Almost immediately following a megathrust earthquake, the uplifted region drops by as much as several meters 1 meter is about 3 feet , causing sea level to rise by amounts that would take hundreds of years if due to climate change. Land Level Change. Schematic diagram of a subduction zone, showing the location of the outer rise and tensional stresses within the subducting plate. Trenches form where the subducting plate begins its descent and can be as much as 11 kilometers 7 miles deep.
Thick layers of sediment may accumulate in the trench, and these and the subducting plate rocks contain water that subduction transports to depth, which at higher temperatures and pressures enables melting to occur and 'magmas' to form. The hot buoyant magma rises up to the surface, forming chains of volcanoes. Note: There is a third setting of destructive boundary: continent-continent. Here, the pushing together of two continental plates results in the mountain forming processes that shaped, for example, the Alps and Himalayas.
Volcanoes can also form above a column of superheated magma called a mantle plume. This may happen in areas that are distant from plate boundaries.
It is also referred to as hot spot or intraplate volcanism. Heat from the mantle plume causes melting and thinning of the crust, which leads to volcanic activity at the surface. Over millions of years, the Pacific Plate has moved over the hot spot, creating a chain of volcanic islands.
This simple schematic diagram shows the movement of the tectonic plate light brown over a mantle plume, or hot spot, to produce a chain of volcanic islands. The superheated magma rises through the mantle yellow , melts the crust above brown and flows on to the surface forming a volcano. Diagram not to scale. Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages.
The Earth beneath our feet is constantly shifting and moving, and violently with catastrophic and immediate results. Find out more about earth hazards. We have a team of volcanologists that works on various research projects in locations around the world to help governments and local people to understand volcano behaviour. Different types of volcano are formed according to the properties of the magma that creates them. Volcanic eruptions can be explosive, sending ash, gas and magma high up into the atmosphere, or effusive, producing lava flows and domes.
Find out about the different types of volcanic hazards that put human lives, livelihoods or infrastructure at risk of harm. Another example, forming the northern rim of the Ring of Fire , is the Aleutian trench shown here:. The Pacific plate descends into the mantle at the site of the Aleutian trench.
Subduction zone volcanism here has generated the Aleutian island chain of active volcanoes. Courtesy of NOAA. The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater and greater depths, it progressively encounters greater temperatures and greater pressures which cause the slab to release water into the mantle wedge overlying the descending plate.
The second important tectonic setting where many volcanoes occur is along or near converging plate boundaries. At most such boundaries, where two plates collide, the heavier of the two - usually an oceanic one - sinks or is pulled under the other plate, a process called subduction.
Subduction consumes lithosphere and since the surface of the earth is a constant, it compensates for the amount of lithosphere created at divergent plate boundaries. Illustration of a subduction zone. Oceanic-oceanic plate collision, subduction and formation of an island arc. Oceanic-continental plate collision, subduction and formation of a volcanic arc.
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