Page 124 - Fundamentals of Physical Volcanology

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TRANSIENT VOLCANIC ERUPTIONS 101

just discussed (although sustained hydromagmatic to the water (gases are bad conductors of heat).
eruptions can also occur). The wide variety of styles Outside the thin steam film the water does not boil,
of hydromagmatic eruptions reflects the complex- and the film is quite stable, only being broken over
ity involved in the way magma and water can mix a small area for a brief moment if a bubble of mag-
and the diversity of settings in which interactions matic gas escapes from inside the pillow.
occur. Vulcanian explosions fueled by external water When the volume of water available is small
are thought to result from interactions between compared with the magma volume, the water is
magma and groundwater on land, but interactions thoroughly heated by the magma and is all con-
can also occur in deep and shallow submarine set- verted to highly compressed steam. If this subse-
tings, for example where lava enters the sea, and quently expands, the large volume increase (by a
during eruptions through lakes or beneath glaciers, factor of nearly 3000) causes an explosion, but the
etc. (see Chapter 1). small volume of water available restricts the size
of the resulting event. For example, when lava
flows over wet ground it sometimes happens that
7.3.1 Types of hydromagmatic eruption
pockets of water trapped as the front of the flow
The type of interaction which occurs in a hydro- advances over them boil faster than the steam can
magmatic eruption depends upon the relative escape along the contact between the flow and the
volumes of water and magma involved and on the ground. The steam bursts through the flow, throw-
extent to which the two can mix. When water is ing out clots of lava and creating a crater, but this
abundant, for example in a submarine setting, the crater is never much wider than the flow is thick.
interaction between magma and water may not A crater formed in a lava flow in this way is one
even be explosive. When lava erupts under water it example of a rootless vent, this term being used
commonly forms flow lobes known as pillows. The because, although lava is thrown out from this loca-
buoyancy force of the water supports some of the tion, it is not a place where fresh lava is coming
weight of the lava and so pillows are thicker and directly out of the ground. Another example of water
more rounded in cross-section than subaerial flow being trapped and unable to escape fast enough
lobes (Fig. 7.3). The surface of the growing pillow occurs when lava flows which have formed tubes
is covered by a layer of steam about 1 mm thick reach the sea. Waves break over the front of the
which acts to control the heat transfer from the lava flow and water is driven into the tube. The steam
formed as the water boils is trapped by the inertia
of the inrushing seawater and again a relatively
small explosion can occur, blowing the top off the
tube.
Between these two extremes, it sometimes hap-
pens that more nearly equal volumes of magma and
water will mix together rapidly with all of the water
being converted to steam. The magma cools as the
water is heated and the final temperature of the
mixture is at least one-third of the initial magma
temperature, say 500 K. The density of water is
−3
very close to 1000 kg m , but the density of steam
−3
at 500 K is 0.43 kg m . Thus the conversion of
water to steam in this way causes a more than 2000-
Fig. 7.3 Lobes of pillow lava erupted on the ocean floor.
fold increase in volume, and this expansion is what
Displacement of the surrounding water effectively reduces
drives a hydromagmatic explosion. This kind of
the weight of the lava and makes pillows less flattened
interaction gives rise to major hydromagmatic
than pahoehoe lobes erupted in air. (Image courtesy of
the Monterey Bay Aquarium Research Institute. (c) 2001 explosions that produce craters called maars. A
MBARI.) well-documented example of this occurred at