Rainbow Falls is the highest
waterfall on the Middle Fork of the
San Joaquin River, in the eastern
Sierra Nevada
The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...
mountains of
California, in the
United States. Plunging to the turbulent water below, the falls are named for the rainbows that appear in their mist on sunny summer days.
Rainbow Falls is located within the boundaries of
Devils Postpile National Monument.
About 75,000 years ago,
lava erupted from a vent just east of the present-day falls. The eruption occurred in two stages. The first pulse of lava flowed about westward and pooled in the Middle Fork of the San Joaquin drainage. The second pulse of lava insulated the first, allowing it to cool slowly and to
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
vertically. The layer above cooled under different conditions and fractured horizontally. Geologists describe these rocks as platy
rhyodacite; they are visible in the cliffs that surround Rainbow Falls.
As water rushes over the cliff, it erodes the vertically fractured rhyodacite more easily. A small
cavern begins to form at the base of the falls, and the overlying rock loses its support and collapses. This process, known as undercutting, causes the waterfall to recede slowly upstream. To date Rainbow Falls has retreated about due to undercutting.
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References
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Landforms of Madera County, California
Waterfalls of California
Plunge waterfalls