Page 86 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 13 — Hyaloclastite varieties and feeder dykes
1. Trachytic hyaloclastite with quenched lava "rags"
The large, ragged and blocky shaped clasts in this
breccia have complete, formerly glassy, chilled
margins. The smaller clasts are fragments derived
from disintegration of the larger clasts. Both consist
of aphanitic, finely vesicular trachyte. Although
quenched, the large ragged clasts have not
necessarily been formed by quench fragmentation —
their shapes suggest formation by tearing apart of
lava, perhaps during mild, subaqueous fountaining,
prior to quenching (cf. water-chilled bombs).

Trachytic hyaloclastite, Middle Miocene; Porto
Santo, Madeira Archipelago.
2. Altered andesitic(?) hyaloclastite
The prominent pale, irregular shapes are carbonate-
altered cores of blocky clasts in hyaloclastite breccia.
The margins of' the blocky clasts are chloritically
altered and dark green, as are clasts in the apparent
matrix. The true clast outlines are in fact very subtle
and much less obvious than the colour contrasts
caused by alteration.

Mount Read Volcanics, Cambrian; Newton Dam
Spillway, western Tasmania.
3. Andesitic feeder dyke and incipient concentric
pillow
This outcrop shows an apophysis extending from a
feeder dyke (D), cut by overlapping curved joints
(arrow) and surrounded by in situ hyaloclastite (H).
Disintegration of the stems of such apophyses
generates large clasts with round shapes and
concentric joints (concentric pillows; 13.4).

Andesite, Miocene; Cape Notoro, Hokkaido, Japan.

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