Historical Geology/Physical properties of rocks

In order to fully understand plate tectonics and the evidence for it, it is necessary for the reader to know a little about the physical properties of rocks. In this article we provide a brief introduction to the concepts involved.

Stress and strain
In ordinary English, stress and strain are more or less synonymous. In physics, they refer to different though related quantities.

Stress is a measure of the force per unit area exerted on a surface of a deformable body. That is, roughly speaking, stress is to solids what pressure is to gasses, and like pressure it is measured in pascals (Pa); that is, in newtons per meter squared.

Strain is the deformation of a body as a result of stress. In geology, strain is given by the length by which a rock expands or contracts divided by the length it was originally: because this is the ratio of a length to a length no units are associated with it.

Tension, compression, and shear
The stress on a rock (or any other material, for that matter) can be classified as tension, compression, or shear, as illustrated in the diagram to the right.

Rock is strong under compression but relatively weak under tension and shear. This is a result of the microscopic structure of rock: it contains microscopic cracks which are forced open and enlarged by tension and shear but which are forced closed by compression.

This is why a small overhang on a cliff will easily break under its own weight (being subjected to shear) whereas the rock at the foot of the same cliff will bear the much greater weight of all the rock above it, as in that case it is being subjected to compression.

Elastic and plastic behavior
A material is said to be elastic if it recovers from stress — that is, if, having been bent or extended or compressed under shear stress or tension or compression, it snaps back into its original configuration when the stress is released.

A material is said to be plastic if, on the contrary, once stress has squeezed it into a certain shape, it retains that shape; plasticine, for example, is plastic at room temperature and surface pressure.

When a solid is placed under stress, its behavior is at first elastic; then (with increasing stress) plastic; then with the addition of enough stress it fractures.

A solid which undergoes very little plastic deformation between elastic behavior and fracturing is said to be brittle. In colloquial English we usually reserve this work for things which are both brittle in the technical sense and also require little stress to break, such as egg-shells; in its technical use in physics, however, a substance such as diamond is also brittle in the technical sense: diamond may not break easily, but it will break before it undergoes any significant plastic deformation.

The opposite of brittle is ductile.

A material will have greater resistance to fracture if it is under a high surrounding pressure; and it will be more ductile at higher temperatures.

The reader should also bear in mind that the rate at which stress is applied may be significant: a force rapidly applied may produce fracture which, if more slowly applied, may produce deformation. The material known as Silly Putty is famous for clearly demonstrating this property: it deforms under gentle pressure from one's fingers but shatters if hit with a hammer.

Application to rocks
We should now explain how all this applies to rocks in particular.

Rocks on the surface will exhibit elastic and brittle behavior, since they are cold and at low pressure. At depth, the pressure will be greater, increasing their brittle strength (that is, their resistance to fracture) and the temperatures will be higher, decreasing their ductile strength (that is, their resistance to plastic deformation).

Below the depth at which the ductile strength is less than the brittle strength, the rocks will be fully ductile and plastic. Some people describe the rocks below this depth as molten, but this is not accurate: they are not a liquid, but rather a ductile solid, like plasticine.

As a result, rocks near the surface tend to fracture under stress creating geological faults; more deeply buried rocks tend to fold.

Earthquakes are also a phenomenon of the upper, brittle, elastic part of the rock. When two pieces of the Earth's crust try to move past one another, their mutual friction impedes them and they bend very slightly. Earthquakes are caused when the potential energy of the bent rocks is sufficient to overcome the resisting friction and they snap back, releasing the stored energy in the form of kinetic energy. This is only possible if the behavior of the rocks is elastic rather than plastic. Consequently we do not expect (and do not find) deep earthquakes except when they are associated with subduction (as will be discussed in a subsequent article).

How do we know?
The behavior of rocks at surface temperatures and pressures are easy to verify. To find out how they would behave at greater temperatures and pressures requires special equipment.

In a simple experiment often repeated with different kinds of rocks, scientists can take a cylinder of rock and compress it with a piston with varying degrees of confining pressure. With little or no confining pressure, the rock will fracture, as you would expect. However, under greater degrees of confining pressure (such as would be experienced by a rock buried at a depth of one or two kilometers) the rock does not fracture, but rather it deforms like plasticine: or, to put it another way, it is ductile rather than brittle.

More recent experiments have reached greater levels of sophistication. By using lasers to heat rock samples, and a device known as a diamond anvil cell to exert pressure on them, it is possible to simulate temperatures and pressures such as are found deep within the Earth.

Such methods do not tell us everything we would like to know. Reproducing conditions in the very core of the Earth would require some sort of breakthrough in materials technology. Another thing that is hard to simulate is the effect of time. We know that materials are more likely to deform and less likely to shatter when stress is applied gradually: so what happens if you apply a gentle stress to a rock over a period of millions of years?

Such questions can to some extent be answered with reference to established notions in physics; but clearly if all such questions could be answered with total accuracy with reference to purely theoretical considerations, then geologists wouldn't spend so much money on diamond anvil cells and lasers.

That being said, what we do know is sufficient for us to understand plate tectonics; certainly it is quite enough for an introductory course such as this one.