Bathymetry data collected with a multibeam echo sounder around Pico Island, Azores (Portugal), reveal
a remarkable series of lava flows on the islandŐs shelf with a variety of pristine structures that suggest how
lava behaves on entering water. Many flows are dendritic in plan view, some with channels and tumuli.
Dendritic geometries are interpreted to arise from flow fronts repeatedly arrested by enhanced cooling and
magma pressure subsequently causing new breakouts. Cascades of elongated flow fingers also occur, with
individual fingers of comparable diameters to the largest known megapillows. Some flows have wide
transverse clefts, in cases separating flows into segments, which are interpreted as caused by their 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 on a low gradient immediately below a steep
nearshore gradient) suggest that, after initial emplacement and development of a crust by cooling, some
flows pressurized. Once movements ceased and viscous stresses dissipated, magma static pressure
developed from the weight of flow interiors passing over cliffs and nearshore gradients. One group of
flows traverses the islandŐs submarine slope, so direct supply of lava to the slopes is possible, although
volumetrically how important it is to the islandŐs internal composition is difficult to tell from these data. On
the basis of observed strong surf erosion of historical flows, these delicate structures probably could not
have survived passage through a moving sea level unmodified by erosion so they are unlikely to be pre-
Holocene subaerial flows. They are interpreted to have formed in the Holocene from flows penetrating sea
level or possibly some from nearshore tube openings or vents. Such flows and abundant clastic deposits are
ephemeral features that become remobilized by surf during times of lower 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.
Mitchell, NC, C Beier, P Rosin, R Quartau, F Tempera, Submarine lava flows around the coasts of Pico Island, Azores, Geochem. Geophys. Geosyst., 9, Q03024, doi:10.1029/2007GC001725, 2008.