PALEOMAGNETISM OF AN INFLATED LAVA FLOW: KILAUEA, HAWAII

Pahoehoe lava flows emplaced on flat ground inflate and expand vertically from less than a half a meter to over five meters in thickness. Inflation results as flow advance slows and the flow’s outer crust cools, and becomes rigid, while the volume continues to increase. In order to better understand the inflation process, we studied the paleomagnetism of basalt samples from a 1990 Kupaianaha flow on Kilauea Volcano just west of Kalapana, Hawaii. Several samples were drilled and oriented from the horizontal top of the flow and the flank, which was dipping about twenty degrees. If the flow cooled below the Curie Temperature (~570º Celsius for basalt) before inflation, there should be a difference in magnetic inclination from cores collected from the horizontal top of the flow and the dipping flank. If the flow cooled below the Curie Temperature after inflation, samples from all over the flow should preserve the same magnetic inclination. A Molspin spinner magnetometer was used to measure inclinations and declinations. Randomly selected samples, demagnetized up to 60 mT using an alternating field demagnetizer, showed no magnetic overprints. Results document differences of at most a few degrees in inclination between horizontal samples from the top of the flow and the dipping samples from the flank. The magnetic minerals of some inflated lava flows must not have acquired inclinations until after the flow inflated, thus the data from these flows is an accurate representation of earth’s magnetic field. For a small number of flows, inflation rotated the magnetic data a few degrees but these changes are minimal and do not reduce the utility of paleomagnetic data. Therefore, it is safe to assume that paleomagnetic research of pahoehoe flows does not need to account for the process of inflation except in rare cases.