Page 53 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Subaerial basaltic scoria
Basaltic scoria lapilli dominate this near-vent
subaerial fallout deposit. The red colour is a result of
thermal oxidation, a process that reflects cooling of
the hot lapilli while in contact with air. The irregular
shapes of the lapilli are the result of tearing apart of
vesiculating magma.

Basaltic lapilli breccia, Miocene; Porto Santo,
Madeira Archipelago, Portugal.

7. Altered scoria in thin-section

Ragged scoria grains are abundant in this sample of
metamorphosed basaltic volcaniclastic sandstone.
The scoria grains and ovoid vesicles (V) within them
are outlined by a fine, opaque mineral. The vesicles
are infilled by delicately fibrous sericite or by albite.
Apart from scoria grains, the sandstone contains
scattered crystal and lithic fragments and fine,
recrystallized matrix. Plane polarized light.

Mount Read Volcanics, Cambrian; specimen 91-
109B, Henty River, western Tasmania.

8. Near vent bomb and juvenile block
These juvenile basaltic pyroclasts are part of a bomb
and block field, deposited near vent by ballistic
fallout. The spindle bomb (S) has been shaped by
surface tension during flight. Adjacent is a large,
angular block with a partly bread-crusted surface (B).
Bread-crust fracturing is caused by vesiculation and
expansion of the hot clast interior beneath a chilled
crust.

Telica composite volcano, Nicaragua; deposits from
the AD 1982 eruption.

Plate 7 — Shards, lithic fragments and accretionary lapilli

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