Page 164 - Fundamentals of Physical Volcanology

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LAVA FLOWS 141

Fig. 9.21 Lava cascading into the
Alae pit crater during the eruption of
Mauna Ulu, Hawai’I, in 1969, showing
how the widths of lava flows do not
change greatly as they move over
ground of variable slope. (Photograph
by J. Judd, Hawaiian Volcano
Observatory, U.S. Geological Survey,
courtesy of the National Oceanic and
Atmospheric Administration, National
Geophysical Data Center.)

rate at which the lava in the middle cools. Never- rootless flows, and when very short rootless flows
theless, the critical Grätz number criterion (eqn form from very viscous magma they are called
9.9) is still found to control the maximum growth rheomorphic flows.
of these features. • Many flows organize themselves into a central
At the other extreme we need to consider what channel, within which lava is moving, and a sta-
happens to a fluid basaltic lava flow when the slope tionary bank or levée on each side of the flow,
of the ground on which it is traveling changes. It which forms when cool material from the flow
is found that if the slope gets very much steeper front is pushed aside as the flow advances. Other
(e.g., Fig. 9.21) the width of the flow does not flows, called sheet flows, have minimal levées,
change much, and the combination of eqns 9.6 and are wider, and advance over a broad front.
9.8 shows that the depth of the flow decreases (by • Some lava flow units stop advancing because the
the cube root of the slope increase) and the speed supply of magma from the vent stops. These are

increases by the same factor. Conversely, if the volume-limited flows. Other flows stop because
slope gets much shallower it is found that the flow the lava near the front has become too cool to
spreads sideways a great deal as well as advancing deform even though the vent is still delivering
at the front of the flow. Both the depth and the magma. These are cooling-limited flows. If the
speed of the lava decrease the further it spreads, supply of magma continues, either a new flow
and the cooling criterion soon causes the advance unit forms at the vent, or a breakout occurs from
to cease. Continued arrival of lava from the steeper an existing flow, in which part of the levée col-
part of the flow upslope results in a series of “waves” lapses and magma emerges to form a new flow
of lava spreading out over the same ground, forming unit. In this case the magma flowing within
a characteristic perched lava pond (Fig. 9.22). the original flow unit is thermally insulated by
the cooled upper surface and the earlier flow
becomes a lava tube. A third possibility is that
9.10 Summary all of the levées of an existing flow unit form
fractures into which small amounts of lava move,
• Lava flows can form either directly, by overflow thus allowing the upper surface to rise so that
of liquid magma from a vent, or indirectly, from the whole flow inflates.
the accumulation of clots of fragmented magma • New lava flow units form in other ways. The sup-
falling from a fire fountain. The latter are called ply rate from the vent may suddenly increase,