All About Plates
From EdHelper.com
Have you ever ridden the bumper cars at the
amusement park? You drive around on a slippery floor in a
car with a huge rubber bumper around it. You move
forward and backward, bumping into anyone in your way.
The earth's crust is a lot like those bumper cars.
The earth's crust is made up of several large plates
and some smaller ones. They all float along on the
asthenosphere. This is a layer of partly molten rock. It lies under the upper mantle.
As these plates move, they bump into one another. Sometimes they move away from
each other. The places where they meet are called plate boundaries.
There are several types of plate boundaries. Most of the plate "action" occurs
within the lithosphere, which includes the crust and upper mantle. The crust is the
solid outermost layer of the earth, and the mantle is the layer under the crust where the
plate movement occurs. All of this makes the lithosphere one restless place. The
places where the plates meet in the lithosphere are called plate boundaries.
Earthquakes usually occur in plate boundaries. There are three types of plate
boundaries: divergent, convergent, and transform.
Oceanic crust is denser than continental crust. As an oceanic plate bumps into
a continental plate, the oceanic plate moves under the continental plate. The oceanic
plate sinks into the mantle under the continental plate, this area is called a subduction
zone. It is forced down into the trench. The continental plate is folded upward.
Because the plates are coming together, or converging, we call this area where the
lithospheric plates move toward each other a convergent boundary.
This boundary is usually referred to as a subduction zone. As the subducting
plate continues to move, the other plate is pushed downward toward the mantle where
it will start to melt. The melting causes the plate to be reabsorbed into the earth. It is
also the cause of some of the world's most destructive earthquakes. The northwest
coasts of the United States, western Canada, and southern Alaska and Aleutian Islands
have a subduction zone plate boundary.
So, when oceanic crust and continental crust meet, the dense oceanic plate
moves under the continental, but what happens when two continental plates that have
the same density collide? When they collide, neither plate is forced downward. The
edges of these plates fold upward. Mountains are formed.
Unlike convergent boundaries, divergent boundaries or "spreading zones" are
found where two plates are moving away from each other. Most divergent boundaries
are found in oceans where seafloor spreading occurs. The ocean floor has mid-ocean
ridges and underwater mountain chains. In the centers of these mid-ocean ridges lie
underwater volcanoes and other "hot spots." These hot spots are areas where magma
rises from the asthenosphere. This rising mantle is referred to as an upwelling of
magma. As the magma rises, plates in the mid-ocean ridges are forced apart. New
earth material is added to the edges which means new oceanic lithosphere is born.
The third boundary is known as a transform fault. Plates in this boundary are
sliding horizontally past each other without much upward or downward movement.
There is also no crust destroyed or produced by the movement of these plates. This
boundary is also referred to as a strike-slip fault or fracture zone. The famous San
Andreas Fault along the coast of California and northwestern Mexico is a transform
fault. Earthquakes in this boundary tend to happen at shallow depths.
As we stand on the earth's crust, we are actually riding on a giant raft. The earth's
surface is made up of huge plates that float on molten material. Most of the time, we
can't feel the motion because it happens very slowly. If these plates collide, however,
we feel and see the effects. Earthquakes and volcanic eruptions are the result of these
rafts bumping into each other. These events change the surface of the earth.