Geology Reference
In-Depth Information
crystallisation
welding
absent
Vapour phase
crystallisation
(except in
densely
welded zone)
+/- devitrification
partial
dense
partial
absent
Figure 5.18
Variation in degree of welding within a single ignimbrite unit. The
section is about 70-100m in thickness.
5.4 Water/Magma and Sediment/Magma Interactions
5.4.1 Hydroclastic deposits
When eruptions occur under water or into ice then dynamic fragmentation of
the magma can result in
phreatomagmatic
eruptions. Eruption of magma into,
or through, water results in another suite of characteristic volcanic products
which include the
hyaloclastites
(described in Chapter 4). The more violent
pyroclastic eruptions in shallow water form small low-profile cones (h/d
=
0.1 - 0.2) composed of fine-grained ash-grade pyroclastic material termed
ash-
or tuff-ring
s (Figure 5.11). Similar pyroclastic material is formed by eruption
from larger composite volcanoes having crater lakes. In comparison with dry
subaerial pyroclastic fall deposits, such material is generally finer-grained (Md
generally below 2 mm) and less well-sorted (
). The individual layers
are generally thin, commonly 1 mm to 1 cm, in comparison with much coarser
subaerial pyroclastic layers (layers about 1 - 5 cm). Characteristic features of
pyroclastic material formed by interaction with water are
accretionary lapilli
and
impact structures
(for example, Figures 5.10 and 5.11).
σϕ
,1.5
>
