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USGS/Cascades Volcano Observatory, Vancouver, Washington

America's Volcanic Past

"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994

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Map, Location of Colorado

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Colorado features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Colorado.]

  • Colorado
  • Colorado Regions
  • Black Canyon of the Gunnison National Park
  • Colorado National Monument
  • Curecanti National Recreation Area
  • Dome Rock Natural Area
  • Dotsero Crater
  • Elephant Rocks Natural Area
  • Eleven Mile State Park
  • Florissant Fossil Beds National Monument
  • Front Range
  • Glenwood Springs Area, Garfield County
  • Great Wall Dike
  • Green River Formation
  • Jefferson County
  • Mini-Wheeler Natural Area
  • Mount Antero and White Mountain
  • Needle Rock Natural Area
  • Ouray Hot Springs
  • Pikes Peak
  • Raton Mesa
  • Red and White Mountain
  • Rocky Mountain National Park
  • Saddle Mountain Research Natural Area
  • Saint Mary's Falls
  • San Juan Mountains
  • Silverton Caldera
  • Slumgullion Earthflow Natural Area
  • Spanish Peaks
  • Specimen Mountain Research Natural Area
  • Wheeler Geologic Natural Area


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.

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.

Excerpt from: U.S. National Park Service, Black Canyon of the Gunnison National Park Website, 2002

Colorado Regions

The Interior Plains:7
The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.

Colorado Plateau:7
The sculptured beauty and brilliant colors of the Colorado Plateau's sedimentary rock layers have captured the imaginations of countless geologists. This is a vast region of plateaus, mesas, and deep canyons whose walls expose rocks ranging in age from billions to just a few hundred years old. Ancient Precambrian rocks, exposed only in the deepest canyons, make up the basement of the Colorado Plateau. Most are metamorphic rocks formed deep within the Earth while continental collision on a grand scale produced the nucleus of the North American continent well over a billion years ago. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement. These deeply-formed rocks were uplifted, eroded, and exposed for eons. By 600 million years ago North America had been beveled off to a remarkably smooth surface. It is on this crystalline rock surface that the younger, more familiar layered rocks of the Colorado Plateau were deposited.

Rocky Mountains:7
The Rockies form a majestic mountain barrier that stretches from Canada through central New Mexico. Although formidable, a look at the topography reveals a discontinuous series of mountain ranges with distinct geological origins. The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains.


Black Canyon of the Gunnison National Park

Black Canyon of the Gunnison:2
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.

Formation of the Black Canyon of the Gunnison:2
The canyon's landscape was formed by the slow, but continuous and unyielding process of erosion -- the effect of one drop of water at a time or the scouring by a seasonally flood-swollen river, the rush of mud-laden side streams after heavy rains, occasional rockfalls from high cliffs, and the relentless creep of landslides. The river first established its course over soft volcanic rock. It then cut through this rock to the harder and older crystalline rock of the present canyon that had been thrust up in an earlier dome-shaped formation known as the Gunnison Uplift. Once committed to its course, the stream had no alternative but to continue to cut through this once-buried hard core, taking about two million years to carve the gorge. The excavating process is still going on, but at a slower pace because of the dams upstream.

Black Canyon Gneiss:2
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. These rocks were once buried deep below the Earth's surface where they encountered extreme heat and pressure.

Painted Wall:2
The Painted Wall is the result of molten material forced under great pressure into the cracks and joints of the base rock. Those sheer walls are an ideal home for the peregrine falcon.

Black Canyon Pegmatite:2
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.

Colorado National Monument

Colorado National Monument and nearby Grand Mesa:3
The Uncompahgre Highland rises to mountainous heights, then is worn down over millions of years to a nearly level plain. (These ancient crystalline rocks - granite, gneiss, schist - which were at the core appear today in the canyon floors.) Great bodies of water follow, depositing layer upon layer of soft, sedimentary rocks as distant mountains give themselves up, grain by grain, to be reformed. Becoming entombed within the rock are the remains of dinosaurs, fish and shellfish, early mammals, and many other life forms that lived within a span of more than 100 million years. Finally the Rocky Mountains begin their slow rise, and the surrounding land is raised as well. Water falling as rain and snow invades rock crevices, expands as ice, and begins prying pieces loose from solid material. As erosion continues, the streams and rivers sort themselves out into the ancestral Colorado River system. Volcanic eruptions, the latest series of geological events, spread molten lava over much of the land nearby. The remnant lava today forms a resilient cap on Grand Mesa to the northeast.

