The Earth's Shifting Crust by Charles Hapgood - wire of information

188
EARTH'S SHIFTING CRUST
would be impossible if the asthenosphere had any consider-
able strength. In his opinion mountain formation requires
the existence of a zone of easy shear.
Thirdly, Daly urges the importance of the general body
of evidence of igneous geology:
. . . The existence of a liquid or approximately liquid astheno-
sphere is strongly suggested by the countless facts of igneous geology.
The hypothesis that the lithosphere is crystalline, a few scores of
kilometers in maximum thickness, and everywhere underlain by a
hot vitreous substratum provides what appears to be the best working
theory of the chemical nature of magmas and their modes of
eruption. ... In general, no petro-genetic theory that does not recognize
a specific world-encircling asthenosphere of this kind has
been found to explain so many facts of the field. . . . (97:399-400).
Among specific evidences that point, in Daly's opinion, to a
really liquid asthenosphere are the plateau basalts, which,
as we have seen, were formed by immense floods of liquid
magma that engulfed hundreds of thousands of square miles
of the surface at one time.
Jeffreys opposes Daly's view of the asthenosphere, and argues
for continuation of considerable strength to a depth of
several hundred miles. Daly notes his argument, and an-
swers it.
Jeffreys's argument is based on the fact that an analysis of
earthquake waves shows that some earthquakes originate at
depths up to 420 miles below the surface. Presumably, they
can originate only as the result of fracture in a solid substance,
which necessarily would have some strength. Daly remarks:
For example, Jeffreys deduces, from the reality of deep-focus shocks,
a strength of about 1000 kilograms per square centimeter for the
material reaching down to the 7oo-kilometer level at least. This
is about the strength of good granite in the testing machine.
To reconcile that conclusion with the demonstrated degree of
isostatic equilibrium, Jeffreys suggests that isostasy is a highly ex-
ceptional condition of the earth. He assumes the condition to have
been established during major orogenic disturbances, and preserved
for only a relatively short time after each paroxysm of mountain-
making (97:400-01).