How Hot is Hot? 4,000 degrees K, according to new study. Researchers at the Magma and Volcanoes Laboratory (CNRS U Blaise Pascal) and the European Synchrotron (ESRF) have collided to make sense of the really hot temperatures at the core-mantle boundary. The CNRS-ESRF study, “New Insight into the Temp- erature of Deep Earth,” has mimicked the conditions at 2,900 kilometres below surface. Plate tectonics theory tells us that our ocean crust is subducted towards that mantle-core interface. CNRS-ESRF tells us that at the boundary temperatures rise rapidly to 4,000 K and the basaltic former crust produces a silica-rich liquid that reacts with the surrounding mantle material. As one approaches the lower mantle (hopefully in an insulated jump suit), one finds that the temperature gradient increases more rapidly than it does at shallower mantle depths. The scientists tell us that the temperature increases about 1,000 in the last few hundred kilometres.
The point of the study was to observe what happens to basalt as it approaches lower-mantle conditions. This was achieved in the lab. Discovering the traits of the basalt seems to have shed some light on mysteries surrounding seismic wave propagation in the D” (D” being geo-speak for the mantle-core region). By recreating a pressure of 46 GPa (That’s right, 46-gazillion pascals of pressure!) the team found that they had created a basaltic slurry that could be giving the seismic anomalies others have observed. They realized that the temperature would have to be between 3800 and 4150 K for this to occur, hence their prediction that this is the D” zone’s temperature. You can read the paper by Denis Andrault et.al. in the journal Science by following this link.
It is interesting that we are still working out the temperatures down there. In The Mountain Mystery, I told part of the story of Joseph Fourier and his thought-experiment that placed a piece of the Sun 12 miles below our surface. He said it would take 200,000 years before the dissipating heat reached the surface through conduction. Fourier pre-dated Lord Kelvin with heat-transfer calculations and his math was better, but Kelvin was a powerful force and his errors about the Earth’s temperature, heat dissipation, and the planet’s age are what we remember today. Kelvin thought the Earth’s internal heat was largely residual and the Sun’s heat was due to gravitational pressure. (When radiation and X-rays were discovered, the elderly Kelvin declared them to be hoaxes.) Consequently, he opted for a young-aged Earth of some 20 million years. He also soundly rejected the idea of mantle convection when it was presented to him by his colleague and assistant, John Perry in 1898. Perry published his idea in Nature and Kelvin ruined his career for his insubordination. And yet, we calculate the core-mantle boundary temperature as 4,000 K. Four thousand Lord Kelvins – now that’s a disturbing mental image!
Read the book, The Mountain Mystery.