When two adjoining tectonic plates suddenly slip, this can cause devastating earthquakes.
Tectonic plates are large sections of the Earth's crust that extend about 100 kilometers below Earth's surface. These plates interlock like the pieces of a jigsaw puzzle and slowly move over the course of millions of years. When two adjoining plates move relative to each other, this can result in mountain formation, earthquakes and a host of other geological phenomena.
Tectonic motion is caused by several forces, including gravity, Earth's rotation and heat convection. A tectonic plate moves when the denser, more fluid layers of the Earth that it lies upon move as a result of these forces.
The Tectonic Plates and Their Boundaries
Mount St. Helens is part of the Pacific "ring of fire," an area of intense volcanoes and earthquakes due to tectonic movement.
There are seven major (primary) tectonic plates and numerous smaller secondary and tertiary plates. These plates are part of the lithosphere, a layer of comparatively light and brittle rock that lies on top of a layer called the aesthenosphere, which is rendered semi-fluid due to the immense temperature and pressure it is subjected to.
The aesthenosphere moves up to several centimeters a year, causing the overlying tectonic plates to move as well. When two plates converge, they collide and form a mountain range. Similarly, when they diverge, they form a valley, rift or ridge. Additionally, tectonic plates can move laterally relative to each other, as is the case at the San Andreas fault.
Volcanic activity and earthquakes are common at these boundaries. The Earth's crust is often thinner in these areas and sudden slippage of adjoining plates can occur when pressure builds up.
Mantle Convection
The middle layers of the Earth between the core and the crust are called the mantle. The mantle is heated from below by the radioactive decay of Uranium and cooled from above by passive dissipation and volcanic activity. Some of this heat is also transformed into movement. Differences in heat and density act on the mantle like a giant conveyor belt moving several centimeters per year. This is perhaps the greatest contributor to tectonic movement.
Gravitational Forces
The violent eruption of magma at weak spots in the planet's crust illustrates the massive pressures experienced deep in the Earth.
The Earth's gravity exerts tremendous pressure upon the mantle and core layers. Due to local differences in density and thickness, however, this pressure isn't completely uniform. This difference in gravitational stress on the deep layers determines the direction that the aesthenosphere beneath the tectonic plates will move, and hence determines the general direction of tectonic movements.
Tidal Forces
The moon causes ocean tides by slightly raising sea level through its gravitational pull.
In addition to the Earth's internal gravity, other celestial bodies can affect tectonic motion through gravitational attraction. The moon and, to a lesser extent, the sun both pull upon the Earth, slightly deforming its shape over the course of rotation. This deformation results in internal friction, which translates to heat. Thus, tidal forces contribute to a third, albeit minor, source of tectonic movement.
The Importance of Mantle Ductility
The mantle is a layer of semi-liquid rock under intense heat and pressure that lies above the liquid metal layer of the Earth's outer core.
Tectonic plates exist only because the Earth's upper crust is hard and brittle while the mantle is ductile and flows like a very slow-moving fluid. Without a dynamic, moving mantle, the plates would stop moving despite the continued presence of some of these forces.
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