Page 237 - Fundamentals of Physical Volcanology

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214 ANSWERS TO QUESTIONS

can no longer deform fast enough, it starts to volcanoes in particular grow laterally by sending
behave as a brittle material, and a fracture forms in out dikes radially into rift zones. We would there-
it. The low-viscosity fluid flows into the fracture to fore look for the now-frozen remains of these fea-
form a dike. tures: laccoliths or very large sills as evidence of the
3 Diapir movement is controlled by the slow defor- main intrusions, and swarms of dikes in the vicinity.
mation of the high-viscosity mantle rocks surround- 3 Basaltic volcanoes at intraplate hot-spots are fed
ing the diapir. Dike tips propagate as brittle more or less continuously on a time scale of years
fractures and so can potentially travel at very high by magma from the mantle, and so any given batch
speeds; the actual speed is controlled by the ability of magma is not likely to stay in the magma res-
of the low-viscosity magma within the dike to flow. ervoir beneath the summit for long enough to
Mantle rock viscosities are ∼10 20 times larger than undergo a great deal of chemical evolution. In con-
molten basalt viscosities, hence the enormous trast, basaltic volcanoes at mid-ocean ridges receive
difference in speed. major inputs of magma much less frequently, per-
4 If the magma is rising mainly as a result of buoy- haps at intervals of several decades, and so have
ancy, it can become trapped when it is no longer time to develop more evolved melts in their storage
less dense than the rocks surrounding it. The base zones as a result of cooling and fractional crystal-
of the crust, where there is generally a significant lization of stagnant magma.
decrease in rock density from mantle to crust, is a 4 In the order given in Table 4.1 (Pinatubo, Fer-
likely density trap for many magmas in continental nandina, Katmai, Krakatau, Rabaul, Taupo, Santorini,
areas and for high-density magmas in oceanic envi- Toba, Yellowstone and La Garita), the caldera areas
ronments. If magma is rising in dikes the situation is are about 4.9, 24, 8, 50, 118, 960, 55, 1890, 2200,
2
more complex because, not only must the magma and 2100 km , the vertical depths of erupted
have a net positive buoyancy integrated over the magma are 0.92, 0.004, 1.52, 0.2, 0.093, 0.036,
entire vertical length of the dike, but also the stress 0.45, 0.8, 0.68, and 2.42 km, and the percentages
intensity at the propagating dike tip must be large of the total magma content are 18, 0.08, 30, 4, 1.9,
enough to overcome the effective fracture tough- 0.7, 9, 16, 14, and 48. Only Fernandina is marginal;
ness of the host rocks. all of the others are well above 0.1% and would
have been expected to show some amount of
caldera collapse, as was observed.
CHAPTER 4

1 First, a caldera may be present, implying that at
some stage the surface rocks collapsed or subsided CHAPTER 5
into space left by an eruption from the shallow 1 Carbon dioxide is much less soluble than water,
magma reservoir. Second, the composition of the and its solubility decreases faster as the pressure
erupted magma may suggest that it has been cooled decreases, so magmas rising from the mantle start
by a significant amount since it left the mantle. to exsolve mainly carbon dioxide long before they
Third, tilt-meters around the summit of the volcano exsolve much water.
may record patterns of deformation in the form of 2 The main factor is the presence or absence of
inward or outward tilting that imply deflation or crystals to act as nuclei on which vapor molecules
inflation of a body of fluid beneath the surface. can congregate to avoid the limitations of surface
Fourth, seismometers on the volcano recording tension.
very distant earthquakes may detect the surface 3 The three processes are: (i) diffusion of volatile
shadow of a zone beneath the volcano through molecules through the liquid to reach gas bubbles;
which S waves cannot travel, implying the pres- (ii) expansion of bubbles as the pressure decreases
ence of fluid. in response to Boyle’s law; (iii) coalescence, i.e.,
2 We very strongly suspect that many kinds of joining together of bubbles that come into contact.
volcano have some sort of magma storage system at 4 The lower viscosity of basaltic magma allows
fairly shallow depth beneath them, and that basaltic bubbles to rise more quickly through the liquid