Page 168 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Volcanic debris-avalanche deposit and slide
blocks
Collapse of the summit area of Socompa (6051 m)
2
generated a vast (600 km ) debris-avalanche deposit and
created a 10 km wide amphitheatre that has been partly
healed by post-collapse lava flows and domes (F). Large
slide blocks (S) form an elongate massif at the mouth of
the amphitheatre. Irregular fields and linear trains of red
weathering lava boulders (36.6), dominate the subdued
hummocky (H) topography of the debris-avalanche
deposit (foreground). The "Campo Amarillo" flow
deposit (C) mantles the proximal parts of the avalanche
deposit and suggests that failure triggered a pyroclasic
eruption (Francis et al., 1985; Francis and Wells, 1988).

Socompa volcano and the 7200 a volcanic debris-
avalanche deposit, northern Chile.
5. Relict source stratigraphy in a debris avalanche
block
This view shows a large debris avalanche block
comprising a mound of pumiceous ignimbrite mantled
by clasts of grey dacite lava. The ignimbrite plus dacite
are allochthonous parts of the original source
stratigraphy, and have been transported more or less
together and intact by the debris avalanche. The
ignimbrite in the blocks is pervasively internally sheared
but has behaved in a relatively ductile fashion during
flowage, whereas the dacite lava has broken up into
abundant separate clasts. Note person for scale (arrow).

WNW margin of the primary debris avalanche
deposit, 7200 a; Socompa volcano, northern Chile.

6. Prismatically jointed dacite lava block in a
volcanic debris-avalanche deposit
This large (>20 m) prismatically jointed, glassy dacite
lava block occurs within the primary debris-avalanche
deposit on Socompa. Such blocks were probably
transported hot and developed the pattern of prismatic
cooling joints only after depositon. They may have been
derived from an active lava flow or dome on the summit
of the volcano at the time of collapse (Francis et al.,
1985; Francis and Wells, 1988). Most of the joints are
curviplanar and divide the block into equant polygons.

Volcanic debris avalanche deposit, 7200 a; Socompa
volcano, northern Chile.
7. Hummocky debris-avalanche deposit
Late Pleistocene collapse of the SW sector of Parinacota
volcano (6348 m) generated a debris avalanche which
altered local hydrology by damming earlier drainages
(Francis and Wells, 1988; De Silva and Francis, 1991).
Lake Cotocotani (LC) and many small lakes are nested
on the hummocky terrane (H) of the debris-avalanche
deposit and are maintained by seepages through it. Post-
collapse andesitic lava flows have healed the avalanche
detachment scarp. The summit of Pomerape, another
composite andesitic volcano, is evident in the
background.
Volcanic debris avalanche deposit, 13.5 ka;
Parinacota volcano, northern Chile.

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