In 1963, John Tuzo Wilson proposed a theory that the Hawaiian Islands resulted from the slow movement of a tectonic plate across a hot region beneath the earth’s crust, with thermal convection rising from deep within the earth’s mantle. The scientist called this hot region a volcanic hotspot. The number of generally recognised hotspots varied, as the theory gained credibility. Most geologists accept the existence of about sixty. Wilson’s hypothesis explains the geological origin of, for example, Hawaii, Yellowstone, Iceland and the Galápagos.
In 1971, William Jason Morgan postulated that the hotspot is caused by something called a thermal mantle plume. It starts around 2,900 km (1,800 mi) deep, on the border between Earth’s core and mantle. The mantle plume consists of rock containing radioactive elements. As in a nuclear power station, the active material releases energy and heats the rock. The pressure in the depth keeps the superheated matter in a solid state, however, the rock becomes softer and less dense than its surroundings. This fuels the thermal plume continually upwards through the earth’s mantle.
The pressure diminishes as the heat rises up the mantle. This causes the surrounding rock to become even softer and hotter. When this reaches the outskirts of the earth’s mantle at depths of about 100 km (60 mi), its temperatures rise to 1,400°C (2,500°F). Solid rock at this depth melts and changes into magma. The magma then punctures the fragile earth’s crust, creating a massive volcanic reservoir, which is trapped in chambers and cracks in the brittle uppermost shell of the earth, known as the lithosphere. The hot magma heats the lithosphere, which expands upwards.
© 2023 Josef Litt
The tops of the magma reservoirs lay only 2–5 km (1.2–3.1 mi) beneath many of the volcanoes at the Galápagos. They function as a staging area, where magma accumulates and begins to cool. When its temperature drops to about 1,150°C (2,100°F), the magma partially crystallises, forming large crystals of green olivine, feldspar and other minerals. The mantle plume feeds the reservoirs with more and more magma. As the arriving magma increases the pressure in the reservoir, a volcanic eruption follows. Once the magma reaches the earth’s surface, it changes composition by cooling and losing much of its gas content, when its name changes to lava. On seabed, the expelled magma forms underwater mountains. Outburst by outburst, the mountains grow until they reach the surface of the sea and become volcanic islands.
Illustration © 2017 Jan Choura.
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