La Tarta volcanic outcrop, Tenerife. This outcrop contains different layers of volcanic rock in the form of lapilli, rounded pebbles of rock formed when blobs of molten lava cool in mid-air after a volcanic eruption. The black and white lapilli here have different origins. The underlying white lapilli were ejected in the Pleistocene epoch, which ended some 11,550 years ago. The layer of black lapilli is of later origin, being ejected as part of the Canadas series III eruptions, in historical time. Photographed at Portadillo, Tenerife, in the Canary Islands. These are volcanic islands that are formed entirely from rock produced in undersea eruptions.