Constructive Margins: Oceanic Ridges
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Plates are moving apart (Divergent)
Slow rate: 15mm per year, produces a wide ridge axis and inward facing faulting scarps (MID-ATLANTIC RIDGE)
Intermediate rate: 90mm per year, produces a smoother outline ridge (GALAPAGOS RIDGE)
Rapid rate: +90mm per year, produces a smooth crest with no ridge (EAST PACIFIC RISE)
Can form volcanoes such as Surtsey, Iceland
Volcanoes are of low viscosity, no build-up of pressure so frequent gentle eruptions
Transform faults can occur at right angles to the margin (potential for earthquakes)
Rift valleys can form – African Rift Valley
Destructive margins: Oceanic/Continental
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Denser oceanic plate is forced underneath the continental plate (subduction)
Down warping of the plate causes an ocean trench to develop
South American and Nazca plate (subducting) forming the Peru-Chile trench
Sediments accumulate on the continental shelf which are deformed and made into fold mountains e.g. Andes
Benioff zone may produce shallow focus earthquakes as the plate melts
As the plate melts the magma becomes less dense than the surrounding asthenosphere and so rises causing
plumes of magma and volcanoes
Lava is of viscous nature and forms explosive composite volcanoes
If eruptions take place offshore, island arcs can be formed
Oceanic/oceanic Convergence:
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One plate is forced underneath the other
Earthquakes can occur in the benioff zone
Melting of the pate creates plumes which form volcanoes in an island arc
Deep ocean trenches are formed due to down pulling of the plate –PACIFIC AND PHILIPPINE PLATE FORMING
GUAM ISLANDS
Continental/continental Convergence:
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As the pates have much lower density than the underlying layers, little subduction occurs
The geosyncline is forced to fold forming synclines and antisynclines
TETHYS SEA WHEN THE INDIAN AND EURASIAN PLATES FORMED THE HIMALAYAS
Little volcanic activity due to little subduction
Movement of plates can cause shallow focus earthquakes – Nepal 2015
Material is forced downwards to form deep mountain roots
Conservative Margins:
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Plate s can move in the same or different directions
Two crustal plates slide past each other
No creation or destruction of crust
No subduction or volcanic activity
Earthquakes likely
SAN ANDREAS FAULT WITH THE NORTH AMERICAN AND PACIFIC PLATE