
about 5-15 m high (section C-C’) similar to those
found at the Osceola lahar (Mount Rainier).
4. Old Debris
flows
5. Andesite
block and ash flows
6. Cretaceous
limestone
Green square
indicates location of next slide.
Veneer deposits
suggest peak levels of the flow passing through the barranca.
The Tetelzingo deposit forms relatively flat terraces. It is 12-20
m thick on average, but in a few places is up to 100 m
thick. Here overlying a 9 ky pyroclastic flow deposit forming a lower terrace level.
Comparison of the grain size characteristics of different lahars
from Mount Rainier and Citlaltépetl volcanoes.
Cumulative plot of grain size characteristics of Tetelzingo and
other lahars from Mt. Rainier.
Ternary diagram of matrix components. Notice how cohesive lahars
can be distinghished by their higher clay content.
Carrasco-Núñez et al. (1993) proposed that the presence of glacier
ice and a very active hydrothermal system during late
Pleistocene time provided a constant supply of pore
water, which enhanced the hydrothermal alteration of the summit of
Citlaltépetl and was the origin of most of the water
for the lahar. The intense hydrothermal alteration
seems to be related to an acid-sulfate leaching process where sulfates are added, while mobile elements are removed from the surroundings
rocks to form clay, silica, and sulfate minerals.
Plot of the degree of
hydrothermal alteration (area and intensity) versus and the area covered by glacial ice for the Cascade volcanoes and Citlaltépetl
(from Carrasco-Núñez et al., 1993). 0-no alteration;
1- small areas of moderate alteration; 2- moderate alteration;
3- large areas of moderate alteration; 4- large intensely-altered rocks. C-Citlaltépetl, GP- Glacier Peak; MA-Mount Adams; MB-Mount Baker;
MH-Mount Hood; MJ- Mount Jefferson; ML- Mount Lassen;
MR-Mount Rainier; MS-Mount Shasta; MSH- Mount St.
Helens; TS-Three Sisters.
A.Height
to lenght ratio (H/L) versus volume (V) or vertical drop to travel distance: It is used to predict maximum runout of debris avalanches.
B. H/L versus
area for volcanic and non-volcanic
debris avalanches, and lahars.
You can see that
cohesive lahars have in general lower H/L ratios than volcanic avalanches, implying longer runouts.
Two different terrace levels are observed on more distal areas from
Citlaltépetl volcano, but only one massive deposits is
observed in proximal localities.
Distribution of the Jamapa debris avalanche deposit, the largest
collapse event from Citlaltépetl volcano.
The intense orange colour shows the primary avalanche, which is
followed by a light colour zone, indicating lateral
transformation to lahar deposit and finally to hypoconcentrated
deposit.
Green square indicates approximate
location of next photograph. Blue circle indicates the
hummocky area, which are shown two slides later.
Inferred source area for the Metlac debris avalanche deposit.
The Metlac debris avalanche is an indurated, massive,
bouldery-rich, matrix-supported, heterolithologic
deposit, which is dominated by andesitic clasts.
Jigsaw fracture clasts are hardly observed on the
lower photograph among the students.
View of the interior of Citlaltépetl´s crater showing areas of
fresh and altered rocks, and some large subvertical
fractures. Alteration is mainly controled by deep
fractures into the crater.
Let´s move to Las Cumbres avaklanche.
Las Cumbres is an eroded stratovolcano consisting of thick and
massive hornblende-bearing andesitic lava flows. The
present summit rim (3,800 m asl) marks the boundary of
a 4 km diameter collapse caldera that is breached to the east. The maximum height of the pre-collapse stratovolcano could have been
similar to that of the present Citlaltépetl volcano
(5675 m asl) because these two volcanoes have a similar
base diameter of about 20 km.
Photographs of Las Cumbres Avalanche deposit in a proximal area,
about 12 km from source, showing the chaotic
distribution of clasts, oversized boulders and highly altered
areas.
The deposit contains less proportion of boulders and is greatly
dominated by matrix facies, sometimes showing still
some coloured areas. The photograph on the left show some
fragmented clasts due to shearing during transport.
Distribution of the 1920 seismogenic debris flow along the
Huitzilapan river, traveling nearly 30 km.
The Huitzilapan debris flow was triggered by an estimated 6.5
magnitude earthquake in 1920, which was preceded by
ten days of heavy rainfall. Multiple small landslide are
the source of most of the material forming the lowermost terraces. Photographs
are not exactly from the same site.
Photograph showing the 1920 Huitzilapan deposit overlaying the
5,860 +/- 60 yr. B.P. debris flow deposit along the
Huitzilapan river-valley. Dashed line indicates the contact
between these two deposits.
Now, let´s talk about Cofre de Perote volcano. See it not oriented
in the same N-S aligment followed by Citlaltépetl and
La Gloria volcanoes.
Peculiar morphology of this cone showing gentle slopes and emisions
of lavas through different vents instead of a single
vent as typical for stratovolcanoes. Last activity was recorded on 200 ky, so it is regarded as an extint volcano;
however, collapsing events have been occurred on 40 ky
and 10 ky, and were apparently not associated with any eruptive activity.
Cofre de Perote is characterized by a
prominent set of scarps that as a group show a spectacular
horseshoe shape that may be linked to repetitive flank failures. So far, at least two main debris avalanche deposits (Xico and Los Percados)
have been confirmed on the eastern lower slopes of
Cofre de Perote towards the Gulf of Mexico
Photographs of the Xico avalanche. Outcrop at about 10 km from
source showing the massive and chaotic nature of the
Xico avalanche deposit with large blocks within a silty
matrix, scarce wood partially charred was dated at 10-13 ky.
Los Pescados deposit consists of at least two different units that
crop out along Los Pescados river valley forming a
central terrace slightly dipping to the east. The deposit forms the lowermost channel-filling terrace deposit of Los Pescados
River, and is therefore the youngest deposit in the
area. It overlies a basaltic lava plateau, which has been
dated at 0.26 +/- 0.03 Ma by the 40Ar/39Ar method.
Some blocks show jigsaw-fractures typical of debris avalanches, but
there are no hummocks at the surface of this deposit.
This deposit may have been originated from a
catastrophic edifice collapse and rapidly transformed to a lahar that flowed
to a distance of at least 50 km from the Cofre de
Perote source.
Los Pescados deposit consists of massive, heterolithologic mixture
of boulders and coarse gravels within a silty-clayey
matrix.
Avalanche or lahar ? For discussion.
Distribution of the limestone basement rocks and main structural
patterns. In the Serdán-Oriental basin they are
partially buried by pyroclastic and lacustrine deposits and have a higher altitude in comparison with equivalent rocks
outcropping on the coastal plain.
Nakamura (1977) model can be applied to Cofre de Perote, generating
avalanches in a perpendicular direction with respect
to the inferred maximum horizontal stress.
A) Schematic profile showing the contrasting relief of the
Altiplano, the coastal plain, and the outcrops of
limestone basement sloping eastward. B) Structural section of the Veracruz coastal plain (modified from Viniegra, 1965).
Map showing the potential areas of instability based on the
combination of unstable conditions including: slope,
alteration areas, fractures, seismic data, and present morphology
This map considers 3 different levels of
hazard and it is based on field data and computer simulations.
Hope you enjoyed this presentation.