Learn about the geology of Black Canyon of the Gunnison National Park.
Black Canyon of the Gunnison National Park is located in western Colorado, near the city of Montrose. The park was created in 1999 and encompasses roughly 30,750 acres of wilderness. The Gunnison River is 48 miles long, but only 14 miles run through the national park. The park is known for its incredibly narrow canyons, which were carved over millions of years by the Gunnison River cutting through ~1.7 billion year-old Precambrian metamorphic rock. Towering cliff faces of foliated gneiss and schist line the canyon walls and create beautiful rock exposures. Painted Wall, the most famous of these outcrops, stands at ~2247 feet (685 meters) high and is the tallest cliff in Colorado. The park was named for its cliffs that are often darkened by shadows, making the walls appear black. The canyon itself is so vertical that the Gunnison River drops an average of 43 feet per mile, and 240 feet per mile at its steepest point at Chasm View, making some of the steepest mountain descents in North America.
The iconic metamorphic rocks were formed almost two billion years ago during the Precambrian Era, when the Earth was a much more volatile place. Mafic (iron and magnesium-rich) and felsic (feldspar and silica-rich) rocks were buried deep in the Earth. With burial came intense heat and pressure that altered these minerals by flattening and elongating the crystals, eventually turning them into the metamorphic rocks gneiss and schist. Faulting, folding, and other tectonic activity further warped the rock, creating the famous dark (mafic) and light (felsic) gneissic banding. Some igneous rocks such as pegmatite are also found in the canyons but made their appearance is quite different from the metamorphic rocks. For intrusive igneous rocks like pegmatite, very slow cooling times of magma intruding into the surrounding rock allowed very large minerals to form. In places like Painted Wall, the magma had such a slow cooling rate that the crystals were able to grow up to six feet long! Extensive uplift and erosion caused these deeply formed rocks to be exposed at the surface. Mountain-building was accompanied by thousands of feet of erosion by the Gunnison River. After cutting through younger layers of rock, the Gunnison River reached the basement metamorphic and igneous rocks, which are much more resistant to erosion than the surrounding sedimentary rocks.
Similar uplift in the region also raised the Sawatch, West Elk, and San Juan Mountains, all of which drain into the Gunnison Basin. Only a river with such a large water supply and high velocity could erode through such resistant rock to create cliffs as steep as the Black Canyon. It is estimated that the Gunnison River eroded the metamorphic rocks at a rate of one inch per 100 years. At this rate, it would have taken over two and a half million years just to erode from the top to the bottom of Painted Wall!
Metamorphism at Depth
Metamorphism occurs when heat and/or pressure changes the texture and/or composition of the original rock. Metamorphism can occur on different scales. Regional metamorphism commonly occurs over large areas, such as at convergent plate boundaries where compressional forces occur when tectonic plates collide. Gneiss, the rock that makes up a significant portion of the Black Canyon, is a high-grade, foliated metamorphic rock whose protolith was originally granite or schist.