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Theory of Plate Tectonics
People noticed that continents fitted together.
Geologist Alfred Wegener published his theory in 1912 that all the continents were
once joined together.
He named this ancient supercontinent Pangaea.
Wegener proposed that at some time the land masses had drifted apart until they
occupied their current positions on the globe.
Wegener's Evidence
CONTINENTAL FIT:
The continents seemed to fit together nicely if placed besides each other.
This is especially true if the continental shelves are taken into account.…read more

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SEA FLOOR SPREADING:
Surveys recorded very young ages for places near or on the ridges.
This is because older crust is continuously being pushed aside by new crust.
This raised a new issue there was no evidence to suggest the earth was
growing in size.
Therefore, ocean crust must be consumed elsewhere.
This led to the discovery of large ocean trenches where ocean floor was being
subducted.
Subduction provided the mechanism for sea floor spreading and the drifting of
continents.…read more

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Where spreading occurs beneath a major land mass, the valley sides move
apart.
As this happens, the central sections drop down to form rift valleys.
Destructive Plate Boundaries
OCEANIC ­ CONTINENTAL PLATE CONVERGENCE:
Oceanic crust is subducted because it is denser than continental crust.
As the oceanic crust descends, friction with the overlying continental crust
builds up and causes major earthquakes.
Destructive boundaries are some of the most seismically active in the world.…read more

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FISSURE ERUPTIONS:
Occur where elongated cracks in the crust allow lava to spill out over a large
area.
Typically these are found around spreading ridges when tension pulls the
plates apart.
Basaltic rock and lava type.
Gentle, persistent eruptions.
SHIELD VOLCANOES:
Basaltic rock and lava type.
Less viscous lava.
Gently sloping cones.
Located at hot spots where oceanic crust meets oceanic crust.
Eruptions are gentle and predictable.
COMPOSITE VOLCANOES:
Most common type found on land.…read more

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Minor Extrusive Features
GEYSERS:
Develop when groundwater held in rocks is heated by underlying rocks at
temperatures well above boiling point.
As this water rises towards the surface it dissolves silica from the surrounding
rocks and lines rock crevasses.
This creates a complex underground plumbing system of pipes and
reservoirs.
Constrictions (the process of something becoming tighter) cause the
superheated water to accumulate temporarily in underground reservoirs.…read more

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Major Intrusive Features
BATHOLITHS:
Formed by huge masses of magma that cooled slowly within the Earth's crust.
Subsequent erosion stripped away the overlying rocks exposing part of the
igneous mass at the surface.
Because batholiths compromise hard, resistant rock, they form upland areas.
Bosses are the same as batholiths but on a smaller scale.
Minor Intrusive Features
DYKES:
Smallscale vertical intrusions that influence the relief on a local scale.
They are thin sheets of igneous rock.
Some extend over long distances.…read more

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Impact of Eruptions
Human impacts can be affected by the following:
Exposure. The scale and frequency of the natural event and the number of
people in an affected area.
Vulnerability. The preparedness of a country or population to cope with a
hazard.
Physical impacts can be affected by the following:
Nature of volcanic ejecta and its violence. Gentle eruptions pose little threat to
human life although lava flows may destroy farmland, buildings and
infrastructure.…read more

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