Rocks and Minerals

 

No Collecting Rock Specimens

Leave what you find! Collecting rock specimens in Black Canyon of the Gunnison National Park or Curecanti National Recreation Area is illegal.

 

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 the north rim of Black Canyon. 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 Black Canyon and nearby vicinity. Precambrian gneiss and schists are shown in pink, light brown, and grey in the canyon.
Credit: U.S. Geological Survey

Right image
Topography of Black Canyon and nearby vicinity
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

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 intrusions along the Painted Wall

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 layers along the canyon rim. Sediments were deposited on top of the Precambrian basement rocks during the Jurassic to Cretaceous periods. During the Gunnison Uplift, soft sediments eroded off the higher areas to reveal the basement rocks.

 

Dig Deeper into Rocks

• 

  Metamorphic Rocks

  Learn more about metamorphic rocks and what NPS sites have them.

• 

  Igneous Rocks

  Learn about igneous rocks, their formation, and the difference between intrusive and extrusive.

• 

  Sedimentary Rocks

  Learn about sedimentary rocks, their formation, and where to find them.

 

Rocks and Minerals FAQs

 

How do we know how old the rocks in Black Canyon are?

The age of these rocks is determined through radiometric dating techniques (radio from the word “radioactive” and metric from “to measure”). The specific techniques used are Uranium-Lead dating and Rubidium-Strontium dating.

Black Canyon is so old that it formed before the existence of present-day North America. Additionally, the Precambrian rock was not formed in what we know as present day Colorado. This rock was formed somewhere else on the globe and moved into its current position by over a billion or more years of plate tectonic activity. The basement rock most likely initially formed in the southern hemisphere.

What is underneath or below the "basement rock" at Black Canyon?

The simple answer is more basement rock, which composes the earth's crust in this area. This crust may be a portion of the ancient continental craton (the crust and upper-most mantle).

What differentiates schist from gneiss?

Gneiss and schist are both metamorphic rocks, which means they started out as one kind of rock (known as the parent rock) and changed into another kind of rock as a result of geological processes.  Both started out on the surface of the earth, were pushed deep into the earth, and were then brought back to the surface.

The primary physical characteristic that differentiates gneiss and schist is the degree of compositional banding in the rock (alternating light and dark bands), known in geological terms as “foliation." Foliated schist is produced when rock is exposed to mid-to-high temperatures and pressures, whereas more foliated gneiss is the result of parent rock being exposed to high temperatures and pressures.

There are many types of schist and gneiss; several different types are present throughout the Black Canyon. Quartz-mica gneiss is the most common type of gneiss in the Black Canyon but others such as migmatite are also present. Schist types include micaceous and hornblende.

What is the difference between pegmatite and granite?

Both granite and “pegmatite” are igneous, intrusive rocks. Pegmatite is not a type of rock, but a term that can be used to describe the texture of rock. 

Granite is primarily composed of feldspar (40 to 80 percent), quartz (20 to 60 percent), and small amounts of other minerals, such as mica.  These are the same minerals that make up the rock in the park that is referred to as “pegmatite.” Granite has small crystals and a fine grain, which resulted from rapid cooling of the molten rock. Pegmatite has much larger crystals and a coarser grain. Unlike true granites, pegmatites may have a combinations of crystals of many different sizes: large, medium, and small

Where did all the weathered and eroded rock that was once in Black Canyon go?

Most of the rock that once filled the canyon was transported by the Gunnison River, to the Colorado River, and eventually washed out to sea. The mouth of the Colorado River is on the Gulf of California (also known as the sea of Cortez) which lies between the Baja Peninsula and mainland Mexico. The river has not reach the ocean naturally since the late 1990s. The sediment transported by the river eventually settles on the ocean floor (the oceanic crust). Over geological time the oceanic crust is subducted back into the earth, where it is compacted and lithified, melts, or is metamorphosed, and comes out somewhere else on the earth as new rock.

This erosion process has been altered by the construction of dams along the Gunnison River. Eroded sediment no longer flows freely to the ocean and is collected and deposited behind the dams. This gradually fills the reservoirs downstream (i.e. Lake Powell and Lake Mead) with sediment.

 

Resources

Blakey, Ron, and Ranney, Wayne (2008). Ancient Landscapes of the Colorado Plateau. Grand Canyon Association, Grand Canyon, AZ. 

Chernicoff, Stanley, and Whitney, Donna (2002). Geology 3^rd Ed. Houghton Mifflin. Boston, MA.

Hansen, Wallace R. 1965. THE BLACK CANYON OF THE GUNNISON: Today and Yesterday. U.S. Geological Survey Bulletin 1191. USGS: Geological Survey Bulletin 1191 (nps.gov)

Hansen, Wallace (1987). The Black Canyon In-Depth. Parks and Monuments Association.

Hansen, W. R., and Peterman, Z. E. (1968). Basement-rock geochronology of the Black Canyon of the Gunnison, Colorado. U.S. Geological Survey Professional Paper. 400: C80-C90.

Jessup, Micah, et al. (2006).Three Proterozoic orogenic episodes and an intervening exhumation event in the Black Canyon of the Gunnison region, Colorado, The Journal of Geology, 114, p. 555-576.

King, Vandall, T. (2010).  Collector’s Guide to Granite Pegmatites.  Schiffer Publishing.Atglen, PA. 

Plummer, Charles and McGeary, David (1993). Physical Geology. Wm. C. Brown. Dubuque, IA.

Robertson, Eugene C., n.d. The Interior of the Earth. The Interior of the Earth (usgs.gov)

Thornberry-Ehrlich T. 2005. Black Canyon of the Gunnison National Park & Curecanti National Recreation Area Geologic Resource Evaluation Report. Natural Resource Report. NPS/NRPC/GRD/NRR—2005/001. NPS Geologic Resources Division. Denver, Colorado

Tweet, J.S., Santucci, V.l., Connors, T., and Kenworthy, J.P.  ( 2012).  Paleontological resource inventory Monitoring: Northern Colorado plateau network. Natural resource technical report. National Park Service, Fort Collins Colorado. 

U.S. Geological Survey. Natural Gemstones Version 1.1. https://pubs.usgs.gov/gip/gemstones/