Saturday, November 19, 2011

Memorable Places Here and There on the Colorado Plateau: Ribbon Falls

About eight miles down the North Kaibab Trail from the Grand Canyon's North Rim, a short detour off to the right beckons sun-parched backpackers to Ribbon Falls. Its irresistible mist is near impossible to forgo on a typically hot and dry day in the canyon, making this side excursion a necessity to visit. But what’s truly fascinating is the geological structure that the falls have produced. The action of ground water, by virtue of its mineral composition, has resulted in the formation of a spectacular travertine dome that's over thirty feet tall.

How did this colossal structure form? Water from the falls makes a 120 foot free-fall landing precisely at the apex of the moss-covered travertine dome. Calcium carbonate is in solution, being made soluble by the absorption of atmospheric carbon dioxide, which makes the water mildly acidic. Its acidity allows the carbonate to be “acquired” from limestone formations at higher elevations such as the Redwall and Muav. Subsequently, carbonate is “released” from the mineral-rich dripping water when it plunges over the falls and releases the carbon dioxide held in solution. The change in water chemistry causes the re-deposition of the carbonate in the form of travertine or tufa (softer and more porous) from the mineral-laden water. Gradually, the mound grows by re-crystallization, molecule by molecule. This landform is called karst, made possible by the dissolution of soluble bedrock. The identical process forms the more familiar stalagmites and stalactites in subterranean limestone-caverns.

Ribbon Falls is located in an amphitheater bounded by dark red cliffs of Shinumo Quartzite. The falls plunge over the ledge of a resistant diabase sill. Diabase is the intrusive equivalent of basalt. This sill is part of a system of Cardenas conduits and a massive basaltic outpouring of the same name that fed magma to the Earth’s surface. These rock formations, along with three others, are members of the Unkar Group, which comprise the lower Grand Canyon Supergroup. Beginning 1.2 billion years ago, the formations of the Unkar Group were deposited over a span of 100 million years and appear to have been associated with a continental collision event that culminated in the formation of the supercontinent of Rodinia.

This view is taken from behind the falls, looking out at the top of its verdant, mossy travertine dome. Vegetation such as the moss, and golden columbine, maidenhair fern and scarlet monkeyflower thrives in the oasis of the fall’s unique microclimate. These plants are not indigenous to the hot, arid climate of the Grand Canyon only a few feet away.

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