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known as shields due to the predominently low viscosity lava, that erupts from
them. The smallest volcanic forms result from a single short-lived eruption
and comprise a variety of cones and extrusions. These include pyroclastic/
scoria cones comprised of material of basic and intermediate composition, and
steep sided flows and domes of more viscous acid lava. Such volcanic forms are
generally 1 - 2 km in diameter and may form upon, or near to, larger volcanic
forms, when they may be termed parasitic volcanoes .
From this description of volcanic forms and products, it is clear that a single
volcanic area may be characterised by a variety of deposits. These may be
contemporaneous and, for a terrestrial composite volcano, may be interpreted
in terms of variation of associations of deposits with distance from the volcano
(for example, Figure 1.2). The central zone (within circa 2 km of the central
vent) is characterised by lava conduits (later exposed as volcanic plugs, dykes
and sills) associated with coarse, poorly-sorted pyroclastic materials which
have been deposited near to the vent. The proximal zone (circa 5 - 15 km
from the central vent) has a higher proportion of lava flows, with a variety of
pyroclastic flow deposits, and the distal zone (beyond circa 5 - 15 km from the
central vent, and extending beyond the volcano) is characterised by pyroclastic
flow deposits associated with fine air-fall deposits dispersed by wind away
from the volcano. These may be interbedded with sedimentary rocks such as
lacustrine deposits and reworked volcaniclastics (epiclastic) rocks. Pyroclastic
Central zone Proximal zone Distal zone
Figure 1.2 Schematic internal architecture of a compostie volcano. See text for
descriptions of central, proximal and distal zones.
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