Ocean Drilling Program, United States

The overall good core recovery in the basaltic sequence and the dense recording of physical and magnetic properties in Hole 990A are well suited to investigate the lava pile in detail, although no wireline measurements could be achieved during the entire Leg 163. The goal of the investigation is to show the different lava flow types with their physical and magnetic properties, based on reliable measurements of density and susceptibility with a sampling interval of 5 cm, and velocity and magnetic intensity with a sampling interval of 10 cm. The top of the volcanic sequence at Site 990 is deeply weathered and oxidized, and all of the lava flow units have oxidized flow tops. In combination with the recovery of soil horizons, this shows that the eruption and emplacement of the lava flows occurred under subaerial conditions. This subaerial emplacement is also indicated by the physical properties with characteristic low density and velocity values at the flow tops, which is a function of the high degree of subaerial flow-top alteration. The lava pile was divided into aa, pahoehoe, and a transitional form as a result of optical core observations as well as by groundmass crystal texture measurements. Highest density and velocity values are found in the aa flows; in particular the velocities of the pahoehoe flows are smaller than in the aa flows. In general, the pahoehoe flows show the highest magnetic intensities, whereas the magnetic intensities for the aa flows are 3.5 times smaller. The magnetic susceptibility shows an opposite trend.