It can be seen that the contours of Moho depth are basically concentric and the thicker
crustal areas are always to be found towards the centres of continents, the thinnest
at their peripheries. This is due to the original thick crust being stretched and
‘necked’ successively thinner until it reaches failure point. The continental shelves
off the coasts of continents mark the lines of failure. Here we have the interface
between continental crust and oceanic-bed crust.
To understand this phenomenon of crustal stretching, we need to study how different
materials behave under strain. We know that a bar of toffee, when stretched, becomes
thinner at its central region, before it splits apart. Metals behave in a similar
manner but their ductile properties vary considerably. Useful information on this
subject is given in the book, The Science and Engineering of Materials by D R Askeland.
When a metal in the shape of a rod is stretched during tensile strength tests, a
point is reached where deformation and necking results. Depending upon the ductility
of the material, this necked region extends further and gets thinner in cross-section
to a point where the material breaks into two pieces. This is described as a pull
to failure test.
Metals are different to rocks, however, in that they are crystalline and have grains
of lattice structures built from one element. Rocks have varying chains of silica
and alumina and these do not form lattice structures. Toffee with its chains of polysaccharides
is therefore closer in its form to the structure rock.