Page 82 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 11 — Hyaloclastite
1. Feeder dyke-hyaloclastite in rhyolite

This exposure shows the gradational contact between
dark grey, coherent obsidian (Ob) in a feeder dyke
and adjacent matrix-rich, in situ hyaloclastite (Hy) in
rhyolitic lava erupted on the sea floor. The transition
zone consists of altered, tan, in situ hyaloclastite
(Ahy). The matrix-rich hyaloclastite is unaltered and
composed of dark grey rhyolite blocks embedded in
white, sand-sized hyaloclastite matrix. Rucksack
foreground for scale.

Rhyolite, Pleistocene; Chiaia di Luna, Ponza, Italy.
2. In situ rhyolitic hyaloclastite

There is a gradation from comparatively coherent,
flow-banded lava (below the pen) into matrix-rich in
situ hyaloclastite. Clasts in the hyaloclastite have
curviplanar margins (Cp) and groups of clasts
display jigsaw-fit texture (dashed line). The granular
matrix (M) is composed of glassy particles and
phenocryst fragments derived from thorough quench
fragmentation of the lava.

Glassy rhyolite, Pleistocene; Cala del Gaetano,
Ponza, Italy.

3. In situ rhyolitic hyaloclastite
A brecciated lobe of flow-banded rhyolite comprises
jigsaw-fit to slightly rotated blocks of dark grey lava
separated by narrow seams of finer matrix. The lobe
has sharp contacts with enclosing pale grey, matrix-
rich in situ hyaloclastite (Hy). Autobrecciation and
quench fragmentation probably operated together
during disintegration of the lobe. It may represent a
pulse of lava emplaced into cogenetic, freshly formed
in situ hyaloclastite.

Glassy rhyolite, Pleistocene; Cala del Gaetano,
Ponza, Italy.

4. In situ andesitic hyaloclastite
This monomict andesitic breccia shows the clast
shapes and jigsaw-fit texture typical of in situ
hyaloclastite. White, silicic pumiceous sandstone that
fills the spaces between the andesite clasts filtered
down from overlying pumiceous volcaniclastic
deposits.

Matsuzaki Volcanics, Late Miocene—Pliocene;
Matsuzaki, Izu Peninsula, Honshu, Japan.
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