Page 240 - Fundamentals of Physical Volcanology

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ANSWERS TO QUESTIONS 217

through a lava fountain. In doing so the clots must 5 First, in general, the larger the volume of a magma
have lost some heat by radiation, so the resulting reservoir, the larger the maximum volume of magma
flow will be cooler and more viscous, will flow more that can be erupted from it; but a major caveat is
slowly, and will be thicker. However, if it is fed at the that if the rocks above the reservoir fail in an inelas-
same volume flux it will travel a similar distance. tic way, so that caldera collapse can occur, then a
2 Although lava flowing in a tube is heavily insu- much larger fraction of the reservoir contents can
lated against heat loss, it still loses some heat. So be erupted than when the rocks behave elastically.
the tube-fed flow units will probably not advance to Second, the larger the reservoir, the longer it is
the same distance that they would if they were fed likely to take after an earlier eruption before the
directly from the vent. Thus the flow field involving conditions needed to trigger a later eruption are
the tube-fed flows will probably have a slightly reached. Thus there tends to be a direct relation-
smaller area (although of course some of its flow ship between repose time and erupted volume.
units will be thicker to compensate).
3 Bubbles, like crystals, give a lava an inherent
CHAPTER 11
yield strength. They also make it less dense.
Equations 9.3 and 9.4 show that both factors mean 1 It would be safer to live 2 km away from a
that the vesicular lava will form much deeper and basaltic volcano than a similar distance away from
wider levées. If the lava fills the central channel to a rhyolitic volcano. This distance is greater than
the top of the levées, eqns 9.6 and 9.7 show that it the range of most fire fountain deposits and is far
will flow faster (the greater depth squared will enough away that you should have an adequate
more than outweigh the lower density in eqn 9.6 warning of any approaching lava flow. However,
and the density does not matter in eqn 9.7). If 2 km is well within the range of even a small pyro-
the lava is deeper and is flowing faster, then clastic density current, and is close enough that a
eqn 9.8 shows that for the same mass flux the width thick and quite coarse air fall deposit could accu-
of the channel will be smaller in the case of the mulate from even a modest eruption column.
vesicular lava. 2 These types of eruption produce large volumes
of relatively fine-grained materials (whether as fall
or flow deposits) which can be mobilized by rain or
CHAPTER 10
snow to cause mudflows from months to years after
1 The high pressure of the overlying water reduces an eruption.

gas exsolution, and the presence of exsolved gas is 3 The main problem is that to monitor the whole
the single most important factor driving explosive planet the satellites have to be in near-polar orbits,
activity. but the ∼90-minute orbital period means that a
2 Although there are many factors affecting the given satellite cannot overfly the same point on the
details, the fact that evolved magmas in general Earth’s surface until about 16 days after its earlier
contain more dissolved volatiles than basaltic mag- visit. Near-continuous monitoring of one location
mas is the basic reason. would require very large numbers of satellites.
3 The main issue is whether gas can become con-
centrated into parts of the magma from which it has
CHAPTER 12
exsolved. An example is when coalescence of gas
bubbles takes place in a low-viscosity magma rising 1 The high eruption cloud carries small particles
slowly enough for large bubbles to overtake smal- to great heights, and these take a long time to fall to
ler ones and grow very large by sweeping up many the ground, so their opportunity to have physical
small bubbles. and chemical effects is maximized. Co-ignimbrite
4 More than one scheme has evolved because clouds from pyroclastic flows rarely reach such
sometimes it is the rate at which magma is erupted great heights as Plinian eruption clouds.
that matters most and in other cases it is the volume 2 High eruption clouds can deposit water vapor,
of material. sulfur dioxide and small silicate particles at great