M A S T E R P A G E
The plates are in very slow but constant motion, so that seen from above by a patient observer, Earth’s surface
might look like a slowly moving spherical jigsaw puzzle. The plates move at rates of 5 to 15 cm (2-7 inches) in a
year—about as fast as our fingernails grow. On a human scale, this is a rate of movement that only the most
sophisticated instruments can detect, but on the scale of geological time, it is a dizzying speed. The oldest rocks in
the crust, formed 3.8 billion years ago, could have traveled around the Earth 14 times at this rate.
Three Kinds of Plate Movement, Four Kinds of Boundaries
Plate movement is generally one of three kinds: spreading, colliding, or sliding. Earthquakes can accompany all
A.
B.
C.
three kinds of movement, but
the most forceful quakes
generally result from colliding
and sliding movements.
When plates are spreading, or
separating from each other,
earth scientists call their move-
ment divergent. The type of
boundary that results (A) is a
ridge, like the Mid-Atlantic
ridge system.
When plates are colliding, or
pushing against each other,
scientists call the movement
convergent. If an oceanic plate
converges with a continental
plate or another oceanic plate,
the oceanic plate is forced down
into the mantle, creating an
ocean trench (B). The Alaskan
trench and the Aleutian Islands
were formed in this way; so
D.
were the Andes Mountains. If two continental plates converge, neither will be forced into the mantle. Instead,
their edges will crumple and a large mountain chain may result (C). The Himalayas arose when India collided
with Asia.
Movement in which plates slide past each other is called lateral (or transform) plate movement (D). This kind of
movement is occurring along the San Andreas Fault in California.
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F E M A
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