Colorado Geology

If you would like to have a wonderful primer on Colorado’s unique geology, order a print copy of the award-winning Messages in Stone: Colorado’s Colorful Geology, a perennial favorite of locals and visitors alike.

On all the broad extent of these United States, certainly no region can be found which presents more facts of interest, more opportunities for investigation, and greater possibilities, than the State of Colorado. — Samuel F. Emmons, geologist on the King Survey of the 40th Parallel from California to Colorado from 1867 to 1872; Director of the Rocky Mountain Division of the United States Geological Survey; and the first president of the Colorado Scientific Society, from his inaugural address.

I do not know of any portion of the West where there is so much variety displayed in the geology as within a space of ten miles square around Colorado City (today’s Colorado Springs). Nearly all the elements of geological study revealed in the Rocky Mountains are shown on a unique scale in this locality. — F. V. Hayden, geological expedition leader to the Colorado region between 1869 to 1876, from the first expedition report.

The geology of Colorado is written in the rocks. From this great book are here presented a few translations of a few paragraphs. The scenery of Colorado is a gallery incomparable. Words lack form and light – the essence and soul of scenery. At best they can but call attention to the elements associated in the picture. They cannot convey the beauty and harmony of the assemblage. — The first director (1908-26) of the Colorado Geological Survey, Russell D. George, in the preface to his 1927 book, Geology and Natural Resources of Colorado.

General geological interest

Many of these are out of print but may be found on Amazon or other online sources.

Chronic, Halka. Roadside Geology of Colorado. Miscellaneous Investigations. Missoula, MT: Mountain Press Publishing Company, 1980.

Foutz, Dell R. Geology of Colorado Illustrated. Grand Junction, CO: Dell R. Foutz, 1994.

Hopkins, Ralph Lee, and Lindy Birkel Hopkins. Hiking Colorado’s Geology. 1st ed. Seattle, WA: Mountaineers, 2000.

Johnson, Kirk R, Robert G. H Raynolds, Jan Vriesen, Donna Braginetz, Gary Staab, and Denver Museum of Nature and Science. Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range. Denver, CO: Denver Museum of Nature & Science, 2003.

Johnson, Kirk R., and Richard Keith Stucky. Prehistoric Journey: A History of Life on Earth. Golden, CO: Fulcrum Publishing, 2006.

Matthews, Vincent and Colorado Geological Survey. “SP-57 Tourist Guide to Colorado Geology.” Special Publication. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2009.

Matthews, Vincent, Katie KellerLynn, and Betty Fox, eds. SP-52 Messages in Stone: Colorado’s Colorful Geology. Second. Special Publications, SP-52. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2009.

Murphy, Jack A. Geology Tour of Denver’s Buildings and Monuments. Historic Denver Guides. Denver, CO: Historic Denver and the Denver Museum of Natural History, 1995.

Murphy, Jack A. Geology Tour of Denver’s Capitol Hill Stone Buildings. Miscellaneous 65. Denver, CO: Historic Denver, Inc, 1997.

Osterwald, Doris B. Rocky Mountain Splendor: A Mile by Mile Guide for Rocky Mountain National Park. 1st ed. Lakewood, CO: Western Guideways, 1989.

Raup, Omer B. Geology along Trail Ridge Road: Rocky Mountain National Park Colorado. Estes Park, CO: Rocky Mountain Nature Association, 2005.

Reed, Jack, and Gene Ellis. Rocks Above the Clouds: A Hiker’s and Climber’s Guide to Colorado Mountain Geology. The Colorado Mountain Club, n.d.

Taylor, Andrew M. Guide to the Geology of Colorado. Golden, CO: Cataract Lode Mining Co., 1999.

ON-004-02 — Colorado POGI Map — Reviving this popular map of Colorado Points of Geologic Interest was a high priority, and here it is as a shared Google Map. Comments welcome!

