Jökull - 01.01.2001, Page 50

Ármann Höskuldsson
Figure 1. Simplified geological map of Central Mexico showing the main volcano-tectonic structures includ-
ing the Trans-Mexican Volcanic belt and the location of the research area. Inset to the right, shows the four
volcanoes and the volcanic complex forming the easternmost part of the Trans Mexican Volcanic Belt. Las
Cumbres is situated about 7 km north of Pico de Orizaba. Of the four volcanoes Pico de Orizaba is currently
most active, including the parasitic vent Sierra Negra. – Staðsetning rannsóknarsvæðisins. Myndin til hægri
sýnir þau fjögur eldfjöll sem mynda austasta hluta mexíkanska eldfjallabeltisins. Las Cumbres er staðsett um
7 km norður af eldfjallinu Pico De Orizaba, en það eldfjall hefur verið virkt á sögulegum tíma. Suður af Pico
De Orizaba er eldstöðin Sierra Negra sem tilheyrir sama kerfi.
The Cerro Las Cumbres rhyolite contains abundance
of macroscopic biotite phenocrysts that distinguishes
it from other rhyolite products in the region (Verma
and Lopez, 1982). At the time of the eruption the vol-
cano was capped by a glacier, which had important
effects on the eruption mechanics and produced a un-
usual depositional pattern of the eruptive products.
DISTRIBUTION OF THE DEPOSITS
Volcaniclastic deposits formed in the 16,980

870 yr
B.P. eruption are distributed asymmetrically around
Cerro las Cumbres (Figures 2a and 2b). The great-
est volumes of deposits are observed on the west
flank, where they cover the slopes of the volcano
from about 3,400 m altitude down to the Basin Ori-
ental (Figure 2a). Barrancas cut into the deposits re-
veal coarse boulder-rich fluvial sediments at the base
of the volcaniclastic deposits. On the west flank of
the volcano, in areas proximal to the vent ( 9 km)
the Cerro Las Cumbres deposits consist of 4 erup-
tive units. The lowest unit of the deposits is laharic
and is overlain by pyroclastic-flow deposits (Figure
3). The pyroclastic–flow deposits form a sheet that
is thick in topographic lows and thins out toward to-
pographic highs. These deposits are overlain by a
pumice–fall deposit, that thins regularly away from
the volcano in accordance with a WNW-oriented dis-
persion axis (Figure 2a). Within a distance of 9 km
from the caldera the pumice–fall deposit is split into
two units by a series of pyroclastic–surge and ash–fall
deposits. Deposits of similar lithologic character cap
the sequence (Figure 3).
50 JÖKULL No. 50