Page 99 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

7. Crystal-rich and crystal-poor volcaniclastic
sandstone

Above the rhyolitic pumice- and crystal-rich breccia
(18.5, 18.6) is diffusely stratified, crystal-rich (lower)
and crystal-poor (upper) sandstone. The lower part
shows wavy bedding (W) and cross bedding (C). The
two facies are interpreted to be water-settled,
pyroclast-rich deposits from shallow subaqueous
explosive eruptions that followed dome effusion (Cas
et al., 1990). Their contrasting compositions (crystal-
rich versus crystal-poor) and grain size (coarse versus
fine) reflect efficient hydraulic sorting of particles
during settling through the water column.

Facies 4 and 5, Bunga Beds, Late Devonian;
Aragunnu Bay, New South Wales.
Plate 19 — Subaerial lava flows and domes

1. Subaerial rhyolitic lava dome
The diameter of the rhyolitic lava dome shown here
is 380 m, and it is 65 m high. A carapace of
autobreccia covers the flat top and an apron of talus
breccia (T) has accumulated around the steep
margins. The dome is partly encircled by a ring of
pumice deposits (C) erupted immediately prior to
dome extrusion.

Novarupta lava dome, about 80 a; Valley of Ten
Thousand Smokes, Alaska, USA.

2. Subaerial rhyolitic obsidian lava flow
A cliff-section through the margin of a rhyolitic lava
flow reveals large, regular columnar joints (J) at the
base, and tightly contorted flow layering (F) near the
top. The flow foliation is defined by trains of
spherulites. The rhyolite is devoid of phenocrysts
indicating eruption at a temperature above the
liquidus and all but the interior was quenched to glass
(obsidian). The top of the flow is pumiceous. Talus at
the foot of the cliff comprises angular boulders and
hides the basal contact.

Obsidian Cliff rhyolite flow, 0.18 Ma; Yellowstone
National Park, Wyoming, USA.

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