Page 81 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Subaerial andesitic autobreccia
The interior of this 4 m thick andesite lava flow is
coherent and enveloped by autobreccia composed of
annealed, spinose, twisted, scoriaceous clasts
("clinker"). The contact between brecciated and
coherent andesite is highly irregular, and
autobrecciated clasts (C) occur locally within
massive coherent lava. Clasts in the autobreccia are
red as a result of thermal oxidation, a process diat is
usually confined to subaerial environments.

Whakapapa Formation, <15ka; Ruapehu volcano,
New Zealand.

6. Talus associated with a basaltic andesite lava flow

A. Incandescent lava blocks cascade down the talus
slope at the front of the 1981 basaltic andesite blocky
lava flow on Arenal volcano. Lava blocks are spalled
from an advancing lava lobe (L), and roll or bounce
downslope, disintegrating further while in transit.
Width of field of view is about 30 m.

Basaltic andesite lava flow, Arenal volcano, AD
1981; Costa Rica.

B. Clasts in the talus apron at the front of the active
blocky lava flow are partly formed by
autobrecciation but during transport, clast shapes are
modified by abrasion and new fragments are created
when large blocks bounce downslope and collide.

Basaltic andesite lava flow, Arenal volcano, AD
1981; Costa Rica.

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