Page 55 - Sedimentology and Stratigraphy

42 Biogenic, Chemical and Volcanogenic Sediments

tens, hundreds or even thousands of kilometres away. ! "
Volcaniclastic material may therefore occur in any

depositional environment and hence may be found
associated with a wide variety of other sedimentary , '
rocks (Chapter 17). Volcanic rocks are also of consid- ;8
erable value in stratigraphy as they may often be , ( >
'
dated radiometrically (21.1), providing an absolute
time constraint on the sedimentary succession. ;8

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3.7.1 Types of volcaniclastic rocks .
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The composition of the magma affects the style of .
eruption. Basaltic magmas tend to form volcanoes
that produce large volumes of lava, but small
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amounts of volcanic ash. Volcanoes with more silicic
magma are much more explosive, with large amounts
of the molten rock being ejected from the volcano as
particulate matter. The particles ejected are known as
pyroclastic material, also collectively referred to as
tephra. Note that the term pyroclastic is used for
material ejected from the volcano as particles and >
volcaniclastic refers to any deposit that is mainly
composed of volcanic detritus. Pyroclastic material
may be individual crystals, pieces of volcanic rock
(lithic fragments), or pumice, the highly vesicular,
chilled, ‘froth’ of the molten rock. The size of the %
pyroclastic debris ranges from fine dust a few microns
across to pieces that may be several metres across.
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3.7.2 Nomenclature of volcaniclastic rocks
Fig. 3.12 (a) The classification of volcaniclastic sediments
and sedimentary rocks based on the grain size of the material.
The textural classification of volcaniclastic deposits (b) Nomenclature used for loose ash and consolidated tuff with
(Fig. 3.12) is a modification of the Wentworth scheme. different proportions of lithic, vitric and crystal components.
Coarse material (over 64mm) is divided into volcanic
blocks, which were solid when erupted, and volcanic
bombs, which were partly molten and have cooled in 3.7.3 Recognition of volcaniclastic material
the air; consolidated into a rock these are referred to
as volcanic breccia and agglomerate respectively. The origin of coarse-grained volcaniclastic sediments
Granule to pebble-sized particles (2–64 mm) are called is usually easy to determine if the lithology of the
lapilli and form a lapillistone. Accretionary larger clasts can be recognised as an igneous rock
lapilli are spherical aggregates of fine ash formed such as basalt. The tephra particles are usually angu-
during air fall. Sand-, silt- and clay-grade tephra is lar, with the exception of rounded volcanic bombs,
ash when unconsolidated and tuff upon lithification. well-rounded accretionary lapilli found in some air
Coarse ash/tuff is sand-sized and fine ash/tuff is silt- fall ashes, and the distinctive shape of fiamme, glassy
and clay-grade material. Compositional descriptions pumice fragments that may resemble a tuning fork
hinge on the relative proportions of crystals, lithic when compacted. Another useful indicator is the uni-
fragments and vitric material, which is fragments of form nature of the material, as mixing of tephra with
volcanic glass formed when the molten rock cools other types of sediment occurs only by subsequent
very rapidly, sometimes forming pumice. reworking. In general, volcaniclastic sediments with