Fig. 2

Paleomagnetic data and contact tests. Orthogonal vector plots of stepwise thermal demagnetization of basalt (a) and soil (b) samples. Red is inclination, blue is declination (cores are azimuthally unoriented). c Stratigraphy adjacent to thick soil horizon in the Reef Hole (Supplementary Fig. 1). Here the unit feet is retained as it was the unit of record. Red, soil or weather flow top. Dark grey, basalt. Light grey, unrecovered. d Characteristic remanent magnetization inclination. Blue, basalt samples. Red, soil samples. The uppermost soil sample from the thick soil horizon (circled) records the polarity of the overlying lava flow (reversed polarity) whereas, the lower samples record a different (normal polarity). This constitutes a positive contact test indicating that a primary magnetization has been preserved. Similarly, the uppermost sample of the weathered lava flow top of the reversed flow (circled) records the direction of the overlying lava flow (normal polarity), constituting a second positive contact test