Page 162 - Fundamentals of Physical Volcanology

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LAVA FLOWS 139

The origin of these various lava textures again
involves the rheological properties of the cooling
lava surface. As a flow moves, its surface is con-
stantly being stretched. In general, the outer part
of the flow consists of a brittle outermost layer
beneath which the lava is viscoelastic. If the rate of
deformation is small, as in all of the surface of a
small or slow-moving flow unit, the lava behaves as
a plastic and stretches. The lava near the surface
flows less easily than that beneath it and as a result
it buckles to form the characteristic folds; but
nowhere do major fractures occur in the lava. If the
deformation rate is greater than some critical value,
Fig. 9.18 A series of pahoehoe toes forming a compound
however, the response of the lava changes to that lava field. (Photograph by Pete Mouginis-Mark, University of
of a brittle solid and fractures form, first on a small Hawai’I.)
scale, giving rise to angular facets, and then, as cool-
ing penetrates deeper, on a larger scale, producing see eqn 9.8) of the flow unit will be small. These
loose, irregular blocks. The exact sizes of the blocks, facts combine to ensure that each flow unit does
and the details of their small-scale roughness, not advance very far, and consists entirely of pahoe-
depend in a complex way on the ways in which the hoe lava. These flow units are called pahoehoe
strain rate varies with time and the rheological pro- toes (Fig. 9.18). Exactly the same process can occur
perties change with depth below the surface. in compound lava flow fields on the ocean floor,
This dependence of texture on deformation rate but there the presence of the surrounding water
explains why, for example, both pahoehoe and 'a'a provides buoyancy that offsets some of the weight
textures may be found on the same channelized of the lava, making the little flow units thicker, for
basaltic lava flow. The edges (the levées) of the flow a given width, than they would be on land. The re-
are at rest and the lava in the middle of the central sulting bulbous structures are then called pillows
channel is flowing fastest. The shearing stresses, (Fig. 9.19) and we speak of fields of pillow lava.
and hence the rates of deformation, are very large There is a final lava flow texture that deserves
at the front of the flow where channel material is mention. This is really a variation on 'a'a lava, but it

being diverted into the levees, which explains why occurs when lava emerges from a tube to form a
levees commonly consist of 'a'a, especially on large new flow unit. If the tube is completely full of lava,
flows where the speeds, and hence deformation then there is contact between the roof of the tube
rates, are largest. Near the middles of the central and the lava that emerges to form the top layer of
channels of large flows, and everywhere on small, the flow. This lava, having been in good contact
slow-moving flow units, however, the deformation with the tube roof, has suffered some cooling and
rates are small enough that the pahoehoe texture so is viscoelastic. If conditions are just right, the
persists. upper skin of the lava is pulled up by the frictional
This correlation of texture with deformation rate drag into a series of tilted spikes that form a char-
leads to a common type of compound lava flow acteristic pattern on the flow surface (Fig. 9.20).
field, especially on basaltic volcanoes. When a lava The regularity of this pattern is reminiscent of the
flow field begins to develop on a region with only a grooves that sometimes form on toothpaste as it
shallow slope, the speed of the lava in any one flow emerges from a tube, and gave rise to the name
unit will be small (see eqns 9.6 and 9.7). Also, toothpaste lava; however, there is no other simi-
because the total lava supply will be divided up larity between toothpaste lava and toothpaste, and
through the network of tubes within the growing this type of lava, although only likely to be present
flow field, each flow unit will have only a small mass in small amounts on any given lava flow field, is
flux and so the size (both the width and thickness – more destructive of boots (and skin if you fall on it)