Page 239 - Fundamentals of Physical Volcanology

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216 ANSWERS TO QUESTIONS

and so ejected clasts will travel to much smaller dis- clasts that were released from lower in the eruption
tances under water. Also, if the explosion happens cloud but blew further. These small clasts must
under deep water, the weight of the water will now cover a larger area than if they had not been
reduce the pressure differences between the blown so far (or moved at all) and so there will be
compressed trapped gases and the surroundings, fewer of them per unit area on the ground. Thus, in
thus reducing the violence of the explosion. the downwind direction at least, the deposit will be
4 The steady case just needs us to insert M = 2 × relatively rich in larger clasts near the vent and rela-
f
5
−1
10 kg s into eqn 6.7 to find H = 4.99 km. For the tively rich in small clasts far from the vent.
intermittent case we see that if the average rate is 3 The kinetic energy per unit mass represented by
−1
2
5
2 × 10 kg s −1 and explosions occur every 10 sec- the eruption speed is 0.5 × 100 = 5000 J kg . The
6
onds, the mass per explosion must be 2 × 10 kg. added potential energy per unit mass that the flow-
Inserting this for M in eqn 7.5 we find H = 1.58 ing material gains due to moving vertically by
e
km, a much lower plume. 1000 m under an acceleration due to gravity of
4
−1
5 A “real” vent is the site of volcanic activity and 10ms −2 is 10 × 1000 = 10 Jkg . Adding the two
3
−1
is directly physically connected to the dike feeding energies gives 15 × 10 Jkg . Converting to the cor-
2
3
magma to the surface. A rootless vent is the site of responding velocity U, i.e., 0.5 U = 15 × 10 , gives
−1
activity (e.g., explosive activity when lava flows U = 173ms . This is a maximum possible speed
over wet ground) but is not directly connected to because it neglects any energy lost due to friction
any magma pathway beneath the surface. with the ground while traveling down the slope.
6 The key issue is that the hot magma and cold 4 If the pyroclastic density current travels out over
water interact in the right proportions to maximize the water its bulk density is presumably less than
the conversion of the thermal energy of the magma the density of the water. The hot particles at the
to the kinetic energy of the eruption products. base make contact with the water and boil some
of it to steam that enters the body of the flow. The
clasts at the base of the current that make the best
CHAPTER 8
contact with the water get cooled, and some liquid
1 If you measured the product of the size and den- water may get sucked into the vesicle spaces within
sity of the largest clast and plotted this on Fig. 8.6, them as the gas in the vesicles chills and contracts,
you could infer a mass flux and hence an eruption so these clasts may get waterlogged and may sink
cloud height. However, you would not know if the into the water. Meanwhile, the throughput of

exposure site was downwind from the vent or in added hot steam makes the clast concentration in
some other direction. So you might have plotted your the body of the flow smaller, so large clasts are less
data point at too great a distance from the vent, and well supported by grain–grain contacts and begin
this would have caused you to overestimate the to migrate to the bottom of the flow, preferentially
eruption rate. So what you get from your single coming into contact with the water. Also as the
exposure is a “maximum” estimate of the eruption steam escapes it will carry some of the smallest
rate and the corresponding eruption cloud height. clasts away with it into the overlying phoenix
2 If there is no wind, all the clasts released at a cloud. Thus the body of the flow may lose large
given height in the eruption cloud (meaning all clasts at the base and small clasts at the top and get
clast sizes up to the largest that can be supported at thinner, eventually becoming so thin that all that is
that height) and hence at a given distance from the left is a layer of cool pumice clasts floating on the
vent drift vertically down to the ground to accu- water surface and a phoenix cloud dispersing on
mulate in the same place. If a wind is blowing, the wind.
smaller particles are moved further downwind than
larger clasts because they fall at a lower speed. A
CHAPTER 9
given location on the ground will now accumulate
some relatively large clasts released high in the 1 The rootless flow must form by the accumulation
cloud and not blown very far, and some smaller of clots of magma that have been transported