We just got finished discussing the four eons of geologic time, those being the Hadean, Archean,
Proterozoic, and Phanerozoic. Now let’s get more specific with the geologic structures
that span those eons. We’re going to explore how rocks on Earth can change their shape, thereby
creating dramatic landforms over the millennia that field geologists love to study today.
Understanding how rocks deform requires a basic understanding of the mechanical
properties of geologic materials and how these mechanical properties can change under different
pressure-temperature conditions. Since both pressure and temperature always increase with
depth, rocks in the shallow crust are deformed differently than deeper ones, thereby forming
different structures than rocks in the deep crust. We live at the very top of the crust, and to us,
rocks are very rigid objects that are often used as a building material. However,
if the surface of the Earth was at a temperature of 1400°C and a pressure of 100,000 atmospheres,
we would find rocks behaving a bit like silly putty, or at least like glacial ice. Glaciers
are composed of solid ice crystals; and in spite of this, they are able to flow,
even if at a very slow rate. This is how rocks behave in the mantle. The term viscosity relates
to a material’s resistance to flow, and it is a very important concept in structural geology.
Now, to better understand the mechanical behavior of rocks, we need to understand the
concepts of stress and strain. Stress describes the quantity of the forces that cause deformation
and is expressed in terms of force per unit area, or Pascals. Stress is equivalent to the pressure
of a solid. There are three normal components of stress, with normal meaning acting perpendicular
to a surface, and each one describes the amount of stress in a particular direction
in three-dimensional space. If a force pulls on an object in an outward direction, causing the
object to stretch, that is called tensile stress. Conversely, if a force pushes inward on an object,
causing it to squash, that is called compressive stress. Both compressive and tensile stresses are
normal stresses. Forces that act parallel to a surface are called shear stress.
If an object experiences sufficient stress to cause deformation, whether it is stretched,
squashed, or sheared, it undergoes strain. Strain is a unitless measure
of the change in shape or volume of an object. The relationship between stress and strain, or
how a material responds to stress, is represented by a value called Young’s modulus. Young’s modulus
is dependent on the physical properties of a material and is determined in the laboratory by
subjecting it to a series of different pressures and measuring the accompanying strain. These
values are recorded and used to create a graph for the material’s stress-strain relationship.
The slope of the best-fitting line through the data is the material’s Young’s modulus.
Since minerals are the building blocks of rocks and each mineral has a specific value for Young’s
modulus, the mineralogy of a given rock will exert a strong control on how that rock will deform.
The relative tendency of a rock to strain in response to stress
is called its competence, which can also be defined as the relative viscosity of a
rock. Incompetent rocks are composed of soft, weak minerals like clay,
while competent rocks are composed of strong, rigid minerals like garnet or feldspar.
Other than competence, the manner in which a rock responds to stress is controlled by the conditions
that it is under, mainly temperature and pressure, but other factors like the deformation rate and
presence of fluids are also important. As the pressure and temperature of a rock are increased,
its strength, or viscosity decreases, making it more willing to deform.
We now understand that rocks respond to stress by becoming strained. The precise response to stress
is described by four different mechanisms: brittle deformation, elastic deformation, plastic
deformation, and viscous deformation. Viscous deformation mainly describes the deformation
of fluids. Rocks that deform in a brittle manner break instead of bend, and this behavior is most
common at the surface. In fact, most rocks in the upper 10 kilometers or so of crust tend to deform
via mechanisms of brittle deformation. This is called the brittle regime. Here, fracturing and
faulting are the main expressions of stress, though elastic deformation is also very common.
Elastic deformation is the manifestation of the bending of atomic bonds that occurs when an object
is stressed. When a force is exerted on an object, its atomic bonds stretch parallel to the maximum
tensile force and contract parallel to the maximum compressive force. If the force is increased,
the material experiences more strain, and once the force is removed, the material reverts back
to its original shape when the atomic bonds relax. However, it is possible to exert a force so great,
that the rock fails and forms a fracture. When a rock fails under stress, all of the accumulated
stress that was responsible for pulling the atoms apart is released, causing an earthquake.
This is like how if you overstretch a rubber band, which deforms elastically,
the released energy snaps the broken ends of the rubber band back toward your hand when it breaks.
Therefore, we would expect the largest earthquakes to occur in the hardest and most competent rocks.
Subduction zones are known for their massive earthquakes. This is because they are able to
transport a very competent material, oceanic crust, to great depths and therefore great
pressures. Normally, materials at these depths are too weak to cause earthquakes due to their
high temperature, causing them to bend instead of break. This brings us to plastic deformation,
which describes how weak materials deform, like silly putty. In contrast with elastic deformation,
plastic deformation occurs when atomic bonds are broken under stress. Under
perfect plastic deformation, materials do not accumulate energy as they’re stressed.
Again, this is because the energy is released when the atomic bonds are broken. Materials that deform
plastically do not revert to their original shape once the stress is removed. Plastic deformation
occurs mainly under the enormous pressures and temperatures of the lower crust and mantle, though
it can occur closer to the surface if either the temperature is high enough or the material
is weak enough, for instance, beneath volcanoes. That covers an introduction to stress and strain,
as well as the types of rock deformation. But we have a bit more to talk about in this realm,
so let’s move forward and look at some specific structures.