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).

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.

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

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

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 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: N. Atlantic Ridge3cm/yr S. Atlantic 5cm/yr N. Pacific12.5cm/yr E. Pacific17.5 cm/yr

Deep, narrow troughs the border many ocean basins. Oceanic Trenches

Thousands of kilometers long, 50 to 100 km wide and several kilometers deep (below sea level). Longest trench: Peru-Chile trench at 5,900 km. Deepest trench: Mariana trench (western Pacific); over 11 km deep.

Trenches are termed Convergent Plate Margins because they are locations where plates converge on, or push against, each other. Where oceanic crust is subducted back into the upper mantle.

Crust melts as it descends, beginning at 100 to 200 km depth and has melted completely by 700 km depth. The zone over which melting takes place is termed the Benioff Zone. Crust descends at angles from 35 to 90 degrees.

Melting crust rises and penetrates overlying crust to form volcanoes. Material (sediment and basaltic rock) is scraped off the subducting crust and accreted to the over-riding crust – termed the subduction complex.

Island Arcs parallel many oceanic trenches: arc- shaped chains of volcanic islands (e.g., Japan) due to the rising magma from melting subducted crust.

Oceanic Crust-Oceanic Crust Volcanic islands develop at the surface of the over- riding crust (forming Island Arcs). The oldest, densest crust normally descends beneath the younger crust. Convergent Plate Margins

Magma material rises from descending slab and builds volcanoes in the rising mountains. Coastal mountain chains develop due to compressive forces and volcanics (e.g., the Andes of South America). Oceanic Crust – Continental Crust Basaltic oceanic crust descends beneath lighter continental crust.

Continental Crust-Continental Crust Compressive forces driving plates fold and thrust the continental margins forming an extensive mountain belt (e.g., the Himalayan Mountains). Neither plate subducts (both too light).

Plate margins along which the plates slip by each other. Termed: Transform Faults On either side of a transform fault plate motions are in opposite directions. Transform Plate Margins

Transform faults displace the oceanic ridge.

Spacing of transform faults is proportional to the rate of spreading. The faster the rate of spreading the greater the distance between transform faults.

The San Andreas Fault is a transform fault. The land east of the fault is on the North American Plate; the land west of the fault is on the Pacific Plate. The eastern side of the fault moves southeast and the western side moves to the northwest. Total movement along the fault has been 564 km over the past 30 million years (1.9 cm per year).

Rupturing of the ground surface along the San Andreas Fault.

Wallace creek crossing the San Andreas Fault

To summarize……

But what drives plate tectonics? Two main hypotheses: 1. Convection Cells within the upper mantle (first postulated by Arthur Holmes a year before Wegener died). and 2. Ridge push and slab pull.

Giant convection cells within the upper mantle drag the plates along laterally. Where convection rises sea floor spreading takes place. Where the convection cells descend they drag crust down, causing subduction. Mantle Convection

Here’s a link to an animation showing how convection might drive plate tectonics.link

Where new, young crust forms its weight pushes down slope to drive the plates laterally. Once the crust has cooled, having been pushed away form the ridge, it sinks into the upper mantle and helps to pull adjacent crust along. This pushing and pulling provides the forces that drive plate tectonics. Ridge push and slab pull