Page 238 - Fundamentals of Physical Volcanology

9780632054435_5_end.qxd 12/10/2007 12:35PM Page 215

ANSWERS TO QUESTIONS 215

than in more evolved magmas so the large bubbles energy is traded for potential energy and the mixture
can overtake smaller ones more easily. slows down as it approaches its maximum height.
5 The main difference is the extent to which the 6 The main control is the rate at which heat is
gas bubbles derived from the volatiles in a given added to the atmosphere: the plume height is pro-
volume of magma stay uniformly distributed within portional to the fourth root of the heat release
that volume as it rises. Intermittent activity occurs rate. Heat release rate is proportional to mass erup-
at the surface vent when large numbers of small tion rate and magma temperature. There is also an
bubbles coalesce into giant bubbles or slugs filling influence of atmospheric temperature profile and
almost all of the width of the dike or conduit. humidity (hence latitude and season).
7 Large pyroclasts have a large terminal fall veloc-
ity through the gases in an eruption cloud and so
CHAPTER 6
can never be carried to great heights. Small clasts
1 First, the continuity eqn 6.1 shows that any can easily be carried up to great heights, but also
increase in gas volume, and hence decrease in the some of them get swept to the edge of the eruption
bulk density, of the magma in a dike of constant cloud, where the rise speed is small at all heights,
shape can be compensated for only by an increase and fall out from there.
in speed. Second, the energy eqn 6.4 shows that 8 Two factors encourage this evolution: erosion
any decompression of the magma, especially of the of the conduit and vent, resulting in a larger mass
gas, provides energy that contributes to increasing eruption rate, causes less air entrainment to occur,
the speed. providing less buoyancy to the eruption column.
2 The main effect is to reduce friction with the Also late-erupted magma may be poorer in volatiles
dike walls. Before fragmentation the fluid in con- than magma erupted at the start of the eruption,
tact with the wall is mainly liquid magma; after and thus will have a smaller eruption speed, also
fragmentation the fluid in contact with the wall is reducing air entrainment.
mainly gas which has a vastly smaller viscosity.
3 First, because gas expansion is the main driving
CHAPTER 7
force, the larger the proportion of gas in a magma
the higher its eruption speed. Second, a high gas 1 The first way involves only the gases released
content causes bubble nucleation to occur earlier, from the magma and occurs when the gas becomes
and hence deeper in the dike, ensuring that the nonuniformly distributed in the magma as a result

gas experiences a larger pressure change which of small gas bubbles joining together to form large
releases more energy. Third, earlier bubble nuc- bubbles which burst intermittently at the surface
leation means earlier fragmentation of the magma of the magma column. The second way involves the
and hence more of the magma rise takes place magma coming into contact with and boiling exter-
under low wall-friction conditions. nal fluids – essentially always water on the Earth.
4 Many explosively erupting magmas accelerate in The water may infiltrate a column of magma that
the dike to the point where the rise speed is equal has stalled in the conduit or the magma may flow
to the speed of sound in the gas–pyroclast mixture. over wet ground or into shallow water.
Unless there is some part of the dike where the 2 The speed at which pyroclasts are ejected depends
walls slope outward toward the surface to make a on the pressure reached in the trapped gas that is
de Lavalle nozzle the mixture cannot go any faster driving the explosion before the “lid” trapping it
and so it cannot decompress down to atmospheric breaks and also on the relative amounts of gas
pressure until it gets out of the vent. and solid material. The greater the pressure, and the
5 At first the magmatic material shares its momen- greater the proportion of gas, the greater the speed.
tum with the air that it entrains and so slows down. 3 The distances traveled by pyroclasts thrown out
But heat from the magma warms the air and pro- from an explosion at a given speed depend on the
duces buoyancy that drives the mixture upward sizes of the drag forces acting on them. Water has
and increases its speed. Eventually all the thermal both a larger density and a larger viscosity than air,