Geoscience Reference
In-Depth Information
Chapter 9
A laminated lumpy mantle
What can be more foolish than to
think that all this rare fabric of heaven
and earth could come by chance,
when all the skill of art is not able to
make an oyster!
is done. Now an oyster has hardly any
more reasoning power than a man
has, so it is probable this one jumped
to the conclusion that the nineteen
million years was a preparation for
him. That would be just like an oyster.
Mark Twain
Anatole France
Even so, it was 99,970,000 years
getting ready . . . because God first
had to make the oyster. You can't
make an oyster out of nothing, nor
you can't do it in a day. You've got
to start with a vast variety of
invertebrates, belemnites, trilobites,
jebusites, amalekites, and that sort of
fry, and put them into soak in a
primary sea and observe and wait
what will happen. Some of them will
turn out a disappointment; the
belemnites and the amalekites and
such...butallisnotlost, for the
amalekites will develop gradually into
encrinites and stalactites and
blatherskites, and one thing and
another...andatlast the first grand
stage in the preparation of the world
for man stands completed; the oyster
Gravitation structures the earth in
concentric shells, or geospheres,
according to their specific gravity.
Gal-or
Standard geochemical and geodynamic models
of the mantle involve one or two layers. Global
tomographic models tend to be fairly simple;
they show continental roots, slabs, shallow mid-
ocean ridge structures and a few very large fea-
tures in the deep mantle. Regional and high-
resolution seismological models of the mantle
are more complex. High-resolution seismic tech-
niques involving reflected and converted phases
show about 10 discontinuities in the mantle, not
all of which are easily explained by solid--solid
phase changes. They also show some deep low-
velocity zones, which may be eclogite layers. The
upper mantle is highly attenuating, anisotropic
and scatters short period seismic energy.
The opposite extreme of a well-stirred
homogenous mantle is a mantle that is strat-
ified by intrinsic density. Convection can be
expected to homogenize the mantle
if
the various
