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DIVERGENT BOUNDARIES
Divergent boundaries are areas in which
plates move
The majority of these divergent boundaries
are located along the crests of
New oceanic crust (lithosphere) is
continuously being
between the diverging plates. The older
crust moves away from the boundary, as it
does so, it cools and contracts hence
increasing in
This is why the older and cooler oceanic
crusts are deeper. This average rate of
spreading is about
.
This is rapid enough to account for the
creation of all Earth’s ocean basins within
the last 200 million years.
However, not all divergent boundaries are found in the
middle of large oceans. The East African rift valleys
represent the initial stage in the breakup of a
.
The volcanic activity associated with divergent
boundaries is represented by volcanic mountains such as
Kilimanjaro and Mount Kenya. The rift valleys are
caused by the associated faulting taking place. As the
spreading continues, the rift valley will
Eventually, it will spread out into the ocean.
The
is the site of a recently
formed divergent boundary. Here the Arabian plate
separated from the Africa plate a few million years ago
and began to
move in a
northeast
direction.
CONVERGENT BOUNDARIES
1. Oceanic-Continental
When a continental plate collides with an oceanic plate,
the less dense
plate will continue
on its course while the denser
plate will sink under the continental plate and into the
. As the descending plate
increases in depth, the heat generated causes partial
melting of the mantle creating magma which will
accumulate beneath the overlying continental crust. This
can result in
Mountains
which are produced by volcanic activity associated with
the
of oceanic lithosphere are
called continental volcanic arcs. Examples of these are
the Andes in South America and the Cascade Range in
northwestern USA.
2. Oceanic-Oceanic Convergence
When two oceanic plate converge, one will
. This will
trigger volcanic activity similar to that which occurs at
an oceanic-continental convergence. The difference
here is that the volcanoes form on the ocean floor. If
the activity continues the volcanoes will emerge as
in the ocean. This newly formed
land consists of an arc-shaped chain of small volcanic
islands is called a volcanic island arc. The Tonga,
Aleutian and Mariana islands are examples of volcanic
island arcs. These island arcs are usually located about 200 to 300 km from a trench axis
3. Continental-Continental Convergence
When two plates carrying continental crust converge there
will be a
Since both plates are of low
density and buoyant by the nature of the continental rock,
neither will
Instead, there will be
folding and deforming of sediments along the continental
margins. These collisions produce the most spectacular
on Earth, such as the
Himalayas which are a result of the Australian-Indian plate
ramming into the Eurasian plate. Other mountain ranges
produced by this type of convergence are the Alps, Urals
and Appalachians.
TRANSFORM BOUNDARIES
A transform fault boundary is where two plates
grind past each other without the
or
of
lithosphere. Most transform faults join two
segments of a mid-ocean ridge. Along these
boundaries the seafloor moves in opposite
directions. However, transform faults are not
solely located in the ocean basin. Probably the
most famous transform fault is
Fault in California. Here
the Pacific plate is moving toward the
northwest grinding past the North American
plate which is traveling in the opposite
direction. Of major importance are the
that are triggered along fault boundaries.
http://www2.stkc.go.th/LOEarthScience/
HOT SPOTS
Hawaii is an example of how plumes create
islands. Hot, solid rock rises to the
from greater depths.
Due to the lower pressure at the shallower
depth, the rock begins to melt, forming
magma. The magma rises through the
Pacific Plate to supply the active volcanoes.
The
islands were once located above the
stationary hot spot but were carried away as
the Pacific Plate drifted to the northwest.
Yellowstone, like Hawaiʻi, is
believed to lie on top of an
area called a
where light,
hot, molten mantle rock rises
towards the surface. The
hotspot's apparent motion is to
the east-northeast. In reality,
the North American Plate is
moving
over the
stationary hotspot deep
underneath.