Mechanism of Formation of Pillow Lava

Abstract

Much of the ocean floor is covered by lava of a distinctive character. The lava appears to be made up of closely packed ellipsoidal masses about the size and shape of pillows - hence the term pillow lava. Only within the last few years has the abundance of pillow lava on the ocean floor been fully recognized. Ocean-bottom photographs and dredge samples have shown that the great bulk of new ocean floor created at diverging plate boundaries (such as the Mid-Atlantic Ridge) is composed of pillowed basaltic lava flows. Closeup observations from submarines at depths of 2.7 km in the rift valley of the Mid-Atlantic Ridge have verified that virtually all the lavas erupted at this plate boundary are pillowed. The submarine portions of the great oceanic volcanoes, such as the ridge beneath the Hawaiian Islands, are also known to be built largely of pillow lava, and it is widespread in outcrops of uplifted ancient lava. Pillow lava is probably the most abundant form of volcanic rock on earth, though most of it is hidden beneath the world's oceans and mantled by younger sediments. Most investigators agree that the pillows form when fluid lava chills in contact with water, either when it erupts directly into water (or beneath ice) or when it flows across a shoreline and into a body of water. However, prior to our study, the process of pillow formation had never been directly observed. The recent eruptions of Kilauea Volcano in Hawaii provided an unparalleled opportunity to study the movement and cooling of lava beneath the sea. In June 1969, lava from the new Mauna Ulu vent on the east rift zone of Kilauea spilled into the sea after flowing 12 km down the south flank of the volcano. This pattern was repeated, with lava flowing into the sea for a few weeks each year through 1973. In April 1971 scuba divers for the first time investigated lava flowing underwater and learned that in favorable circumstances the lava could be approached closely. Despite heated water, explosive concussions, vigorous convective currents, and poor visibility due to suspended sediment, valuable observations were made.