Very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow

A new paleomagnetic result from a lava flow with a distinctive, two‐part remanence reinforces the controversial hypothesis that geomagnetic change during a polarity reversal can be much faster than normal. The 3.9‐m‐thick lava (“Flow 20”) is exposed in the Sheep Creek Range (north central Nevada) and was erupted during a reverse‐to‐normal (R‐N) geomagnetic polarity switch at 15.6 Ma. Flow 20 began to acquire a primary thermoremanence while the field was pointing east and down but was soon buried, reheated, and partially‐remagnetized in a north‐down direction by the 8.2‐m‐thick flow that succeeded it. A simple conductive cooling calculation shows that the observed remagnetization could not have occurred unless Flow 20 was still warm (about 150°C near its base) when buried and that the 53° change from east‐down to north‐down field occurred at an average rate of approximately 1°/week, several orders of magnitude faster than typical of secular variation.