Geology Reference
In-Depth Information
However, since the pressure of sea water at a depth of 3000 m is sufficient to
prevent exsolution of dissolved gases, the vesicularity of pillows varies from
about 10 - 40% when they formed at shallow depths (less than 500 m) to below
about 5% at 1000 m, and this may be used as a crude estimate of the depth of
water into which the pillow lavas were erupted.
Pillows may be spheroidal, ellipsoidal or so flattened (even when undeformed
tectonically) that they may be difficult to distinguish from other similar struc-
tures. For example, many ancient basaltic lavas contain pillow-like structures,
such as lobes of (subaerial) pahoehoe lava, that might be confused with true pil-
low lavas (Figure 4.17b). In view of the palaeogeographic significance of pillow
lavas (particularly in the identification of ophiolite complexes, see Chapter 8)
and their value as 'way-up' indicators, you should examine the outcrop carefully
for the features described above.
A further product of eruption of basaltic magma into water results from
large-scale fragmentation of glassy lava (for example, pillow rinds and pillow
lava fragments - Figure 4.18). Such fragmentation produces a deposit composed
of glassy basaltic particles of sand size mixed with separated pillows. Such
deposits are termed
hyaloclastite
or
palagonite-tuff
when the basaltic glass
has been palagonised. Hyaloclastite commonly forms massive poorly-bedded
deposits associated with subaerial lava, or submarine to emergent volcanoes
(for example, Figure 4.19). In some cases hyaloclastite overlain by lava may be
interpreted in terms of advance of (subaerial) lava from a shoreline into water to
build a lava 'delta' that progrades the shoreline out into the water (Figure 4.20).
Sometimes the hyaloclastite shows distinct bedding and this might then reflect
reworking of the fine-grained glassy material by waves and currents, so that the
deposits may contain sedimentary structures reminiscent of sand deposited in
shallow water.
