North-south–trending dikes exhibiting well-developed columnar jointing have been visually observed on a series of submersible dives to the submarine extension of the southwest rift zone of Mauna Loa Volcano, Hawaii. These dikes are exposed along a west-facing 1,900-m-high scarp that is the sea-floor expression of the southwest rift zone. They form prominent walls, 10 to 60 m high and 1 to 3 m thick at their crests, separated by sediment- and rubble-mantled benches. The talus at the base of each dike consists of faceted blocks of basalt broken from the face of the wall.

Major-element analyses of samples collected in situ show the rocks to be of “normal” Mauna Loa tholeiite composition.

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