Rainbow Falls
At Rainbow Falls the Middle Fork of the San Joaquin River plunges
101-feet over a cliff of volcanic rock known as rhyodacite. When
the sun is overhead, rainbows highlight the spectacular falls.
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 1 km 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 vertically.
The layer above cooled under different conditions and fractured
horizontally. Geologists describe these rocks as platy rhyodacite
and are visible in the cliffs that surround Rainbow Falls.
So what is currently happening beneath 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 500 feet
due to undercutting.
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