Papua New Guinea rock cooled by about 100 degrees C as it sank past 90 km

Averaged from the surface down, the change is large. About 800 degrees C at 45 km ^[2] is roughly 18 degrees per kilometre ^[13]. At more than 90 km the rock was near 700 degrees ^[12], or at most about 8 degrees per kilometre ^[14]. Those averages ignore how temperature varies along the path, so they are only a rough guide. The paper is titled "Geothermal gradient change during subduction recorded by ultrahigh-pressure eclogite" ^[9].

Both stages were unusual. "At about 45 kilometers (28 miles) deep, the rock was surprisingly hot, but as it traveled deeper, it entered an environment that was cooler than we would normally expect at that depth," Schmitt said ^[6]. The phys.org story leads on the heat, opening with "an unexpectedly hot rock" ^[11].

The history comes from tiny crystals of coesite and zircon trapped inside garnet ^[5]. Coesite forms under extremely high pressures, so finding it puts the rock at 90 km or more ^[5]. Zircon supplies the dates ^[5]. The phys.org account does not report those ages or explain how the two temperatures were estimated.

Schmitt offered two explanations. "One possibility is that intense shearing where the tectonic plates meet generates additional heat at relatively shallow depths. Another is that the subduction zone had not yet cooled to the lower temperatures expected," he said ^[7]. In the first case, the anomaly is extra heat at 45 km. In the second, the rock recorded a zone that was changing while it sank, so the reversal is partly about time as well as depth ^[7]. The zircon ages bear on that directly. A long gap between the hot and cool stages leaves time for the zone to cool; a short gap would weaken that explanation ^[5].

Schmitt's summary of the stakes is measured. The thermal structure of subduction zones "may be more complex than previously thought," he said ^[10]. I think that is the strength the evidence earns. The depth is well fixed, and the cause of the cooling is still open ^[7].