Geography Reference
In-Depth Information
quite passive features of these moving plates since they just ride on
top of them and, unlike ocean floors, they are not consumed into
the mantle. Earthquakes occur because while plates slowly move
you can imagine them having rough or bumpy sticking points.
Over time an enormous force builds up and eventually the plates
move in a jolt which is experienced as an earthquake.
Movements at the boundary between two plates can explain the
nature of landforms found in these areas. Where plates are moving
apart there are
divergent plate boundaries
(e.g. at the mid-
ocean ridge) where new crust is formed. The lava formed at mid-
ocean ridges is hot and very runny, forming gently sloping shield
volcanoes. Volcanic eruptions with this type of lava (e.g. on Iceland
which straddles a mid-ocean ridge) tends not to be explosive
because gas bubbles can easily escape through the runny liquid,
although occasionally large gas bubbles do emerge creating a scene
with runny lava flying into the air. Eruptions may be in the form
of walls of molten lava issuing from a linear crack in the Earth.
Most divergent boundaries are in the mid-ocean but there are some
within continents. The Syrian-African Rift Valley is a good
example of a divergent plate boundary on land. As the valley has
continued to deepen, it is now below sea level and some of it has
filled with water (e.g. the Dead Sea is 339
metres below sea level).
Transform faults occur where plates slide past one another (e.g.
San Andreas Fault, California). Here there is often little creation or
destruction of lithosphere and there are few volcanoes at transform
boundaries. However, these boundaries can be associated with fre-
quent major and destructive earthquakes. The rates of movement
can be from a few centimetres in a small earthquake to two metres
in a large event.
When two plates move towards one another, at a
convergent
plate boundary
, major physical features are formed. If one of the
plates slides beneath the other, a
subduction zone
is formed. This
happens where two ocean crusts collide or where denser ocean
crust meets less dense continental crust and ocean crust then
becomes part of the mantle at this point. This is why ocean crust is
relatively young by geological timescales. Often this process also
creates mountain belts as the crust thickens at the subduction zone.
For example, the Nazca plate collides with the South American
plate and is subducted below it creating the Andes Mountains and
