Most of the rocks exposed along the length of the Black Canyon are Precambrian in age (older than 500 million years) and are either metamorphic or igneous. Some sedimentary layers are also evident along U.S. Highway 50 from Red Creek to Elk Creek. Rocks found in the canyon have a wide variety of minerals. Here is a brief look at some of them and where they may be found.
Left image
Geologic map of the Upper Black Canyon, Crystal Reservoir, Morrow Point Reservoir, and the west side of Blue Mesa Reservoir
Credit: U.S. Geological Survey
Right image
Topography of Upper Black Canyon and nearby vicinity of Curecanti
Credit: Image courtesy of Google Earth
A sample of gneiss
NPS/A. Fitzgerald
Metamorphic Rocks
The word "metamorphic" originates from the Greek language and means to transform or change. Metamorphic rocks usually start out as sedimentary, or igneous rock, but when buried deep in the earth, intense heat and pressure "cooks" or bakes" them into a completely new rock.
Heat and pressure are the two most important parts of this process, but time also plays a role; the longer the rock has been baked and squeezed, the greater the changes. Rock is usually buried deep within the Earth's crust (6–8 miles or 9.5–12.9 km) before temperatures and pressures are high enough to melt and change their physical and chemical composition. Black Canyon's metamorphic rocks have been altered to the point that little trace of the original rock remains. However, geologists suspect that the original rocks (protoliths) were sands, mud, and volcanic debris, that accumulated on the floor of an ancient sea. The time of metamorphism is estimated at 1.7 to 1.9 billion years ago.
Gneiss and schist are examples of metamorphic rocks found in the Black Canyon. These rocks blend from one to another because of variations in the heat and pressure which occurred when some rocks were buried deeper than others
Gneiss
Along the road from Cimarron to the Morrow Point Dam Overlook (Curecanti National Recreation Area), one can find examples of gneiss, the predominant rock in the Black Canyon. Gneiss (pronounced "nice") has bands, layers, or even lenses of blocky crystals such as feldspar, alternated with bands of a flat, plate-like mineral such as mica.
Gneiss represents some of the most advanced stages of metamorphism—where the most intense temperatures and pressures are exerted upon the rock. Original rocks were buried even deeper and were hotter, almost to the point of melting. Places where the rock has been partially melted and the melt was injected, or squeezed into the layers of the remaining solid portions of the gneiss, create a type of gneiss known as migmatite. Migmatite gneiss is a rock that almost melted and is an intermediate between igneous and metamorphic.
The gneiss has been so highly transformed, meaning that the temperatures and pressures were so extreme, that there is little evidence of what the original sedimentary layers of rock were. The large amount of mica, with a silica content of nearly 85 percent, suggests that the protolith was an impure sandstone or chert.
Schist
Schists are the other metamorphic rocks found in the Black Canyon. The protoliths were not buried as deeply, so there was less heat and pressure. These schists have been altered less than gneiss because of the lower pressures and temperatures. The main difference between schist and gneiss is the thickness of their internal layers, known as lamellae (pronounced "la MEL lee"). Gneiss has thick lamellae and schist have very thin, fine layers.
There are two kinds of schists found in the canyon: either a mica-rich schist, that has a lot of biotite and muscovite mica, or a schist rich in hornblende, a type of mineral called amphibole. A zone of schist crosses the canyon between Gunnison Point and Pulpit Rock Overlook. Rare minerals such as garnets, sillimanite, staurolite or andalusite can be common in places.
Curecanti Needle, the formation Curecanti Quartz Monzonite is named after
NPS Photo
Igneous Rocks
Igneous rocks (meaning "fire-born") cooled from a molten rock, or magma, deep beneath the surface of the earth. If magma cools before it reaches the surface, it is called intrusive. Magma that reaches the surface, as in a volcanic eruption, is referred to as extrusive. Examples of igneous rocks in the Black Canyon are intrusive rocks. Here, the magma was pushed into the existing metamorphic rock and never reached the Earth's surface. Igneous intrusive rocks in Black Canyon leans mostly towards a granite composition - meaning it is more silicic (high in silicia). Curecanti has both intrusive and extrusive rocks visible.
