Topographic maps disclose the areas of big earthquakes and also indicate the limits of the 12 major tectonic plates. Because that example, the Pacific key is bounded by the earthquake zones of brand-new Zealand, new Guinea, the Mariana Islands, Japan, Kamchatka, the Aleutian Islands, western phibìc America, the east Pacific Rise, and the Pacific-Antarctic Ridge.

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Major capital for this expedition was provided by NOAA Ocean expedition Program and also NOAA Vents Program; video clips edited by invoice Chadwick, Oregon State University/NOAASee all videos because that this article

The volcanoes top top the western and also northern margin of the Pacific plate (New Zealand, brand-new Guinea, Mariana Islands, Japan, Kamchatka, and also the Aleutian Islands) are all subduction volcanoes. The rift volcanoes space largely covert along the submarine comb of the east Pacific Rise and also the Pacific-Antarctic Ridge at depths of 2 to 3 km (1.2 to 1.9 miles) listed below sea level. The Cascade volcanoes in the northwestern unified States and also the volcanoes in Mexico and main America are regarded the subduction under the phibìc American bowl of the little Juan de Fuca and also Cocos plates, which are on the east side that the Pacific Plate. Similarly, the volcanoes that the Andes are related to the subduction that the Nazca Plate in ~ the south American Plate.

Conceptual models of exactly how subduction and rift volcanoes may type are displayed schematically in the diagram and also in the video. Of the 1,450 volcanoes noted in the table of landform types, 80 percent occur along subduction zones, and also 15 percent happen along rift zones. This percentages are somewhat misleading, however, since most of earth’s rift zones are around 2 to 3 kilometres (1.2 come 1.9 miles) listed below sea level, where volcanic activity is tough to detect. In ~ those depths energetic submarine volcanoes have actually yet to be observed, though many hydrothermal areas have been discovered along submarine rift zones by research study submersibles. Iceland, a segment that the Mid-Atlantic Ridge the emerges above sea level, has 70 volcanoes that have erupted during the previous 10,000 years. If this is a usual number for a rift system, there may be several thousand potentially active volcanoes follow me the oceanic ridges that room the surface expressions of the human being rift system.

Subduction volcanoes

As an oceanic plate is subducted beneath a continent plate, seafloor sediments wealthy in water and also carbon dioxide are brought beneath the overriding plate. This compounds may act as fluxes, to reduce the melting temperature the magma. Return the process is not plainly understood, magma apparently forms and rises by buoyancy indigenous a depth of 100 to 200 kilometres (60 to 120 miles). Subduction-zone volcanoes take place on the overriding plate and are balance out inland native the actual bowl boundary along the s trench.

The increasing subduction-zone magma is more than likely basaltic in composition and is developed by the partial melting of mantle rocks. Together the rising magma moves slowly up through the continental crust that the overriding plate, however, two things may happen to increase considerably the silica content of the magma. Crystallization the olivine and pyroxene minerals from the basalt have the right to leave the residual melt enriched in silica and depleted in magnesium, iron, and calcium. This process is called fractional crystallization. Also, basaltic magmas have sufficient excess heat to partially melt the continental host rocks through which they space ascending. Because continental rocks are generally greater in silica, potassium, and sodium than are oceanic rocks, this process of assimilation and mixing can additionally play an essential role in developing the wide range of compositions that take place in rocks indigenous subduction volcanoes.

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The additional gas content of many magmas in ~ subduction volcanoes (which, coupled through their regularly high-viscosity magma, makes them dangerously explosive) might be defined by more than one process. Extr water and carbon dioxide might come native both subducted seafloor sediments and assimilated crustal rocks. Furthermore, any kind of fractional crystallization has tendency to concentration volatile aspects in the residual melt. If volcano gases form separate fluid phases within batches the ascending magma (as carbon dioxide gas is most most likely to do), these fluid phases may ascend an ext rapidly 보다 the as whole magma body and also be concentrated in the upper portion. Sudden growth of these hot volcanic gases at atmospheric press is the obvious reason for the very explosive nature of countless subduction volcanoes.