Emplacement and inflation structures of submarine and subaerial pahoehoe lavas from Hawaii

Abstract

Features of subaerial pahoehoe tumuli from Kilauea and Mauna Loa Volcanoes in Hawaii and subaqueous flow lobes from Loihi Seamount off Hawaii and north of Oahu Island document the controlling factors of flow-lobe formation. Studied subaerial flow-lobe tumuli consist of uplifted pahoehoe crust, formed from coalesced flow lobes. The south rift zone of Loihi has abundant conical lava mounds and terraces with flat tops. Steep flanks (>4°) of these mounds are covered with elongate pillows, but the flat tops are overlain by lobate sheets and inflated pahoehoe flows. The pahoehoe lobes have an elongate dome-like shape 2 x 0.5 m to 20 x 20 m and up to 5 m in height. Hollow pahoehoe lobes, which are rare among subaerial pahoehoe flows on coastal plains in Hawaii Island, are common on Loihi. Similar hollow lobes and a subaqueous tumulus were found on a 2200-m deep terrace offshore of Oahu Island. In contrast to corrugated pillow lobes, these subaqueous flows have smooth surfaces covered with fine streaks of thin glass flakes. We suggest that subaqueous flow lobes grow like a slowly advancing subaerial pahoehoe flow. The plastic crust stretches, thins, and ruptures at the flow front, where new lava is exposed, and subsequently is quenched by water to form thin flaky glass. The Graetz number, giving a dimensionless temperature distribution within lava, varies flow by flow but remains almost constant for subaqueous flow lobes. This means that the lateral extension of a flow lobe is cooling limited, which results in a linear correlation between the rate of lava supply and the volume of lobes. Flow lobes stop advancing due to heat loss to the environment, as brittle crust thickens and hinders the lobes from deforming freely.