CGS Field trip guides:

Birkeland, Peter W., Daniel C. Miller, Penny E. Patterson, Alan B. Price, and Ralph R. Shroba. “OF-96-04-27 Soil-Geomorphic Relationships near Rocky Flats, Boulder and Golden, Colorado Area with a Stop at the Pre-Fountain Paleosol of Wahlstrom.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Blair, Rob. “OF-96-04-09 Geology of the Western San Juan Mountains and a Tour of the San Juan Skyway, Southwestern Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Collins, Donna Bishop. “SP-27 Scenic Trips into Colorado Geology.” Touristic Geology, 1:250,000. Special Publication. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1985.

Colorado Geological Survey. “OF-96-04-20 History, Geology, Hydrogeology, Summitville Mine and Downstream Effects, and Other Nearby Mines of the San Luis Valley, Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

———. “RockTalk V02N1, January 1999 – Annual Review.” RockTalk, January 1999.

———. “RockTalk V11N1, Spring 2008 – 2007 Summary.” RockTalk, Spring 2008.

Erslev, Eric A., and Joe D. Gregson. “OF-96-04-25 Oblique Laramie Convergence in the Northeastern Front Range: Regional Implications for the Analysis of Minor Faults.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Fleming, Robert W., Rex L. Baum, and William Z. Savage. “OF-96-04-11 The Slumgullion Landslide, Hindsdale County, Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Henry, Thomas W., Emmett Evanoff, Daniel Grenard, Herbert W. Meyer, and Jeffrey A. Pontius. “OF-96-04-24 Geology of the Gold Belt Backcountry Byway, South Central Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Hynes, Jeffrey L. SP-31 Proceedings of the 1985 Conference on Coal Mine Subsidence in the Rocky Mountain Region. Special Publications, SP-31. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1986.

Kirkham, Robert M., Bruce Bryant, Randall K. Streufert, and Ralph R. Shroba. “OF-96-04-08 Geology and Geologic Hazards of the Glenwood Springs Area, Central Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Krutak, Paul R. “OF-96-04-23 Depositional Environments of Codell-Juana Lopez Sandstones and Regional Structure and Stratigraphy of Canon City and Huerfano Areas and Northern Raton Basin, South-Central Colorado.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Matthews, Vincent and Colorado Geological Survey. “SP-57 Tourist Guide to Colorado Geology.” Special Publication. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2009.

Matthews, Vincent, Katie KellerLynn, and Betty Fox, eds. SP-52 Messages in Stone: Colorado’s Colorful Geology. Second. Special Publications, SP-52. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2009.

Noe, D. C., Jonathan L. White, and G. Zabel. “SP-56 Geology and Geologic Hazards along the I-70 Corridor, Vail to Glenwood Springs, Colorado.” In Field Trip Guidebooks, 1st North American Landslide Conference, edited by Jeffrey A. Coe and D. C. Noe. Special Publication, SP-56. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2007.

Penn, Brian S., and David A. Lindsey. “OF-96-04-28 Tertiary Igneous Rocks and Laramide Structure and Stratigraphy of the Spanish Peaks Region, South-Central Colorado: Road Log and Descriptions from Walsenburg to La Veta to Aguilar.” Fieldtrip. Open File Report. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 1996.

Schwochow, Stephen D., ed. RS-08 Proceedings of the Fifteenth Forum on Geology of Industrial Minerals – Industrial Minerals in Colorado and the Rocky Mountain Region. Resource Series, RS-08. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1980.

“SP-19 Colorado Tectonics, Seismicity and Earthquake Hazards: Proceedings and Field Trip Guide of a Symposium Held in Denver, Colorado, June 4-6, 1981.” Tectonics, Seismicity, and Earthquake. Special Publication. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1981.

Thompson, Ren A., Mark R. Hudson, and C. L. Pillmore, eds. SP-44 Geologic Excursions to the Rocky Mountains and beyond: Field Trip Guidebook for the 1996 Annual Meeting, Geological Society of America, Denver, Colorado, October 28-31. Special Publications, SP-44. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1996.

