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, with some sedimentary layers evident along the North Rim. The rocks in the Black Canyon have a wide variety of minerals. Here is a brief look at some of them and where they may be found.
The word "metamorphic" has its origins in 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 "cook" or "bake" 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 (six to eight miles, for instance) 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, or 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 represents some of the most advanced stages of metamorphism, with the most intense temperatures and pressures exerted upon the rock. That means the original rocks were buried deeper and were hotter, almost to the point of melting. In places the rock has been partially melted and the melt was injected, or squeezed into the layers of the remaining solid portions of the gneiss, creating a type of gneiss known as migmatite. Migmatite gneiss is a rock that almost melted and is an intermediate between igneous and metamorphic. More information »
Schists are the other metamorphic rocks found in the Black Canyon and are at the other end of the heat and pressure scale. The original rocks (protoliths) were not buried as deeply so there was less heat and pressure. Although still considered metamorphic, these schists have been altered less because of the lower pressures and temperatures. More information »
Igneous rocks are those that 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. The striking, pinkish banding evident throughout the canyon walls is intrusive-igneous rock.
Quartz Monzonite may sound intimidating, but it's 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.
Pegmatite is another type of granite with the three main minerals of quartz, feldspar, and mica added to it. It has unusually large, intergrown crystals and is the last and most water-rich portion of a magma to cool. 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 huge - up to 6 feet in length. There were three episodes of pegmatite intrusion in the Black Canyon rocks. The most spectacular example of these intrusions is the Painted Wall, at 2,250 feet, it's the tallest cliff in Colorado. More information »
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Last updated: June 27, 2023