Page 175 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

6. Accretionary lapilli-bearing, rhyolitic ash fall
deposit

There are abundant, close packed, accretionary lapilli
(A) in this ash fall deposit. The bed has diffuse upper
and lower contacts and the accretionary lapilli decrease
slightly in size from the base to the top. The deposit
largely comprises glassy, rhyolitic ash pyroclasts.
Abundant accretionary lapilli and fine grain size are
typical features of deposits from phreato-magmatic
eruptions.

Oruanui Formation, 26.5 ka; Whangamata Road,
North Island, New Zealand.
7. Subaerial basaltic fallout: very thick, clast-
supported scoria lapilli deposit
This cutting exposes the proximal fallout deposits of a
basaltic scoria cone. The principal components are ragged
and angular basaltic scoria lapilli, together with minor
fluidally-shaped lava bombs (behind the lens cap). The
deposit is very thick but only weakly stratified. Locally the
scoria pyroclasts display incipient welding. Some scoria
lapilli and the rims of the lava bombs are thermally
oxidised. These features arise during periods of rapid
accumulation in proximal settings that promote effective
heat retention.

Mount Ecdes scoria cone, Tertiary; western Victoria.

8. Subaerial basaltic fallout: poorly sorted scoria
lapilli, bomb and block deposit

Coarse grained, poorly sorted fallout is in most cases
confined to relatively proximal settings. This scoria fallout
was deposited about 50 m from the vent and forms the rim
beds of a scoria cone. Fluidally-shaped lava bombs are
separated by closely packed, blocky, angular scoria lapilli.
The lava bombs have chilled, poorly vesicular margins,
moderately vesicular interiors and are internally jointed.
Out of context, the outcrop could be mistaken for pillows
and pillow fragment breccia. However, the bombs do not
show the distinctive surface textures of pillows, and the
blocky scoria lapilli do not have quenched rims.
Eppelsberg scoria cone, Quaternary; Wannenkopfe,
Germany.
Plate 40 — Textures and structures in volcaniclastic deposits from suspension, flotation and traction

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