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138 CHAPTER 9

is no longer moving, its interior may still be molten.
Then what matters is the strength of the cooled
crust, largely a function of the crust thickness given
by eqn 9.1. If the age of a flow and the total thick-
ness of the flow unit are known we can usually
make a reliable estimate of the advisability of walk-
ing on it; however, this estimate may be influenced
by circumstances if one finds oneself unexpectedly
surrounded by flows in the middle of a developing
compound flow field!
If one is walking on recent lava flows, another
practical issue is the texture and roughness of the
surface. Lavas of basaltic and similar composition
can develop two main textures, usually described
by the Hawaiian words 'a'a and pahoehoe. The Fig. 9.16 The classic ropy texture of a pahoehoe lava flow
surface. (Photograph by Lionel Wilson.)
pahoehoe texture consists of a series of folds in an
otherwise smooth surface as shown in Fig. 9.16.
The folded surface may itself become folded, lead- to shreds the soles of the strongest boots after just a
ing to a very complex texture, but apart from the few minutes’ walking. Worse still, the blocks are
fact that the very outermost skin of the lava tends not well packed on the surface of the flow and tend
to shatter, on cooling, into tiny glassy platelets, any to move or rotate under one’s feet, so that a broken
normal footware will be enough to protect one’s leg or ankle is a real possibility.
feet on a pahoehoe lava flow. On flows of lava of intermediate composition,
In stark contrast to this, exactly the same lava more viscous than basalt, the distinction between
composition may form an 'a'a surface (Fig. 9.17), pahoehoe and 'a'a texture becomes lost and instead
consisting of irregular blocks a few to several tens the flow surface consists of loose, irregular blocks,
of centimeters in size, with the individual blocks larger than those on 'a'a flows and lacking the
having extremely rough, sharp facets that can tear extreme roughness. These are called block lavas.

Fig. 9.17 The rough, clinkery
texture of an 'a'a lava flow surface.
(Photograph by Pete Mouginis-Mark,
University of Hawai’I.)