Exploring Plate Boundaries
Transform Boundaries
A transform boundary is a point where two plates slip past each other, moving in opposite directions. Along transform
boundaries, the earth’s crust is neither created nor destroyed. Earthquakes occur frequently along these boundaries.
The San Andreas Fault which causes many of the earthquakes in California is an example of a transform boundary.
Analyze the diagram. Notice the type of plate movement that occurs along a transform boundary.
Crust
Mantle
Transform Boundary
Divergent Boundaries
An area where two plates move apart is known as a divergent boundary. Divergent boundaries occur on land and on
the ocean floor. If two oceanic plates move apart, new ocean crust is created along the boundary. This is known as a
mid-ocean ridge. If two plates move apart on land, a deep valley, called a rift valley, is formed. The Great Rift Valley
in east Africa is a well-known example. If this process continues, the valley could eventually be filled with ocean
water and two new continents would result. Since divergent boundaries can create new crust, they are constructive
boundaries. Analyze the diagram. Notice the type of plate movement that occurs along a divergent boundary.
Crust
Mantle
Divergent Boundary
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Convergent Boundaries
The area where two plates come together is called a convergent boundary. This movement of plates causes a collision.
Collisions can occur between oceanic crust and oceanic crust, oceanic crust and continental crust, or continental crust
and continental crust. Collisions between two oceanic plates result in the formation of trenches. Trenches also form
when oceanic and continental plates collide. A collision between two continental plates squeezes the crust into
mountain ranges. Convergent boundaries are also called destructive boundaries because crust is being destroyed. The
Himalayan Mountains is an example of a convergent boundary between two continental plates. This mountain chain
continues to form as the plate carrying India collides with the plate carrying Asia. Analyze the diagram. Notice the
type of plate movement that occurs along a convergent boundary.
Crust
Mantle
Convergent Boundary
---------------------------------------------------------------------------------------------------------------------------------Subduction Zones
Subduction zones occur at some convergent boundaries. When a plate carrying oceanic crust collides with a plate
carrying continental crust, subduction occurs. Ocean crust, which is denser than continental crust, sinks beneath the
continental crust forming a deep ocean trench. As the trench is formed, the oceanic crust is forced back into the
mantle. As the rock material in the ocean crust begins to melt in the mantle, some of the molten rock is forced upward
producing volcanoes. Subduction zones are common in an area known as “The Ring of Fire.” In this region, the ocean
crust of the Pacific plate is colliding and subducting with the many continental plates that surround it. Due to this
process, there is much earthquake and volcanic activity. Analyze the diagram. Notice the type of plate movement
that occurs along a subduction zone.
Continental Crust
Oceanic Crust
Mantle
Subduction Zone
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