Page 49 - Fundamentals of Physical Volcanology

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26 CHAPTER 2

Table 2.1 Summary of the main types
Magma type Eruption temperature Viscosity Water content
of magma and the trends of their
(°C) (Pa s) (%)
physical properties. The viscosity
tends to increase as the water
Basalt 1050–1200 30–300 0.25–2
content increases.
Basaltic andesite 950–1200 100–1000 0.5–2
Andesite 950–1100 300–1000 1–4
4
Dacite 800–1100 10 –10 6 2–4
5
Rhyolite 700–900 10 –10 10 3–6
can be characterized quite well in terms of the eruptions. Xenoliths brought up in kimberlites are
magma silica contents (Table 2.1), although the derived from depths of at least 100–200 km. Direct
amounts of other chemical compounds in the min- samples of this sort show that the mantle consists
erals forming the rock are important too. The key dominantly of peridotite, a crystalline rock com-
issue is that the differing chemical compositions of posed of up to 50% olivine, ∼40% ortho- and clino-
the magmas lead to them having very different pyroxene and ∼10% garnet, spinel or plagioclase,
abilities to contain dissolved volatile compounds the exact mixture depending on the pressure.
such as water and carbon dioxide, and very differ- Next we need to consider how the mantle
ent abilities to flow under a given set of stress deforms when it is stressed, i.e., when a force is
conditions, i.e., they have very different viscosities. applied to it: this introduces the concept of
rheology, the way materials change shape when
stressed. On very long time scales (tens of millions
2.4 Melting and melt segregation in of years) the rocks forming the mantle can deform
the mantle very slowly in a plastic manner, typically at rates
of centimeters per year, and can be considered to
We have seen that basaltic magmas generated by behave as a liquid. The slow deformation rate in
melting of upwelling parts of the mantle are by far response to the applied forces means that this
the most common magma type on Earth, and it is their liquid has a very high viscosity. On the very short
production that ultimately leads, however indirectly, time scales it takes a seismic wave to pass through
to the diverse range of magmas found in the various the mantle (tens of minutes), both the compres-

tectonic settings on Earth. To understand a volcanic sional (p) and shear (s) waves are transmitted.
system, then, we have to start by understanding When seismic waves encounter a normal liquid
how basaltic melts form in the mantle and how they such as water, however, only the p waves are trans-
segregate from the region in which they form. mitted, because the shearing force of the s waves
causes the liquid to flow; the viscous resistance
to the shearing transforms kinetic energy to heat
2.4.1 Nature of the mantle
and the s waves are rapidly damped out. So, on the
The typical composition of the mantle is known short time scale of passage of a seismic wave, the
from various lines of evidence. Most significantly, mantle behaves as a solid. This is called an elastic
certain geological processes bring samples of man- solid because, although it deforms under stress, the
tle to the surface. Ophiolite complexes are parts shape returns to normal after the stress is removed
of the oceanic lithosphere which, as a result of (interestingly, what we call “elastic bands” are very
tectonic processes, have been uplifted, rotated, inelastic – they do not go back to exactly the origi-
and incorporated into continental crust, and where nal length after a stretching force is removed). It is
subsequent erosion sometimes exposes them at the the interaction between these properties of the
surface. Such complexes contain sections of rock mantle, apparently liquid on long time scales and
from the upper mantle. Other samples of the man- apparently solid on short ones, that controls the
tle are brought up as xenoliths during volcanic melting process within it.