Divergent boundary
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In plate tectonics, a divergent boundary (divergent fault boundary or divergent plate boundary), (but also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents but eventually form ocean basins. Divergent boundaries within continents initially produce rifts which produce rift valleys. If the rifting process stops, a failed rift results. Therefore, most active divergent plate boundaries exist between oceanic plates and are often called oceanic rifts. Divergent boundaries are a type of stress called tension. It is when two bodies of land are spread apart.
It is thought that convection currents in the Earth's mantle rise to the base of the lithosphere where the divergent plate boundary exists. This supplies the area with vast amounts of heat and pressure that melts rock from the asthenosphere (or upper mantle) that then travels to the rift area forming large flood basalt flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other. The average rate of movement is comparable to how fast human fingernails grow, about 2 cm a year.
Over millions of years the plates have moved many hundreds of kilometers away from both sides of the divergent plate boundary. Because of this, rock closest to the boundary is younger than rock further away on the same plate.
Continental crust is often split along divergent plate boundaries.
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[edit] Divergent boundaries
- the Mid-Atlantic Ridge
- the boundary between the African Plate and Arabian Plate (the Red Sea)
- Great Rift Valley
- West Antarctic Rift
- East Pacific Rise
- Pacific-Antarctic Ridge
- Galapagos Rise
- Gakkel Ridge
It is now considered highly unlikely that convection currents within the mantle drive the movement of tectonic plates (as the mantle is in fact solid). It is now thought that seafloor spreading is caused by the 'drag' from the plate subducting at the other end. The mantle melts because when the plates move apart pressure is reduced, leading to a reduction in the melting point of the mantle close to the surface.
[edit] Other plate boundary types
[edit] See also
[edit] References
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