A volcanic belt is a large volcanically active region. Other terms are used for smaller areas of activity, such as volcanic fields. Volcanic belts are found above zones of unusually high temperature (700-1400 °C) where magma is created by partial melting of solid material in the Earth's crust and upper mantle. These areas usually form along tectonic plate boundaries at depths of 10–50 km. For example, volcanoes in Mexico and western North America are mostly in volcanic belts, such as the Trans-Mexican Volcanic Belt that extends 900 km from west to east across central-southern Mexico and the Northern Cordilleran Volcanic Province in western Canada.
The deeply deformed and eroded remnants of ancient volcanic belts are found in volcanically inactive regions such as the Canadian Shield. It contains over 150 volcanic belts (now deformed and eroded down to nearly flat plains) that range from 600 to 1200 million years old. These are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that form what are known as greenstone belts. They are thought to have formed at ancient oceanic spreading centers and island arc terranes. The Abitibi greenstone belt in Ontario and Quebec, Canada is one of the world's largest greenstone belts.
Volcanic belts may be formed by multiple tectonic settings. They may be formed by subduction zones, which is an area on Earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the mantle, at rates typically measured in centimeters per year. An oceanic plate ordinarily slides underneath a continental plate; this often creates an orogenic zone with many volcanoes and earthquakes. In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle and plates are moving apart. An example of a subduction-zone related volcanic belt is the Okhotsk-Chukotka Volcanic Belt in northeastern Eurasia, which is one of the largest subduction-zone related volcanic provinces in the world, stretching some 3200 km and comprising about 2 million km3 of volcanic and plutonic material.
Volcanic belts may also be formed by hotspots, which is a location on the Earth's surface that has experienced active volcanism for a long period of time. These volcanic belts are called volcanic chains. Canadian geologist John Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectonic plate across a "fixed" hot spot deep beneath the surface of the planet, thought to be caused by a narrow stream of hot mantle convecting up from the mantle-core boundary called a mantle plume. But more recently some geologists, such as Gillian Foulger view upper-mantle convection as a cause. This in turn has re-raised the antipodal pair impact hypothesis, the idea that pairs of opposite hot spots may result from the impact of a large meteor. Geologists have identified some 40-50 such hotspots around the globe, with Hawaii, Réunion, Yellowstone, Galápagos, and Iceland overlying the most currently active. An example of a hotspot volcanic belt is the Anahim Volcanic Belt in British Columbia, Canada, which was formed as a result of the North American Plate sliding westward over the Anahim hotspot.
Most hotspot volcanoes are basaltic because they erupt through oceanic lithosphere (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, which have high water contents. Where hotspots occur under continental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts to form rhyolites. These rhyolites can be quite hot and form violent eruptions, despite their low water content. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history.
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