In
geology, a
hotspot is a location on the Earth's surface that has experienced active
volcanism for a long period of time.
J. 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. Hotspots are thought to be caused by a narrow stream of
hot mantle convecting up from the Earth's
core-mantle boundary called a
mantle plume,
[1] although some geologists prefer upper-mantle convection as a cause.
[2][3] This in turn has re-raised the antipodal pair impact hypothesis, the idea that pairs of opposite hotspots may result from the impact of a large
meteor.
[4] Geologists have identified some 40–50 such hotspots around the globe, with
Hawaii,
Réunion,
Yellowstone,
Galápagos, and
Iceland overlying the most currently active.
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, in which water is trapped under the overriding plate. 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. However, when rhyolitic magma is completely erupted, it may eventually turn into basaltic magma because it is no longer trapped in the less dense continental crust. An example of this activity is the Ilgachuz Range in British Columbia, which was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.[5]
As the continents and seafloor drift across the mantle plume, "hotspot" volcanoes generally leave unmistakable evidence of their passage through seafloor or continental crust. In the case of the Hawaiian hotspot, the islands themselves are the remnant evidence of the movement of the seafloor over the hotspot in the Earth's mantle. The Yellowstone hotspot emerged in the Columbia Plateau of the US Pacific Northwest. The Deccan Traps of India are the result of the emergence of the hotspot currently under Réunion Island, off the coast of eastern Africa. Geologists use hotspots to help track the movement of the Earth's plates. Such hotspots are so active that they often record step-by-step changes in the direction of the Earth's magnetic poles. Thanks to lava flows from a series of eruptions in the Columbia Plateau, scientists now know that the reversal of magnetic poles takes about 5,000 years, fading until there is no detectable magnetism, then reforming in near-opposite directions.
Hotspot volcanoes should not be confused with island arc volcanoes. While each will appear as a string of volcanic islands. Island arcs are formed by subducting, converging tectonic plates. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench. This plate releases water into the base of the over-riding plate as it is subducted, and this water cause some rock to melt, and it is this that fuels a chain of volcanoes, such as the Aleutian Islands near Alaska.
