Quote: Originally posted by B(a)P  |
Curios about your theory on volcanoes? How they form, where does all that molten rock come from? |
Volcanoes are all either at or near tectonic plate boundaries or above plumes of hot rock in the mantle (so-called "hotspots").
The boundary variety come about because one plate has collided with another and is "diving" beneath it. This is called subduction. The plate being
subducted is under the ocean, and the plate it is subducting under is continental crust. The oceanic plate thus carries lots of water with it. This
water acts to reduce the melting point of the rocks being carried down. The flowing rock of the plate is solid at first, and it doesn't melt until it
has been carried some distance under the continental plate.
Once the rock has been brought deep enough, though, some of it does liquify forming magma. This magma has lower density than the still solid rock
around it, so it slowly begins to rise through the rock of the overlying continental margin. Eventually it rises so far that it exerts enough pressure
on the thinning crust to form cracks and erupt to the surface. The first lava flows at the surface begin to create the body of a classic conical
volcano. The details differ depending on the exact chemistry of the rocks melted to form the magma, but this is why active volcanoes are found mostly
on the margins of continents. Mt St Helens is a prime example.
The other type of volcano is responsible for building up new islands in ocean plates travelling over those mantle plume hotspots mentioned earlier.
Their geochemistry is such that they form shield volcanoes where the magma has much lower viscosity and spreads more horizontally than is the case
with continental volcanoes. The Hawaiian Island chain is a good example which illustrates that the hotspot plume responsible seems to stay more or
less stationary while the marine plate travels over it, leaving a chain of extinct island volcanoes no longer directly above the plume.
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