A divergent boundary occurs when two tectonic plates move away from each other. The Mid-Atlantic Ridge is an example of divergent plate boundaries. When two plates come together, it is known as a convergent boundary. The impact of the colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges or one of the plates may bend down into a deep seafloor trench. A chain of volcanoes often forms parallel to convergent plate boundaries and powerful earthquakes are common along these boundaries.
The Pacific Ring of Fire is an example of a convergent plate boundary. At convergent plate boundaries, oceanic crust is often forced down into the mantle where it begins to melt. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction recycling of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks is subducted under another.
The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved.
Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates. If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor.
Trenches are the deepest parts of the ocean floor and are created by subduction. Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent.
Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in this region. Even though the Nazca Plate as a whole is sinking smoothly and continuously into the trench, the deepest part of the subducting plate breaks into smaller pieces that become locked in place for long periods of time before suddenly moving to generate large earthquakes.
Such earthquakes are often accompanied by uplift of the land by as much as a few meters. On 9 June , a magnitude This earthquake, within the subduction zone between the Nazca Plate and the South American Plate, was one of deepest and largest subduction earthquakes recorded in South America. 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. Convergent plate boundaries are locations where lithospheric plates are moving towards one another. The plate collisions that occur in these areas can produce earthquakes, volcanic activity, and crustal deformation.
When continental and oceanic plates collide, the thinner and more dense oceanic plate is overridden by the thicker and less dense continental plate. The oceanic plate is forced down into the mantle in a process known as "subduction. At a depth of about miles km , materials in the subducting plate begin to approach their melting temperatures and a process of partial melting begins. This partial melting produces magma chambers above the subducting oceanic plate.
These magma chambers are less dense than the surrounding mantle materials and are buoyant. The buoyant magma chambers begin a slow ascent through the overlying materials, melting and fracturing their way upwards.
The size and depth of these magma chambers can be determined by mapping the earthquake activity around them. If a magma chamber rises to the surface without solidifying, the magma will break through in the form of a volcanic eruption.
The Washington-Oregon coastline of the United States is an example of this type of convergent plate boundary. Here the Juan de Fuca oceanic plate is subducting beneath the westward-moving North American continental plate. The Cascade Mountain Range is a line of volcanoes above the melting oceanic plate. The Andes Mountain Range of western South America is another example of a convergent boundary between an oceanic and continental plate.
Here the Nazca Plate is subducting beneath the South American plate. Visit the Interactive Plate Boundary Map to explore satellite images of convergent boundaries between oceanic and continental plates.
Two locations are marked to show this type of plate boundary - the Cascade volcanoes along the Washington-Oregon coast of North America and the Andes mountain range on the western margin of South America. Effects of a convergent boundary between an oceanic and continental plate include: a zone of earthquake activity that is shallow along the continent margin but deepens beneath the continent; sometimes an ocean trench immediately off shore of the continent; a line of volcanic eruptions a few hundred miles inland from the shoreline; destruction of oceanic lithosphere.
When a convergent boundary occurs between two oceanic plates, one of those plates will subduct beneath the other.
Normally the older plate will subduct because of its higher density. The subducting plate is heated as it is forced deeper into the mantle, and at a depth of about miles km the plate begins to melt. Magma chambers are produced as a result of this melting, and the magma is lower in density than the surrounding rock material.
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