Page 55 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Lithic fragments are typically only very minor
components of coherent lavas. They may be volcanic
or non-volcanic and, in many cases, can be linked
with conduit wall-rock or substrate lithologies. This
photograph shows a basalt inclusion in rhyolite lava.
The inclusion may be a foreign lithic fragment but
another possibility is that it is derived from
incomplete mingling of mafic and rhyolitic magmas
prior to eruption.

Boyd Volcanic Complex, Late Devonian; Bunga
Head, New South Wales.

6. Accretionary lapilli

The conspicuous ovoid shaped features in these
samples are accretionary lapilli. The lapilli are only a
few millimeters across and closely packed in the
bottom right sample. In the sample on the left side,
the lapilli are flattened and compacted. All three
samples come from different parts of a widespread
unit of subaerial rhyolitic fallout tuff.

Cana Creek Tuff, Currabubula Formation, Late
Carboniferous; Cana Creek, New South Wales.

7. Reworked accretionary lapilli
Accretionary lapilli are composed of concentric
shells of fine and coarse ash. Fresh accretionary
lapilli can be rapidly cemented or indurated, and
remain intact during erosion and reworking. The
scattered accretionary lapilli in this volcaniclastic
sandstone have been eroded from primary deposits
and their fine-grained outer rims are partly abraded.
The host sandstone is a subaerial mass-flow deposit
generated during the explosive eruption that also
produced the accretionary lapilli-bearing fallout
deposits (7.6; McPhie 1987).
Cana Creek Tuff, Currabubula Formation, Late
Carboniferous; Cana Creek, New South Wales.

8. Armoured lapilli
These armoured lapilli consist of nuclei of black
basaltic scoria lapilli coated by pale, coarse ash of the
same composition. They occur in phreatomagmatic
surge deposits in the rim beds of a tuff cone.

Cape Bridgewater volcano, ~4 Ma; Cape
Bridgewater, Victoria

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