Page 139 - Fundamentals of Physical Volcanology

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116 CHAPTER 8

Fig. 8.8 A thin (∼1 m thick) nonwelded ignimbrite flow
unit, part of a deposit on the island of Terceira in the Azores. Fig. 8.9 Cross-bedding in surge deposits from
This unit shows a good example of a basal layer which is hydromagmatic eruptions at the prehistoric Hana Uma vent,
depleted in coarse clasts and an upward concentration in O'ahu, Hawai’I. (Photograph by Elisabeth Parfitt.)

larger pumice clasts. (Image courtesy of Stephen Self.)

They tend to be of small volume and are mainly emplacement events at a given location may allow
associated with events in which lava domes or lava one or more flow units to be laid down in quick
flow fronts collapse. succession and to cool significantly before an-
The term flow unit is used to describe the other emplacement event occurs. This process
deposit from a single pyroclastic density current at produces cooling units which may be recognized
a particular location. Any given eruption may pro- by changes in color of the deposit, reflecting dif-
duce a number of separate pyroclastic density cur- ferences in oxidation rate as the mass cools, or
rents resulting in a compound deposit consisting of differences in the amount of flattening or welding
several flow units. Many ignimbrites are emplaced of clasts.
hot enough that some welding between clasts The morphological features of the deposits from
occurs. Also there may be some compaction of the pyroclastic density currents that do not completely
deposit under its own weight. This causes clasts bury the pre-existing topography suggest that the
that are hot enough to deform in a plastic fashion bulk of the flowing material has a density greater
to be flattened to form structures called fiamme than that of the surrounding atmosphere. Thus the
(Italian for flames). The gaps in time between distributions of the deposits indicate that the cur-