Jökull - 01.01.2001, Side 58

Ármann Höskuldsson
Figure 8. Final phase of the eruption: base surge layers interbedded with vulcanian–like ash– and scoriae–fall
deposits. Note low angle crossbedding, bomb–sags of ballistic ejecta, and plastic deformation of the underly-
ing beds (scale is 25 cm). – Gusthlaupaset víxlað með loftborinni gjósku af vulcanian gerð myndað í lokafasa
eldgossins. Takið eftir lághorna skálögun, bombusekkjum og plastískri ummyndun undirliggjandi laga (sem
bendir til blauts umhverfis). Kvarðinn á myndinni er 25 cm langur.
phibole (0.1–0.5 mm). The phenocrysts are subhedral
to euhedral. The glassy groundmass contains euhedral
zircon, apatite and titanomagnetite microlites
The pyroclastic-surge deposits
Pyroclastic-surge deposits are interbedded with the
vulcanian ash-fall deposits in sections from the west
flank of Cerro Las Cumbres (Figure 3). The surge
deposits consist of coarse to medium–grained ash
(Md=1.5 mm) arranged in sets of low–angle cross-
beds (Figure 8). Ballistic bomb impacts have caused
plastic deformation of the surge beds as well as the
ash–fall layers (Figure 8). In the caldera, the strati-
graphical equivalent of these surge deposits consist of
lapilli–tephra (2–64 mm), featuring sandwave struc-
tures with amplitudes up to 1 m. These coarse grained
surge sequences are intercalated with layers of coarse
grained (100–1000 mm) airfall breccia deposits rich
in lithic– and obsidian blocks (Figure 3, Q78). The
intra–caldera sequences are deformed and uplifted by
later dome extrusion onto the caldera floor.
The Dome
The caldera of Cerro las Cumbres was partly filled
by the el Cumbre Grande lava dome, from which the
mountain takes its name. Emplacement of this dome
complex was the last episode of the eruption.
The dome rocks show porphyritic fluidal texture in
thin section and contain abundant groundmass plagio-
clase and pyroxene. Phenocrysts of resorbed anhedral
quartz (0.5–1 mm), subhedral resorbed biotite (0.5–
1 mm), zoned subhedral to euhedral plagioclase dis-
playing reaction rims (1–2 mm), subhedral to anhedral
sanidine with reaction rims (1–2 mm), subhedral id-
dingsitized olivine (0.1–1 mm), subhedral clinopyrox-
ene (0.1–1 mm), and euhedral titanomagnetite (Figure
58 JÖKULL No. 50