A paleomagnetic study of the Pohue Bay flow and its associated coastal cones, Mauna Loa volcano, Hawaii: constraints on their origin and temporal relationships
Detailed paleomagnetic and rock-magnetic studies of the Pohue Bay flow and associated cones on its coastal flat were made to determine if the origin of the cones was due to primary (volcanic) or secondary (littoral) processes. We used paleomagnetism to determine the temporal relationships between the flow and cones. If the flows and cones are from the same eruption, the littoral origin of the cones is strongly favored. A total of 530 specimens from 232 core samples were collected and studied from the Pohue Bay flow and from lava ponded within the cones. Remanent magnetizations are very stable to stepwise alternating field demagnetization and show small angular dispersion and well-defined characteristic magnetizations. Magnetic carriers correspond to members of the titanomagnetite series with single or pseudo-single domain states. The overall mean directions for the Pohue Bay flow (Dec = 10.8°, Inc = 23.6°, k = 287.4 and α95 = 2.3°) and the lavas ponded within the cones (Dec = 12.8°, Inc = 25.2°, k = 353 and α95 = 4.9°) are statistically indistinguishable and have been drawn from a common Fisherian distribution, supporting a close age relationship. Additionally, the angular dispersion for the combination of the main flow and lavas ponded in cones is small, with a paleosecular variation (PSV) estimate of Sf = 4.2°. This small PSV value supports the temporal association between the Pohue Bay flow and associated cones. They can thus be assigned to the same eruption timeframe. Because the source vent of the Pohue Bay flow is far upslope, the cones must therefore be littoral in origin, formed when the Pohue Bay flow entered the ocean. From the secular variation curve and comparison with age-dated flows with similar paleomagnetic directions, we estimate that the Pohue Bay flow was erupted approximately 1300 years ago. We were also able to distinguish both a possibly younger lava flow that later utilized the main tube of the Pohue Bay flow and an earlier sub-set of cones that were possibly formed before the Pohue Bay eruption.