LYCOS RETRIEVER 
Plates: Boundaries
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At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The genesis of divergent boundaries is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot believed to have created the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century. Such hot spots can be very productive of geothermal power and Iceland is actively developing this resource and is expected to be the world's first hydrogen economy within twenty years.
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Sometimes, when there is a convergent boundary between two continental plates, subduction cannot occur. Since continental crust is more bouyant, or less dense, than oceanic crust, one plate does not easily override the other. Instead, the plates crumple as they plow into one another, and a very high mountain range is created. This is a special type of convergent boundary called a collisional boundary. The Himalayas in India are the result of two continental plates (the Indo-Australian and Eurasian plates) colliding head on.
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Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest.
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The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side.
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When two plates collide (at a convergent plate boundary), some crust is destroyed in the impact and the plates become smaller. The results differ, depending upon what types of plates are involved.
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