1 Structure of the Earth Plate Boundaries Driving Mechanisms of Plate Tectonics Plate Tectonics The Earth can be considered as being made up of a series of concentric spheres, each made up of materials that differ in terms of composition and mechanical properties. Structure of the Earth Crust and Lithosphere Crust: the outermost layer of the earth, a hard outer shell. Crust beneath the oceans and the continents is different: Oceanic crust: relatively thin, varying from 5 to 8 km (but thinner at Oceanic ridges). Has the average composition of basaltic rock that is rich in silica and magnesium (Sima). Continental Crust: thicker and more variable in thickness than oceanic crust: Thickness ranges from 20 km to about 75 km (beneath mountain ranges). Has the average composition of granitic rock that is rich in Silica and Aluminum (Sial).
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Structure of the Earth
Plate Boundaries
Driving Mechanisms of Plate Tectonics
Plate Tectonics
The Earth can be considered as being made up of a series of concentric spheres, each made up of materials that differ in terms of composition and mechanical properties.
Structure of the Earth
Crust and Lithosphere
Crust: the outermost layer of the earth, a hard outer shell.
Crust beneath the oceans and the continents is different:
Oceanic crust: relatively thin, varying from 5 to 8 km (but thinner at Oceanic ridges).
Has the average composition of basalticrock that is rich in silica and magnesium(Sima).
Continental Crust: thicker and more variable in thickness than oceanic crust:
Thickness ranges from 20 km to about 75 km (beneath mountain ranges).
Has the average composition of graniticrock that is rich in Silica and Aluminum (Sial).
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The lithosphere has the composition of the upper mantle (Iron and Magnesium Silicates) but is rigid like the crust.
Lower temperatures and pressures allow the Lithosphere to be rigid.
Very thin (a couple of km) at the Oceanic Ridge; extends to 80 km depth beneath old oceanic crust that is well away from the Ridge.
Lithosphere:The topmost layer of the upper mantle.
The crust and lithosphere “float” on the underlying mantle.
Where the crust and lithosphere are thick (e.g., beneath continental mountains) they extend deeper into the upper mantle.
Beneath the continents the lithosphere extends to up to 300 km beneath mountain ranges.
USGS
The crust and lithosphere are broken up into 25 Lithospheric Plates
The Mantle includes the Lithosphere.
Unlike the crust, the mantle is dominated by Iron and Magnesium Silicate minerals.
Upper Mantle: near its melting point so that it behaves like a plastic (Silly Putty is a reasonable analogy); the upper mantle material flows under stress.
Lower Mantle: solid material, rather than plastic.
Upper mantle material flows by convection; transfers heat from within the Earth towards the surface.
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The Core: the metallic portion of the Earth; Iron mixed with small amounts of Nickel.
Outer Core: probably liquid (based on studies of shock wave passage through the Earth).
Inner Core: solid, made up of cooled liquid core material.
USGS
Plate Boundaries
Plate Boundaries
The types of boundary between plates are distinguished by the type of relative plate motion along the boundary:
Oceanic Ridge – Divergence
Transform Margins – Horizontal slip
Oceanic Trench – Convergence
Oceanic Ridge
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More-or-less continuous volcanic mountain chain throughout the world's oceans.
65,000 km long.
Average width approx. 1,000 km.
Average depth approx. 2.3 km below sea level.
Rise up to 3 km above the surrounding sea floor.
A kilometre deep valley runs along much of the length of the ridge.
The ridge is a Divergent Plate Margin and divergence takes place by Sea Floor Spreading.
From http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html
New crust is added from upwelling magma (molten rock) from the upper mantle.
Older crust is pushed laterally away from the ridge axis – so that the sea floor spreads away from the ridge axis.
Oceanic crust becomes older with distance from the oceanic ridge.
Spreading rates (distance per year that two points on either side of a ridge move apart) vary: