Geology

Vernal Mesa Quartz Monzonite

The term "Vernal Mesa Quartz Monzonite" was coined by J. F. Hunter in 1925 during his research in the Black Canyon of the Gunnison. The steepness of the walls of the Black Canyon owe their existence to this rock. The hardness of the rock and the down cutting force of the Gunnison River have worked in conjunction for approximately 2 million years to carve the canyon. The quartz monzonite is extremely resistant to erosion, so as the river cuts down and makes the canyon deeper, the hard rock keeps the walls from eroding and widening.

The quartz monzonite does not run through the entire canyon. It covers an area of approximately three square miles and stretches from Rock Point to Warner Point on the south rim. This is also the most dramatic section of the park, containing plunging cliffs, imposing monoliths and the "Narrows" at the river.

Vernal Mesa Quartz Monzonite derives its name from Vernal Mesa which encompasses the majority of the South Rim of the Black Canyon. Quartz monzonite contains small amounts of sodium and potassium and more calcium than granite. It's similar in appearance to granite, but is different chemically and mineralogically. It's dark gray and course grained. Its companion rock, called Curecanti Quartz Monzonite, named for the Curecanti Needle in the Morrow Point Reservoir, differs from the Vernal Mesa Monzonite by being lighter and finer grained.

Both these rocks originated about 1.4 billion years ago. This activity post dated the creation of the surrounding metamorphic rock, dated 1.9 million years ago, by approximately half a million years.

Moving as a large body, called a pluton, miles deep below the surface, magma intruded into the existing schists and gneisses, devouring the rock in its path, folding them in and half-way digesting them. At times this blob of molten rock would push the metamorphic rock out of the way. Slowly the magma cooled and individual crystals solidified. This type of event occurred three times over the ages while the metamorphic rock was still deep in the earth. (In addition to rock at the Curecanti Needle and Vernal Mesa, another pluton intruded in the vicinity of Pitts Meadow in the Gunnison Gorge National Conservation Area).

Quartz
Quartz is composed purely of silicon dioxide (Si3O2) and is colorless when pure. Impurities such as iron oxide (rose quartz) or many tiny bubbles (milky quartz can give it many colors). This hard mineral breaks with a smoothly curved surface, and is one of the most common minerals in the Earth's crust.

• Quartz occurs in every class of rock and in every conceivable form.
• It can be useful as a geologic thermometer because the type of crystalline form indicates specific temperatures at formation.
• Used in glass manufacture, and as an oscillator to control frequencies in radios or watches.
• The name originates from the German "quarz," or crystal.
• You can find quartz in nearly any of the rocks at Black Canyon.

Black Canyon Dimensions

The Black Canyon is incredibly deep and sheer, with plunging cliffs, soaring buttresses and a thundering river. The following lists will help you understand the physical size of the canyon in comparison to other canyons and man-made structures.

COMPARISONS TO OTHER CANYONS

The Painted Wall is the highest cliff in Colorado. From river to rim it stands 2250 feet (685 meters).

4: The Cenozoic Era

The Cenozoic Era began about 70 million years ago and extends to the present. In our book of 1000 pages of Earth history, the Cenozoic represents only the last thirteen pages! Most of these pages fall into the Tertiary Period. The Tertiary represents a critical time in the geologic history of Colorado.

The beginning of the Tertiary coincides with the birth of the Rocky Mountains. The event is known as the Laramide Orogeny (orogeny means "mountain building"). The cause of the Laramide Orogeny reaches back more than 200 million years.

At the end of the Triassic period, the great supercontinent known as Pangea began to break apart, and North America began to separate from Europe. Far to the west, the North American crustal plate began colliding with and over-riding the Pacific-Farallon Plate. The collision between the two plates caused the crust to buckle and fold -- just like the fenders of two cars in a head-on collision! This folding started in California and gradually moved its way eastward, finally reaching Colorado about 60 million years ago.

During the Tertiary, the stresses caused by the colliding plates to the west forced several Precambrian crustal "wedges" upwards, forming the Colorado Front Range and the Southern Rocky Mountains. In some areas, the mountain building was accompanied by volcanic eruptions and magma emplacement.

