Rapid Field Change in a Single Lava Flow
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A paleomagnetic survey conducted on a basaltic lava flow in the Sheep Creek Range of Northern Nevada revealed what appears to be a 50? change in the direction of the magnetic field that occurred during the cooling of the flow. Previous studies of this flow have indicated that it erupted 15.6 million years ago, during a transitional to normal state of the geomagnetic field when the field may have been changing as rapidly as a few degrees per day. As the flow cooled from 630 to 300?C over several months, the ferromagnetic minerals recorded the direction of the earth?s magnetic field. This interpretation is highly controversial and more work is needed to prove its verity. Samples were taken every 20 cm vertically through the 3.5 m flow and their remnant magnetizations were measured. Thermal demagnetization steps ranging from 150?C to 630?C show the variation of the field direction as the flow cooled. Most samples taken higher than 0.5 m above the flow base display a direction like that of the overlying flow. Closer to the flow base, where the flow cooled quickly, a high temperature component similar to the direction of the underlying flow was also in the sample. Two possible explanations for this change in field direction are that either the flow recorded a rapid field change while cooling 330?C or that the upper 3 meters of the flow was remagnetized by heat from the overlying lava flows.