White, Jonathan L., N. C. Dessenberger, W. L. Ellis, J. D. Higgins, and S. Gaffney. SP-56 First North American Landslide Conference Field Trips, Vail, Colorado, June 3-10, 2007. Edited by D. C. Noe and Jeffrey A. Coe. Special Publication, SP-56. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2007.

———. “SP-56 Geology and Geologic Hazards along the I-70 Corridor, Glenwood Springs to Grand Junction, Colorado.” In Field Trip Guidebooks, 1st North American Landslide Conference, edited by D. C. Noe and Jeffrey A. Coe. Special Publication, SP-56. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2007.

Blown across the land by wind or carried along by water and ice as the land continued to remake itself, loose sediments eventually compressed and cemented into rock and left messages in stone for us to decipher. Sediments include the mud at the bottom of streams, the sand dunes at the foot of the mountains, the chemical precipitates of salt in shallow seas, the beaches at the edge of inland seas, and the graveyards of tiny fossils at the bottom of tropical oceans. In these sedimentary layers, such as the Book Cliffs in western Colorado, the imprints of changing life forms in an ancient world are faithfully recorded.

Sediments that were originally particles of rock or former living organisms were transported and later sculpted by wind and water, pressed, and finally lifted to prominence as some of the state’s most imposing landmarks. The sandstones of Colorado National Monument, the reddish-brown siltstones and mudstones of Owl Canyon, and the Flatirons that flank Boulder are all sedimentary rocks. Other sedimentary deposits include massive limestone formations around Leadville, the evaporites of the Eagle Valley, chalks of the eastern plains, coals near Trinidad, oil shale in western Colorado, and the thick shale of eastern Colorado.

How do geologists know that a particular sedimentary rock formed in a specific environment? In 1795 geologist Sir Charles Lyell developed a concept that is the next best thing to being there: “The present is the key to the past.” The idea is that by studying the characteristics found in modern depositional environments and comparing them to similar features found in ancient rocks, one can solve the mystery. For example, in Colorado, we can study the features of modern dunes in the Great Sand Dunes National Park and compare them to the ancient deposits of the 250 million-year-old Lyons sandstone found along the eastern flank of the Front Range. Using the same theory allows us to decipher the rock in the images below to be mudcracks, ripple marks, coarse-grained conglomerate, and raindrop impressions.

As the name indicates, metamorphic (meta = change, morph = form) rocks are pre-existing igneous, sedimentary, or metamorphic rocks that have been altered, or metamorphosed, deep within Earth’s crust. The rocks changed form in response to intense fluctuations in temperature, pressure, shearing, stress, or chemical environment. During Colorado’s mountain-building events, the intrusion of igneous bodies increased the crustal temperature to result in contact and regional metamorphism. The dominant metamorphic rock types in Colorado are gneiss, schist, amphibolite, and quartzite.

Contact metamorphism occurs when hot magma intrudes into cooler rock. The intrusion heats the surrounding rock, making the low-temperature minerals unstable. These minerals change to minerals that are stable at the new, higher temperatures. Contact metamorphism of the Leadville limestone created the Yule Marble.

Regional metamorphism occurs because pressure and temperature change over a broad area. Different pressure and temperature combinations create a variety of minerals. In the course of a drive through Big Thompson Canyon from Loveland to Estes Park, you traverse all of the mineral zones of regional metamorphism. The presence of biotite near the mouth of the canyon signals an area of low-grade metamorphic rocks. Farther up the canyon are the garnet and staurolite zones indicative of formations subjected to higher temperatures. Within five miles, you reach the community of Drake in the highest (sillimanite) temperature and pressure zone of regional metamorphism. Exposed in the hills above Drake are coarse-grained pegmatites that are vein-like offshoots of the batholith. The high-grade rocks and pegmatites are indicators that you are approaching the 600-square-mile granitic batholith that surrounds Estes Park.

Around Estes Park and Rocky Mountain National Park, the metamorphic rocks were raised to temperatures and pressures at or near their melting point. This gave rise to migmatites, an intimate mixture of igneous and metamorphic rocks. Migmatites are found throughout much of the Front Range.

Achaean Metamorphic

The Archaean rocks (of the Owiyukuts Complex) are the oldest in Colorado. They were metamorphosed about 2.7 billion years before present. That means that the original rocks were even older. Unfortunately, there aren’t very many of them, only about 40 acres outcrop in extreme northwest Moffat County, Colorado along Beaver Creek.

Metamorphic folding

Rocks are subjected to intense pressures and temperatures during metamorphism. This makes them quite ductile and allows them to fold into weird and wonderful shapes. Sometimes they are folded multiple times such that early formed folds will be re-folded. The metamorphic rocks in Colorado endured three, and perhaps four, periods of ductile deformation.

Refolded Folds

The oldest folds in Colorado are in Precambrian metamorphic rocks. The layers were folded during different regional metamorphic events. When previously folded rocks were again subjected to heat and pressure, they were refolded and became refolded folds.

Early geologists studying the Precambrian structures of the Front Range found many clues indicating two periods of folding, but were unable to find a place where they could see both sets of folds in the same outcrop. Finally, they found an outcrop in Clear Creek Canyon east of Blackhawk that has both sets of folds.

It is fairly easy to pick out large-scale (many sq km) anticline and synclines. However, it requires careful searching to discern the smaller-scale (cm-sized or less) tight folds that were folded once during an earlier period of folding, then folded again and tightened by a second period of folding.

Preserved Original Features

Commonly the original features occurring in rocks are completely altered or destroyed by the recrystallization and/or deformation of the metamorphic process. However, there are places in Colorado where the original features can be discerned, even in high-grade metamorphic rocks.

Igneous rock is formed from magma that has cooled and become solid. Molten rock is extraordinarily hot, sometimes exceeding 2,000 degrees Fahrenheit. If this molten and partially crystallized material (magma) solidifies underground before it reaches the surface, the rock is intrusive or plutonic. If magma travels up through Earth’s crust and reaches the surface, the resulting rocks are extrusive or volcanic.

Magma that reaches the surface forms a variety of volcanic landforms and deposits. Today, resistant volcanic flows cap mesas such as Grand Mesa, White River Plateau, Raton Mesa, and those near Basalt, Colorado. In the southwestern part of the state, ash-flow tuffs cover thousands of square miles. The tuff is created from the ash that is blown from the volcano to blanket the landscape.

An ash-flow eruption creates a roughly circular depression called a caldera. There are at least nineteen calderas in Colorado, making the state one of the world’s best outdoor laboratories in which to study their formation.

Plutonic Rocks

The Rocky Mountains of Colorado boast spectacular views of numerous plutonic (or intrusive) rocks. These rocks were formed long ago as magma rose from deep sources and solidified before making it all the way to the surface. As the magma rose, in many places, it brought with it precious minerals such as gold, silver, lead, and molybdenum (used in hardening steel). After millions of years, erosional processes stripped off the overlying rocks, exposing them as we see today.

Alkalic Complexes

Alkalic complexes in Colorado’s Wet Mountains and the Powderhorn district near Gunnison contain carbonatite and a variety of other exotic rock types. Both complexes contain abundant rare earth elements. Indeed, Powderhorn (also known as Iron Hill) contains the nation’s largest volumetric deposit of rare earths as well as the world’s largest titanium deposits.

Batholiths

A batholith is an intrusive mass of solidified magma usually granitic in composition and defined as having a minimum size greater than 40 mi² (100 km²) and no known floor. These are emplaced deep in the crust and are irregular in shape.

Proterozoic granitic batholiths are common throughout Colorado’s Rocky Mountains. It was generally thought that the batholiths fell into three age groups: the Routt plutonic suite at about 1.7 billion years ago, the Berthoud plutonic suite at about 1.4 billion, and the Pikes Peak batholith at about 1.0 billion. With more age dating, and more advanced dating techniques, we are discovering that the intrusions were more spread out in time.

