Page 41 - Fundamentals of Physical Volcanology

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2 Magma generation and

segregation

2.1 Introduction T (°C)
0 1000 2000 3000
0 0
We saw in Chapter 1 that a volcanic eruption can be
viewed as the culmination of a series of physical Geotherm
B
and chemical processes (Fig. 1.21). The initial stage 100
P (GPa) 50 Depth (km)
in the sequence of events which ultimately cause a
volcanic eruption is always the generation of magma 200
deep within the planet (Stage 1 in Fig. 1.21) – without
A Liquidus
magma there can be no eruption! In this chapter we Solidus
300
will look at what we know about magma genera- 100
tion and then go on to consider the very earliest
Fig. 2.1 The temperatures and pressures (and hence depths
stages of magma movement in which the magma
below the surface) at which mantle rocks begin to melt (the
first starts to segregate and move away from the
curve labeled solidus) and at which they are completely
region in which it formed.
melted (the curve labeled liquidus). At any point in the
shaded zone a rock is a mixture of unmelted minerals and
liquid. The typical variation of temperature with depth in
brings a body of rock up from a depth corresponding to A
2.2 Rock-melting mechanisms the Earth is the geotherm. If convection in the mantle
to a depth corresponding to B, it cools slightly, but even so
All rocks contain a mixture of different minerals
reaches a state in which it starts to melt. If the same body of
and, as a result, they melt over a range of tempera-
rock continues to rise to shallower depths, the melt fraction
tures rather than at one specific temperature. The
increases.
temperature at which melting first starts in a rock is
called the solidus temperature; the temperature at
rock does not change much, it is nevertheless at a
which the last bit of solid vanishes and all of the
much higher temperature relative to its solidus.
rock is liquid is called the liquidus temperature.
This kind of melting is known as decompression
There are three main processes by which melting
melting or pressure-release melting.
of rock can occur within the Earth.
3 By changing the composition of the rock, usually
1 By heating the rock and raising its temperature by the addition of water. The “dry” melting tem-
above the solidus temperature. perature of a rock is considerably higher than its
2 By reducing the confining pressure on the rock “wet” melting temperature, i.e., the temperature at
while keeping its temperature nearly constant. which it will melt when abundant water is present
In most cases reducing the pressure acting on a (Fig. 2.2). As a result, the addition of water to an
rock reduces the solidus and liquidus temperatures initially dry rock can induce melting if the initial
(Fig. 2.1). So although the actual temperature of the temperature of the rock is sufficiently high (Fig. 2.2).