The dramatic mountains and valleys of Earth’s landscape, as well as the continuous occurrence
of global earthquakes, provides evidence that Earth’s thin, brittle outer shell of rock
is under a constant state of stress. Let’s explore how this stress impacts the formation of small
local faults, and broader tectonic plate boundaries. First, what IS stress? Stress,
is the force that. can deform or move rock. Here, stress is equal to the amount of force, in this
case compressional or squeezing force, acting on the contact between the two blocks shown.
The amount of stress is described as a force per unit area. For example, a 150-pound person
standing on the ground exerts about six pounds per square inch on the sole of their foot.
The type of stress describes the direction of the forces acting on it. For example,
Extension pulls, compression pushes, and opposing horizontal forces shear. Thirty miles deep, rock
undergoes a compressive stress in all directions that is nearly 200,000 pounds per square inch.
In response to such extreme stress, rocks bend and/or break through a process called deformation.
The following factors control whether the deformation is elastic, plastic, or brittle.:
High pressure deep in Earth inhibits breaking,
High temperatures deep in Earth make rock more flexible, and
the minerals making up some rock makes it more flexible than others with a different makeup .
Let’s explore the process of Elastic Deformation first. We’ll use a strand of uncooked spaghetti.
You can deform the spaghetti by applying a compressive force to the ends. When the force
is removed it returns to its original shape: a process called elastic rebound. We can also
demonstrate that solid rock can deform elastically by using a split piece of granite or marble:
if i squeeze this i can actually squeeze the sides of this rock together and when i let go
it comes back again rocks are elastic actually all solids are elastic it's just that we don't
really notice the elasticity as much as in something like a rubber that's very very loud
but still the rocks are elastic enough that when plates move they can bend elastically
like huge springs and then when the earthquake happens boom they release that elastic energy
and turn it into kinetic energy and into heat and into the waves record seismometers
Blocks of rock, 100s, or even 1000s of miles across require immense slow, steady stress
to deform elastically. For example, crustal material deforms with a steady bending near
either side of a pre-existing, locked fault. Sudden movement along the fault occurs
when accumulated stress overcomes friction and stored elastic energy is released as the
ground returns to its original shape. • Ductile deformation occurs when
rocks undergo bending that remains even after the stress is removed.
Using the spaghetti analogy again, an uncooked noodle undergoes] ductile deformation when placed
in boiling water and compressed against the bottom of the pan. When the stress is removed
we see that the deformation remains. Similarly, the same rock that can deform elastically
could, under different temperature and pressure conditions found deep in Earth,
undergo ductile deformation. We can see evidence of ductile deformation when rock layers,
deformed at depth are later uplifted and revealed at the surface, as in this road cut.
• We have already explored how uncooked spaghetti can deform elastically. However, when the force
applied to the spaghetti exceeds its strength limit, the spaghetti will bend and then break.
This type of permanent deformation is called brittle deformation. In rock bodies,
a fracture is a break that has experienced no movement along the crack. In contrast, a
fault initiates on a fracture but one side shifts relative to the other side. Most faults are locked
by friction, unable to move except by elastic deformation or the abrupt jerk of an earthquake
Throughout the rest of this animation, we’ll ignore ductile deformation, and focus on
earthquakes that occurs in Earth’s uppermost cool layers of rock, called tectonic plates.
Faults can be separated into three broad classes based on the direction
of the stress that caused the fault offset. When extensional stress is applied, the rocks
extend, initially forming fractures usually at an angle of less than 70 degrees to the surface.
When the hanging wall drops relative to the foot wall, it is called a normal fault.
If compressive stress is applied to rock, the effect is to shorten it.
The fractures that form look a lot like a normal fault, but the motion is in the opposite
direction. These are called reverse faults. When rock is pushed horizontally in opposite
directions, the shearing produces strike-slip faults. Most strike-slip faults are close to
vertical and involve little to no vertical motion. The stress regimes that act locally on faults,
also act at a larger regional scale to create the three basic types of plate boundaries.
ExTensional stress occurs at divergent plate boundaries where two plates move away from
each other. Examples include oceanic spreading ridges that form the longest mountain ranges in
the world. As the plates pull apart, hot, and therefore lower-density, mantle rock, rises to
support the 3000-9,000-foot high spreading ridges. As the plates move away from the ridge, they cool,
become brittle and break into normal faults. Compressive stresses occur at convergent plate
boundaries where two plates move toward each other. If two continental plates collide,
they produce a broad uplift like the Himalayan Mountain Range with reverse faults parallel to
the plate boundary. If an oceanic plate subducts beneath a continental plate,
most of the faults occur within the overlying plate as it is compressed and buckles forming
coastal mountains ranges parallel to the plate boundary. The sloping contact between the plates
is under high friction as the subducting plate shoves the overlying plate backwards.
Shear stress occurs at transform boundaries where two plates slide horizontally past each other
with strike-slip motion. Transform faults link offset boundary segments. Most transform faults
are found in the ocean basin where they connect offsets in the mid-ocean ridges.
Transform faults can also connect a spreading ridge with a subduction zone,
such as the San Andreas Fault that separates the Pacific and North American Plates.
Geologists have defined plate boundaries by the dominant forces acting on the plates. These are
usually shown on maps as continuous connected segments. While shown as lines, they are actually
broad zones of deformation that are affected by the regional stress from constant plate motion.
For example, the San Andreas Fault is not a simple long strike-slip fault, but is actually a Fault
Zone that includes over 200 related faults that generate thousands of earthquakes every year.
In conclusion, stress is a force that acts on rocks to change their shape or volume.
How the rock responds, depends on the type of stress
and the conditions the rock is being subjected to when it encounters stress.
It is this change in Earth’s crust that generates different types of faults and plate boundaries
and results in the dramatic mountains and valleys that we see in Earth’s landscape.