The edges, or boundaries, of tectonic plates meet up in different ways. There are three main types of boundaries: transform, convergent, and divergent. At transform boundaries, plates move past each other. This is one of the most common causes of earthquakes.
At convergent boundaries, plates move toward each other. They can push together and cause mountain ranges to form. Ocean-to-continent convergence occurs when oceanic crust converges with continental crust, forcing the denser oceanic plate to plunge beneath the continental plate.
This process called subduction , occurs along oceanic trenches called subduction zones where lots of intense earthquakes and volcanic eruptions can occur. The denser, subducting plate begins to heat up under extreme pressure near the mantle and melts to create causes melting in the volcanoes. These coastal volcanic mountains are found in a line above the subducting plate.
The volcanoes are known as a continental arc. The movement of crust and magma causes earthquakes. Click here for a map of earthquake epicenters at subduction zones. The Juan de Fuca plate is created by seafloor spreading just offshore at the Juan de Fuca ridge. If the magma at a continental arc is felsic, it may be too viscous thick to rise through the crust. The magma will cool slowly to form granite or granodiorite. These large bodies of intrusive igneous rocks are called batholiths, which may someday be uplifted to form a mountain range.
An oceanic-to-oceanic plate boundary occurs when two oceanic plates converge, causing the older, denser plate will subduct into the mantle.
An ocean trench marks the location where the plate is pushed down into the mantle. The line of volcanoes that grows on the upper oceanic plate is an island arc. The Ring of Fire is a ring around the Pacific Ocean of subduction zones, which most are oceanic-to-oceanic convergence. Here is an animation of an ocean continent plate boundary. Along these subduction zones, volcanic islands also called volcanic arcs form. Fortunately, even though this powerful earthquake was felt as far away as Minnesota and Toronto, Canada, it caused no major damage because of its great depth.
Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest. The eruptive activity is clearly associated with subduction, but scientists vigorously debate the possible sources of magma: Is magma generated by the partial melting of the subducted oceanic slab, or the overlying continental lithosphere, or both? As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed.
The Marianas Trench paralleling the Mariana Islands , for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate.
The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior nearly 11, m than Mount Everest, the world's tallest mountain, rises above sea level about 8, m. Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes.
Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano.
Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.
The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion.
Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow continuous convergence of these two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.
Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8, m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4, m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.
Below: Cartoon cross sections showing the meeting of these two plates before and after their collision. The reference points small squares show the amount of uplift of an imaginary point in the Earth's crust during this mountain-building process.
The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers divergent plate boundaries or, less commonly, trenches convergent plate boundaries. You will learn more about volcanoes in the following lessons. As the volcano erupts it may build a mountain.
The lava along with ash and other pyroclastic material will continue to build the mountain higher with each eruption. This is a cross section of the Earth in the Southern Hemisphere. The map shows a subduction zone that has created the Peru-Chile Trench at the western edge of South America.
This subduction zone has produced the Andes Mountains which run along the entire west coast of South America.
It also shows you the Mid-Atlantic Ridge which is spreading the Atlantic Ocean making it wider and wider. The cross section shows two processes at work;. The pink lines on this map of the Pacific Ocean represent deep ocean trenches. These trenches are some of the lowest points on the crust of the Earth.
Marianas Trench north of New Guinea is the deepest point on the Earth's surface at 36, feet below sea level. Marianas Trench is 7, feet deeper than Mount Everest is high!!!! Trenches surround almost all of the Pacific Ocean. There are trenches wherever continental plates and oceanic plates collide. The Java Trench in the Indian Ocean is the deepest point of that ocean at 24, feet below sea level.
Write the answers to the following questions in complete sentences on a piece of paper. Use the page titles located directly under the questions to move your way through the lesson to locate the answers.
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