Page 133 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 21 — Three types of pyroclastic flow deposits
1. Rhyolitic block and ash flow deposit

The very thick, poorly sorted, volcanic breccia shown
here is the deposit from a block and ash flow
generated during growth of a subaerial rhyolite lava
dome. It comprises angular, blocky rhyolite clasts in
ash matrix, and displays weak reverse grading.
Although non-welded, primary transport and hot
emplacement is indicated by the presence of charred
and carbonised wood. The exposure is about 5.4 km
from the source.

Kaharoa Pyroclastics, AD 1300; Crater Road,
Tarawera, New Zealand.

2. Texture of a block and ash flow deposit
Fresh exposures of the rhyolitic block and ash flow
deposit in 21.1 show characteristic poor sorting and
matrix-supported fabric. The polyhedral, white
rhyolite lapilli and blocks are reversely graded.
Below the block and ash flow deposit are stratified
ash and pumice lapilli layers from earlier cogenetic
explosive eruptions. The section in the photograph is
about 2 m thick.

Kaharoa Pyroclastics, AD 1300; Crater Road,
Tarawera, New Zealand.

3. Andesitic scoria and ash flow deposit
These small volume, scoria and ash flow deposits (S)
consist of vesicular andesitic blocks and lapilli, and
only minor coarse ash. The deposits were emplaced
on relatively steep slopes, and have well-developed
surface ridges and levees. This suggests that prior to
deposition the pyroclastic flows had appreciable
yield strength.

Deposits from the AD 1975 eruption of Ngauruhoe;
Tongariro Volcanic Centre, New Zealand.

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