Geology Reference
In-Depth Information
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Solid, yielding mantle
The mantle is solid and deforming, not molten. How do we know?
Viscosity. Estimating the viscosity of the mantle. Temperature depen-
dence of deformation of solids. Inevitable convection.
Prior to the twentieth century, geologists had deduced that the interior of the
Earth had to be yielding, in order to accommodate the uplift of mountains. Some
geologists assumed the interior was molten, but others acknowledged that a yielding
solid would be sufficient. The clearest expression of the latter viewpoint is in the
famous 1855 paper by Airy [35] in which he proposed an explanation for mountains
being approximately in isostatic balance, namely that mountains have thickened
crust beneath them. However, Airy also cited a key observation to support the idea
of a deformable, solid interior [35]:
This fluidity may be very imperfect; it may be mere viscidity;
it may even be little more
than that degree of yielding which (as is well known to miners) shows itself by changes in
the floors of subterraneous chambers at a great depth
[emphasis added] when their width
exceeds 20 or 30 feet [7 or 10 m]; and this degree of yielding may be sufficient for my
present explanation.
There had been debate about whether the interior is presently fluid or whether it
had only been fluid when the Earth was forming. It was Hall [36] in 1859 who
established that continuing deformation was required, through his observations
that sediments throughout thick sedimentary formations had all been deposited in
shallow water. This required continuous subsidence as the sediments accumulated.
The development of seismology late in the nineteenth century led to the discovery
that the mantle transmits two kinds of waves that were identified as compressional
and shear. Liquids have no shear strength, and so cannot transmit shear waves.
The mantle must be solid to transmit both compressional and shear waves. The
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