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TOPIC: New Earth seismic layer
Blobrana


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Core-mantle boundary
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The Earth's hidden weakness

Three thousand kilometres beneath our feet, the Earth's solid rock - known as the mantle - gives way to the swirling liquid iron of the outer core (the 'core-mantle boundary'). The last few hundred kilometres of the lowermost mantle is also known as D" (pronounced 'dee-double-prime').
D" is one of the most enigmatic parts of the Earth which scientists have struggled to understand for decades; it can only be measured remotely, using seismic waves from earthquakes.
Geoscientists from the University of Bristol and University College London, report this week in the journal Nature that some of the poorly understood properties of D" might be explained by a sudden softening of the main mineral which makes up the mantle, due to the enormous pressures and temperatures near the Earth's core.
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Blobrana


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Earth's deep mantle
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A new analysis of the processes that constantly stir the Earth's deep mantle is helping to explain how the mantle holds onto a portion of ancient noble gases that were trapped during the Earth's formation.
The research, which appears recently in the journal Nature, takes aim at a question that has vexed geoscientists for years: how to reconcile leading theories about the convection of Earth's mantle with observations of ancient noble gases in volcanic rocks. Researchers at Rice University and Harvard University developed a new model to explain how noble gases -- elements like helium, neon and argon -- are lost from the Earth's interior during mantle convection.

"Most existing models find that convection should have left the mantle extensively depleted in ancient noble gases, unless part or all of the lower mantle has been somehow isolated. We set out to see if there was a mechanism that could both preserve ancient noble gases in the lower mantle and still be consistent with the existing framework for whole mantle convection" - study co-author Helge Gonnermann, assistant professor of Earth science at Rice.

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Blobrana


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RE: New Earth seismic layer
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Geologists at the University of Illinois have confirmed the discovery of Earths inner, innermost core, and have created a three-dimensional model that describes the seismic anisotropy and texturing of iron crystals within the inner core.

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Blobrana


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The arrangement of electrons in iron-containing minerals deep within Earth influences how those minerals behave, according to new lab results. That could affect our understanding of the composition of rocks deep underground, researchers say.
Jung-Fu Lin at the Lawrence Livermore National Laboratory, California, and his colleagues took some iron-containing minerals known as ferropericlases and subjected them to the brutal high temperatures and pressures of the lower mantle the layer of the inner Earth lying just above the core, in some places more than 2,000 kilometres beneath the surface.
By studying these minerals in minute detail, the team watched the electronic properties of single atoms change as they are squeezed and heated. This affects large-scale properties of the minerals, such as how fast sound travels through the rock.

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Blobrana


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Physicists in the US and France claim to have shown that there is a previously-unknown layer in the Earths lower mantle where the spin states of irons electrons switch, causing a change in the mantles density. The researchers discovered the spin transition zone by subjecting laboratory samples of lower-mantle material to high temperatures and pressures. Their findings could affect our understanding of how seismic waves propagate through the Earths interior (Science 317 1740).
Lying between 650 and 2800 km beneath the Earths surface, the lower mantle makes up over half of the Earths volume and is made mostly from compounds containing the elements oxygen, magnesium and silicon. Roughly 5%, however, is iron, contained in compounds such as ferropericlase (an iron-magnesium oxide) and silicate perovskite (an iron-magnesium silicate).

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Blobrana


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Keeping the Earth’s plates oiled
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Findings on water in the asthenosphere presented at the first EuroMinScI conference
Earth's surface is a very active place; its plates are forever jiggling around, rearranging themselves into new configurations. Continents collide and mountains arise, oceans slide beneath continents and volcanoes spew. As far as we know Earth's restless surface is unique to the planets in our solar system. So what is it that keeps Earth's plates oiled and on the move?
Scientists think that the secret lies beneath the crust, in the slippery asthenosphere. In order for the mantle to convect and the plates to slide they require a lubricated layer. On Mars this lubrication has long since dried up, but on Earth the plates can still glide around with ease.
If you could pick up a rock from the surface of Mars, then the chances are it would be magnetic. And yet, Mars doesn't have a magnetic field coming from its core. These rocks are clinging to the signal of an ancient magnetic field, dating back billions of years, to the times when Mars had a magnetic field like Earth's.
So how have these rocks hung onto their magnetic directions and what do they tell us about Mars? Strangely, the answer to these questions might be sitting here on Earth.
Most continental rocks on Earth align their magnetic moments with the current magnetic field - they are said to have 'induced' magnetism.

