Page 158 - Fundamentals of Physical Volcanology

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LAVA FLOWS 135

Fractured, no strength
Brittle but unfractured
Viscoelastic, non-Newtonian

Uncooled Newtonian core
Fig. 9.15 Illustration of the changing
properties of the lava in a lava flow
lobe as a result of the variation of
temperature with distance from
the flow’s cooling boundaries.

formed. It is possible that the single Bingham rheol- fluid it will flow if given the slightest chance to
ogy model is an adequate approximation for these do so. Under any given set of stresses, it may be
features. the brittle-elastic layer, the viscoelastic layer or the
However, the realization that the majority of lava Newtonian layer that controls the apparent rheol-
flows contain a very hot central core, at least until ogy of the flow as a whole. Working out the distri-
the time when they stop flowing, implies that there bution of stresses inside a lava flow unit to decide
is no good reason to expect the lava deep in the how, and if, it will move is a problem that has not
channel, which is responsible for the advance of yet been completely solved.
the flow, to have the same properties as the cool
lava forming the levées. The cross-sectional profile
of a channelized flow is more likely to be as shown 9.6 Lava flow motion
in Fig. 9.14b, with the channel containing a layer
of lava chilled against the original ground surface, It is reasonable to assume that, at least in basaltic
above which a raft of cooled lava, distorted and lava flows, the hot lava in the central channel has
sheared at its edges where it experiences friction a negligible yield strength. In that case we can
with the stationary levées, is carried along on top assume that the lava in the central channel can
of a Newtonian fluid core. be described by just one parameter, its constant
The fact that there are very great differences Newtonian viscosity, η. It can then be shown that
between the physical properties of the different if the lava in the channel has depth d , which we

c
parts of a lava flow is central to understanding flow assume (as is generally true) to be much less than
shape and movement. Figure 9.15 illustrates the the width of the channel, its average flow speed U
key facts. The material at the front and sides of a is given by
flow consists, in general, of at least three layers. The
2
very cool outer layer contains many cracks that U = (ρ gd sin α)/(3 η) (9.6)
c
have formed due to the stresses of cooling, and the
presence of the cracks means that this layer has no as long as the motion of the lava is smooth and
strength at all. Inside this is a layer where the lava is laminar. If the flow speed is large enough, the
below its solidus temperature, but has not devel- motion of the liquid in the channel may become
oped a network of interconnecting cracks, so that disordered and turbulent, in which case the speed
it has some brittle strength. Further still from the is given by
surface is a layer that has a temperature above the
solidus. This material is viscoelastic and will deform U = [(2 gd sin α)/f ] 1/2 (9.7)
c
in a plastic fashion if stressed slowly but, like the
brittle layer, will develop cracks if it is stressed where f is a friction factor equal to about 0.01. It
suddenly. Finally the lava furthest from the surface should be stressed that most of the types of lava
is well above its solidus temperature and has no flow yet observed in eruptions on Earth have been
significant strength, so that like any Newtonian laminar, although carbonatite flows are commonly