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The Hawaiian Islands are successively older toward the northwest and younger toward the southeast. Northwestern islands, like Laysan, are hardly more than bits of volcanic rock now. Southeastern islands, including the major Hawaiian Islands, are still significant chunks of land. In geologically recent times, including today, the big island of Hawaii is the youngest and the farthest southeast of the major islands; Kauai is the oldest and the farthest northwest 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. Erosion has sculpted the exotic landscapes we associate with tropical islands. Waves pound the volcano’s edges, undercutting them and, where the volcano slopes more steeply, forming cliffs like those of the Na Pali Coast. Streams take material from higher on the volcano, cutting valleys into its flanks and depositing the material they carry as alluvium, like the alluvial apron at the mouth of Kalalau Valley. 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.