Page 32 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 27

Figure 2.4
Plate separation
creates hot spots
in Iceland. (From
Kious and Trilling,
1996.)

plate, which is composed of denser rocks. This pushes up mountain ranges along
the leading edge of the continent, coupled with volcanism and flanked by an
offshore ocean trench.

2.4.4 Plate Tectonics and Earthquakes

The movement of one plate over or past another seldom is smooth, but involves
a frictional phenomenon called stick-slip. A common example is the unseemly
screech of chalk on a blackboard. Sticking at plate margins does not prevent
plates from moving, so elastic energy is stored in rocks on both sides of the fault.
When slip finally does occur that energy is released like a spring and causes an
earthquake. Since the stress relief is local, at what is termed the epicenter, stress
is transferred to flanking areas that did not slip, making them more susceptible
to slipping and causing an earthquake. Hence a ground rule is that the longer the
period of inactivity, the more stress builds up, and the more severe the earthquake
will be when it comes. The most dangerous areas are not where an earthquake
has recently occurred, but along active faults where they have not recently
occurred, in what is termed a ‘‘window’’ of inactivity.

2.4.5 What Drives Drift?
The deep interior of the earth is largely a mystery, with clues coming from the
velocity of sound waves following a direct path from an earthquake epicenter
through layers of the globe. The interior of the earth is hot because of radioactive

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