geothermometer

noun2

1. •
   A proxy that allows for geothermometry, the determination of the temperature of a geological material from the past or existing at great depth.

   • The geothermometer is often used to calculate the deep temperatures of geothermal reservoirs on the basis of water-rock equilibrium in the exploration of geothermal resources. — 1966
2. •
   (no longer used) A thermometer designed for measuring extremely high temperatures below ground level.

   • Walferdin's registering thermometer and the geothermometer of Magnus are constructed on this principle. — 1880