Tertiary volcanism is responsible for one of the most notable geologic features in Curecanti National Recreation Area. The Dillon Pinnacles tower above the northern shore of Blue Mesa's Sapinero Basin. The rock forming the pinnacles is called the West Elk Breccia (pronounced bretch'-yuh or bretch'-ee-yah). It formed from a huge volcanic mud flow (called a lahar) of ash and volcanic debris that spewed from violent, pyroclastic eruptions in the West Elk Mountains about 30 million years ago. You probably know that pyro refers to fire. The term clastic is used to describe bits and pieces of broken rock.

The West Elk Breccia contains a jumble of angular rock fragments that vary in size and shape. These fragments are imbedded in a matrix of fine volcanic ash and mud. Many of the larger clastic fragments are more resistant to the effects of erosion and weathering than the soft, mud-ash matrix. These larger rock fragments provide an "umbrella of protection" against the elements, sheltering the rock immediately beneath it. The result is the mysterious spire-like form of the pinnacles.

The West Elk Mountains were not the only volcanoes erupting during the Tertiary. About 28 million years ago, a series of volcanic ash flows that originated from the San Juan Mountains blanketed much of southern Colorado. The tremendous caldera eruptions of the San Juans were characterized by turbulent, flowing clouds of hot incadescent ash, gasses and tiny shards of volcanic glass. Such plinian-type eruptions are sometimes referred to as nuée ardentes or "glowing avalanches". As the turbulent ash clouds settled out, the burning-hot ash and glass shards welded together to form a dense, erosion-resistant rock called welded tuff. The various layers of welded tuff serve as cap rocks that protect the softer rocks beneath them and give the mesas of Curecanti their flat top (mesa means "table" in Spanish).

Schist

Schists are strongly foliated, and look like a stack of paper. Some of the best places to see schist are among the spires at Gunnison Point or Kneeling Camel View. The Great Pillars, seen here, are among the largest examples of schist in the canyon. 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.

Plagioclase
Plagioclase is a whole family of minerals that make up what is called a solid solution series. That means there are two ends of a scale with "pure" members at each end, with blended members of the family in between. "Pure" yellow is at one end and "pure" green is at the other with many shades between them. If you mix a few other elements in you find white, reddish-gray or black colors as well, making this hard mineral an important part of most metamorphic (and igneous) rocks, and is one of the most common rock-forming minerals you will find.

• The most common variety in Black Canyon is Albite. It is rich in sodium.
• Albite is used as a gemstone, also known as "moonstone," but gem quality finds are rare in the United States.
• Is used commercially in the ceramics industry as well.
• The name albite comes from the Latin "albus" or white.
• Albite is found in schists, gneisses and quartz monzonite.

Hornblende
Like the essential building block of plagioclase, hornblende is actually a member of the solid solution series for amphiboles. Hornblende is the most common occurring amphibole in the canyon and is black or dark green in color. When it fractures, it tends to break at angles of either 60 or120 degrees. It contains significant amounts of potassium and fluorite in its chemical formula, which helps determine the color of the mineral.

• It sometimes forms large masses known as amphibolite or hornblende schists that are made up of thin, black parallel aligned, needle-like crystals.
• It is difficult to find any large individual crystals.
• The name comes from the German miner's term "horn" possibly in reference to the color of horn and "blenden" which means to deceive. This seems to have come from the fact that the mineral mimics the more valuable metallic ores, but doesn't produce a commercial viable metal.
• Hornblende makes up one of the types of schists in the canyon.

5: From Past to Present

The uplift and volcanism of the early to mid-Tertiary established the highland that would serve as the headwaters for the Gunnison River. Snowmelt from the Sawatch Range to the east, the West Elk Mountains to the north and the San Juans to the south provided an ample supply of water to what would eventually become the Gunnison Basin. Geologists believe that the modern Gunnison River became established in its current course about 10 to 15 million years ago, just after the last eruptions in the San Juans and West Elks. This coincides with the beginning of a period of rapid uplift of the Great Basin and Colorado Plateau provinces that lie between the Rockies and the Sierra Nevada Range in California. To date, geologists are at a loss to explain the forces behind the uplifting of such an immense region.

Whatever the cause, the uplift allowed the early Gunnison River to easily cut its way down through the thick layers of Tertiary volcanics and Mesozoic sedimentary rocks. Then about two million years ago, the river began to expose the much harder Precambrian basement rocks of the Gunnison Uplift, a block of crust that had been forced upwards during the Laramide Orogeny. Trapped in its own canyon, the Gunnison had no other choice but to battle the rocks beneath it. At the rate of about one inch per every hundred years (or the width of a human hair each year), the Gunnison slowly worked its way through the resistant rock, forming the narrow, steep-sided Black Canyon of the Gunnison. Only a high volume, high-velocity river like the Gunnison could produce such a breath-taking canyon!

Spring meltwaters continue to feed the Gunnison today as it makes its way through both parks on its journey to the Pacific Ocean. The difference today is that you won't see dinosaurs, erupting volcanoes, or lush tropical terrain along the banks of the Gunnison. The climate is not the only thing that has changed. The Gunnison River no longer flows freely through canyon. Three dams hold back its seasonal flood, reducing its former glory to a feeble shadow. Yet even in its diminished state, the Gunnison continues to add to the geologic story of Black Canyon and Curecanti drop by precious drop.

Pegmatite

Pegmatite is abundant throughout the 48 mile canyon, and is found in the form of stripes on the cliffs as well as large bodies. The largest of these are found upstream from East Portal, but some outcrops are surprisingly close to the edge, such as the Kneeling Camel on the north rim of the canyon. Some pegmatites contain spodumene, a mined lithium mineral. Less common minerals found in pegmatite include magnetite (or hematite), beryl, tourmaline and garnet.

Microcline
This silica-rich (SiO2) member of the feldspar family is enriched in potassium and can be white, or pink with hints of yellow. Mircrocline is a hard mineral that breaks at right angles, and is the feldspar of choice for any pegmatite.

• This mineral is used as an opaque gemstone when it occurs as the bright green variety known as Amazonite.
• Microcline is also quarried for ceramics, ceramic glazes, and as a scouring powder.
• Microcline comes from the Greek "micros" or small, and "klinein," to incline.
• Like some of the other common minerals, this one is found in many places in both the metamorphic and igneous rocks.

Garnet
Garnets are a family of minerals with at least 3 different varieties found in the canyon. They form at high pressures and are better known as gemstones than rock forming minerals. In fact, in medieval times, garnets were thought to cure depression and other ailments. The specimen at right was making a 12-sided crystal but ended up solidifying and struggling against the other crystals that didn't yield in the cooling process. The three varieties in the canyon are:

• Almandine - is iron-rich, violet-red in color and sometimes called the common garnet. It can have weak magnetic properties.
• Pyrope - is magnesium-rich, yellowish red in color, and can be used in a variety of sandpaper valued for its better cutting qualities. The name comes from the Greek "pyropos," meaning fire-eye. Most of the gems are of this variety.
• Spessartine - is manganese-rich, brown with red or pink tones, and comes from the Spessart district of Bavaria in Germany.inc

Gneiss

The road at Cimarron in Curecanti National Recreation Area leads to the Morrow Point Dam Overlook, and travels through some fine 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 metamorphosis with some of the most intense temperatures and pressures. In fact, in some places, the rock has actually 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.

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%, suggests that the original rock (protolith) was an impure sandstone or chert.

Rare minerals such as garnets, staurolite, or sillimanite can be abundant locally. The presence of such minerals acts as a marker for exactly how much pressure and temperature the original rocks were exposed to during metamorphism.

Muscovite Mica
Muscovite belongs to the mica family and is composed of thin elastic, silvery-white sheets found in "books," so called because on edge they appear as the pages of a book. It is very soft and pliable, and is another of the most common rock building minerals. Muscovite mica is very commonly found in small pieces on both rims and throughout the canyon.

• Large specimens are most commonly found with pegmatite.
• It is mined in some places in Colorado.
• Was once valued for making isinglass for heat resistant windows.
• Today it is used as insulation for electrical equipment or for dry lubricants like graphite, and is classed as a strategic mineral.
• The name originates from the Russian city of Moscow, where much of the window material came from in earlier times.

Biotite Mica
Biotite is an important and common mineral in both igneous and metamorphic rocks. This soft and pliable mineral is composed of elastic brownish-black sheets that are semi-transparent. Similar to Muscovite mica, the coloring comes from the inclusion of iron and magnesium into its chemical make-up.

• It is commonly found with Muscovite mica, but there are larger deposits on the north rim of the park.
• The name comes from J.B. Biot, a French physicist, astronomer and mathematician.

Pothole Ecology - Ephemeral Pools

Throughout much of the Colorado Plateau, a region covering much of Western Colorado and southern Utah, rain is sporadic during the summer months. With surface temperatures reaching 120 degrees, obtaining moisture sometimes becomes top priority for the animals that make this region their home. The little rain that does fall tends to run off over the rocky surface, or gets absorbed quickly into the dry soil. Some of the moisture however gets trapped in the shallow depressions in the rocks. Collected in pools, this water may last perhaps only a few days, leading researchers to the name "ephemeral." This water is essential for the survival of many desert creatures.

Although most animals use these potholes as a wayside stop in their daily journeys, some species of aquatic life make these ephemeral pools their permanent home. Since potholes have several wet-dry phases, the organisms that live in them have made a number of special adaptations that help them survive in this ever-changing environment.

Not all of the rock at Black Canyon readily forms pot holes, but the sandstone and igneous rocks weather away into pits or pools that gather water and support life. Because the rim of the canyon lies between 7,700 feet (2,347 meters) and 8,300 feet (2,530 meters) above sea level, the temperature ranges can be extreme from season to season. The organisms that live in these potholes not only have to contend with often insufficient amounts of water, but also a very short window of time for growth and development. The high elevations and cold temperatures found at the Black Canyon provide a more hostile environment for pothole creatures when compared to much of the Colorado Plateau.

The biggest challenge facing any of the organisms living in the pools is desiccation (drying out) after the water has evaporated. These animals have three main ways of dealing with drought:

Drought Resisters
Some animals have a tough, waterproof exoskeleton that prevents dehydration. Burrowing into the fine mud that lines the bottom of the pool further reduces their exposure to the sun. After the pool has dried completely, the surface of the mud that surrounds them may be baked solid, this however helps to seal in the moisture the animal will need to survive until the pool fills again.

Drought Escapers
Some organisms use potholes to lay eggs and develop, but as adults, they cannot tolerate desiccation. After their drought-resistant eggs are laid the adults must move on to a permanent water source, or more typically, they die as the pool dries. However, the next generation is now ready to hatch during the next fill cycle.

Drought Tolerators
Other organisms have developed ways to lose up to 90% of their total body moisture for long periods of time and as a pool fills again, rehydrate and become fully functional. This process is know as cryptobiosis and is accomplished by a command center located in their nervous system that remains hydrated and can carry out the basic life functions of the dehydrated cells. Other tolerators have only one stage in life, such as egg or larva that can survive desiccation, but will die if the pool dries out during another phase.

Pothole organisms face many challenges during the wet or active phase of their life cycle as well. Since there is no guarantee that once a pool fills there will be enough water for the organism to reach adulthood, to survive most pothole creatures have adopted a very short life span. For example, tadpole shrimp can reach maturity in as little as 24 to 36 hours, mate and lay eggs. Another method of survival employed by pothole organisms is delayed hatching. If all the eggs of a particular species hatch at the first sign of moisture, but the pool dries out before they can reach maturity, no adults would survive to ensure the propagation of the species. To compensate, not all eggs in a given pothole hatch at the same time. Some eggs will remain dormant even after several wet-dry cycles. This spacing apart insures, by random chance, that at least one hatch will receive enough moisture to reach adulthood.

Pothole ecosystems and the species that live within them are extremely sensitive and can be greatly affected by the slightest climate change or a disturbance to their environment. These pools do not have the ability to counteract sudden shifts in their pH levels. These changes can be brought about by many means, including industrial pollution which may bring acid rain, or a careless washing of our hands or pots and pans while we enjoy the back-country of our parks and other public lands. The animals living in these shallow pools may not be able to adjust to sudden changes and perish. Taking extra care when around these pools will help ensure the continued survival of these unique and fragile ecosystems.

Canyon Minerals

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.

METAMORPHIC ROCK
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.

PLEASE REMEMBER... Collecting rock specimens in Black Canyon of the Gunnison National Park is illegal.

Gneiss
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. 

Schist
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.

IGNEOUS ROCKS
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
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.