"I consider induced rocks to have 'Alzheimers'. These are the rocks that forgot where they were born and how to get home" - Suzanne McEnroe from the Geological Survey of Norway at a European Science Foundation (ESF), EuroMinScI conference near Nice, France this year.

However, not all of Earth's continental rocks have an induced magnetisation. Some rocks stubbornly refuse to swing with the latest magnetic field, and instead keep hold of the direction they were born with. These rocks are said to have a remnant magnetisation.
McEnroe and her colleagues have been studying some of Earth's strongest and oldest remnant magnetic rocks, to find out why they have such good memories. Understanding these rocks may give us clues as to what kind of rocks lie on Mars.

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Blobrana


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Earth's lower mantle
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New picture of Earth's lower mantle emerges from laboratory studies
Laboratory measurements of a high-pressure mineral believed to exist deep within the Earth show that the mineral may not, as geophysicists hoped, have the right properties to explain a mysterious layer lying just above the planets core.
A team of scientists, led by Sébastien Merkel, of the University of California, Berkeley, made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite. The work appears in the June 22 issue of the scientific journal Science.
The team included Allen McNamara of Arizona State University's School of Earth and Space Exploration, part of the College of Liberal Arts and Sciences. McNamara, a geophysicist, modelled the stresses the mineral would typically undergo as convection currents deep in Earth's mantle cause it to rise and sink. Also on the team were Atsushi Kubo and Thomas S. Duffy, Princeton University; Sergio Speziale, Lowell Miyagi and Hans-Rudolf Wenk, University of California, Berkeley; and Yue Meng, HPCAT, Carnegie Institution of Washington, Argonne, Ill.

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Blobrana


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Molten rock layer
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ASU geophysicists detect molten rock layer deep below American Southwest
 A sheet of molten rock roughly 10 miles thick spreads underneath much of the American Southwest, some 250 miles below Tucson, Arizona From the surface, you can't see it, smell it or feel it.
But Arizona geophysicists Daniel Toffelmier and James Tyburczy detected the molten layer with a comparatively new and overlooked technique for exploring the deep Earth that uses magnetic eruptions on the sun.
In 2003 two Yale University geoscientists published a hypothesis about the composition and physical state of rocks in Earth's mantle. They proposed that mantle rock rising through a depth of 410 kilometres  would give up any water mixed into its crystal structure, and the rock would then melt.

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Blobrana


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Complex structure in Tonga mantle wedge
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The subduction zones where oceanic plates sink beneath the continents produce volcanic arcs such as those that make up the "rim of fire" around the Pacific Ocean. The volcanoes are fed by molten rock rising within a wedge of the Earth's mantle above the subducting plate. Although geologists have a pretty good picture of the processes that produce volcanic arcs, a new study finds that the structure of the mantle wedge may be far more complex than anyone had imagined.

"Geology textbooks show simple cartoons of the processes happening in these mantle wedges--a sinking slab and some melting that comes up in volcanoes--but our results suggest that those cartoons are grossly inadequate" -  Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.

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Blobrana


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Earth's internal structure
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High-resolution images herald new era in Earth sciences
Ultrasound-like technique spots earthquakes, oil supplies
High-resolution images that reveal unexpected details of the Earth's internal structure are among the results reported by MIT and Purdue scientists in the March 30 issue of Science.
The researchers adapted technology developed for near-surface exploration of reservoirs of oil and gas to image the core-mantle boundary some 2,900 kilometres, or 1,800 miles, beneath Central and North America.

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Credit  Robert van der Hilst, MIT

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