Revelations about the center of the Earth

Jul 22, 2005
Earth

Recently, seismologists have observed that the speed and direction of seismic waves in Earth’s lower mantle, between 400 and 1,800 miles below the surface, vary tremendously. “I think we may have discovered why the seismic waves travel so inconsistently there,” stated Jung-Fu Lin.* Lin was with the Carnegie Institution’s Geophysical Laboratory at the time of the study and lead author of the paper published in the July 21, issue of Nature. “Until this research, scientists have simplified the effects of iron on mantle materials. It is the most abundant transition metal in the planet and our results are not what scientists have predicted,” he continued. “We may have to reconsider what we think is going in that hidden zone. It’s much more complex than we imagined.”

Image: Under the extreme pressure and temperature conditions of Earth's lower mantle, electrons in oxide materials are forced to pair-up in the same orbits. The electronic transformation causes a jump in bulk wavespeeds that may explain why seismic waves in this region of the deep mantle behave so peculiarly. (Image courtesy S. Jacobsen, M. Wysession, and G. Caras.)

The crushing pressures in the lower mantle squeeze atoms and electrons so closely together that they interact differently from under normal conditions, even forcing spinning electrons to pair up in orbits. In theory, seismic-wave behavior at those depths may result from the vice-gripping pressure effect on the electron spin-state of iron in lower-mantle materials. Lin’s team performed ultra high-pressure experiments on the most abundant oxide material there, magnesiowüstite (Mg,Fe)O, and found that the changing electron spin states of iron in that mineral drastically affect the elastic properties of magnesiowüstite. The research may explain the complex seismic wave anomalies observed in the lowermost mantle.

As co-author of the study Viktor Struzhkin elaborated: “This is the first study to demonstrate experimentally that the elasticity of magnesiowüstite significantly changes under lower-mantle pressures ranging from over 500,000 to 1 million times the pressure at sea level (1 atmosphere). Magnesiowüstite, containing 20% iron oxide and 80% magnesium oxide, is believed to constitute roughly 20% of the lower mantle by volume. We found that when subjected to pressures between 530,000 and 660,000 atmospheres the iron’s electron spins went from a high-spin state (unpaired) to a low-spin state (spin-paired). While monitoring the spin-state of iron, we also measured the rate-of-change in the volume (density) of magnesiowüstite through the electronic transition. That information enabled us to determine how seismic velocities will vary across the transition.”

“Surprisingly, bulk seismic waves travel about 15% faster once the electrons of iron are spin-paired in the magnesium-iron oxide,” commented co-author Steven Jacobsen. “The measured velocity jump across the transition might, therefore, be detectable seismically in the deep mantle.” The experiments were conducted inside a diamond-anvil pressure cell using the intense X-ray light source at the nation's third-generation synchrotron source, Argonne National Laboratory near Chicago.

“The mysterious lower mantle region can’t be sampled directly. So we have to rely on experimentation and theory. Since what happens in Earth’s interior affects the dynamics of the entire planet, it’s important for us to find out what is causing the unusual behavior of seismic waves in that region,” stated Lin. “Up to now, earth scientists have understood Earth’s interior by only considering pure oxides and silicates. Our results simply point out that iron, the most abundant transition metal throughout the entire Earth, gives rise to very complex properties in that deep region. We look forward to our next experiments to see if we can refine our understanding of what is happening there,” he concluded.

Source: Lawrence Livermore National Laboratory

Explore further: SDO captures images of two mid-level flares

add to favorites email to friend print save as pdf

Related Stories

Does dark magma lurk in deep Earth?

Nov 13, 2014

(Phys.org) —A key to understanding Earth's evolution is to look deep into the lower mantle—a region some 400 to 1,800 miles (660 to 2,900 kilometers) below the surface, just above the core. Data have ...

The Earth's hidden weakness

May 28, 2010

(PhysOrg.com) -- Three thousand kilometres beneath our feet, the Earth's solid rock gives way to the swirling liquid iron of the outer core.

Recommended for you

SDO captures images of two mid-level flares

17 hours ago

The sun emitted a mid-level flare on Dec. 18, 2014, at 4:58 p.m. EST. NASA's Solar Dynamics Observatory, which watches the sun constantly, captured an image of the event. Solar flares are powerful bursts ...

Why is Venus so horrible?

23 hours ago

Venus sucks. Seriously, it's the worst. The global temperature is as hot as an oven, the atmospheric pressure is 90 times Earth, and it rains sulfuric acid. Every part of the surface of Venus would kill you ...

Image: Christmas wrapping the Sentinel-3A antenna

Dec 19, 2014

The moment a team of technicians, gowned like hospital surgeons, wraps the Sentinel-3A radar altimeter in multilayer insulation to protect it from the temperature extremes found in Earth orbit.

Video: Flying over Becquerel

Dec 19, 2014

This latest release from the camera on ESA's Mars Express is a simulated flight over the Becquerel crater, showing large-scale deposits of sedimentary material.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.