Rainbow Falls (California)
Encyclopedia
Rainbow Falls is the highest waterfall on the Middle Fork of the San Joaquin River
, California
, United States
. Plunging 101-feet down to the turbulent water below, the falls are named for the rainbows that appear in its mist throughout sunny summer days.
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 kilometre (0.621372736649807 mi) 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.
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 (152.4 m) due to undercutting.
San Joaquin River
The San Joaquin River is the largest river of Central California in the United States. At over long, the river starts in the high Sierra Nevada, and flows through a rich agricultural region known as the San Joaquin Valley before reaching Suisun Bay, San Francisco Bay, and the Pacific Ocean...
, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Plunging 101-feet down to the turbulent water below, the falls are named for the rainbows that appear in its mist throughout sunny summer days.
About 75,000 years ago, lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
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 kilometre (0.621372736649807 mi) 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
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
vertically. The layer above cooled under different conditions and fractured horizontally. Geologists describe these rocks as platy rhyodacite
Rhyodacite
Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate...
and 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 500 feet (152.4 m) due to undercutting.