Jökull - 01.01.2001, Blaðsíða 49

Late Pleistocene subglacial caldera formation at Cerro las
Cumbres, eastern Mexico
Ármann Höskuldsson
Náttúrustofu Suðurlands
South Iceland Institute of Natural History
Strandvegur 50, 900 Vestman islands, Iceland
Abstract — A violent plinian eruption in the Cerro las Cumbres volcano about 16,980

870 yr B.P. led to a
formation of a caldera 3.5 to 4.5 km in diameter. At the time of the eruption the volcano was capped by a
glacier, which had dramatic effects on the eruption mechanics and produced an unusual depositional pattern
of the eruption products. When in contact with water, the eruption became phreatomagmatic whereas devoid of
meltwater it remained plinian in character. Two such cycles were observed. During the initial phreatomagmatic
phase, while glacier melting was high, a series of lahars and ash flows flowed down the west flank of the
volcano. A plinian phase followed. A collapse of the central caldera block enabled water to regain access to
the vent and the eruption again became phreatomagmatic. When this second batch of water had evaporated
the eruption regained its plinian character. The intensity decreased towards the end of the eruption and the
activity became phreatomagmatic again. A dacitic dome at the caldera floor marks the final eruption phase.
The magma expelled ranged from high calcium rhyolite to dacite in composition. The rhyolite pumices contain
abundant biotite phenocrysts. After the eruption, all deposits layed down on the glacier were washed away
as it melted. A semi–circular zone around the caldera, devoid of eruption deposits, thus reflects the extent of
the glacier at the time of the eruption. Judging from the volume of the caldera depression, the total volume of
magma extruded by the eruption was on the order of 7–9 km


(DRE). Of this volume 4 km


(2.1 km


DRE) were
expelled in the initial phase of the eruption.
INTRODUCTION
The eastern Trans–Mexican volcanic belt is character-
ized by a N-S–trending chain of volcanoes, the Cofre
de Perote–Pico de Orizaba mountain range. The chain
consists of four major volcanic edifices and one vol-
canic complex, which from north to south are Cofre de
Perote, La Gloria complex, Cerro las Cumbres, Pico
de Orizaba, and Sierra (Figure 1, Negendank et al.,
1985).
Cerro las Cumbres (3950 m) at 19  10’N and
97  16’W, was first recognized as a caldera structure
by Werle (1984) and later identified as an individ-
ual stratovolcano by Negendank et al. (1985). It
was largely constructed during the early to middle-
Pleistocene epoch. Activity declined in the late Pleis-
tocene when extensive erosion of the edifice took
place (Negendank et al., 1985).
Environmental effects of the late Pleistocene Wis-
consin glaciation in Mexico are not fully understood.
White (1987), White and Valastro (1984) and Heine
(1988) studied the glacial cover on the large stratovol-
canoes; Ajusco, Malinche, Popocatépetl and Pico de
Orizaba. They concluded that during the late Wiscon-
sin, all volcanoes over 4,000 m altitude were capped
by glaciers. Glaciers are reported to have reached as
low as 3,200 m altitude during the glacial maximum
(White, 1987; Heine, 1988).
This article describes the most recent eruptions at
Cerro las Cumbres volcano, which were associated
with a summit collapse forming a caldera 3.5  4.5 km
in diameter. Carbon–14 dating of charred plant/wood
remains found in the erupted products yields an age of
16,980

870 yr B.P. (sample CIF8537, dated at Gif sur
Yvette ). Magmas expelled by the eruptions range in
composition from dacite to high-K rhyolite (Table I).
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