842 Encyclopedia of the Solar System
a given initial velocity. The result is that the largest clasts
could travel up to 50 km. This means that the roughly circu-
lar deposit from a localized, point-source explosion would
be spread over an area 100 times greater than on Earth, be-
ing on average 100 times thinner. Apart from the possibility
that the pattern of small craters produced by the impact
of the largest boulders on the surface might be recognized,
such a deposit, with almost no vertical relief and having very
little influence on the preexisting surface, would almost cer-
tainly go unnoticed in even the latest spacecraft images, and
indeed no such features have yet been identified. However,
if the explosion involves a larger, more complex, and espe-
cially elongate vent structure, there would not be such large
differences. In the Elysium region of Mars, a large, water-
carved channel, Hrad Vallis, has a complex elongate source
depression that appears to have been excavated by a Vulca-
nian explosion when a dike injected a sill into the ice-rich
permafrost of the cryosphere—the outer several kilometers
of the crust, which is so cold that any H 2 O must be present
as ice. As heat from the sill magma melted the ice and boiled
the resulting water in the cryosphere, violent expansion of
the vapor forced intimate mixing of magma and lumps of
cryosphere, encouraging ever more vapor production. Soon
all the cryosphere above the sill was thrown out in what is
called a fuel-coolant explosion (here the fuel is the magma
and the coolant is the ice) to produce a deposit extending
about 35 km on either side of the 150 km long depression.
Residual heat from the magma melted the remaining ice
in the shattered cryosphere rocks so that for a while, un-
til it froze again, there was liquid water present to form a
characteristic “muddy” appearance in the deposit (Fig. 11).
A Vulcanian explosion on Venus would also be very dif-
ferent from its equivalent on Earth. In this case, however,
the high atmospheric pressure would tend to suppress gas
expansion and lead to a low initial velocity for the ejecta,
and the atmospheric drag would also be high. Pyroclasts that
would have reached a range of 5 km on Earth would travel
less than 200 m on Venus. On the one hand, this should con-
centrate the eruption products around the vent and make
the deposit more obvious; however, the resolution of the
best radar images fromMagellanis only∼75 m, and so
such a deposit would represent only three or four adjacent
pixels, which again would probably not be recognized.
On the Moon, a number of Vulcanian explosion products
have been identified. The dark halo craters on the floor of
Alphonsus have ejecta deposits with ranges up to 5 km.
Since the Moon has a much lower atmospheric pressure
than Mars (essentially zero), the preceding analysis sug-
gests at first sight that lunar Vulcanian explosions should
eject material to very great ranges. However, the Alphon-
sus event seems to have involved the intrusion of basaltic
magma into the∼10 m thick layer of fragmental material
forming the regolith in this area, and the strength of the re-
sulting mixture of partly welded regolith and chilled basalt
was quite low. Thus, only a small amount of pressure buildup
occurred before the retaining rock layer fractured. As a
FIGURE 11 Part of the Hrad Vallis depression in the Elysium
Planitia area of Mars. The depression is surrounded by a
“muddy” deposit and is interpreted to have formed when a
volcanic explosion excavated the depression and threw out a
mixture of hot rocks and overlying cryosphere—cold rocks
containing ice. (NASAMars Global Surveyorimage.)result, the initial speeds of the ejected pyroclasts were low,
and their ranges were unusually small.4.4 Hawaiian Activity
In some cases, especially where low-viscosity basaltic
magma travels laterally in dikes at shallow depth, enough
gas bubble coalescence and bubble rise occurs for much of
the gas to be lost into cracks in the rocks above the dike.
Magma then emerges from the vent as a lava flow. However,
when basaltic magmas rise mainly vertically at appreciable
rates (more than about 1 m/s), some gas bubble coales-
cence occurs but little gas is lost, and the magma is released
at the vent in a nearly continuously explosive manner. A
lava fountain, more commonly called a fire fountain, forms
over the vent, consisting of pyroclastic clots and droplets of
liquid entrained in a magmatic gas stream that fluctuates
in its upward velocity on a timescale of a few seconds. The
largest clots of liquid, up to tens of centimeters in size, rise
some way up the fountain and fall back around the vent
to coalesce into a lava pond that overflows to feed a lava
flow—the effusive part of the eruption—whereas smaller
clasts travel to greater heights in the fountain. Some of the
intermediate-sized pyroclasts cool as they fall from the outer
parts of the fountain and collect around the lava pond in
the vent to build up a roughly conical edifice called an ash