Page 101 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Subaerial basaltic pahoehoe lava in cross-section
The upper unit (A) in this stack of spongy pahoehoe lava
flow units shows an inward increase in vesicle size, and has
a medial gas blister (G) lined by disrupted vesicles. The gas
blister formed when large, closely spaced vesicles near the
centre of the unit coalesced, parting the floor and roof.
Flow unit (B) also had a median gas blister but it has been
filled by a younger lava tongue (C). Lava in the floor and
roof of flow unit (D) displays concentric layers of differing
vesicle size and abundance. These result from shearing
during intermittent periods of flowage after the lava has
begun to cool. Spongy pahoehoe develops as a result of
vesicle growth, mostly by coalescence, in static lava
(Walker, 1989b).
Mauna Iki pit crater, Holocene; Kilauea volcano,
Hawaii, USA.
7. Subaerial basaltic a'a and pahoehoe lava flows
Surface features of subaerial a'a (left) and pahoehoe (right)
lava flow types contrast markedly. The a'a flow surface is a
mass of spinose, twisted blocks, some of which are loose
and easily dislodged. In contrast, the pahoehoe flow has a
smooth, lobate surface of shiny glassy lava folded into ropy
patterns.

Lava flows from the AD 1969-74 eruption of Mauna
Ulu; Chain of Craters Road, Kilauea volcano,
Hawaii, USA.

Plate 20 — Textures in glassy, subaerial rhyolitic lava
1. Finely vesicular pumiceous rhyolite
A. Pale grey, glassy, finely vesicular pumice forms a
carapace about 10 m thick that covers much of the
Little Glass Mountain rhyolite lava flow. Vesicles
make up around 30% by volume and are generally
elongate, occasionally spherical and largely less than
1 mm in diameter. Subtle flow banding is defined by
variations in vesicle size and abundance.

Little Glass Mountain rhyolite flow, 1100 a;
Medicine Lake Highland volcano, California, USA.

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