Page 87 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Andesitic hyaloclastite: concentric pillow breccia
Concentric pillow breccia is a distinctive variety of
hyaloclastite formed by disintegration of finger-like
lava apophyses along overlapping concentric joints
(13.3). The larger clasts have rounded, pillow-like
shapes and concentric joints, hence the term
"concentric pillow breccia" (Yamagishi, 1987).

Yoshida Formation, ~3 Ma; Senjojiki, Izu Peninsula,
Honshu, Japan.

5. Submarine lava lobe enclosed by in situ
hyaloclastite
This outcrop shows an andesite lava lobe (L)
surrounded by in situ hyaloclastite (H). The lobe is
dissected into polyhedral joint blocks and locally has
finger-like apophyses (outlined) extending into the
hyaloclastite. Within the hyaloclastite, groups of
clasts with jigsaw-fit texture are enclosed by areas
where clast rotation and separation are evident.

Andesite, Neogene; Rebun Island, Hokkaido, Japan.

6. Andesitic feeder dyke
The 2-4 m wide, dark grey, andesitic dyke here
intrudes white, bedded pumiceous sandstone and
conglomerate. Adjacent to the dyke, the host
sandstone is slightly indurated and bedding is
destroyed. The margins of the dyke are black and
glassy, and locally protrude into the sandstone. Joint
patterns within the dyke suggest it has a complex
shape in three dimensions. The irregular shape of the
dyke implies that it was intruded when the host
sandstone was only poorly consolidated.

Ishiki Formation, ~3 Ma; Senjojiki, Izu Peninsula,
Honshu, Japan.

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