Curecanti National Recreation Area

Dillon Pinnacles and West Elk Mountains:2
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.

San Juan Mountains:2
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 incandescent 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).

Dome Rock Natural Area

Dome Rock:9
This scenic area is dominated by massive exfoliating outcrops of Pikes Peak granite. Dome Rock, among the most spectacular of the formations, rises 800 feet from the canyon floor. The site also contains good-quality examples of ponderosa pine, limber pine, Douglas-fir and montane grassland plant communities. The area provides winter browse and lambing grounds for a herd of bighorn sheep.

Dotsero Crater

Dotsero Crater:11
One of the youngest eruptions in the continental U.S. produced an explosion crater, a lahar, and a 3-kilometer long lava flow in north-central Colorado near the town of Dotsero. The eruption is dated at 4,140 years before present, based upon a carbon-14 date from wood under the scoria. The Dotsero crater is 700 meters wide and 400 meters deep.

Dotsero Crater, Triangle Peak, Willow Peak:1
Igneous activity has occurred as recently as 4,150 years ago at Dotsero volcano, the youngest known volcano in Colorado. Volcanic activity has also occurred during the Quaternary at other locations in this area, including Triangle Peak and Willow Peak. However, much and perhaps all the demonstrable Tertiary deformation within the area, where evaporitic rocks are at shallow depths and in the adjoining Grand Hogback Monocline, appears to be directly due to salt tectonism and salt dissolution.

Elephant Rocks Natural Area

Elephant Rocks:9
Elephant Rocks are a weathered remnant of the Fish Canyon tuff (late Oligocene, about 28 million years old). The rocks themselves support an occurrence of the rare parsley Neoparrya lithophila, (rock-loving neoparrya). An interesting mixture of shortgrass prairie and foothill shrubland vegetation occurs on the site.

Eleven Mile State Park

Eleven Mile State Park:9
Eleven Mile State Park features a 3,405 surface-acre reservoir lying on the South Platte River at the southern edge of South Park. Formed behind Elevenmile Canyon Dam, other drainages emptying into the reservoir include Cross, Prudence, Union, Balm-of-Gilead, Simms and Spring creeks. The South Platte River Valley is mantled with Wisconsin-aged glacial outwash material, Pleistocene alluvium, which covers older formations. The southeastern portion of the park has been covered by Como-age surface deposits. Most of the park is underlain by Precambrian rocks: Silver Plume granite east and south of Howbert Point and Pikes Peak granite along the river west of Howbert. The western and southern edges consist of Thirtynine Mile andesite deposits; igneous and metamorphic rocks of Tertiary origin. Cross Creek cuts through another Tertiary deposit, Trachyte (also igneous and metamorphic rock) Along the southwestern reservoir margin, Wall Mountain tuff, part of a south-dripping sequence of welded tuffs that incorporated volcanic ash and pumice is exposed as steep cliffs. Thirtynine Mile andesite has been deposited over both the granite and tuff formations on the southern reservoir edge.

Florissant Fossil Beds National Monument

Florissant Fossil Beds:4
Today, Florissant Fossil Beds National Monument is a wonderland of meadows, forests, and wildflowers. Yet 34 to 35 million years ago, Lake Florissant, stretching 15 miles through an ancient forest valley, dominates the scene. Nearby a volcano rumbles. In the past, volcanic mudflows blanketed parts of the forest surrounding Lake Florissant, killing entire trees. Now as the volcano again erupts violently, the devastation is widespread. The exploding volcano showers the countryside with million of tons of ash, dust, and pumice. Caught in this deadly cloud, insects, leaves, fish (anything that cannot escape) die, and many fall to the lake bottom, where they are buried. These eruptions occur again and again for perhaps as long as 700,000 years. Each time, fragments of life are trapped in a layer of volcanic sediments at the bottom of the lake. Eventually these sediments become a finely layered shale and transform the buried plan and animal life into fossils. Florissant Fossil Beds National Monument preserves this site.