Dikes

Dikes are formed when magma (a mixture of molten material and crystals) rises from below and cuts across pre-existing strata. The magma may follow pre-existing cracks or faults, or may create its own path upward. The magma crystallizes underground and becomes a dike, which is a plutonic or intrusive rock.

Erosion cuts into the earth and allows us to observe the dikes. The magma in a dike may or may not have reached the surface. If the magma pours out onto the surface then it becomes a volcanic or extrusive rock. All extrusive rocks must of necessity have intrusive feeders, usually dikes or plugs.

Colorado is home to examples of every known type of dike structure.

Laccoliths

Laccoliths are mushroom-shaped bodies with a flat floor and a domed roof. Thus, they appear to have begun forming in the same way as sills; however, as magma continued to intrude, it pushed up the overlying layers rather than continuing to spread out laterally. Colorado has many laccoliths and is the first place in the world where they were recognized and named. Corry (1988) lists more than 80 by name and location, and it is estimated that there are between 8-10,000 globally.

Magma that moves upward and cuts across pre-existing layers of rock forms what are generally known as plutons. The largest plutons are batholiths, such as the granitic rocks of Pikes Peak, which are part of a 1,300-square-mile batholith. Smaller plutons take a variety of shapes, each with its own name, such as stocks, plugs, dikes and sills.

Plugs

A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. If rising volatile-charged magma is trapped beneath a plug, the resulting extreme build-up of pressure can sometimes lead to an explosive eruption. Eons later, erosion can reveal the more resistant plug forming residual landforms. For example, Edinburgh Castle in Scotland is built upon an ancient volcanic plug.

Sills

Sills form by magma squeezing between, and parallel to, the layers of the host rock, rather than cross-cut existing layers of strata as dikes do.

Colorado has sills of many different sizes and ages intruded into an astonishing variety of surrounding rocks. One of the largest is Archuleta Mesa in southern Colorado near the New Mexico border.

Volcanic Rocks

Volcanic (or extrusive) rocks have cooled from molten material that either flowed out onto the surface of the earth or were blasted up into the air and settled back onto the surface of the earth. Volcanic igneous rocks are widespread throughout Colorado. At one time, about sixty percent of Colorado was covered by volcanic rocks but much of that coverage has since eroded.

Volcanic rocks come in a variety of forms, depending on the type of eruption from which it originated. For example, in a violent eruption ash fall, ash flow, and lava flow may occur, all of which produce different types of volcanic igneous rock.

Many of these rocks originated in the San Juan volcanic field, which is in the southwestern region of the state. There, many large caldera eruptions generated hundreds of cubic miles of pyroclastic debris.

Ash Falls

When volcanic debris is ejected into the atmosphere it eventually settles back down to the surface as ash fall. Volcanic igneous rocks that result from this process are usually of uniform in grain size, which are tiny flakes of glass. Some ash ejected into the atmosphere may float for years before settling back down to the surface. Ash falls provide very good age datums that may be correlated for thousands of miles in some cases. A very important ash fall deposit in Colorado is the Lava Creek B ash that was created by Yellowstone Caldera around 640,000 years ago. This ash has been instrumental in providing age constraints on numerous formations around Colorado, such as the Browns Park Formation.

Ash Flows

Ash flows follow explosive volcanic eruptions and occur when dense ash sinks and flows down the flanks of an erupting volcano. These scorching hot flows can travel hundreds of miles per hour, knocking down trees and obliterating anything else in their path. Ash flows can form volcanic igneous rocks such as welded tuffs, where the incredible temperature of the flow causes ash to fuse together. Ash flows may also contain various sizes of other pyroclastic material.

Following an epic period in the region’s geologic history, Colorado now has ash-flow tuffs that cover thousands of square miles in the southwestern part of the state. Between 25 and 37 million years ago when ash flows were erupting with gusto, it is estimated that two-thirds of the state was covered with ash flow deposits. The Fish Canyon Tuff surrounding the present-day site of Creede, Colorado is one of the largest ash flows in the world. It contains approximately 1,200 cubic miles of material that was deposited red hot and has a welded zone more than a half-mile thick.

