Download Earth`s Stresses Convection currents fuel continental

Earth’s Stresses
Convection currents fuel continental drift. Because the plates are
constantly moving around, crashing into each other, sideswiping each
other, and pulling away from each other, the surface of the Earth is
under several different types of stress. Stress is the amount of force
per unit area that is put on a given material.
When a rock changes its shape due to stress this reaction it is called
deformation. Deformation causes rocks to break, tilt and fold
Rock responds to stress differently depending on the pressure and
temperature (depth in Earth) and composition of the rock.
Elastic deformation: For small differential stresses, less than the yield
strength (amount of force a rock can withstand before it breaks), rock
deforms like a spring. It changes shape by a very small amount in
response to the stress, but the deformation is not permanent. If the
stress could be reversed the rock would return to its original shape.
Brittle deformation: Near the Earth's surface rock behaves in its familiar
brittle fashion. If a differential stress is applied that is greater than the
rock's yield strength, the rock fractures. It breaks. Note: the part of the
rock that didn't break springs back to its original shape. This elastic
rebound is what causes earthquakes.
Ductile deformation: Deeper than 10-20 km the enormous lithostatic
stress makes it nearly impossible to produce a fracture (crack - with
space between masses of rock) but the high temperature makes rock
softer, less brittle, more malleable. Rock undergoes plastic deformation
when a differential stress is applied that is stronger than its yield
strength. It flows. This occurs in the lower continental crust and in the
mantle
There are three basic types of stress, compression, tension, and
shearing.
Tension- This stress pulls on the crust where plates are moving apart,
stretching rock to become thin in the middle. Tension occurs at
divergent boundaries.
Compression- This stress squeezes rock together where plates are
moving toward one another, causing them to fold or break. Huge
mountain ranges can be the result of compression.
Shearing- This stress pushes rock in different directions, causing them
to slip apart or change shape.
Over millions of years, flat land can change shape in many ways.
Folding occurs when rocks bend due to stress but do not break. There
are several different types of fold, but the two most common are
synclines and anticlines.
A fold in a rock that bends upward is called an anticline. A fold in a rock
that bends downward to form a valley is called a syncline.
Faults are created where breaks in the rock of the crust occur. Faults
usually occur at plate boundaries where the forces of pushing and
pulling are greatest.
There are four main types of faults, normal faults, reverse faults, thrust
faults, and lateral or strike slip faults,
Normal Faults- occurs at an angle, where the crust is being pulled apart,
one block lies above the fault (hanging wall), while the other lies below
the fault (footwall)
If you think of the footwall as a slide, in a normal fault after an
earthquake it looks like the hanging wall slid down in relation to the foot
wall. That could very well be the case but it is just as likely that during
an earthquake the footwall could have been push up in relation to the
hanging wall. The key to remembering a normal fault is to remember
that it looks normal, the hanging wall looks like it slid down the
slide(footwall).
Reverse Fault- occurs at an angle, where the crust is being pushed together. The
hanging wall slides up and over the foot wall. The hanging wall could have been
pushed up the foot wall or the footwall could have been pushed down in relation to
the hanging wall. The key to remembering the reverse fault is to remember it
does not look normal, it looks like the hanging wall slid up the slide (footwall).
The third type of fault is called either a strike slip fault or lateral fault.
These types of faults occur when forces cause rock to break and move
horizontally. There is very little vertical movement.
The fourth type of fault is a thrust fault. A thrust fault occurs when one
side of a fault gets thrust on top of the other side of the fault. Thinks of
this like putting one layer on top of the other like putting a blanket on
someone.
Folded mountains are actually formed by crust which has been
squeezed together, folded and pushed together by compressional
forces. This occurs along convergent plate boundaries where 2 plates
move towards each other, between continental plates or between an
oceanic and a continental plate. The crust and the rocks get bent and
crumpled, and massive layers of Earth's crust gets uplifted as a result,
forming spectacular fold mountains.
Examples of folded mountains are the Himalayas in Asia and the Andes
in South America
Fault-block mountains are formed by the movement of large crustal
blocks when forces in the Earth's crust pull it apart.
Large blocks of rock along the sides of faults can be uplifted and tilted
sideways by incredible forces. On the opposite sides of the faults, the
ground can slid downward. The blocks of rock pushed upward or the
blocks of rock left standing in its original position while the surrounding
rocks move downward are called fault block mountains.
The Sierra Nevada Mountains in California are an example of a faultblock mountain range