The Pacific War Online Encyclopedia
|
| Previous: Spratly Islands | Table of Contents | Next: Spruance, Raymond A. |
According to the theory of plate tectonics, the major features of the Earth's crust result from convective currents in the Earth's mantle, where a slow plastic flow of hot rock to the surface from deep in the earth is balanced by a similar slow flow of cooler rock down into the depths. This is possible because the solid mantle rock is hot enough to slowly creep under pressure, like metal that has been softened in a forge prior to being worked.
The regions where hot mantle rock is slowly creeping towards the surface are known as spreading centers. The earth's crust is pulled apart here, and liquid magma generated by the upwards flow periodically erupts through a long series of parallel fissures. This magma is usually poor in silica and gas, so the lava is very hot and fluid, and the eruptions are not very explosive. The lava solidifies as mid-oceanic ridge basalt (MORB).
The largest spreading centers are
located in ocean basins
and were unknown to geologists before the era of modern
oceanography. Only a
few small spreading centers are found on land, and their significance
was not
at once recognized. The largest is the East African Rift
Valley, where the
continent of Africa is being torn apart. This spreading
center is still very
young and the characteristic basalt eruptions have not yet
begun. Instead,
such magma as reaches the surface is heavily contaminated with melted
crust and
is richer in silica and volatiles. This allows large volcanic
peaks such as
Kilimanjaro to form. The Basin and Range Province in the
western United States
may lie over another spreading center, but this is less certain.
The Pacific has many oceanic spreading centers. The largest is the East Pacific Rise, from which ocean crust slowly creeps towards the northwest, forming the characteristic patterns of hot spot island chains. Much smaller spreading centers lie behind island arcs such as Tonga and Japan. These back-arc spreading centers probably arise from a rather shallow flow in reponse to the subduction along the fore-arc subduction zone.
The Pacific War Online Encyclopedia © 2007, 2010 by Kent G. Budge. Index
