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Deep valleys surrounded by steep cliffs are typical landforms sculpted by the erosion of Hawaiian volcanoes—here, windward Oahu.

The Hawaiian Islands are successively older toward the northwest and younger toward the southeast. Northwestern islands, like Necker, are hardly more than bits of volcanic rock now. Southeastern islands, including the major Hawaiian Islands, are still significant chunks of land. Kauai and Niihau are the oldest and the farthest northwest of the major islands. Oahu is younger, and Maui is younger yet. The Big Island of Hawaii is the youngest and the farthest southeast of the major islands.

The molten material—lava—characteristic of Hawaiian volcanoes is relatively fluid. The fluidity of the lava allows it to spread widely, and repeated eruptions produce broad-based, rounded volcanoes called shield volcanoes. The volcano expels not only flowing lava but volcanic fragments such as cinder and ash. Alternating layers of these materials build up during periods of volcanic activity.