We just learned about the different types of rock deformation, but there are two very
important structures pertaining to this concept that must be discussed in detail, those being
faults and folds.
Faulting is the main type of brittle deformation in rocks.
Faults are classified based on the type of deformation that creates them.
Normal faults are caused by extension, reverse faults by compression, and strike-slip, or
transverse faults by shearing.
Normal and reverse faults are called dip-slip faults because fault motion, or slip, is parallel
to the fault’s dip.
This is in contrast with strike-slip faults where slip is perpendicular to the dip.
In geology, dip refers to the direction that the fault plane is inclined.
To conceptualize this, imagine that you pour a cup of water onto an inclined surface.
The direction that the water flows is the dip, which has two components, an azimuth,
representing a horizontal direction, and an angle, representing the steepness of the plane.
In any dip-slip fault, there is a hanging wall and a footwall.
If you imagine drilling a tunnel longitudinally through the fault, when standing in this tunnel,
you would be standing on the footwall and the hanging wall would be above you.
In reverse faults, the hanging wall moves upward, and in normal faults, it moves downward.
Reverse faults primarily form in areas where the tectonic plates are converging, like along
subduction zones and continent-continent collisions.
There also areas where the plates are converging at an angle.
These transpressional faults experience both reverse and strike-slip faulting.
A fault with both dip-slip and strike-slip motion is called an oblique fault.
An example of this is the San Andreas Fault.
Here, the Pacific and North American plates are converging at about a 10-degree angle.
Though predominately a strike-slip fault, it is also slightly compressional, as is evidenced
by the Coastal Mountain Ranges of California.
Normal faults are most common where the plates are moving apart and the crust is being stretched.
This is currently occurring in East Africa, the Basin and Range Province of North America,
and at the center of every ocean basin in the world.
As the crust thins during extension, its weight above the underlying mantle is reduced and
spread out over a larger surface area.
This causes the crust to bob up in a process called isostatic adjustment, or more specifically,
isostatic rebound.
It’s like walking off a boat.
When your weight is removed, the boat bobs upward and rises in the water.
Get back in, and the boat bobs down in response to your weight.
Back to the geological case, the crust is analogous to the boat and the mantle is analogous
to the water, with the crust bobbing up and down in response to the thickening and thinning
of the crust.
The same thing occurs in areas that are experiencing compression.
The compressional forces thicken and shorten the crust, causing subsidence, because the
now heavier crust will sink into the mantle.
This is what leads to the development of foreland basins, or large areas of depressed land located
adjacent to large mountain ranges.
The weight of the mountain pushes the crust down into the mantle, not only beneath it,
but also adjacent to it, and it is these adjacent lowlands that sink below sea level during
isostatic adjustment, which in this case, is called subsidence.
Basin subsidence is what makes the formation of ultra-thick sedimentary deposits possible.
For example, as much as 10,000 feet of sediment accumulated in the Catskill Foreland Basin
over a period of only about 20 million years during the Upper Devonian Period.
This sediment was shed from the now-eroded Acadian Mountains that existed along the eastern
seaboard.
Subsiding foreland basins like this have fostered deposition of some of the largest sedimentary
formations in the world.
In some areas, compressional forces cause rocks to become folded, like a car that gets
crumpled during an accident.
Folding occurs when a rock is being compressed and the temperature and pressure are just
high enough for plastic deformation to occur.
Folds are classified based on their geometry and stratigraphy, or arrangement of strata.
Folds that are concave up are called synforms, and folds that are concave down are called
antiforms.
When layered rocks are folded into a synform, the youngest rocks will usually be at the
hinge of the fold, which is similar to the local maximum or minimum of a parabolic function.
When this is the case, the fold is called a syncline.
Anticlines have the oldest rocks at the hinge zone.
Geologic folds can range in wavelength from millimeters to kilometers, as can the amplitude.
If a rock is folded in a way that compresses it equally in two perpendicular directions,
it can form domes and basins.
Domes that are capped with an impermeable rock like shale make excellent traps for oil
and gas.
Hydrocarbons are positively buoyant in the Earth, causing them to rise toward the surface
through a rock’s pore space, until they are trapped by the impermeable shale dome.
Folds can also be tilted over on their sides during deformation, becoming either plunging
or inclined.
The zig-zag ridges of the Appalachian Mountains in Pennsylvania perfectly represent plunging
folds that have had their tops planed off during erosion.
The Appalachian Mountains, which formed around 300 million years ago, were completely eroded
into a flat plain near sea level by the beginning of the Cenozoic Era.
The ridges of today are caused by recent uplift due to isostatic adjustment that is hypothesized
to have been caused by a piece of the bottom of the crust breaking off and sinking into
the mantle, in a process called delamination.
Remember, whenever the crust loses mass, it bobs up.
In this way, they are unlike the younger Rockies and Himalayas whose towering peaks were thrust
upward by the force of two colliding plates.
In the modern Appalachians, the rocks that are most resistant to erosion, like sandstone,
form the ridges, while the weakest and most soluble rocks like limestone and shale, form
the valleys.
The most striking folded rocks are found in areas that have experienced the collision
of two plates, such as the Himalayas, Alps, Rockies, and Appalachians.
Large reverse faults called thrust faults are found in these locations.
In some cases, the overriding plate can be thrust hundreds to thousands of kilometers
over the plate it is colliding with.
The immense pressures and temperatures created by the collision of two plates also cause
the rocks to recrystallize and become metamorphic rocks.
Different minerals have different pressure-temperature stability ranges and can therefore be used
to estimate the pressure and temperature conditions during metamorphism.
When the pressures are very high with respect to temperature, as in subduction zones, the
blue mineral glaucophane forms.
These rocks are called blueschist.
When the pressure and temperature follow the normal geothermal gradient, as in the typical
continent-continent collision, the green mineral chlorite forms.
These rocks are called greenschist.
When both the temperature and pressure are extreme, minerals tend to become segregated
during recrystallization, and the light and dark minerals are banded in the rock, which
is called gneiss.
Increasing the temperature much more will cause the rock to begin melting.
In metamorphic rocks, platy minerals like muscovite tend to align themselves perpendicular
to the maximum stress-creating foliation, or a planar fabric within the rock that is
called cleavage.
When a rock has cleavage, it breaks into planar, parallel pieces, like slate.
The extent of cleavage development depends on the intensity of the pressure and the quantity
of platy minerals.
Features such as cleavage, folding, and faulting are often present in and contribute to the
formation of some of the largest and most impressive geologic structures on Earth.
Now that we know a little bit about how rock can react to different stresses, we are ready
to learn about the structure of the Earth in considerable detail, from the surface all
the way down to the core, so let’s move forward and do just that.