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The magma ascends to form an arc of volcanoes parallel to the subduction zone. Volcanic arcs are distinct from volcanic chains formed over hotspots in the middle of a tectonic plate. Volcanoes often form one after another as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other.
Volcanoes that occur above subduction zones, such as Mount St. Helens, Mount Etna, and Mount Fuji, lie approximately one hundred kilometers from the trench in arcuate chains called volcanic arcs. Plutons, like Half Dome in Yosemite National Park, generally form 10–50 km [ 65 ] below the volcanoes within the volcanic arcs and are only visible ...
The arc formed due to subduction along the Cascadia subduction zone. Although taking its name from the Cascade Range , this term is a geologic grouping rather than a geographic one, and the Cascade Volcanoes extend north into the Coast Mountains , past the Fraser River which is the northward limit of the Cascade Range proper.
Subduction zones are marked by oceanic trenches. The descending end of the oceanic plate melts and creates pressure in the mantle , causing volcanoes to form. Back-arc basins can form from extension in the overriding plate, in response to the displacement of the subducting slab at some oceanic trenches .
The Tonga–Kermadec Ridge is an oceanic ridge in the south-west Pacific Ocean underlying the Tonga–Kermadec island arc.It is a result of the most linear, fastest converging, and seismically active subduction boundary on Earth, the Kermadec–Tonga subduction zone, and consequently has the highest density of submarine volcanoes.
Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes. The latter form over subduction zones, at converging plate boundaries. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench.
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved. [1]
The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Volcanic Arc) above the subduction zone. [32]