Geoscience Reference
In-Depth Information
Volcanoes as a
Hazard
CHAPTER 11
the western Pacific Ocean, where the Pacific Plate is
subducting beneath the Eurasian Plate (Figure 9.1).
Basaltic lavas have temperatures of between 1050 and
1200°C, while andesitic ones are 100-150°C cooler.
For each 500°C decrease in temperature, fluidity
decreases tenfold. For this reason, and because of their
higher silica content, andesitic lavas are 200-2000
times more viscous than basaltic lavas. Andesitic lavas
are also gas-rich, with 50-90 per cent of the gas
consisting of water assimilated from dissolved crustal
material. This high water content and high
viscosity
makes andesitic volcanoes, located over supposed
subduction zones, much more explosive than basaltic
volcanoes sitting over hot spots in the center of crustal
plates.
Magma is not necessarily extruded at the Earth's
surface by pressure originating in the mantle below.
Instead, lava penetrates through tens of kilometres of
weakened, fractured crust because of gas pressure. All
magmas, be they andesitic or basaltic, contain various
amounts of dissolved gas. These magmas can become
oversaturated in gas due to crystallization of the
magma, its cooling down, decreases in hydrostatic
pressure, or enrichment of volatile molecules from
outside the magma. The gas in oversaturated lavas
begins to vesiculate or form bubbles, a process that
immediately makes the magma lighter or more
buoyant. If the magma is forced quickly upward by
pressure from below, then the
hydrostatic pressure
INTRODUCTION
(Bolt et al., 1975; Tazieff & Sabroux, 1983)
Of all natural hazards, volcanoes are the most complex.
Whereas a tropical cyclone has a predictable structure,
or a drought can generate a predictable sequence of
events in rural communities, such predictions cannot
be made in respect of volcanoes. There is a multitude
of volcanic forms, and each event appears unique in
the way that it behaves, and the physical and human
consequences it produces. This chapter will examine
the different types of volcanoes and the secondary
phenomena associated with their occurrence. It will
conclude with a detailed description of some of the
more spectacular volcanic disasters that have occurred
in recorded history.
Volcanoes are conduits in the Earth's crust through
which gas-enriched, molten silicate rock magma
reaches the surface from beneath the crust. The origin
of magma is still debated, but it is generally believed
from seismic evidence that the mantle is partially
liquefied 75-300 km below the Earth's surface. There
are two types of magma. The first type consists of
silica-poor material from the mantle, and forms
basaltic volcanoes. The second type consists of silica-
rich material originating from either the melting of the
crust in subduction zones or the partial differentiation
of liquefied mantle material. This second type forms
'andesitic' volcanoes, the largest group of which rings
