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Plate Tectonics
Plate Boundaries
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Plate Boundary Interactions
• As the lithospheric plates move their boundaries interact
with each other in one of three ways:
1. Divergent Boundaries(
)
• – Two plates moving away
from each other. Divergent
boundaries are usually found in
oceans (they can form on
continents, but water flows in
as the land pulls apart) and
forms ridges.
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Divergent boundaries and notes
The typical example of a divergent boundary is the mid-Atlantic ridge. As
the plates spread apart a long (“crack”) forms and magma is able to flow to
the surface.
This magma forms new oceanic crust, volcanoes (and islands like Iceland)
and shallow-focus earthquakes. These divergent boundaries are not as
common on the thicker continental plates.
But if a divergent boundary does occur the gap is called a “rift valley”.
Note: Symbols of Plate Boundaries:
You should become familiar with the
symbols which are used to represent
the indicated information on a
plate tectonics map.
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Convergent
2. Convergent Boundaries (
) – Two plates move
towards each other. At the area of collision, one plate is
goes down and the other is pushed up.
This is dependent on density and thickness of the plates.
The collision forms a long trench which can be 8 to 10 km
deep.
Convergence can occur in one of three combinations:
 An ocean plate and a continental plate
 Between two ocean plates
 Between two continental plates
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Convergence: Ocean and continental
Recall that oceanic plates are less dense than continental plates. As the
oceanic plate is subducted under the continent the collision forms
mountains and volcanoes.
The mountains form as the continental plate is compressed and forced
upward.
The volcanoes form as
the ocean crust melts
into magma and as the
Asthenosphere is
Punctured.
The high pressures are
then able to break
through the week spots
in the crust above.
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Convergence: Two Ocean Plates
The Marianas Trench occurs between the Pacific plate and the
Philippine plate and is 11km deep at a location called “Challenger
Deep”. This is deeper than Mt Everest is tall!
Volcanoes also form along the upper plate and parallel to
the trench much like they do from ocean–continental
collisions. When large enough, these volcanoes become
islands.
Earthquakes are also
frequent and powerfull as
they are closer to the
surface of the thinner
oceanic crust.
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Convergence: Two Continental
Continental plates are relatively buoyant and tend to crumple and lift
against each other instead of being subducted. This results in extreme
plateaus and mountains.
The collisions between the Eurasian plate and the Indian
plate form the world’s highest continental mountains called
the Himalayas. The Tibetan plateau, formed next to the
Himalayas, is higher than most of the mountains in Europe or
North America. The depth of
subducted plates can cause
earthquakes to occur deep
(300 to 700 km) within the
earth. Earthquakes with a
focus at these depths are
called “deep-focus” earthquakes.
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Transform Boundaries (
) – Two plates move beside
each other.
Transform boundaries often form
near ridges in the ocean. However,
the most recognized transform
boundary occurs along the west side
of the North American plate.
Notice:
A “zig-zag” pattern forms as the two
plates slide past each other.
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Consequences of Plate Movement
Look at the cross section below which shows the formation of a ridge like
the mid-Atlantic ridge. The material being forced out at the ridge is
distributed both to the left and the right simultaneously.
The diagram above reveals two factors that contribute to plate movement:
• Ridge “push” – the new Material added to the plate from the ridge
increases the weight of the plate.
The force of gravity will cause it to
“slide” downhill.
• Slab “pull” – as a plate is
subducted the convection currents
help draw the plate in. Much like
pulling on a roll of toilet paper.
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Hot Spots
There are many locations on the earth that appear to have
continual volcanic or intense geothermic activity, but are
not part of a plate boundary. These locations are called
hot spots.
It appears that the source of the hot spot is dependent on
the mantle. The hot spots may be a result of strong
convection currents at
one location, or they
may be the results of
magma plumes (like
the blobs you see rise
in a “lava” lamp).
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Hot Spots
As the plates drift over top the hot spot a “chain” of
volcanoes are made. Hawaii is one of the best known hot
spot chains.
1. Look at the diagram of the Hawaiian hot spot chain and
answer the questions:
a) Which island on the diagram is the oldest?
Answer – Kaui as it is furthest away
b) Which island on the diagram is the youngest?
Answer – Hawaii as it is still above the hot spot
c) If the hot spot is fixed in location, how are the islands
spread apart?
Answer – The Pacific Plate is moving.
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