Page 163 - Fundamentals of Physical Volcanology

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140 CHAPTER 9

Fig. 9.20 The very rough toothpaste-texture lava surface
forming on the south flank coastal plain of Kilauea volcano,
Hawai’I. (Photograph by Christina Heliker, courtesy U.S.
Geological Survey, Hawaiian Volcano Observatory.)

disturbed by, for example, inflation, looks dull. We
have seen that the surface texture is a function of
the rheology and the flow conditions, and so radar
observations of flows from aircraft or satellites,
especially for flows that are very inaccessible on the
ground, can be a useful tool.
Fig. 9.19 A sequence of pillows formed by undersea
eruptions, now exposed in an uplifted cliff at Acicastello,
Sicily. (Photograph by Lionel Wilson.)
9.9 Effects of ground slope and lava viscosity

than any other type. Equally, because the spikes are The formation of channelized lava flows and com-
distributed over a relatively flat surface, this lava pound lava flow fields is particularly characteristic

type is much easier to walk over than 'a'a provided of basaltic lavas. If we consider the viscosities
one does have stout footware. (Table 2.1) of more evolved lavas such as andesites,
These lava surface textures have a significance dacites, and rhyolites, eqn 9.6 shows that, on a given
that goes beyond the damage that they do to volca- slope, these lavas must be “very” much thicker if
nologists and their boots. Surfaces with different they are to move with a given speed. When account
textures scatter sunlight and radar waves in dis- is taken of the fact that the speed and thickness are
tinctive ways. The rougher the surface the more also related via the erupted mass flux (eqn 9.8) it is
uniformly the radiation falling on it is scattered, found that viscous lava flows are thicker, wider,
whereas the smoother the surface the greater the and move much more slowly than basaltic flows on
chance that the reflection will be more like that similar slopes. Indeed, the most viscous lavas, rhyo-
from a mirror. Radar works by detecting energy lites, have so much difficulty in moving away from
scattered back to a detector mounted on the same the vent that, unless the surface on which they
platform as the transmitter, and so with energy scat- erupt is very steep, they essentially accumulate in
tered in all directions a significant amount returns all directions around the vent to form a steep-sided
to the receiver from an 'a'a surface whereas much dome (Fig. 1.3), and it is more the steep slope of
less is likely to do so from a pahoehoe surface. Thus the dome surface than the slope of the underlying
'a'a lava always looks bright to radar whereas ground that drives the lava to move. This great
pahoehoe, unless the surface of the flow has been thickness of viscous flows and domes reduces the