Lake Florissant:5
During the Oligocene (35-34 million years ago) a complex of composite volcanoes, 18 miles to the southwest, began to erupt. Early eruptions sent volcanic mudflows into the Florissant area and buried forest of giant redwoods and other trees. A later mudflow formed a dam across the ancient drainage, resulting in the formation of a large lake. The fine grained muds and volcanic ash deposited in this ancient Lake Florissant eventually became the shales that contain the exquisitely preserved fossils of insects, leaves and flowers. Sedimentation in the lake varied from slowly deposited, thin layers of organic-rich muds enriched in diatoms to rapidly deposited, thick layers of coarse ash and pumice.

Pikes Peak Granite:5
The Precambrian Pikes Peak Granite, 1.02 billion years old, is the oldest unit at Florissant. This pink granite forms the rounded rocky hills in the monument. The Pikes Peak Granite formed as a large intrusive batholith that covers more than 1,150 square miles and is composed of pink to reddish-tan, medium-to coarsely crystalline, biotite and hornblende-biotite granite. These Precambrian rocks were uplifted during the Laramide orogeny and exposed at the surface by Eocene time (37 million years ago) after erosion of the overlying Paleozoic and Mesozoic units.

Thirty-Nine Mile Volcanic Field and Guffy Volcano:4
Thirty-four million years ago (Eocene) volcanism begins at Thirty-Nine Mile Volcanic Field. Products include: ashflows, lava flows, agglomerates from lahars, breccias, ashfalls, lake formations at Florissant and Antero in South Park. Guffey Volcano and others in the field are formed as composite volcanoes. This indicates a violent type of eruption similar to Mount St. Helens type.

Wall Mountain Tuff:4
Thirty-six million years ago (Eocene) massive ashflows emanating from the Mount Aetna caldera in the Sawatch Range resulted in the formation of the Wall Mountain Tuff. Stream conglomerates containing clasts of the tuff were deposited over the tuff and are found today as the Tallahassee Creek and Castle Rock conglomerates.

Front Range

Front Range Foothills:1
For simplicity, we can think of the geology of the Front Range Foothills areas as falling into three basic categories of rock and soil deposits: "really old," "old," and "young." The "really old" deposits consist of the igneous and metamorphic rocks that make up the mountains in the Front Range itself. These rocks date back to the Precambrian era. Most of them are over 1 billion years old. The "old" deposits consist of sedimentary rock formations: mostly sandstone and shale with some limestone and coal layers. These rocks date back to the Paleozoic, Mesozoic, and Cenozoic eras. They range in age from about 300 million years to 63 million years. These formations have been folded and uplifted by the Rocky Mountains along the foothills, whereas they are flat-lying beneath Denver. The "young" deposits consist of unconsolidated (i.e., soil) sediment deposits that lay atop the older rock deposits. These sediments are alluvial (deposited by streams), colluvial (slope-wash), and eolian (wind). Many of these deposits date back to 1 million to 25 thousand years, when Colorado's mountains experienced several periods of glaciation and melt-off.

North Table Mountain:1
Looking east from Golden, Colorado, we see the broad slopes of North Table Mountain. These slopes are covered with landslide deposits, which are underlain by the Denver Formation of Cretaceous-Tertiary age (the K-T boundary is hidden up there, somewhere). The rimrock cliffs at the top of the mountain are the remnants of a series of 63-64 million-year-old lava flows. The lava originally flowed down a valley. After cooling, it became very hard and resistant to erosion. The surrounding highlands later eroded away leaving the former valley as today's high point. This type of geomorphic feature is known as a topographic inversion.

South Table Mountain:1
Tertiary Volcanic Rocks

Glenwood Springs, Garfield County

Glenwood Springs Basalt:1
The rock in and surrounding the sinkhole at Colorado Mountain College is basalt. Basalt is an extrusive igneous rock. Basalt is dark colored, fine grained and may appear dull, almost velvety on fresh surfaces. Numerous, smooth cavities or vesicles, which represent entrapped gas bubbles released from the rock as it cooled, are common in the upper part of basalt flows. slightly larger crystals within the fine-grained basalt are called phenocrysts. Isotopic dating of basalt flows in the Glenwood Springs area suggest widespread volcanic activity from about 20 to 25 million years ago and 8 to 11 million years ago (Larson and others, 1975). Isolated volcanoes have been active about 4 million, 3 million, 1.5 million, and 4,000 years ago.

