Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 203

under continents or oceans, and they can also occur at the centre of tectonic
plates or at spreading centres such as Iceland. Hot spots found over spreading
centres have the potential to release more basaltic magma than hot spots under
the centreoftectonicplates(Abbott,1999).
Basaltic lava flows commonly exhibit three different types of texture: pahoe-
hoe, aa and pillow lava. Pahoehoe lava is highly liquid and cools to leave a
smooth, ropy surface. Aa lava flows are much more viscous and form a rough,
blocky texture. Pillow lava occurs when lava flows reach water in lakes or the
ocean causing it to cool rapidly into ovoid forms. Lava flows are often associated
with basaltic magmas. Pyroclastic material, normally associated with the more
acidic magmas, forms when gas trapped within the magma explodes resulting
in the projection of debris into the air. This debris can contain magma and rock
material broken into a wide range of sizes from dust, coarse-grained ash and cin-
ders to blocks and bombs. Blocks consist of large fragments that are solid when
airborne while bombs are composed of large fragments that are in liquid form
when airborne. Coarser-grained material settles closer to the volcanic eruption
whereas fine ash and dust can travel considerable distances.
There are three main characteristics that determine the form and structure
of volcanoes. These characteristics are the type of magma feeder channel, the
character of the material emitted and the number of eruptions that occur. The
feeder channel may consist of either a central vent or a fissure. The material
erupting from a volcano plays a significant role in determining the volcano's
morphology. Stratovolcanoes form mainly from andesitic to rhyolitic magmas,
being relatively high in silica, with high viscosity. These volcanoes are steep
sided symmetrical forms composed of alternating layers of pyroclastic debris and
lava flows. Shield volcanoes form from basaltic magmas which, because of their
lower viscosity, flow for longer distances forming a volcano which is very wide
compared to its height. Generally, volcanoes consisting mainly of pyroclastic
material grow taller more rapidly than shield volcanoes (Bell, 1998).
Volcanoes can also be divided into monogenetic and polygenetic forms
depending on whether they produce a single or multiple eruption, respectively.
Monogenetic volcanoes have a central vent, are always small and have a simple
structure where the eruptive centre moves on after an eruption. Monogenetic
volcanoes usually occur in fields rather than individually. Polygenetic central
vent volcanoes are more complicated and larger than monogenetic volcanoes.
Stratovolcanoes are often polygenetic. The simplest form of these volcanoes is
cone shaped, although the shape of such volcanoes may change due to several
factors. These include migration and displacement of the vent to produce two
or more summit craters and the formation of calderas. Calderas are formed
when the volcanic peak collapses onto the magma chamber below. Polygenetic