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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. The Big Island of Hawaii is the youngest and the farthest southeast of the major islands. Southeast of the Big Island, Loihi Seamount is a growing volcano far beneath the ocean. In a few thousand years, Loihi may be the newest Hawaiian island.

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.

Erosion has sculpted the exotic landscapes we associate with volcanic tropical islands. Waves pound the volcano’s edges, undercutting them and, where the volcano slopes more steeply, forming cliffs. Streams take material from higher on the volcano, cutting valleys into its flanks and depositing the material they carry as alluvium. Alluvial deposits form the floors of valleys like Waipio and Waimanu. New episodes of volcanism wholly or partly fill in those landscapes, and erosional forces immediately begin sculpting the new surface as well as the remaining older surface.