The striking, pinkish banding evident throughout the canyon, and most well-known on the Painted Wall, is pegmatite—granite with huge crystals.
Quartz Monzonite
Quartz monzonite, an intrusive-igneous rock, makes up the Curecanti Needle and intrusions from Rock Point to Warner Point. Quartz monzonite may sound complicated, but it is only a type of granite. Granite is a crystalline, igneous rock composed mainly of quartz, orthoclase, and microcline. The name monzonite means that the magma that created the rock had approximately equal amounts of sodium and calcium-rich feldspars. When "quartz" is added to the title, it means that a large amount of silica was present in the magma. Silica, when cooled, becomes quartz.
There are two types of quartz monzonite within Black Canyon of the Gunnison National Park and Curecanti National Recreation Area: Curecanti Quartz Monzonite (lighter, finer grained) and Vernal Mesa Quartz Monzonite (darker, coarse grained). Both rocks originated about 1.2 to 1.4 billion years ago. This activity post dated the creation of the surrounding metamorphic rock, dated 1.9 million years ago.
Pegmatite is visible along U.S. Highway 50 before reaching the Lake City Bridge cutoff.
NPS Photo
Pegmatite
Pegmatite refers to igneous rock with very coarse texture and unusually large, intergrown crystals. It consists mostly of quartz, feldspar, and mica. As magma cools and solidifies, water becomes concentrated. This concentration makes the magma more fluid and easier to squeeze, like toothpaste out of the tube, into the surrounding rock. The crystals can be very large—up to 6 feet (2 m) in length.
Pegmatite is abundant throughout Black Canyon. It is found in the form of large bodies, stripes, and "dikes." Since it is more resistant to erosion, it sticks out boldly against less resistant gneiss and schist. Potassium (K) feldspar gives this pegmatite its pink shade. Large outcroppings are visible on the Painted Wall, upstream from East Portal, or at Kneeling Camel on the north rim of Black Canyon.
Sedimentary Rocks
Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organisms. If buried deeply, sediment becomes compacted and forms rock.
Black Canyon has some sedimentary rock along the canyon rim. Sediments were deposited on top of the Precambrian basement rocks during the Jurassic to Cretaceous periods - creating various sandstones, shales, coal beds, and muds. During the Gunnison Uplift, soft sediments eroded off the higher areas to reveal the basement rocks. In Curecanti, the Wanakah Formation, Morrison Formation, Burro Canyon Formation, Dakota Sandstone, and Mancos Shale are all exposed.
The Wanakah Formation is mostly under Blue Mesa Reservoir and not visible. The Morrison Formation, the remnants of a tropical and wet environment, is famous for abundant dinosaur fossils. It is composed of sandstone cliffs and red, grey, and green shale; the formation is thicker as one moves down the Black Canyon and visible in sections along the Blue Mesa Reservoir shoreline.
The Dakota Sandstone, another river deposit layer, is less clay-like than Morrison and more erosion resistant than Mancos Shale. It caps mesas, hills, and forms part of the canyon rim. It is visible in sections along U.S. Highway 50. Mancos Shale, which forms many sloping hills and badland formations, is the youngest of the visible sandstone layers in the area. It is mostly covered by vegetation in Curecanti.
Learn more about sedimentary rocks and how they form.
Resources
Black Canyon/Curecanti Online Bookstore
A variety of titles dealing specifically with the geology of the Black Canyon and Colorado in general, are available for online purchase.
The Interior of the Earth
Online Edition by Eugene C. Robertson
This publication, available through the U.S. Geological Service, gives an excellent introduction to what lies below the surface of the Earth.
Natural Gemstones
Also from the USGS, this page provides an in-depth look at gemstones from around the country.
