These zones are marked by deep ocean trenches and chains of volcanic mountain ranges or island arcs that form parallel to the plate boundaries. An example of this kind of convergence is happening at the Cascadia subduction zone off the coast of the Pacific Northwest. At divergent boundaries, plates are moving away from each other. Volcanic activity and earthquakes occur at divergent boundaries, but they are not as violent as those at convergent boundaries. Where plates diverge below the ocean, magma molten rock rises from the mantle to fill the space between the plates and solidifies, forming underwater mountain ranges called mid-ocean ridges.
On land, rift valleys form where plates diverge. A good example of this is the East African rift valley. At transform boundaries, plates are sliding past each other horizontally in different directions. Transform boundaries can produce great earthquakes, but volcanoes are rare.
The San Andreas fault, which separates the North American plate from the Pacific plate and is responsible for many of California's earthquakes, is located on a transform boundary. As the plates move, their rough edges can get stuck on each other. This stops movement at the boundary while the rest of the plates keep moving. Stress builds up, and when it becomes too much, the plates suddenly slip past one another, and the rocky, brittle lithosphere cracks.
These cracks are called faults. It is the energy released by the sudden movement on these faults that causes most earthquakes. Plate boundaries are made up of many faults. How plates move relative to each other determines, in part, the type of faults at their boundaries. There are three basic types of faults: reverse or thrust , normal, and strike-slip. Earthquakes are often described based on the type of fault they occur on.
However, earthquakes are not typically so simple. It is common for fault movements to include both up-and-down and side-by-side movement together. In addition, not all earthquakes occur at plate boundaries. Faults located far from plate boundaries also generate earthquakes, but less frequently, and they are difficult to explain. Though smaller in size, the minors are no less important when it comes to shaping the Earth.
The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States. The plates make up Earth's outer shell, called the lithosphere. This includes the crust and uppermost part of the mantle. Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide. The movement of the plates creates three types of tectonic boundaries: convergent, where plates move into one another; divergent, where plates move apart; and transform, where plates move sideways in relation to each other.
Where plates serving landmasses collide, the crust crumples and buckles into mountain ranges. India and Asia crashed about 55 million years ago, slowly giving rise to the Himalaya , the highest mountain system on Earth. As the mash-up continues, the mountains get higher. Mount Everest, the highest point on Earth, may be a tiny bit taller tomorrow than it is today. These convergent boundaries also occur where a plate of ocean dives, in a process called subduction, under a landmass.
As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America.
At ocean-ocean convergences, one plate usually dives beneath the other, forming deep trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth. These types of collisions can also lead to underwater volcanoes that eventually build up into island arcs like Japan. At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. You are using a browser version with limited support for CSS.
To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. An earthquake centred in the Zagros Mountains toppled a building in Darbandikhan, Iraq.
Earth Planet. Download references. By Jeff Hecht. Crust forming faster? But the result is controversial, since previous work seemed to show the opposite. Plate tectonics is driven by the formation and destruction of oceanic crust. This crust forms where plates move apart, allowing hot, light magma to rise from the mantle below and solidify.
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