Page 88 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

7. Feeder dyke to andesitic hyaloclastite
A vertical andesite feeder dyke (D2) intrudes earlier
dykes and related hyaloclastite. The dyke has a
massive core, marginal columnar cooling joints (J),
and undulating bulbous margins. It is about 10m
wide at the base, but thickness varies over the
exposed length. An earlier, less coherent dyke (D1)
has an irregular shape; tongues and apophyses
(arrow) propagate into adjacent in situ hyaloclastite
(H).

Yakumo Formation, Late Miocene; Moiwa, Shakotan
Peninsula, Hokkaido, Japan.
Plate 14 — Peperite (intrusive hyaloclastite)
1. Rhyolite-siltstone peperite
This example of blocky peperite occurs at the base of
a rhyolitic intrusion where it is in contact with
siltstone. The blocks and splinters of rhyolite (R) are
separated by pale grey, massive siltstone. Rhyolite
clasts have curviplanar margins (C) and in places,
groups of clasts display jigsaw-fit texture.

Bedded Pyroclastic Formation, Ordovician;
Llanberis Pass, Snowdonia, northern Wales, UK.

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