Colorado Mountain College Campus:1
Turn left at the Colorado Mountain College Campus. Drive an additional 0.3 miles and park in the dirt parking lot next to the soccer field. Walk 200 feet to rock tower ruin and view large sinkhole in basalt.

Flat Tops and Basalt Mountain Shield Volcano:1
Basalt, olivine basalt, and andesitic basalt flows, and interbedded siltstone, conglomerate, sandstone, and claystone -- over a dozen flows form the caprock for the Flat Tops or can be seen on south flank of the Basalt Mountain shield volcano (thickness approximately 250 feet).

Spring Valley:1
Spring Valley is a salt-collapse half graben into which a large part of the hills to the east are collapsing. The 22 million years old basalt exposed in the roadcut has been downdropped 3,000 to 4,000 feet into the Carbondale collapse center. The floor of Spring Valley was occupied by a lake until being drained by homesteaders prior to 1900.

Tunnel Eastbound I-70:1
This tunnel passes through a unique igneous rock with large crystals called porphyroblastic biotite granodiorite. Samples may be collected along the bike path from Glenwood Springs.

Great Wall Dike

The Great Wall Dike:13
The Great Wall dike north of Spanish Peaks in Colorado, is a spectacular example of a volcanic wall. The well-developed dike pattern in this area has been largely the result of a pre-existing system of joints that developed during the early stages of late Cretaceous orogeny. It is thought that in the formation of dikes, particularly at shallow depths, the magma does not force the fissures apart prior to entry. Instead the fissures were pulled apart by dilation or stretching, and then the magma flowed into the cracks.

Green River Formation

Green River Formation:6
Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years ago: Lake Gosiute, Lake Uinta, and Fossil Lake, the smallest. All are gone today, but they left behind a wealth of fossils in lake sediments that turned into the rock layer known as the Green River Formation, made up of laminated limestone, mudstone, and volcanic ash. The fossils are among the most nearly perfectly preserved remains of ancient plant and animal life in the world.

Jefferson County

Crushed Stone:1
Crushed stone is a construction material used mainly as an aggregate for the manufacture of concrete, and for road base material. Several large quarries are present in the foothills west of the Denver metro area. The hard, dense metamorphic and igneous rocks of Precambrian age (about 1.7 billion years old in this area) that form the core of the Front Range make excellent sources of for aggregate. Hard volcanic rock of Tertiary-age (about 60 million years old) also makes good aggregate, and is being quarried at the Ralston Quarry (Asphalt Paving Co.)

Morrison Quarry:1
This quarry produces crushed stone from Precambrian metamorphic and igneous rock. The quarry site was chosen partially because the hills screen most of the operation from view.

Ralston Quarry:1
Hard volcanic rock of Tertiary-age (about 60 million years old) also makes good aggregate, and is being quarried at the Ralston Quarry (Asphalt Paving Co.)

Mini-Wheeler Natural Area

Mini-Wheeler Natural Area:9
The unusual "Devil's Dunce Cap" formations are the eroded remnants of a 1,200-square-mile volcanic ash flow. This Gribbles Park Tuff was deposited 29 million years ago during the Oligocene. Although located within the Thirtynine Mountain Volcanic Field, the tuff's source is thought to be outside of the field to the west. The exact source is unknown, and research on this may be useful in determining the timing of the opening of the upper Arkansas valley. The more well-known Wheeler Geologic site in Mineral County has higher formations (and is from a different volcanic source), but this site covers a larger area than Wheeler.

Wheeler Geologic Natural Area:9
Wheeler Geologic Natural Area is a mass of pinnacles and domes, some more than several hundred feet high. Erosive forces have sculpted these forms in white, beige and lavender layers of lava and ash. The geologic features occur within virgin forests of Engelmann spruce and subalpine fir.

