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So much for plate collisions.
What about plate separations? In these cases, no
rock is forced down to where it can melt, so no volcanoes form, right?
Wrong! Nature does it again! Where plates are separating at divergent
boundaries, something must flow in to fill the gap or else we would
have giant, open cracks extending hundreds of kilometers into the earth's
interior. The stuff that flows in is hot, soft rock rising up from the
mantle. A small amount of basaltic rock melts and flows to the surface
through cracks. There is little or no dissolved gas in this lava, so the
eruptions are not explosive. The amounts of lava erupting from submarine
volcanoes at any given time are small, but the eruptions are continuous
and occur along all the oceanic ridges, making this type of volcano the
most common in the world. Iceland and the Kenya rift are among the
very few places on land where this type of volcanism occurs.
Sometimes plates do not actually collide, but rather they are just sliding past each other as at transform boundaries. San Andreas Fault is perhaps the most well-known transform boundary. Here plates are rubbing against each other, but no crust is either destroyed or created. Earthquakes are often associated with this kind of boundary, as the rubbing of the plates can cause friction and strain, which is released periodically. However, the earthquakes are unaccompanied by volcanic activity. We have now accounted for most of the volcanoes in the world, but there are a few other types not associated with plate boundaries. These oddballs include some of the most continuously active volcanoes in the world, like Kilauea in Hawaii. The lavas in these volcanoes are basaltic and contain little dissolved gas. This type of volcano is usually found at one end of a long chain of extinct volcanoes. For example, the active volcanoes in Hawaii form a small cluster of active volcanoes at one end of a linked chain of hundreds of extinct volcanoes nearly three thousand miles long. The ages of the volcanoes in these chains increase systematically along the chain from one end to the other. In the Hawaiian chain, their ages range from zero for the active volcanoes on Hawaii to several million years for those on Niihau, the westernmost part of the major Hawaiian islands, to over 100 million years for those at the far end of the chain. The long chains of extinct volcanoes with a "live" one at one end motivated the idea of "hotspot" volcanism: a small, fixed source of lava from deep in the mantle that continuously "burns through" the overlying lithospheric plate as it passes over. We currently think that hot spots are caused by plumes that rise from the lower mantle. Some of the earth's internal heat is transported by mantle convection, but some is also transported by plumes of very hot rock. When a plume reaches the lithosphere, the plume flattens out against the bottom of the lithosphere, heats it, and causes it to bulge and fracture. Some of the plume material rises through cracks to the surface to form a huge flood basalt volcano. Later, as the plate continues to move over the hot spot, it leaves a trail of shield volcanoes. Since the lavas come from the mantle, the plumes are basaltic in composition and low in dissolved gases. [ Locations of Volcanoes:page
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3 / page 4]
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