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800-Mile-Wide Hot Anomaly Found Under Seafloor off Hawaii

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« on: May 27, 2011, 12:34:15 am »

800-Mile-Wide Hot Anomaly Found Under Seafloor off Hawaii
Findings contradict long-held theory that a plume directly fuels Hawaii.

Hot lava spills into the sea from under a hardened lava crust on the Big Island of Hawaii (file picture).
Photograph by Patrick McFeeley, National Geographic
Dave Mosher
for National Geographic News
Published May 26, 2011

Hawaii's long-accepted birth story—that the volcanic islands were, and are, fueled by a hot-rock plume running directly to Earth's scorching core—could be toast, a new study hints.

(See pictures of a recent eruption Hawaii's Kilauea volcano.)

Scientists say they've finally found solid evidence of a giant mass of hot rock under the seafloor in the region. But it's not a plume running straight from the core to the surface—and it's hundreds of miles west of the nearest Hawaiian island.

Until now, the researchers say, good seismic data on the region has been scarce, so there was no real reason to question the most obvious explanation: that a stream of hot rock directly from around Earth's core formed the 3,100-mile-long (5,000-kilometer-long) chain of islands and undersea mountains in the Pacific Ocean.

As Earth's crust slid over the plume, as if on a conveyor belt, the erupting seafloor built mounts, mountains, and islands out of layers of cooled lava over tens of millions of years—or so the conventional wisdom goes.

But after analyzing 20 years' worth of earthquake data, geophysicists say they've found an 800-mile-wide (1,300-kilometer-wide) region of hot rock in the Hawaiian region—but nothing near the Big Island of Hawaii. The island, the youngest in the chain, is traditionally thought to be above the purported plume.

But, despite the new evidence against the giant-plume theory, "we can't rule out a narrow plume below the island, but the main source comes from a different place. It can't be linked directly below," said geophysicist Robert van der Hilst of MIT, co-author of the new study, led by his colleague Qin Cao and appearing online today in the journal Science.

(Related: "Hawaii's 'Gentle' Volcano More Dangerous Than Thought.")

Hawaii Plume Takes Deep Detour?

Volcano formation starts where Earth's mantle—the planet's thickest rock layer—meets the molten outer core, some 1,800 miles (2,900 kilometers) below the surface.

The outer core heats the mantle's bottom rocks into buoyant putty, which rises toward the crust, as if in a lava lamp. Within a few miles of the crust, the rock decompresses, melts, and often oozes—or erupts—out of the Earth's surface.

(See "New Magma Layer Found Deep in Earth's Mantle?")

A technique called seismic tomography uses the sounds of earthquakes rippling through the planet and bouncing around to detect such plumes, or hot spots. But this kind of data has been limited for Hawaii.

"It's been very difficult to image the mantle below Hawaii, simply because it's so far away from any seismic sensor networks," van der Hilst said. Data suggesting a plume directly below the island is very limited and based on a relatively small number of discrete seismic events, he said.

By contrast, Van der Hilst said, the new study analyzed two decades' worth of noisy seismic data and narrowed it down to the clearest signals.

Those signals pointed to the giant anomaly, about 410 miles (660 kilometers) down: a relatively disklike segment of rock between 540 and 720 degrees Fahrenheit (300 and 400 degrees Celsius) hotter than its surroundings and between 370 miles (600 kilometers) and 1,000 miles (1,600 kilometers) west of the Big Island.

The team suspects the plume is pooling at the boundary between the upper and lower mantle, then snaking its way to the crust below the archipelago before rising to feed Hawaii's volcanic islands.

(Related: "Scientists to Drill Earth's Mantle, Retrieve First Sample?")

More evidence is needed before it's known for sure what's stoking the volatile island chain, but at the least, the new study "shows the process of how our planet loses heat is more complicated than we thought," van der Hilst said.

More: Magma Chamber Surprisingly Close to Hawaii's Surface?
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