Page 155 - Fundamentals of Physical Volcanology

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132 CHAPTER 9

are small, and so mostly they behave in the same
way as crystals in acting as obstacles to the liquid
flow. As long as the volume fraction of the lava that
consists of these crystals and small bubbles is less
than about 20%, they have little effect other than
increasing the viscosity of the lava. The bulk be-
havior of the lava is still the same as that of a simple
fluid such as water, in which the rate of deforma-
tion, the strain rate, is directly proportional to the
applied stress. Fluids with this property are called
Newtonian fluids, and an example is shown as the
line labeled N in Fig. 9.12. The ratio of the stress to
the strain rate is called the Newtonian viscosity of
the fluid. However, when the volume fraction occu-
pied by the bubbles and crystals becomes greater
than about 30%, other effects appear. The lava now
begins to acquire a threshold resistance to being
Fig. 9.11 Two scientists from the Hawaiian Volcano
sheared. This means that the stress applied to it
Observatory prepare to take a sample of lava from a lava
tube seen through the skylight shown in the lower center of must exceed a certain level called the yield
the image. The skylight formed by the collapse of the tube strength before the lava will even begin to deform
roof. Note the extensive compound flow field around the and flow. Once this initial resistance to deformation
skylight. Image taken on the south flank coastal plain of has been overcome, the way in which the bulk of
Kilauea volcano, Hawai’I during the early 1990s.
the liquid responds may take one of a number of
(Photograph by Elisabeth Parfitt.)
forms. The simplest of these is a response in which
the strain rate is directly proportional to the applied
stress in excess of the yield strength. This behavior
9.4 Lava flow rheology is shown as the line labeled B in Fig. 9.12 and fluids
with this property are called Bingham plastics. The
The term rheology was introduced in Chapter 2 in slope of the line labeled B in Fig. 9.12 is the plastic
connection with the way the rocks in the mantle viscosity of the fluid, and is a constant that, together

flow when stressed. We need to consider a similar with the yield strength, completely characterizes
set of issues in connection with lava flows, but the the properties of the fluid. Also shown on Fig. 9.12
situation is more complex because in the course are the curves labeled T and D: these show the flow
of its emplacement the lava in a flow unit changes, properties of a thixotropic and a dilatant fluid,
from being almost completely molten when it is respectively. These fluids clearly have a more com-
erupted, to being partly solid and partly molten plex response to stresses trying to make them flow.
when its front comes to rest, to being completely An important property of all of the nonNew-
solid some time later after its core has cooled below tonian (i.e., the Bingham, thixotropic and dilatant)
its solidus temperature. fluids that have a yield strength is illustrated by the
Most lava flows contain significant numbers of lines labeled F1 and F2 in Fig. 9.12. These represent
gas bubbles, and as the lava cools from the liquidus the same fluid moving under two different sets of
to the solidus increasing numbers of mineral crys- conditions. Line F1 represents the flow moving
tals nucleate and grow within it. The presence of down a shallow slope; the stress acting on it is the
both of these components alters the way the purely component of its weight acting along the slope, and
liquid bulk of the material is able to shear and because the slope is small this stress is also small.
deform when stresses are applied to it. Surface ten- In fact this stress is only just greater than the yield
sion forces act to keep the gas bubbles as nearly as strength of the fluid, and so it moves only slowly,
possible spherical in shape, especially when they indicated by the small strain rate (effectively the