A Revolution in Plate Tectonics?

An ophiolite is, therefore, a suite of temporally and spatially associated ultramafic, mafic, and felsic rocks that formed as the products of multiple mantle melting events and magmatic differentiation processes in a particular tectonic environment.

Broad plumes rooted at the base of the Earth's mantle beneath major hotspots

Scott W. French & Barbara Romanowicz

Nature 525, 95–99 (03 September 2015) doi:10.1038/nature14876
Received 12 November 2014 Accepted 19 June 2015 Published online 02 September 2015

Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique—combining accurate wavefield computations with information contained in whole seismic waveforms—that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core–mantle boundary to about 1,000 kilometres below Earth’s surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth’s mantle and guide further research into thermochemical convection.

Scientists Link Moon’s Tilt and Earth’s Gold
By KENNETH CHANGNOV. 27, 2015

The moon’s orbit is askew, and two planetary scientists believe that they have come up with a good reason.


Intriguingly, their idea also explains why gold and platinum are found in the Earth’s crust, well within diggable reach.

The moon is believed to have formed out of a giant cataclysmic collision early in the history of the solar system when an interplanetary interloper the size of Mars slammed into Earth and lofted a ring of debris circling over the Equator. The debris coalesced into the moon.

At its birth, the moon was quite close to the Earth, probably within 20,000 miles. Because of the tidal pulls between the Earth and moon, the moon’s orbit has slowly been spiraling outward ever since, and as it does, Earth’s pull diminishes, and the pull of the sun becomes more dominant.

By now, with the moon a quarter million miles from Earth, the sun’s gravity should have tipped the moon’s orbit to lie in the same plane as the orbits of the planets.

But it has not. The moon’s orbit is about 5 degrees askew.

“That the lunar inclination is as small as it is gives us some confidence that the basic idea of lunar formation from an equatorial disk of debris orbiting the proto-Earth is a good one,” said Kaveh Pahlevan, a planetary scientist at the Observatory of the Côte d’Azur in Nice, France. “But the story must have a twist.”

Writing in this week’s issue of the journal Nature, Dr. Pahlevan and his observatory colleague Alessandro Morbidelli propose the twist.

The moon did indeed form in the Earth’s equatorial plane, the scientists said, but then a few large objects, perhaps as large as the moon, zipping through the inner solar system repeatedly passed nearby over a few tens of millions of years and tipped the moon’s orbit.

A series of computer simulations show that the idea is plausible.

“This mechanism works for a broad range of physical conditions,” Dr. Pahlevan said.

Eventually the crisscrossing mini-planets would have been tossed out of the solar system, swallowed by the sun, or slammed into the Earth or the other planets.

Robin M. Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who wrote an accompanying commentary in Nature, said the thousands of close passes that typically occur before an impact were a “really new realization” by Dr. Pahlevan and Dr. Morbidelli.

“While a single scattering event will only change the moon’s tilt slightly,” Dr. Canup said, “it’s the cumulative effect of these many passes that can produce this tilt.”

The scars of one or more moon-size objects hitting Earth would have long been erased by the tectonics of the shifting surface, but those impacts would explain the gold, platinum and other precious metals in the Earth’s crust but not on the moon.

Metals on the early Earth should have sunk to the interior. Thus, planetary scientists think that after the moon was created, later collisions that provided the last 1 percent or so of the Earth’s mass added a veneer of precious metals.

A dearth of lunar metals argues for a few large metal-rich objects hitting the Earth rather than many small ones.

The computer simulations show that the chances of the moon’s getting hit are low. In the simulations, if there was one object buzzing by, the moon was hit 9 percent of the time. With four objects, the chances of a lunar impact rose to 25 percent.

“Not an overly likely outcome, which is good,” Dr. Canup said.

Scientists including Dr. Canup had proposed other explanations for the tilt. “I would say those relied on certainly more complex processes and required rather narrow sets of conditions for success,” Dr. Canup said. “I think where this has really stepped in is it’s a very simple mechanism.