Submarine lava flows around the coasts of Pico Island, Azores

Bathymetry data collected with a multibeam echo-sounder around Pico Island, Azores (Portugal) reveal a remarkable series of lava flows on the island's shelf with a variety of pristine structures. Many flows are dendritic in plan-view, some with channels and tumuli. Dendritic geometries are reminiscent of lava with more viscous rheology than expected for subaerial flows of these basaltic-Hawaiitic compositions, consistent with viscosity increased by enhanced cooling in water. Screens of elongated flow fingers also occur, with individual fingers of comparable diameters to the largest known megapillows. Some flows have wide transverse clefts, in some cases separating flows into segments. They are interpreted as caused by the flow upper surfaces having solidified, while their still-fluid cores allowed the surfaces to extend. A number of flows moved onto the shelf as large bodies, stopped, and then sourced smaller lobes forming the dendritic patterns. This two-stage evolution and the tumuli (which lie adjacent to a steep near-shore gradient) suggest that, after initial emplacement and development of a crust by cooling, some flows pressurized. Once movements ceased and viscous stresses were dissipated, magmastatic pressure developed from the weight of flow interiors passing over coastal cliffs and near-shore submarine slopes. One group of flows traverses the submarine slope of the island, so direct supply of lava to the slopes is possible, although volumetrically how important such flows are to building the island's internal structure is difficult to tell from these data. Based on observed strong surf erosion of historical flows, these delicate structures could not have survived passage through a moving sea level so they are not pre-Holocene subaerial flows. They were formed in the Holocene from flows penetrating sea level or possibly some from near-shore tube openings or vents. Such flows and abundant sand deposits are ephemeral features that probably become remobilized by surf during times of lowering sea level. The shelves of active volcanic islands are therefore active geologically and are far from being simple products of erosional truncation as was once envisaged. The finding of lava extending below sea level also has implications for assessing the hazards of volcanic islands.