Jökull - 01.01.2001, Síða 56

Ármann Höskuldsson
Figure 6. A secondary explosion crater within the ash–flow deposits. The crater is about 50 m in diameter. In
the background a section through the pyroclastic–fall deposits can be observed. The crater was blasted through
the pyroclastic–fall deposits, which shows that the they were still hot when the pyroclastic–fall layers accu-
mulated on top of them. – Gervigígur, um 50 m í þvermál sem nær í gegnum gjóskulagið frá eldgosinu sem
myndaði öskjuna. Í baksýn sést í snið gegnum loftbornu gjóskuna. Gígurinn er sprengdur upp í gegnum alla
gjóskulagasyrpuna en það bendir til þess að setmyndun hafi verið mjög hröð.
The ash–flow deposits
Ash-flow deposits cover the western flank of Cerro
las Cumbres, attaining a maximum thickness of 16 m
within topographic lows close to the dispersal axis of
the pyroclastic fallout unit (Figure 2a).
The ash–flow deposit is fine grained, with more
than 70% of the material being medium to fine ash
(Figure 4). It forms a single continuous unit be-
tween the lahar deposits to the base of the pumice–
fall deposits above (Figure 3). The upper main part
of the ash–flow deposits is massive, with occasional
lenses of pumice lapilli and lithics (Figures 5a and
5b). This massive part of the deposit is rich in car-
bonized detritus and logs. Its basal layer is stratified
showing antidunes and megadunes similar to those
found in the lahar deposits (Figure 3). Smith and
Roobol (1991) described similar small–volume de-
posits from the Mt. Pelée volcano in Martinique and
suggested that they were formed by “ash hurricanes”
because they featured depositional structures indicat-
ing turbulent flow. The term “ash–flow” is preferred
for the Cerro las Cumbres deposits because they are
of much greater volume (about 1–2 km


, Table I) than
the “ash hurricane” deposits mentioned above. A
rootless crater, produced by a secondary phreatic ex-
plosion, occurs in the distal sector of the ash–flow
deposits (Figure 6). Overlying pumice–fall deposits
were also distributed by the explosions, implying that
the two units were deposited within a short time of
each other. The rootless–crater may have formed dur-
ing dehydration of underlying laharic deposits.
A Scanning Electron Microscope (SEM) study of
grains from the ash–flow deposits was undertaken in
order to understand the intense fragmentation of the
56 JÖKULL No. 50