Solid, yielding mantle - Mantle Convection for Geologists - page 27

apparent contradiction of a material being simultaneously solid and fluid is resolved
by observing that the timescale of seismic waves is very short (tens of seconds)
whereas the timescale of geological processes is very long (millions of years).
Solids that yield over millions of years don't have time to yield in a few seconds.
The term liquid refers to materials in the liquid state, like water and magma. The
more general term fluid can serve to describe the slow yielding of a material in the
solid state.
By early in the twentieth century a picture had emerged of a strong surface layer
about 150 km thick that does not normally deform even on geological timescales,
underlain by a more yielding interior. The strong layer had come to be called the
lithosphere , and Barrell [8] in 1914 proposed the term asthenosphere for the weaker
region below. I will not use the term 'asthenosphere' because its meaning became
quite confused in the early days of plate tectonics. Some people thought there was a
very weak, lubricating layer about 100 km thick immediately under the lithosphere,
whereas others assumed convection was confined to the upper mantle, which by
implication would be the asthenosphere. These days we have clear evidence for
convection extending deep into the lower mantle, so the 'weak layer' includes
much or all of the mantle. Therefore, I will speak only of the lithosphere and the
convecting mantle. Even if there were a weaker layer under the lithosphere, it is
hard to resolve and it would make little difference to the large-scale flows associated
with plates.
The concept of the deformable mantle just described was still a qualitative con-
cept. In order to quantify the 'mere viscidity' or 'degree of yielding', observations
were needed of presently occurring deformations. These came from the 'rebound'
of the Earth's surface following the removal of thick ice sheets at the end of the
Ice Age, about 11 000 years ago. Before we consider those, we need a way to
characterise a fluid's resistance to deformation.
4.1 Viscosity
In mechanical terms, a fluid is a material that can undergo an unlimited amount
of deformation. A solid, on the other hand, may deform to a small extent, but it
will break if you try to deform it too much. Another distinction is that many solids
will deform only by a certain amount under the action of a particular force, and
then return to their original shape if you stop applying that force. Such materials
are called elastic. On the other hand, a fluid will keep deforming as long as a force
is applied to it, and if the force is removed it will simply stop deforming, without
returning to its original shape.
These distinctions are often very clear in our common experience, but in some
circumstances they are not so clear. Thus, for example, some metals are elastic