Mount Antero and White Mountain

Colorado Gemstone:1
The aquamarine was adopted as the official state gemstone on April 30, 1971, by an act of the General Assembly. The mountain peaks of Mount Antero and White Mountain in Colorado are among the finest quality localities known for gem aquamarine. They are also among the highest in elevation, located at 14,000 feet. The granite rock of these peaks contains pegmatite bodies that are characterized by large miarolitic cavities containing the gem quality aquamarine crystals. The cavities are found through a vertical area of a mere 500 feet. The crystals in these cavities range in color from light blue to pale and deep aquamarine green, and in size from very small to 6 centimeters in length.

Needle Rock Natural Area

Needle Rock:9
Needle Rock towers 800 feet above the floor of the Smith Fork of the Gunnison River valley. It originated as the throat of a large volcano about 28 million years ago (Miocene epoch) when molten rock intruded between existing sedimentary formations. As the surrounding country rocks eroded over millions of years, the resistant igneous core was exposed.

Ouray Hot Springs

Ouray Hot Springs:1
The cliffs around Ouray are composed of Paleozoic and lower Cretaceous sedimentary rocks overlain by Tertiary San Juan Formation volcanic rocks. The Ouray hot springs are heated by geothermal waters emanating from faults beneath the alluvium of the valley bottom and are a faint reminder of past volcanic activity.

Pikes Peak

Pikes Peak Granite:4,10
Pikes Peak Granite is to the north of Florissant. Samples collected on top of Pikes Peak are 1,030,000,000 years old, dated by various atomic clock methods.

Raton Mesa

Raton Mesa - National Natural Landmark:14
Las Animas County - Illustration of a mesa preserved by a thick lava cap which has resisted destruction from weathering and erosion. Only significant reference available illustrating the magnitude of erosion involved in developing the land surface of the Great Plains adjacent to the lower foothills of the Rocky Mountains. Owner: Private. DESIGNATION DATE: April 1967

Red and White Mountain

Red and White Mountain Laccolith:13
Less fluid magma may form a sill variation known as a laccolith. These shallow intrusive bodies are actually domes that did not quite break through to the Earth's surface but instead heaved up the overlying beds of older rocks. Both laccoliths and sills may be fed by either dikes or from the central conduit. The rocks overlying large laccoliths are often relative light sedimentary types. This is probably because the lighter sedimentary rock layers are easier for the magma to lift. A more dense rock would instead be broken by the magma in its push toward the surface. ... (One example) Red and White Mountain, Colorado, is of Tertiary age. Overlying layers of rock have been eroded.

Rocky Mountain National Park

Saddle Mountain Research Natural Area

Saddle Mountain:9
Saddle Mountain Research Natural Area supports a variety of pristine plant communities. Included in the natural area are dense stands of Engelmann spruce and subalpine fir, aspen, and bristlecone pine. Extensive meadows of Danthonia parryi (Parry oatgrass) occur on shallower soils derived from basalt.

Saint Mary's Falls

Saint Mary's Falls:8
From Colorado Springs, take either 26th Street or Cheyenne Road to North Cheyenne Canyon Road to Gold Camp Road. About one mile past the intersection of North Cheyenne Canyon Road/Gold Camp Road and High Drive is the trailhead. Foot and horse trail. Motorized vehicles are prohibited. St. Mary's Falls cascades 250-300 feet down a solid granite wall and is about 2.0 miles from Gold Camp Road. The falls elevation is 8,880 feet.

San Juan Mountains

San Juan Mountains:2
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 incandescent 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).

