Unraveling Earth's Mysteries: The Role of Mantle Plumes in Volcanic Chains
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Back in the 1960s, when the theory of plate tectonics was rapidly gaining acceptance, certain geological features seemed to evade explanation. While the theory provided explanations for questions that had long puzzled scientists — where volcanoes appear, where land is born, where ocean basins are carved out, where ancient crust was annihilated — it couldn’t explain something like Hawai’i. Plate tectonics predicts that the boundaries of tectonic plates — where two plates collide, slide over or under one another, grind side by side, or move apart — are where most of the planet’s geologic fireworks can be found. The so-called Ring of Fire, the horseshoe-shaped region that marks the fringes of the many plates surrounding the Pacific plate, is home to 75% of the world’s active volcanoes.
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But despite being nowhere near a plate boundary, Hawai‘i is an archipelago of giant volcanoes. The active submarine volcano L?‘ihi, off the southeastern shore of the island of Hawai‘i, is the youngest member of a warped chain of volcanoes 6,000 kilometers long, one that can be traced all the way to long-expired underwater volcanoes in the northwest Pacific. This phenomenon, known as intraplate volcanism, stood out as a geologic aberration. In 1963, the Canadian geophysicist John Tuzo Wilson suggested that volcanic chains like this are forged when a tectonic plate continuously drifts over a stationary hot spot in the mantle — the scorching rock that makes up 84% of Earth’s volume. This creates a sequence of volcanoes that erupt, grow, then die out as the plate migrates away from the magmatic fuel source. In 1971, the American geophysicist William Jason Morgan proposed that these hot spots were caused by plumes of particularly hot material rising from the lower mantle.
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Over the following decades, geophysicists concluded that plumes are around 200 degrees Celsius hotter than the ambient mantle. When plumes reach the base of tectonic plates, their heat melts their surroundings, making plenty of magma. The plumes also carry mantle material up from Earth’s depths. This material melts at the lower pressures found away from the core, feeding additional magma into the crust. The combined supply of hot magma neatly explains a great number of Earth’s intraplate volcanoes. Chains of volcanoes, also known as hot spot tracks, are difficult to explain without invoking plumes. Hawai‘i is an oceanic example, but they can be found on land, too: The Yellowstone supervolcano is the youngest member of a hot spot track dating back at least 17 million years, one that poured 210,000 cubic kilometers of lava across the Pacific Northwest before blasting out a trail of giant volcanic cauldrons from Oregon to Wyoming — the undeniable scar tissue of an unrelenting mantle plume.
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Chemical evidence also implies the existence of mantle plumes. There are two stable types of helium: helium-3 and helium-4. Helium-3 was trapped deep within Earth during its formation and is decidedly ancient. Several hot spot volcanoes, including Hawai‘i’s K?lauea, erupt lavas with an abundance of the stuff. That, said Godfrey Fitton, a petrologist at the University of Edinburgh, suggests that these volcanoes are mining mantle matter from considerable depth — and a plume is a reasonable explanation.
