The origin of Mauna Loa's Ni¯nole Hills: Evidence of rift zone reorganization

Abstract

In order to identify the origin of Mauna Loa volcano's Ni¯nole Hills, Bouguer gravity was used to delineate density contrasts within the edifice. Our survey identified two residual anomalies beneath the Southwest Rift Zone (SWRZ) and the Ni¯nole Hills. The Ni¯nole Hills anomaly is elongated, striking northeast, and in inversions both anomalies merge at approximately -7 km above sea level. The positive anomaly, modeled as a rock volume of ~1200 km3 beneath the Ni¯nole Hills, is associated with old eruptive vents. Based on the geologic and geophysical data, we propose that the gravity anomaly under the Ni¯nole Hills records an early SWRZ orientation, now abandoned due to geologically rapid rift-zone reorganization. Catastrophic submarine landslides from Mauna Loa's western flank are the most likely cause for the concurrent abandonment of the Ni¯nole Hills section of the SWRZ. Rift zone reorganization induced by mass wasting is likely more common than currently recognized. Key Points 100-200 ka ago Mauna Loa's SWRZ ran through the Ninole Hills Series of mass wasting events caused a geologically instantaneous reorganization of Mauna Loa's SWRZ Reorganization and rapid movement of rift zones is not uncommon; although generally unrecognized