San Juan Mountains:1
The mountains that you see while driving along Red Mountain Pass were shaped over billions of years through multiple episodes of mountain building and uplift, ancient seas, volcanic upheavals, and icy glaciers. In the San Juan Mountains we see evidence of rock formations that span a vast amount of geologic time. From recent landslide features and relatively young volcanic events to billion-year-old basement rocks, we see a large part of the geologic time scale represented in these rocks. The San Juan Mountains are mostly composed of rocks that erupted from Tertiary volcanoes beginning about 40 million years ago. The volcanic activity continued sporadically for another 30 million years. Lava flows covered vast areas and mixed with older rocks to form conglomerates and breccias. Underlying the volcanic rocks, the older geologic sequence is visible in the Ouray area and south along the Uncompahgre River Canyon. Small outcrops are present in Ironton Park. Precambrian rocks (>600 million years before present) in much of Colorado are igneous or extensively metamorphosed rocks, but the Precambrian Uncompahgre Formation near Ouray comprises former sedimentary rocks that have been only moderately metamorphosed, and retain much of their original character. Sandstones have become quartzites; shales and mudstones have become slate. In Ouray and south to the vicinity of Bear Creek, sedimentary rocks from the Paleozoic era outcrop on the canyon walls. The rocks unconformably overlie the Uncompahgre Formation. Paleozoic rocks include the Ouray Limestone of Devonian age, the Leadville Limestone of Mississippian age, the Pennsylvanian Molas and Hermosa Formations, and the Permian Cutler Formation. The horizontally bedded, tan rocks to the east of the hot springs are part of the Ouray Limestone. Unconformably overlying the Paleozoic sequence are the Mesozoic rocks of the Triassic Dolores Formation, Jurassic Wanakah and Morrison Formations, and the Cretaceous Dakota Formation. These rocks are exposed north and west of Ouray, but are not visible south along the Uncompahgre River Canyon. After the Mesozoic sediments were deposited, a time of uplift and erosion ensued and the Tertiary Eocene Telluride Conglomerate was deposited. Another period of erosion removed most of the Telluride Conglomerate in this area. Subsequent volcanic activity began in the area of the San Juan Mountains. The San Juan Formation volcanic material was erupted from stratovolcanoes (like Mount St. Helens) and was deposited unconformably above the older formations exposed at the surface. The San Juan Formation volcanics are predominately andesites. Subsequent to deposition of the San Juan Formation, the Silverton Volcanic Group was deposited. The early members of this volcanic sequence are contemporaneous with caldera development in this area. Its members, from oldest to youngest, are the Eureka Member (rhyolite), Burns Member (andesite, rhyodacite), Henson Formation (andesite), and a pyroxene andesite member. The San Juan, Uncompahgre, Silverton, and Lake City Calderas and their associated volcanic deposits record a 15-20 million-year history of volcanism in the Ouray-Silverton area.

Silverton Caldera

Silverton Caldera:1
Volcanic activity, related faulting, and subsequent erosion formed the landscape and geology of this area. From a stop on the northwestern side of the roughly circular Silverton caldera, to the southeast Red Mountains No.2 and 3 can be seen. They are within the subsided or down-dropped central portion of the caldera. The upper part of these mountains consists of the Henson Formation. The lower part consists of the older Burns Formation. Both units are Tertiary Oligocene in age (approximately 27-28 million years), a time when intense volcanic activity in the San Juan Mountains area caused deposition of thick beds of volcanic rock. Looking west to the outside of the Silverton caldera, the Burns formation is at the surface and the Henson Formation is not present due to erosion.

The stunning red, maroon, orange, and yellow colors of the Red Mountains are primarily due to past hydrothermal alteration of these rocks. Hydrothermal alteration is literally the process of "hot water" circulation through the strata that changes the mineral composition of the original rocks. Hydrothermal fluids commonly contain significant concentrations of metals and sulfur. Hydrothermally altered rocks can contain significant amounts of disseminated sulfide minerals, primarily pyrite (iron sulfide). This process happened within and surrounding the Silverton caldera. Subsequent erosion of the strata exposes the pyrite to oxidation, and as the iron combines with oxygen it turns the rock various colors evident in the Red Mountains.

Stop just south of Red Mountain Pass, in the upper reaches of the Mineral Creek drainage basin, a tributary of the Animas River. We are now standing on the northwest structural boundary or rim of the Silverton caldera. The caldera center is to the east-southeast. Mineral Creek, which State Highway 550 follows to the south, traces the western rim of the caldera. The Silverton caldera formed after the last major eruption cycle of the older San Juan caldera in the middle Tertiary period (approximately 27 million years ago). As the caldera collapsed on itself, vertical faults and fractures formed pathways for the upward movement of gases, ore bearing fluids, and igneous intrusive plugs. The rim faults also became pathways for circulating hydrothermal fluids, which caused extensive alteration and emplacement of sulfide minerals.

