Page 167 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Modern volcaniclastic grain-flow deposits
These lenticular beds comprise matrix-poor, clast-
supported, granule- to pebble-sized volcanic lithic
and pumice fragments. The beds have steep (30°)
primary dip and are reversely graded. They are part
of a modern scree slope and are typical of deposits
emplaced by grain flows in which dispersive pressure
and grain interaction contribute to clast support
during gravity-driven flowage (Lowe 1982).

Camaldoli Hill, Naples; Campi Flegrei, Italy.

2. Volcanic debris-avalanche deposit: very poorly
sorted, volcanic lithic breccia
This cutting exposes the interior of a hummock in a
young volcanic debris-avalanche deposit. There is a
clear boundary (dashed line) between areas of the
deposit in which the colour of the matrix differs.
Each area consists of angular lava clasts, some
showing jigsaw-fit texture (arrow), supported within
sandy matrix. The lava clasts were generated by
brittle fracture and dilation of debris avalanche lava
blocks, and have progressively mixed with finer
components derived from disintegration of softer
lithologies during gravity-driven flow.
Murimotu Lahar Formation, 9500 a; western ring
plain of Ruapehu volcano, New Zealand.
3. Jigsaw-fit texture in a debris-avalanche deposit

Clusters of clasts within the volcanic debris-
avalanche deposit shown in 36.2 display jigsaw-fit
texture characteristic of in situ fragmentation.
Abundant small fragments of dacite lava can be fitted
back together to reconstruct the outline of a larger
clast (within dashed line). During flowage in the
debris avalanche, pieces of in situ fractured blocks
progressively separated and mixed with clasts from
adjacent blocks. Fracturing is the result of dilation of
blocks at the time of failure and brittle shattering due
to collisions during transport (Glicken 1991).

Murimotu Lahar Formation, 9500 a; western ring
plain of Ruapehu volcano, New Zealand.

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