Mantle Plumes Visualized

[BillT]

These plumes underlie volcanic hot spots like Iceland and Hawaii.

http://www.sciencemag.org/content/349/6252/1032.full?utm_campaign=email-sci-ntw&utm_src=email

[Mugwump]

These plumes underlie volcanic hot spots like Iceland and Hawaii.

http://www.sciencemag.org/content/349/6252/1032.full?utm_campaign=email-sci-ntw&utm_src=email

....oops ??.....need to log in...??

but they do even have an organization..    http://www.mantleplumes.org/

[BillT]

Opps, sorry about that.
Here's the text at least (sadly, not the pictures):

Mantle plumes seen rising from Earth's core

    Eric Hand

    Science 4 September 2015

A key internal feature of Earth has come into focus. For more than 40 years, geoscientists have debated whether Earth's hot spots?several dozen volcanic regions that bulge upward as if something hot and insistent were pushing up from below?are fed by mantle plumes, columns of hot rock thought to rise from the core nearly 3000 kilometers below the surface. Proponents cited physical models that suggest that plumes form naturally, like bubbles in a boiling pot, as the mantle is heated from below. And seismologists, using earthquake waves to perform MRI-like tomographies on the planet, have seen signs of plumes in the upper parts of the mantle. But absent evidence of mantle plumes stretching continuously up from the core, a small minority of scientists has argued that hot spots could be fed by shallower reservoirs.

Now, a new study finds that it's plumes all the way down. Using a sophisticated technique to wring unprecedented detail from earthquake records, researchers have found evidence for 28 mantle plumes, most of them beneath volcanic hot spots, rising continuously, and vertically, from the core. ?If our picture makes sense, then we've settled the debate. Plumes exist,? says Barbara Romanowicz, a geophysicist at the University of California, Berkeley, who published the study on 2 September in Nature along with her colleague, Scott French.

The confirmation comes with surprises. The plumes are 600 to 800 kilometers wide, more than three times as big as simple models have predicted, Romanowicz says. As a result, plumes can carry more heat away from the core than was thought. Moreover, their deepest reaches are ramrod straight and do not appear to sway in the currents of rock believed to circulate in the lower mantle. ?It's quite likely that people will need to rethink how the lower mantle is working,? she says.


Hot spots Researchers have identified 28 plumes in the mantle?and nearly all of them correspond with volcanic regions called hot spots. "CREDIT: ADAPTED FROM S. FRENCH ET AL., NATURE 525, 95 (3 SEPTEMBER)"

Other researchers have hailed the study. ?The tomography work they did is a great tour de force,? says Peter van Keken, a geodynamicist at the University of Michigan, Ann Arbor. ?What was really fuzzy in previous models has become sharper.? Van Keken is intrigued by the shearing and bending of the plumes seen at depths shallower than 1000 kilometers. He thinks the change in plume behavior at 1000 kilometers could represent an unknown phase change in the rising rock, which makes it a little less stiff.

As for the surprising size of the plumes, van Keken says it jibes with an emerging notion that the hot plumes may be made of different stuff from the surrounding mantle. If a plume gathers dense basaltic material near the coremantle boundary, says van Keken, ?it helps the plume to slow down, it helps the plume to be broader.?
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From the deep This rendering of the plume beneath Hawaii shows a lateral bending above a depth of 1000 kilometers. The level could mark an undiscovered mineral phase transition in lower mantle rock. "ILLUSTRATION: ADAPTED FROM S. FRENCH ET AL., NATURE 525, 95 (3 SEPTEMBER)"

Guust Nolet, an emeritus geophysicist at the University of Nice Sophia Antipolis in France, says previous studies have indicated the existence of deep mantle plumes (see Science, 16 January 2004, p. 338), but this one sharpens the view at the top and bottom of the mantle. He says the surprising size of the plumes will change the accounting of how Earth sheds internal heat?a flux estimated at 44 terawatts. Perhaps half of that heat flux is thought to come from radioactive elements within Earth, and the other half from residual heat in the core. Geophysicists have assumed that mantle convection, a broad cellular pattern of circulating rock, carried most of the heat away from the core. ?We need to review that view and give the plumes a much larger role,? Nolet says. ?It might be that the lower mantle isn't convecting at all.?

The new result takes advantage of a technique called seismic tomography. Large earthquakes that shake Earth's surface send waves that travel all the way through the planet, bouncing off internal boundaries and slowing down when they encounter anomalously hot, less dense structures, such as plumes. From measurements of many earthquakes, a picture of Earth's interior can be derived.

Many tomographic studies rely on just the arrival times, or initial pulses, of different earthquake waves. A relatively new modeling technique called whole waveform tomography uses the full rupture duration of each earthquake?in effect, most of the squiggles in a seismogram. The process is computationally intensive: Romanowicz and French's analysis using records of 273 large earthquakes required the equivalent of 3 million computer hours on a Department of Energy supercomputer called Hopper.

That result is not enough to persuade Gillian Foulger, a geophysicist at Durham University in the United Kingdom, and a longtime skeptic of deep mantle plumes. Although she applauds the computational effort, she says that the whole waveform technique is still in its embryonic stages. She also notes that most of the plumes found by Romanowicz and French sit under hotspots in the ocean, where seismic data is extremely limited. ?If your data is poor, you see a lot of noise,? she says. Some researchers are pushing for programs to help fill that information gap (see sidebar).

But van Keken says the evidence for deep mantle plumes is strong. ?The naysayers are few and far between,? he says. Not that he wouldn't also like to improve the resolution of the models. If they could see features as small as 100 kilometers across, they could reveal the fate of slabs of ocean crust plummeting into Earth's interior from subduction zones, he says. Some slabs may pool at intermediate depths, whereas others may fall all the way to the core. But to achieve that sort of view, models will need to tap into the higher frequency content embedded in seismograms?and that will require much more computing power. ?You'll need a really big computer if you don't want your grad student to wait forever for their Ph.D.,? van Keken says.
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