Breccias

Breccias a rock formed from angular gravel and boulder-sized clasts that are cemented together in a matrix. The angular nature of the clasts indicates that they have not been transported very far from their source and have not been water worn smooth. Volcanic breccias (agglomerates) comprise blocks of lava in an ash matrix, and are the product of an explosive eruption.

Breccia can be further divided according to:

• Class – breccia can be divided into two broad classes: clast supported – where the clasts touch each other and the matrix fills the voids; and matrix supported – where the clasts are not in contact and the matrix surrounds each clast;
• Clast size – fine (2 – 6mm), medium (6 – 20mm), coarse (20 – 60mm), very coarse (> 60mm);
• Sorting – a breccia comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;
• Lithology – a breccia where the clasts represent more than one rock type is termed polymictic (or petromictic), while one where the clasts are of a single rock type are monomictic (or oligomictic).

Calderas

Calderas form during massive volcanic eruptions where large portions of the surface collapse into the emptying magma chamber below. This type of eruption generates hundreds of cubic miles of ash, which in turn creates tremendous amounts of volcanic rock.

Colorado has experienced at least 20 caldera eruptions. The San Juan volcanic field is one of the most notable areas in Colorado where this type of eruption occurred. Hundreds of square miles of Colorado’s southwest are covered by ash flow tuff generated by these eruptions. The world’s largest caldera, La Garita, is also located in the San Juan volcanic field. This caldera extruded over 1,200 cubic miles of ash flow, which is also the world’s largest ash flow deposit.

Columnar Jointing

Columnar joints forms as volcanic rock cools and contracts and is thus a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing can occur in cooling lava flows and ash flow tuffs (ignimbrites), as well as in some shallow intrusions. Joint growth is perpendicular to the surface of the volcanic flow.

Column diameters vary from a few centimeters to three meters (10 feet), and can be as much as 30 meters (about 100 feet) in length. They are typically parallel and straight (colonnade), but can also be curved (entablature). The number of sides of the individual columns can vary from three to eight, with six sides (hexagonal in cross section) being the most common.

We are used to seeing columnar jointing have vertical joints, particularly in basalt flows. They form at right angles to the isotherms (lines of equal temperature). In a horizontal lava flow the isotherms are normally horizontal because the flow loses heat to the cooler ground below and the cooler air above: the columns thus are vertical.

At first glance here one might think there is a violation of that relationship. However, the dike is intruded into rock and is losing heat to each of its sides as it cools. Therefore, the isotherms are vertical and since columns form perpendicular to isotherms, they are horizontal.

Lava Flows

Lava flows come in a variety of styles depending upon the composition of erupted material. Two major categories of lava flow in Colorado areas basalt (magnesium and iron rich) flows and rhyolite (silica rich) flows.

Cinder Cone Flows

A cinder cone or scoria cone is a steep, straight-sided conical hill of pyroclastic material called tephra (volcanic debris) that accumulates around and downwind from a volcanic vent. Cinder cones range in size from tens of meters to 300 meters tall. The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles “frozen” into place as magma exploded into the air and then cooled quickly. Many cinder cones have a bowl-shaped crater at the summit.During the waning stage of a cinder-cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain, but oozes quietly into the crater or beneath the base of the cone as lava. Lava rarely issues from the top (except as a fountain) because the loose, un-cemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent. Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders. Thus, it often burrows out along the bottom of the cinder cone, lifting the less-dense cinders like a cork on water, and advances outward, creating a lava flow around the cone’s base. When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava. However, if the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent.

Cinder cones are commonly found on the flanks of shield volcanoes, stratovolcanoes, and calderas. Colorado’s youngest cinder cone volcano is located near Dotsero, Colorado and has been dated to be about 4,150 +/- 300 years old. More information is available in the CGS OF-08-14 Geologic Map of the Dotsero Quadrangle, Eagle and Garfield Counties, Colorado report.