Subducted rock from Papua New Guinea defies conventional temperature expectations

Researchers studying a rock sample from northern Papua New Guinea discovered an anomalous thermal history that contradicts standard models of heat distribution in the Earth's interior. The rock reached approximately 800°C at a depth of 45 kilometers but cooled by about 100°C as it descended beyond 90 kilometers, the opposite of the expected gradual temperature increase with depth. This finding, published in Nature Geoscience, has implications for understanding earthquakes, volcanism, and carbon cycling in subduction zones.

Subduction zones occur where tectonic plates converge and one plate is forced beneath another into Earth's interior. Rocks caught in this process experience extreme pressure and temperature changes as they descend. Researchers analyzed microscopic mineral phases locked within garnet crystals to determine the pressure-temperature conditions the Papua New Guinea sample experienced at different depths, with coesite indicating ultra-high pressure environments and zircon providing timing information.

The unexpected cooling pattern suggests subduction zone thermal structures may operate differently than current models propose. Two leading explanations emerged: frictional heating from intense grinding where plates collide could create anomalous heat at shallower depths, or the particular subduction zone may not have reached thermal equilibrium, retaining warmer conditions at intermediate depths before cooling.