Slumgullion Earthflow Natural Area

Slumgullion Earthflow:9
Two major earthflows included in this natural area illustrate a striking example of mass wasting (the movement of large masses of earth material). Slumgullion earthflow is over four miles long and 2,000 feet wide. A huge mass of volcanic rock slumped down the valley about 700 years ago, damming Lake San Cristobal. This flow is gradually being covered by a younger flow which began 300 years ago. Trees on the earthflow are being pushed over by the forward advance of the younger flow.

Slumgullion Earthflow - National Natural Landmark:14
Hinsdale County - Seven hundred year-old, 1,000-acre landflow composed of volcanic rock which formed a dam that created Lake San Cristobal. A younger landflow, currently active, is moving as much as 20 feet per year along the path of the earlier flow. Owner: Federal, Private. DESIGNATION DATE: October 1965

Spanish Peaks

Spanish Peaks:12
The mountainous sections of the Great Plains were formed long before the remaining areas were outlined by erosion. Uplift of the Black Hills and the Central Texas Uplift began as the continental interior was raised and the last Cretaceous sea was displaced, 65 to 70 million years ago. They stood well above the surrounding plains long before any sediments from the distant Rocky Mountains began to accumulate at their bases. In southern Colorado and northern New Mexico, molten rock invaded the sedimentary layers between 22 and 26 million years ago. The Spanish Peaks were formed at this time from hot magma that domed up the surface layers but did not break through; the magma has since cooled and solidified and has been exposed by erosion. Elsewhere the magma reached the surface, forming volcanoes, fissures, and basalt flows. A great thickness of basalt flows accumulated at Raton Mesa and Mesa de Maya between 8 and 2 million years ago. Volcanism has continued intermittently, and the huge cinder cone of Capulin Mountain was created by explosive eruption only 10,000 to 4,000 years ago. Most of these volcanic masses were formed before major downcutting by the streams began. Other igneous intrusions and volcanic areas in the northern Great Plains similarly were formed before the streams were incised.

Spanish Peaks - National Natural Landmark:14
Huerfano County and extends into Las Animas County - One of the best exposed examples of igneous dikes known; dikes are formed when molten igneous material is forced into a fracture or fault before becoming solidified. There may be over 500 such dikes in the area. Owner: Federal, Private. DESIGNATION DATE: January 1976

Specimen Mountain Research Natural Area

Specimen Mountain and Shipler Mountains:9
Specimen Mountain Research Natural Area straddles the Continental Divide in Rocky Mountain National Park and includes Specimen and Shipler Mountains. This natural area is capped with volcanic material from a nearby volcano that erupted approximately 28 million years ago and now supports virgin stands of Engelmann spruce-subalpine fir and pristine alpine meadows. The area is important calving grounds for Rocky Mountain bighorn sheep. Natural salt licks in the "crater" attract elk, deer, and bighorn sheep.

Wheeler Geologic Natural Area

Wheeler Geologic Natural Area:9
Wheeler Geologic Natural Area is a mass of pinnacles and domes, some more than several hundred feet high. Erosive forces have sculpted these forms in white, beige and lavender layers of lava and ash. The geologic features occur within virgin forests of Engelmann spruce and subalpine fir.

Excerpts from:
1) Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources Website, 2002
2) U.S. National Park Service Website, Black Canyon of the Gunnison National Park, 2001
3) U.S. National Park Service Website, Colorado National Monument, 2000;
4) U.S. National Park Service Website, Florissant Fossil Beds National Monument, 2000
5) U.S. National Park Service Website, Florissant Fossil Beds National Monument, 2002, Geology Field Trip Guides by Anabelle Foos
6) U.S. National Park Service Website, Fossil Butte National Monument, 2001
7) USGS/NPS Geology of the Parks Website, 2001
8) U.S. Forest Service Website, Pike's Peak Ranger District, 2002
9) Colorado State Parks Website, 2002
10) William L. Newman, Geologic Time: USGS Online Information Publication, 1997
11) Wood, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America: Cambridge University Press
12) Trimble, D.E., 1980, The Geologic Story of the Great Plains: U.S. Geological Survey Bulletin 1493
13) NOAA National Data Centers, NGDC Website, 2003
14) U.S. National Park Service, National Natural Landmarks Website, 2003

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02/04/03, Lyn Topinka