Soft Mantle
Saturday, 26. January 2008, 15:46:59
Everything on Earth can be explained in terms of 4 states/phases of matter - solid, liquid, gas, and plasma. In this sense the mantle is solid. From the way seismic waves travel through the Earth we know that the mantle is solid. P waves will travel and refract through both fluid and solid materials. S waves, however, cannot travel through fluids. S waves travel through the mantle, but not through the outer core, so the mantle is solid.
Chris at Highly Allochthonous had an excellent post on the solid state of the mantle titled Annoying misconceptions in Geology. But how solid is it after all. It is often described as plastic - the plastic mantle. Other relevant words may be elastic or soft. I find it a telling fact, that you don’t get earthquakes beneath something like 700 km, so down there the mantle cannot be that rigid.
According to an article published in Science of 25 January 2008 laboratory experiments suggest that the lower mantle is softer than previously thought. The lower mantle extends from about 660 km to 2900 km into Earth and sits atop the liquid outer core. Knowledge of this deep and inaccessible region is derived largely from seismic data. 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 1,500° to 3,700° Celsius.
Changes in the electronic configuration of iron at these pressures and temperatures seem to alter the elastic behaviour, making the lower mantle "softer" than previously estimated. The results suggest that scientists may have to go back to the drawing board to model this region of the Earth.
Crowhurst et al. Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle http://www.sciencemag.org/cgi/content/abstract/319/5862/451
• http://www.eurekalert.org/pub_releases/2008-01/ci-eg012208.php
• http://www.scientificblogging.com/news_releases/earth_getting_soft_in_the_middle
• http://www.livescience.com/environment/080124-soft-middle.html
Chris at Highly Allochthonous had an excellent post on the solid state of the mantle titled Annoying misconceptions in Geology. But how solid is it after all. It is often described as plastic - the plastic mantle. Other relevant words may be elastic or soft. I find it a telling fact, that you don’t get earthquakes beneath something like 700 km, so down there the mantle cannot be that rigid.According to an article published in Science of 25 January 2008 laboratory experiments suggest that the lower mantle is softer than previously thought. The lower mantle extends from about 660 km to 2900 km into Earth and sits atop the liquid outer core. Knowledge of this deep and inaccessible region is derived largely from seismic data. 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 1,500° to 3,700° Celsius.
Changes in the electronic configuration of iron at these pressures and temperatures seem to alter the elastic behaviour, making the lower mantle "softer" than previously estimated. The results suggest that scientists may have to go back to the drawing board to model this region of the Earth.
Crowhurst et al. Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle http://www.sciencemag.org/cgi/content/abstract/319/5862/451
• http://www.eurekalert.org/pub_releases/2008-01/ci-eg012208.php
• http://www.scientificblogging.com/news_releases/earth_getting_soft_in_the_middle
• http://www.livescience.com/environment/080124-soft-middle.html
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