Geology Reference
In-Depth Information
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Figure 5.8 The components of a typical air-fall ash of dacite composition. The
diameter of the field is about 2mm. The ash has a fine matrix which has been
omitted and consists of angular glass shards, (a) some showing cuspate forms;
pumice fragments, (b) showing vesicles that are nearly round in cross section in
one direction and much stretched in the direction right angles to the first; and
crystals of hornblende, (c) biotite (d) and feldspar (e) with a little quartz and
lithic fragments (f).
are useful in poorly or un-consolidated deposits (for additional instruction on
recording sedimentary structures refer to Tucker, this topic series).
5.2 Pyroclastic Flows and Ignimbrites
As with lava sequences (Section 4.1), it is very useful to have some conceptual
idea of the processes which are involved in the deposition of pyroclastic rocks,
to help understand how deposits form. Here we will briefly describe some of
the recent developments in our understanding of pyroclastic flows which will
help you to properly describe and record their deposits; ignimbrites and block
and ash flows .
5.2.1 Understanding pyroclastic processes
Ignimbrite deposits are formed from explosive eruptions that result in turbulent
mixtures of fragmented rocks, hot magma, crystals and ash which flow down
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