Page 165 - Fundamentals of Physical Volcanology

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142 CHAPTER 9

Fig. 9.22 A perched lava pond
forming near the prehistoric cinder
cone Pu‘u Kamoamoa on Kilauea
volcano, Hawai’I, in 1983. The pond
forms as lava moves onto nearly flat
ground so that the forward speed
becomes very slow and the flow
starts to spread nearly uniformly in all
directions. (Photograph by J.D. Griggs,
courtesy U.S. Geological Survey,
Hawaiian Volcano Observatory.)

causing lava to surge over an existing levée. Alter- form thick, slow-moving lava flows or to build
nately, the inner part of a levée may break loose up around the vent as domes.
and move down-channel for some distance before • The fact that the outermost surface of any lava
blocking the channel and causing an overflow. flow is so cool that it behaves as a nonNewtonian
In a long-lived eruption, all of these various ways fluid with strain-rate-dependent properties leads
of making new flow units may operate, to create to a range of characteristic textures of lava flow
a compound flow field. surface, from smooth, folded pahoehoe surfaces
• Because channelized lava flows form levées that to extremely rough 'a'a rubble, and to block lava.
prevent indefinite lateral spreading, they behave
in many ways like nonNewtonian fluids, especi-
ally Bingham plastics that have a yield strength as
9.11 Further reading
well as a viscosity. Some cool, viscous lavas, espe-

cially those containing large numbers of crystals
or gas bubbles, may be truly nonNewtonian Calvari, S. & Pinkerton, H. (1999) Lava tube mor-
liquids. For other, more fluid, basaltic lavas it phology on Etna and evidence for lava flow
emplacement mechanisms. J. Volcanol. Geotherm.
is better to model the levées as one liquid, a
Res. 90, 263–80.
Bingham plastic, and the central channel lava as
Fink, J.H. & Anderson, S.W. (2000) Lava domes
a separate, more nearly Newtonian liquid.
and coulees. In Encyclopedia of Volcanoes (Ed.
• Although there are many potential complica-
H. Sigurdsson), pp. 307–19. Academic Press, San
tions, expecially for tube-fed lava flows, it appears
Diego, CA.
that cooling-limited flows stop moving when the
Griffiths, R.W. & Fink, J.H. (1993) Effect of surface
value of a particular dimensionless number, the
cooling on the spreading of lava flows and domes.
Grätz number, decreases from an initially large
J. Fluid Mech. 252, 667–702.
value near the vent to a critical value of about
Hon, K., Kauahikaua, J., Denlinger, R. & Mackay, K.
320. This condition corresponds to waves of (1994) Emplacement and inflation of pahoehoe
cooling having penetrated to the center of the sheet flows: observations and measurements of
flow from the upper and lower boundaries. The active lava flows on Kilauea Volcano, Hawai’I.
same Grätz number limitation on flow advance Geol. Soc. Amer. Bull. 106, 351–70.
applies to flows of evolved composition such as Hulme, G. (1974) The interpretation of lava flow
dacites and rhyolites, despite their tendency to morphology. J. Roy. Astron. Soc. 39, 361–83.