![]() A well-known transform plate boundary is the San Andreas Fault in California, which is responsible for many of the state’s earthquakes. Transform boundaries occur where two plates slide past each other. This creates an opening where molten rock can erupt, forming new crust. The Mid-Atlantic Ridge, a mostly underwater mountain range in the Atlantic Ocean, was created due to a divergent plate boundary. Convergent boundaries are also responsible for creating some of the tallest mountains on earth, such as K2 and Mount Everest.ĭivergent boundaries occur where two plates move away from each other. Typically, when two plates converge, the thinner plate will sink under the thicker plate in a process called subduction. Geologists classify plate boundaries into one of three categories: convergent, divergent, and transform.Ĭonvergent boundaries occur where two plates move toward each other and collide. Moving plates of cold, brittle lithosphere produce most earthquakes, especially where they are contorted and grind against one another along their boundaries.Plate boundaries occur where two (or more) tectonic plates meet. Two things are necessary for earthquakes to occur: 1) motion within the Earth that stresses and deforms material and 2) material that deforms by breaking rather than flowing. The fluids (mostly water) rise, wetting hot rock in their path and causing some of the rock to melt. As one plate dives (subducts) beneath the other, it heats up and releases hot fluids (similar to the way we sweat when we get hot). The second mechanism results in volcanism at convergent plate boundaries. As the hot mantle rises, sudden drop in pressure causes melting, much as taking the lid off a pressure cooker causes super-heated water to flash to steam. The first type of melting occurs at diverging plate boundaries (mid-ocean ridges continental rift zones) and hotspots, where mantle beneath plates is hot but remains solid because it is under great pressure. There are two main ways Earth materials melt: 1) hot mantle rises and decompresses and 2) water flows through hot rock. Volcanic activity occurs above places in the Earth where pressure, temperature, and the presence of water conspire to melt rock. And a mountain chain of volcanoes forms where a plate rides over a hotspot the volcanoes get older and lower as the plate moves them away from the hotspot. Sheared-up mountain ridges and valleys form in a narrow zone where one plate slides past another at a transform plate boundary. The highest mountains on Earth, the Himalayas, are forming where thick blocks of continental crust (India and Asia) collide as a result of plate convergence. One is a coastal range consisting of material squeezed up out of the sea, the other a volcanic chain farther inland above where hot water rises from the descending plate. Where plates converge along an active continental margin, the one with thin oceanic crust descends (“subducts”) beneath the continent, forming two parallel mountain ranges. Plate divergence sometimes rips a continent apart, forming long mountain ranges separated by deep rift valleys. ![]() The largest mountain range on Earth is the system of mid-ocean ridges, a line of underwater volcanoes formed on the seafloor as plates diverge. Most mountain ranges are long and narrow because they form at plate boundaries or hotspots. ![]() The Atlantic and Gulf coasts, though initially formed from plate-boundary activity, are now far from any plate boundaries and accompanying tectonic events. The Pacific coast of the United States coincides with plate boundaries and thus has earthquakes, volcanic eruptions, and developing mountain ranges. ![]() So why is there so much tectonic activity (earthquakes, volcanic eruptions and the formation of mountain ranges) in the western United States compared to the East? Tectonic activity commonly occurs along the boundaries of the moving plates. Convergent boundaries are the big culprits. Divergent boundaries have some, but not tons of vertical motion. Earthquakes at transform boundaries, like the San Andreas fault, involve hardly any vertical motion. Remember the three types of plate boundaries. Lillie, Wells Creek Publishers, 92 pp., 2015, The Action is at The Edges We probably wouldn't expect a big tsunami. Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. The plates rip apart at divergent plate boundaries, crash together at convergent plate boundaries, and slide past each other at transform plate boundaries. Earth’s outer shell is broken into tectonic plates that move relative to one another. ![]()
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