Environmental Engineering Reference
In-Depth Information
Deformations on volcanic media can be clas-
sified in four groups: a) subsidence, b) lifting,
c) lateral movements (dispersion or spreading) and
d) ladder instability. The process of subsidence
might occur due to the deflation of the magmatic
chamber, cessation of the magma's ascend, col-
lapse of a lava tunnel or due to the lateral magmatic
stresses, which can be transform at the surface in
settlements, depressions, or in a larger scale into
the formation of cauldron like volcanic features
like calderas. The ground uplift might be caused
by the intrusion of a magmatic body (cryptodome)
or the inflation of the magmatic chamber due to
presence of new magma or fluids.
The alternation of tephra layers and lava flows,
usually deposited with a critical stability angle,
combined with its fast alteration (weathering and
hydrothermalism), produce impermeable clay silt
soils (mechanically incompetent) and permeable
fractured media in the lavas, with favorable condi-
tions for sliding. We can also add the effects of the
hydrothermal alteration, seismic movements, struc-
tural weaknesses, etc. These and many other factors
are responsible of the sectorial collapses (craters
and calderas) of some of the flanks that originate
mega-slides (23, 27, 29). The work of siebert et al
(40), resumes the great volcanic avalanches in cen-
tral america (prehistoric and historic).
The deformation of the edifice-foundation in a
volcanic structure was not evaluated quantitatively
until a short time ago, even though Milne (32)
was the first one to suggest that the shape of a
volcano is related with its deformation due to its
own weight during its building process, and Van
Bemmelen (46) as well as susuki (45) comment
that a volcano's growing process and its deforma-
tion style depend on the type of basement where it
develops. it is not until the last 30 years, that the
topic was again approach, as a model that could
explain in a different way the shape and form of
volcanoes, the substrates function and the reason
of existence of many tectonic-volcanical structures
(10-12, 15, 31, 47, 48).
The keys to the volcanic dispersion are (9):
a) the existence of a weak basal layer, and b) a suf-
ficiently high mass and magma influx to drive the
process. When a volcano grows, it goes through
5 steps (that can be repeated, omitted and super-
imposed): 1) building, 2) compressing, 3) trusting
at the foot, 4) intrusion due to stress relaxation,
and 5) spreading (lateral movement) that creates
an extensional field in the superior flanks of the
volcano. The geological result is the formation of
reverse faults and propagation pleats. This is why
the identification of the geological conditions for
the dispersion of a volcanic cone and its geological
symptoms can help control and identify the dan-
gerous sectors for the collapse of the volcano.
The studies on volcanic avalanches or volcanic
instability under geotechnical bases (theoretical as
well as data from the field and laboratory), are very
few (51, 52). Recently, there have been published
some new papers that share similar aspects with
the present investigation (2, 5, 16, 17, 56). in the
work of alvarado and coworkers (1, 2, 5), there
were summarize several results of their investiga-
tion, however, due to the lack of diffusion, this
study is presented to the geotechnical community,
in a revised, summarized and updated format.
2
GeoloGical FRaMeWoRk
The arenal volcano grew on the tectonic valley on
altered volcanic rocks (>1 million years old) dis-
placing the arenal river to its actual position. The
oldest rocks of its foundation include tuffs, pumice
and breccias and lava flows, hydrothermally altered
and wetheared in different degrees.
The volcano is at least 7000 years old. its erup-
tive activity has been known for the presence of
explosive eruptions alternated with lava flows. The
historical activity is represented by an initial closed
conduit eruption with the formation of three new
craters (a, B and c), that along with the anteced-
ent (crater D), constitute a fissure system e-W
originated on July 29-31, 1968. From september of
that year to 1973, there was effusive activity in cra-
ter a or inferior, that generated a basaltic andesitic
lava field (with a maximum thickness of 150 m).
in 1974, the effusive activity migrated to crater c,
where there have been hundreds of relatively small
lava flows until now. The total lava field is 7.5 km 2
for an actual volume of 0.6 km 3 (46, 54).
This lead to the transformation of the arenal
cone from a conic shape with a cuspidal crater and
ladders with a 33˚-34˚ inclination in its half supe-
rior part to a double cone that has been growing
(c cone) on top of the antecedent cone (D cone),
disturbing the volcano's symmetry. The eastern
slope is steeper and shorter than the western slope,
because of the prevailing wind directions, that tend
to settle the pyroclastic deposits further west, sof-
tening the slopes in that direction.
From the structural point of view, there is a semi
curvilinear geoform in the west flank of the arenal
volcano, observed in aerial photographs taken
before 1970, given that it was later covered by lava
flows. although it has been interpreted in various
ways, if we assume that the Danta fault (n-s) has
a certain normal-dextral component, according to
the tectonic context (29), we may reinterpret the
generation of a low angle reverse fault in response
to its movement under the conical body. The faults
with holocene displacements are normal faults,
ne-sW in the southern slope, that could affect
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