Plate Margins

Soem detailed notes on the different types of plate margins/boundaries.

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Helena Bonici Geography 04/07/2011
12EA2Plate Tectonics
When continental and oceanic plates collide the thinner and denser oceanic plate is overridden by the
thicker and less dense continental plate. The oceanic plate is subducted.
As the oceanic plate descends it is forced into higher temperature environments. At a depth of about
100 miles (160 km) materials in the subducting plate begin to approach their melting temperatures
and a process of partial melting begins.
This partial melting produces magma chambers
above the subducting oceanic plate. The magma
chambers are less dense than the surrounding
mantle materials and are buoyant. The buoyant
magma chambers begin a slow ascent through
the overlying materials, melting and fracturing
their way upwards. If a magma chamber rises
to the surface without solidifying the magma
will break through in the form of a volcanic
Effects of a convergent boundary between an
oceanic and continental plate include: a zone of earthquake activity that is shallow along the continent
margin but deepens beneath the continent, sometimes an ocean trench immediately off shore of the
continent, a line of volcanic eruptions a few hundred miles inland from the shoreline, destruction of
oceanic lithosphere.
When a convergent boundary occurs between two oceanic plates one of those plates will subduct
beneath the other. Normally the older plate will subduct because of its higher density.
The subducting plate is heated as it is forced deeper into the mantle and at a depth of about 100 miles
(150 km) the plate begins to melt. Magma
chambers are produced as a result of this
melting and the magma is lower in density
than the surrounding rock material. It begins
ascending by melting and fracturing its way
through the overlying rock material. Magma
chambers that reach the surface break through
to form a volcanic eruption cone.
In the early stages of this type of boundary
the cones will be deep beneath the ocean surface but later grow to be higher than sea level. This
produces an island chain. With continued development the islands grow larger, merge and an
elongate landmass is created.
Effects that are found at this type of plate boundary include: a zone of progressively deeper
earthquakes, an oceanic trench, a chain of volcanic islands, and the destruction of oceanic lithosphere.
A powerful collision occurs the two thick continental plates collide and both of them have a density
that is much lower than the mantle, which prevents subduction.
Fragments of crust or continent margin sediments
might be caught in the collision zone between the
continents forming a highly deformed melange of
rock. The intense compression can also cause
extensive folding and faulting of rocks within the

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Helena Bonici Geography 04/07/2011
12EA2Plate Tectonics
two colliding plates. This deformation can extend hundreds of miles into the plate interior.
Effects found at a convergent boundary between continental plates include: intense folding and
faulting, a broad folded mountain range, shallow earthquake activity, shortening and thickening of
the plates within the collision zone.
When a divergent boundary occurs beneath oceanic lithosphere, the rising convection current below
lifts the lithosphere producing a
mid-ocean ridge.…read more

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Helena Bonici Geography 04/07/2011
12EA2Plate Tectonics
Transform Plate Boundaries are locations where two plates slide past one another. The fracture zone
that forms a transform plate boundary is known as a transform fault. Most transform faults are
found in the ocean basin and connect offsets in the mid-ocean ridges. A smaller number connect
mid-ocean ridges and subduction zones.
Transform faults can be distinguished from
the typical strike-slip faults because the sense
of movement is in the opposite direction.…read more


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