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PLATES AND PLATE MOVEMENTS
The lithosphere is broken up into about a dozen different plates (make sure you are able to name the main plates (see diagram below). Each of the crustal plates moves on top of the semi-molten asthenosphere below. The nature of the plates is dependent on the crustal formation (remember there are two types of crust (continental crust and ocean crust - see structure of the earth for the differences between them).
As well as the names of the main plates, you should know their direction of movement and their rate of movement. Crustal plates move only a few cm a year and some plates are moving much faster than others, World book, suggests movements of up to 10cm (4 inc) a year. Rates of plate movement can be calculated using palaeomagnetic research from the sea bed. (An interesting web article on the speed of continental plate motion)
3 Examples of Plate Boundaries and their rates of movement: (see your annotated map from class - or that in Waugh "An Integrated Approach" for extra examples / rates of movement)
- East Pacific Rise - Boundary between Pacific and Nazca / Anatarctic Plates - fastest rate of movement of about 10-17cm per year. (plates are moving away from each other)
- Mid-Atlantic Ridge - Boundary between N American and Eurasian / African - rate of spreading of about 2-4 cm / year (plates moving away from each other)
- Arctic Ridge - the slowest spreading ocean ridge on the earth - less than 2.5cm / year
What is the relative rate of movement of the South American, Nazca, Antarctic, African, and Pacific plates? This link has a good overview of relative rates of plate movement and their direction for the main plates (well worth a look and learn!)
What Causes Plates to Move?
Although the exact mechanism of plate movementis still debated. It is widely accepted that the most likely cause of the movement is due to the existence of convection currents in the asthenosphere. These convection currents are driven by internal heat processes.
What is the source of heat driving convection currents?
The main source of heat driving convection currents is from radioactive decay of material such as uranium and thorium in the core. It is also likely that there is residual primary heat from when the earth first formed and from frictional heating, caused by sinking of dense iron-rich material downwards towards the core.
How do Convection Currents result in plate movement?
Convection currents rise and diverge at mid-oceanic ridges, as the mantle is heated, it becomes less dense than the surrounding material and rises. As the rising convection currents in the asthensophere reach the overlying lithosphere, they move away from each other carrying the overlying crust away from the ridge, allowing the upwelling of fresh magma and the creation of new crust. As the currents begin to cool, the material becomes denser and sinks back down, the falling limbs of convection currents converge and descend at subduction zones where crust is destroyed as one plate (the denser oceanic crust) is "dragged" back down into the asthenosphere. See this excellent animation of convection current motions (link from Wycombe High School)
It is believed that slab pull is also important in the movement of plates. As plate material cools and becomes more dense, easily being dragged down into a subduction zone, the weight of the sinking slab helps to move the plate.
Source of diagram: USGS
See also the following links / resources...
A simple illustrated overview of plate movements due to convection currents (from ThinkQuest)
Another good overview of plate tectonics and an outline of why plates move