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Plate Movement
The theory of plate tectonics
1912, Alfred Wegener published theory of single continent (Pangaea) that existed 300 million
years ago
o Later split into 2 continents
Laurasia in north
Gondwanaland in south
Different evidences to prove single continent
o Geological evidence
Bulge of south America fitting into indent…

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Seen in Hawaii (see picture below)




Features of plate margins

Constructive (divergent) margins
o Plates move apart in oceanic areas
o RIDGE VALLEYS
Longest continuous uplifted features
Precise form influenced by rate at which plates move apart
Slow rate ­ 10-15mm/year, produces wide ridge axis (30-50km) and
deep central rift…

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OCEANIC/OCEANIC CONVERGENCE
Ocean trenches and island arcs are the features associated ­ takes place
offshore

CONTINENTAL/CONTINENTAL CONVERGENCE
Plates forming continental crust have much lower density than underlying
layers, not much subduction where they meet
Due to there being no subduction ­ no volcanic activity. Can create shallow-
focus earthquakes

Conservative…

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Vulcanicity
Distribution
Most volcanic activity associated with plate tectonic processes, mainly located along plate
margins
Such activity therefore found:
o Along ocean ridges
o Associated with rift valleys
o On or near a subduction zones
o Over hot spots


Volcanic Eruptions
Vary in form, frequency and type of volcanic eruption…

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Main types of extrusive volcanic landforms
o Basic/shield volcanoes ­ formed from free flowing lava. Have gentle sides and cover a
large area
o Fissure/lava plateaux ­ extensive lava flows are basaltic in nature, flow great distances
o Acid/dome volcanoes ­ steep sided convex cones, viscous lava that is rhyoltic…

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o Volcanic gases ­ include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur
dioxide and chlorine
Secondary effects :
o Lahars ­ volcanic mud flows
o Flooding ­ melting of glaciers and ice caps
o Tsunamis ­ giant sea waves generated after violent caldera-forming events
o Volcanic landslides
o Climatic change…

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Seismicity
Causes of earthquakes
As crust of Earth is mobile, tends to be a build up of stress within the rocks
When pressure is suddenly released part of the surface experience an intense shaking motion
Point of pressure release is known as focus ­ point immediately above on earth's surface…

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Seismic records enable earthquake frequency to be observed ­ records only date back to 1848
when instrument to record seismic waves was first developed


Effects of earthquakes
Initial effect is ground shaking
Severity depends on magnitude of earthquake, distance from epicentre, local geological
conditions
Secondary effects:
o Soil liquefaction when…

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