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North American
P la te
Eu r a si an Plate
Caribbean
Plat e
Juan De Fuca
Plate
Philippine
Plate
Arabian
Plate
Indian
Plate
Cocos
Plate
African Plat e
Nazca
Plate
So uth American
Plate
Pacific Plate
Australian Plate
Antarctic Plate
Scotia
P lat e
C o nvergent margin
Divergent margin
Figure 6.1. Major tectonic plates of the globe. Major convergent and divergent plate
margins are marked by arrows. Most of the high-magnitude earthquakes occur
along the convergent margins.
amount of energy is required to pull a 70--100 kilometre thick oceanic
plate back into the mantle via a subduction zone. The same is true when
continents push together, such as India pushing into Asia to create the
Himalayas.
Earthquakes tend to be more severe along plate margins dominated by
transform faults and subduction/collision zones compared to spreading centres
(where tectonic plates are moving away from each other). The major tectonic
plate slide past motions on Earth occur along four long transform faults. These
are the Queen Charlotte fault in the Northeast Pacific, the San Andreas Fault in
California, the Dead Sea fault zone in the Middle East and the Alpine fault at
thesouthwestern edge of the Pacific Ocean which cuts across the South Island of
NewZealand. The major global spreading centres are found in Iceland, the Red
Sea, the Gulf of Aden, the Galapagos and Azores Islands, and the East African
Rift Valley system.
Tectonic plates collide with each other in one of three main ways: ocean
plates colliding with ocean plates, ocean plates colliding with continental plates
or continental plates colliding with continental plates. The first two result in
subduction of one plate beneath the other while the latter results in continental
upheaval. Oceanic plates are normally subducted due to their greater density.
Continental plates are usually not subducted as the huge volumes of low-density,
high-buoyancy rocks cannot sink to great depth and be pulled into the mantle
(although the Australian continental plate is being subducted below the Asian
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