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CHAPTER 1 They are produced when ocean floor rocks are
heated by contact with the hotter rocks into which
1 All magmas coming from the mantle appear
they are being pulled down and helped to melt
to contain some dissolved volatiles that can be re-
by the presence of water in ocean floor sediments
leased as gases at low pressure. Explosive activity
being carried along with them.
happens when a sufficiently large number of gas
2 First, melts are almost always less dense than
bubbles are formed. Thus truly effusive eruptions
their pre-melting parent rocks and also the unmelted
require that not too much gas is released in the
solid residues remaining after melting occurs. Thus
erupting magma. On the deep ocean floors the
buoyancy – the effect of gravity on the difference
water pressure is so high that gases stay dissolved in
in density – acts to make them rise. Second, the par-
virtually all magmas, suppressing explosive activity.
ent rocks are commonly subjected to nonuniform
If magma is stored at shallow depth it gets the
stresses by processes such as mantle convection
chance for gas bubbles to escape into the surround-
and plate movements, and these tend to drive the
ing rocks before the magma erupts. If magma comes
melts upward as compaction occurs.
directly to the Earth’s surface the key issue is just
how much gas it contains.
2 Common types of activity on human time scales CHAPTER 3
are Strombolian, Hawaiian, Vulcanian, subPlinian, 1 The rheology of rock typically depends on tem-
Plinian, hydromagmatic, deep marine, and subglacial
perature, pressure, and the strain rate – the rate of
eruptions. Very much rarer are flood basalt, ultra-
deformation – imposed on the rock. It is difficult,
Plinian, ignimbrite-forming, and diatreme-forming
but not impossible, to subject rocks in the labor-
eruptions. This is very fortunate, because the less
atory to temperatures like those in the mantle. It is
frequent eruptions are those that are more violent,
significantly more difficult to reproduce the mantle
and likely to spread larger volumes of volcanic
pressures, and it is impossible to carry out experi-
materials over larger areas more quickly.
ments for the lengths of time needed to reproduce
the mantle strain rates.
2 At great depths, where the mantle is partly
CHAPTER 2
molten in the rising parts of convection cells, both
1 Magmas formed at mid-ocean ridges and hot the host rock and the melt forming within it will be
spots are generally some or other variety of basalt. rising. As melt percolates between mineral grains
They are produced by partial melting of mantle rocks and becomes concentrated in the upper part of a
as a result mainly of decompression of rocks in the convection cell, the region of melt concentration
rising parts of mantle convection systems. Subduc- behaves as a diapir, i.e., a body of relatively low-
tion zone magmas are generally more silica-rich viscosity fluid rising through a layer of relatively
than basalts and have a wide range of compositions. high-viscosity fluid. When the high-viscosity fluid