Technique measures heat transport in the Earth’s crust

March 31st, 2009 - 3:28 pm ICT by ANI  

Washington, March 31 (ANI): A scientist has developed a technique that provides much more accurate data on heat transport through rocks in the Earth’s crust than conventional methods, which brings scientists closer to a better understanding of the planet’s interior.

The scientist in question is Anne M. Hofmeister, research professor of earth and planetary sciences in Arts and Sciences at Washington University in St. Louis (WUSTL).

Temperature is an important driver of many geological processes, including the generation of magmas (molten rocks) in the deepest parts of the Earth’s crust, about 30 to 40 kilometers below the surface.

Yet, until recently, temperatures deep inside the Earth’s crust were uncertain, mainly because of difficulties associated with measuring thermal conductivity, or how much heat is flowing through the rocks that compose the crust.

In conventional methods of measuring thermal conductivity, measurement errors arise as the temperature of a rock nears its melting point.

At such high temperatures, heat is not just transported from atom to atom by vibrations, but also by radiation (light).

Since conventional methods cannot separate heat flow carried by vibrations from that associated with radiation, most measurements of how efficiently rocks transport heat at high temperatures have been overestimated.

Using an industrial laser that is typically used for steel welding, Hofmeister was able to circumvent the problems that plagued the older methods.

Her technique, laser-flash analysis, provides much more accurate data on heat transport through rocks than conventional methods.

In laser-flash analysis, a rock sample is held at a given temperature and then subjected to a laser pulse of heat, allowing Hofmeister to measure the time it takes for the heat to go from one end of the sample to the other.

Since measuring heat transport in the crust itself is impossible, Hofmeister used the laser to measure heat transport in individual rock samples at various temperatures and then averaged across samples to represent the dynamics of the crust.

According to Hofmeister, “Our analysis shows that rocks are more efficient at conducting heat at low temperatures than was previously thought and less efficient at high temperatures. The process of moving heat around really depends on the temperature of the rocks.”

Hofmeister and her collaborators found that the conductivity of rocks in the lower crust, where the external temperature is very high, is much lower - by as much as 50 percent - than was predicted by conventional methods.

These results also suggest that the lower crust may be much hotter than scientists previously recognized.

“The new methods change our understanding of how heat is transported in geological environments,” said Hofmeister. (ANI)

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