http://www.sciencedaily.com/releases/2013/02/130207002002.htm

The Deep Roots of Catastrophe: Partly Molten, Florida-Sized Blob Forms Atop Earth's Core

Feb. 6, 2013 — A University of Utah seismologist analyzed seismic waves that bombarded Earth's core, and believes he got a look at the earliest roots of Earth's most cataclysmic kind of volcanic eruption. But don't worry. He says it won't happen for perhaps 200 million years.

"What we may be detecting is the start of one of these large eruptive events that -- if it ever happens -- could cause very massive destruction on Earth," says seismologist Michael Thorne, the study's principal author and an assistant professor of geology and geophysics at the University of Utah.

But disaster is "not imminent," he adds, "This is the type of mechanism that may generate massive plume eruptions, but on the timescale of 100 million to 200 million years from now. So don't cancel your cruises."

The new study, set for publication this week in the journal Earth and Planetary Science Letters, indicates that two or more continent-sized "piles" of rock are colliding as they move at the bottom of Earth's thick mantle and atop the thicker core some 1,800 miles beneath the Pacific. That is creating a Florida-sized zone of partly molten rock that may be the root of either of two kinds of massive eruptions far in the future:

Hotspot plume supervolcano eruptions like those during the past 2 million years at Wyoming's Yellowstone caldera, which covered North America with volcanic ash.
Gargantuan flood basalt eruptions that created "large igneous provinces" like the Pacific Northwest's Columbia River basalts 17 million to 15 million years ago, India's Deccan Traps some 65 million years ago and the Pacific's huge Ontong Java Plateau basalts, which buried an Alaska-sized area 125 million to 199 million years ago.

"These very large, massive eruptions may be tied to some extinction events," Thorne says. The Ontong eruptions have been blamed for oxygen loss in the oceans and a mass die-off of sea life.

Since the early 1990s, scientists have known of the existence of two continent-sized "thermochemical piles" sitting atop Earth's core and beneath most of Earth's volcanic hotspots -- one under much of the South Pacific and extending up to 20 degrees north latitude, and the other under volcanically active Africa.

Using the highest-resolution method yet to make seismic images of the core-mantle boundary, Thorne and colleagues found evidence the pile under the Pacific actually is the result of an ongoing collision between two or more piles. Where they are merging is a spongy blob of partly molten rock the size of Florida, Wisconsin or Missouri beneath the volcanically active Samoan hotspot.

The study's computer simulations "show that when these piles merge together, they may trigger the earliest stages of a massive plume eruption," Thorne says.

Thorne conducted the new study with Allen McNamara and Edward Garnero of Arizona State University, and Gunnar Jahnke and Heiner Igel of the University of Munich. The National Science Foundation funded the research.

Probing the Deep Earth with Seismic Waves

Seismic imaging uses earthquake waves to make images of Earth's interior somewhat like X-rays make CT scan pictures of the inside of the human body.

The new study assembled the largest set of data ever used to map the lower mantle in the Pacific region by using 4,221seismograms from hundreds of seismometers around the world that detected 51 deep earthquakes originating more than 60 miles under the surface.

Thorne and colleagues looked for secondary earthquake shear waves known as S-waves that travel through much of Earth, hitting the core, and then convert to primary compressional waves or P-waves as they travel across the top of the core. Then they convert back to S-waves as they re-enter the mantle and then reach seismometers. Thorne says the short bursts of P-wave energy are very sensitive to detecting variations in the rock at the core-mantle boundary.

Thorne performed 200 days of supercomputer simulations at the University of Utah's Center for High Performance Computing. He simulated hundreds of possible shapes of the continent-sized piles and state-sized blobs until he found the shapes that could best explain the seismic wave patterns that were observed.

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http://www.sciencedaily.com/releases/2013/02/130207002002.htm