One of the most famous examples of the quartz monzonite is the Curecanti Needle (along Morrow Point Reservoir). The monzonite is harder than the metamorphic rocks and weathers more slowly. The needle was created when the waters of the Gunnison River and Blue Creek carved away the surrounding metamorphic rock, leaving the Needle in their wake. The third side is separated by weathering of a fault system.

The Geologic Story

INTRODUCTION: WRITTEN IN THE ROCKS
Like pages in a book, the rock layers of Black Canyon and Curecanti tell a story of past environments, ancient animals and dynamic processes of change. But unlike a book that we can read in a short time, this geologic book has to be read from a different point of view. Time is thrown out of balance here, and we need to see the land from a very different perspective.

GEOLOGIC TIME
Time is an everyday part of our lives. We keep track of time with a marvelous invention, the calendar, which is based on the movements of the Earth in space. One spin of the Earth on its axis is a day, and one trip around the sun is a year.

While this concept seems rather straightforward, the calendar we use today is very different from earlier versions. It is a great achievement, developed over many thousands of years as theory and technology improved. For centuries, scholars have sought to understand time and its relationship to the age of the Earth. Today, geologists estimate that the Earth is 4.6 billion years old! Who can fathom such an expanse of time?

Geologists have designed a very special type of calendar in order to grasp Earth's long history. This geologic time scale is very different from the familiar calendar we use to keep track of our busy lives. In some ways, the geologic time scale is more like a book, with the rocks as pages. Some of the pages are tattered and torn, and some are missing -- especially the early parts. To make matters worse, the pages aren't numbered. Luckily, geology gives us the tools to help decipher and read this incredible book!

Just like a calendar is divided into months, weeks, days, and so forth, the geologic time scale has its own unique set of time divisions. The largest division is called an eon. Eons, which can span billions of years, are subdivided into eras, which are subdivided into periods, which are subdivided into epochs, which are subdivided into ages, and... well, you get the picture!

The names used to designate the divisions of geologic time may seem bewildering at first glance, but nearly every name represents an historic breakthrough in geologic thought.

1: The Precambrian Era

The Precambrian Era accounts for the history of the Earth from its very beginning up until about 540 million years ago. If we condensed all of Earth's history into a 1000 page book, the Precambrian would fill pages 1 through 880 -- most of the book! The story would reveal a time of harsh and drastic changes in the Earth and show very little to no sign of life.

Because most of Colorado's Precambrain-age rocks have been highly altered by extreme heat and pressures, it is difficult for geologists to interpret what this region may have looked like during this time. So we usually find ways to describe the types of Precambrian rocks, and note where and how they may have been formed.

Precambrian rocks are often called basement rocks since they are usually buried deep beneath the surface. They only become exposed under special circumstances where the overlying younger rocks have been stripped away. Most exposures of these ancient rocks are found in the cores of mountain ranges or in deeply eroded canyons like the Black Canyon.

In Black Canyon of the Gunnison National Park, the Gunnison River cuts through Precambrian rock nearly 2 billion years old! Most of these rocks are metamorphic and show evidence of exposure to extreme pressures and temperatures. Some of the rocks are igneous and formed from magma that pushed its way up into cracks in the Earth's crust, where it cooled and crystallized.

The metamorphic rock that dominates the walls of the Black Canyon is called gneiss (pronounced "nice"), and is blended with schist, another rock that normally has flat or elongated crystals. You might spot the intense folding of the alternating light and dark bands in this adjacent photo. These rocks were once buried deep below the Earth's surface where they encountered extreme heat and pressure.

Pink streamers of pegmatite animate the towering cliffs of the canyon at Painted Wall overlook. This granite-like igneous rock formed as hot magma forced its way into cracks. It cooled slowly, allowing large crystals to form. The pegmatite is loaded with shiny muscovite (mus'-ko-vite) mica and large crystals of a pinkish mineral called potassium feldspar.

The overlooks along the South Rim Drive in the National Park offer excellent panoramic views of the canyon, and a chance for close examination of the Precambrian rocks that form the canyon walls. You might especially enjoy viewing the rocks along the Oak Flat Trail or the walk out to Rock Point.

In Curecanti, the best exposures of these ancient rocks can be found downstream from Blue Mesa Dam. Find outcrops of these rocks along Highway 92 or hike the Pine Creek and Curecanti Creek Trails to see excellent rock examples.