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A Closer Look: Plate Boundaries
What are plate boundaries?
A central concept of plate tectonics is that
large portions of the Earth's lithosphere — the crust and the rigid
part of the upper mantle — move relative to one another. This relative
movement of the plates creates varying conditions at the plate
edges, or boundaries.
Geologists describe three basic types of plate boundaries:
divergent boundaries, convergent boundaries, and transform boundaries.
Divergent boundaries exist where two plates are moving apart. This occurs
most commonly at mid-ocean spreading ridges. At such
ridges, plates move apart and the mantle exposed is melted by
the decrease in
pressure, becoming magma. That magma then rises up to “fill in” as
new oceanic crust. As the plates separate, the geologic feature
known as a “rift valley” is created. Iceland, which sits on
the Mid-Atlantic ridge, is splitting apart on the divergent boundary
between the North American and Eurasian Plates.
Convergent boundaries exist where two plates are moving toward
each other.
Subduction demonstration.
Convergent boundaries can be of three types:
Ocean-Ocean
Convergence: If the plates moving toward each other are both
made of oceanic crust, one of the plates will move downward,
or subduct, under the other plate. This is called a subduction
zone. A deep trench forms on the ocean floor at the location
where one oceanic
plate subducts under another. Also, a volcanic arc (a chain of
volcanoes parallel to the trench) typically develops above the
subduction zone.
These volcanoes are generated as water brought down on the subducting
plate melts the overlying mantle, causing magma to rise through
the mantle and crust, erupting at the surface. The Mariana trench
and volcanic arc
mark where the Pacific and Philippine plates converge. The Aleutians,
Japan, and the Philippines are other examples of volcanic arcs
that exist as a chain of islands.
Ocean-Continent Convergence: If one plate topped by oceanic crust moves toward another topped
by continental crust, the more dense,
oceanic plate will subduct under the less dense, continental
plate and a trench
will develop off the shore of the continent. Off the coast of
South America, along the Peru-Chile trench, the oceanic Nazca
Plate is being subducted
beneath the continental South American Plate. As a result, the
Andes, a mountainous volcanic arc, have developed in South America.
The Cascade
Range, where Mt. St. Helens is found, is another example of a
volcanic arc formed by the convergence of oceanic and continental
plates.
Continent-Continent Convergence: If both converging plates
carry continental crust, neither of the plates fully subduct.
Continental rocks have a relatively low density and, like two
colliding icebergs,
usually
resist downward motion. During a collision, the crust is compressed
and subjected to very high temperatures and pressures. The lithosphere
thickens
and crustal rocks are folded and faulted. Large amounts of uplift
push rock high into the sky, forming mountain ranges such as
the Alps or the
Himalayas.
Transform Boundaries: Transform boundaries exist where
two plates slide past each other. Here, no lithosphere is created
or destroyed. Often these large faults, or fracture zones, connect
divergent or convergent plate boundaries. Most transform faults are found
on the ocean
floor.
Examples include the Alpine fault in New Zealand, which forms
the boundary between the Australian and Pacific plates, the Dead
Sea fault, which forms
the boundary between the African and Arabian plates, and the
San Andreas fault, which lies between the Pacific and North American
plates in California.
Is it that simple?
Not all plate boundaries are as simple as the main
types discussed above. For example, there are several places
on the Earth where
three plate boundaries intersect. These are referred to as “triple
junctions.” An
example of a triple junction exists where the African, Australian,
and Antarctic plates intersect. There, three spreading ridges
intersect. What
does that tell us about this area of the African, Australian,
and Antarctic plates? — the plates are moving apart in this region.
In
some parts of the world, plate boundaries are not well defined.
These regions, called “plate-boundary zones,” are large areas
where the effects of plate interactions are unclear Plate boundary
zones involve at least two large plates and one or more microplates (small
plate
fragments) caught up between the larger plates. The geology of
these areas can be very complex. The Mediterranean-Alpine region between
the Eurasian
and African plates is an example of a plate boundary zone.
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