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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
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