Earth's getting 'soft' in the middle

January 24, 2008
Earth

Since we can’t sample the deepest regions of the Earth, scientists watch the velocity of seismic waves as they travel through the planet to determine the composition and density of that material. Now a new study suggests that material in part of the lower mantle has unusual electronic characteristics that make sound propagate more slowly, suggesting that the material there is softer than previously thought.

The results call into question the traditional techniques for understanding this region of the planet. The authors, including Alexander Goncharov from the Carnegie Institution’s Geophysical Laboratory, present their results in the January 25, 2008, issue of Science.

The lower mantle extends from about 400 miles to 1800 miles (660-2900 kilometers) into Earth and sits atop the outer core. Pressures and temperatures are so brutal there that materials are changed into forms that don’t exist in rocks at the planet’s surface and must be studied under carefully controlled conditions in the laboratory. The pressures range from 230,000 times the atmospheric pressure at sea level (23 GPa), to 1.35 million times sea-level pressure (135 GPa). And the heat is equally extreme—from about 2,800 to 6,700 degrees Fahrenheit (1800K–4000K).

Iron is abundant in the Earth, and is a major component of the minerals ferropericlase and the silicate perovskite in the lower mantle. In previous work, researchers found that the outermost electrons of iron in ferropericlase are forced to pair up under the extreme pressures creating a so-called spin-transition zone within the lower mantle.

“What happens when unpaired electrons—called a high-spin state—are forced to pair up is that they transition to what is called a low-spin state. And when that happens, the conductivity, density, and chemical properties change,” explained Goncharov.

“What’s most important for seismology is the acoustic properties—the propagation of sound. We determined the elasticity of ferropericlase through the pressure-induced high-spin to low-spin transition. We did this by measuring the velocity of acoustic waves propagating in different directions in a single crystal of the material and found that over an extended pressure range (from about 395,000 to 590,000 atmospheres) the material became ‘softer’—that is, the waves slowed down more than expected from previous work. Thus, at high temperature corresponding distributions will become very broad, which will result in a wide range of depth having subtly anomalous properties that perhaps extend through most of the lower mantle.”

Source: Carnegie Institution

Explore further: How to look for a few good catalysts

Related Stories

How to look for a few good catalysts

July 30, 2015

Two key physical phenomena take place at the surfaces of materials: catalysis and wetting. A catalyst enhances the rate of chemical reactions; wetting refers to how liquids spread across a surface.

Mini-Neptunes might host life under right conditions

July 23, 2015

M-dwarfs, which are cooler than our sun, have habitable zones closer to the stars. As such, any habitable planets orbiting these stars would transit frequently, making the chances of discovery better.

Recommended for you

Global index proposed to avoid delays on climate policies

August 4, 2015

Professor David Frame, Director of Victoria's Climate Change Research Institute (CCRI), has co-authored a paper published today in the high profile international scientific journal Nature Climate Change. The paper argues ...

Researchers investigate increased ocean acidification

August 3, 2015

The primary cause of global ocean acidification is the oceanic absorption of CO2 from the atmosphere. Although this absorption helps to mitigate some of the effects of anthropogenic climate change, it has resulted in a reduction ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

ModernMan
2.8 / 5 (5) Jan 25, 2008
The picture of the Earth looked strange to me, and I couldn't figure out why. Then I realized that it's not very often that the eastern hemisphere is pictured in US news stories.
out7x
1 / 5 (2) Jan 25, 2008
P or S waves? anisotropy due to pressure, or heat, or crystal?

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.