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Tectonic Plates
The Structure of the Earth and Tectonic Plates

Tectonic Plates

The earth's surface is broken into seven large and many small moving plates. These plates, each about 50 miles thick, move relative to one another an average of a few inches a year. Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault.


The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot.


At convergent boundaries, plates move toward each other and collide. Where an oceanic plate collides with a continental plate, the oceanic plate tips down and slides beneath the continental plate forming a deep ocean trench (long, narrow, deep basin.) An example of this type of movement, called subduction, occurs at the boundary between the oceanic Nazca Plate and the continental South American Plate.


Where continental plates collide, they form major mountain systems such as the Himalayas.

Mid Atlantic Ridge

At divergent boundaries, plates move away from each other such as at the Mid-Atlantic Ridge. Where plates diverge, hot, molten rock rises and cools adding new material to the edges of the oceanic plates. This process is known as sea-floor spreading.

Transfor-fault boundaries

At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly northwestward relative to the North American Plate on which San Francisco sits.


Plate tectonics, the branch of science that deals with the process by which rigid plates are moved across hot molten material, has helped to explain much in global-scale geology including the formation of mountains, and the distribution of earthquakes and volcanoes.

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