Читать книгу Oahu Trails. Walks Strolls and Treks on the Capital Island онлайн

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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. The cliffs at the extreme west and east ends of Oahu are wave-cut cliffs. These wave-cut cliffs grade into the dramatic, fluted stream-cut cliffs (pali) and lush valleys. Streams take material from higher on the volcano, cutting valleys into its flanks and depositing the material they carry as alluvium. Alluvial deposits cover the floors of the stream-cut valleys. 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.

Oahu is geologically an infant on an Earth more than 4 billion years old. Potassium-argon dating of rocks suggests that lava welled forth to build the Waianae volcano between 3 and 4 million years ago. After three major periods of activity, it finally became quiet about 2.5 million years ago. Since then, erosion has carried away most of it, leaving only a crescent-shaped piece of its east rim standing as the Waianae Range. The Koolau volcano burst forth between 2.6 and 1.8 million years ago, building its shield in a single major period of activity and building up the plateau between the two volcanoes. Now, erosion has worn away most of the volcano’s eastern rim, leaving its long west rim standing as the Koolau Range.