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18.1 Forces that move Tectonic Plates Hot mantle rises at one place, then cools and falls at another. This creates convection currents within the mantle known as mantle convection. Friction between the mantle and lithosphere moves the crust along, similar to how floating herbs are moved to the sides in a pot of boiling soup. Where hot mantle rises, it heats the crust until it expands and floats higher. This makes a ridge and pulls the crust thinner; cracks open in the thinner area and magma comes to the surface. As it cools, the magma wedges the plates apart. As the new sea floor cools, its density increases and it sinks down and away from the mid-ocean ridge, pushing the rest of the plate ahead of it. This push originating from the ridge is called ridge push. Convection currents in the mantle act on surfaces of descending plate, increasing the force pulling the entire plate along. The oceanic plate descends into the mantle, pulling the rest of the plate with it This is called slab pull. About 700 km down, the temperature and pressure soften the plate, recycling it into the mantle. Mountains When tectonic plates converge, the plate with higher density may descend beneath the other, or subduct. The overriding plate bulldozes the material on top of the subducting plate. At an oceanic–continental convergent boundary, the leading edge of the overriding continental plate is compressed, causing folding, faulting, and uplift of mountains along the coast. At a continental–continental boundary, the sedimentary rock in-between the two plates is squeezed, lifted, and folded into mountains (Figure 2). The oceans were never as high as the mountains in Yoho National Park, but you’ll find fossils of marine organisms high up in the Rockies near Field, B.C. (Figure 3). The Burgess Shale found in Yoho is fossil-bearing rock that was laid down underwater more than 500 million years ago at the edge of an oceanic plate. About 200 million years ago, as the North American Plate moved over an oceanic plate, the fossil-bearing rock was pushed up and formed part of the Rocky Mountains. At divergent boundaries, the lithosphere is tearing apart, driven by upwelling magma from the mantle. Earthquakes and volcanoes often occur along a rift. Where the divergent boundary crosses the thicker lithosphere of a continent, blocks of the crust collapse as the crust stretches apart, creating rift valleys. As with undersea rifts, the edges of the rift bulge up, forming ridges. At an oceanic–continental convergent boundary, the margin of the subducting plate forms a deep ocean trench. The trenches are nearly parallel to a chain of volcanoes and the subduction process causes earthquakes. At an oceanic–oceanic convergent boundary, a deep ocean trench is also formed, parallel to a chain of volcanic islands. Trenches are the deepest places in the oceans, descending over 3 km below the ocean floor (Figure 5). Page 521 1 10