In the eastern U.S. we passed over an old mountain system - the Appalachian Belt, now rejuvenated by uplift starting some few million years ago. We flew across the vast lowland of the continental interior. From Central Colorado and along the trend from Mexico to Canada, we encounter an array of geological and physiographic expressions of orogenic (mountain-building) events that began in the late Paleozoic and continue today. This map presents an overview of the major landform classes of the western 1/3rd of the U.S.
The Rocky Mountains is a vast, generally north-south collection of joined or separated ranges that extend from northern New Mexico well into Canada. The Rockies (as they are often called) are shown here, first in a general location map, then within the U.S. as a shaded relief map and finally as a physiographic subdivisions map:
The Colorado-New Mexico-Wyoming segment of the Rockies stands out in imagery taken from space, in part because they are usually heavily wooded (evergreens in particular) and so appear darker:
The major ranges and basins are identified in this map:
Parts of the Rocky Mountains were being built geologically since the Paleozoic. The main orogeny occurred through subduction to the east at the end of the Mesozoic, with periodic rejuvenation since then. This sketch illustrates the general conditions during the Cretaceous Period as uplift was maximized:
The geologic map (below) of Colorado shows that the core of the Rockies is mainly Precambrian metamorphic and some igneous rocks. The San Juans consist largely of volcanic rocks.
The map below shows the distribution of Precambrian rocks in red. These consist mainly of deeper basement rocks in the North American craton that were uplifted and thrust surfaceward from about 70 million years ago (Cretaceous) to 42 million years ago (Eocene) - a time of mountain building known as the Laramide orogeny. Erosion of overlying rocks has exposed them widely throughout the mountains.
The Rockies in the Southern section, which includes nearly all of the western half of Colorado, are made up mostly of old metamorphic basement rock and some younger granitic intrusions, with remnants of the sedimentary cratonic cover at the time of uplift (climaxing about 68 million years ago). One current hypothesis considers the subducting plate to have been shallower than normal for such an orogenic process.
The boundary zone between the Great Plains and the eastern Front of the Rocky Mountains is abrupt, with the transition zone generally less than 1-2 km (0.6-1.2 miles) wide. The 700 km wide scene taken by the Day-Vis channel on the Heat Capacity Mapping Mission (HCMM) sensor shows much of the southern end of the Rockies from northern New Mexico into the southern half of Wyoming. The map below it indicates that the entire state of Colorado is included in the image.
In Colorado, the principal mountain groups are the Front Range, the Sangre de Cristos, the Sawatch Range, and the San Juan Mountains. In this part of the United States uplifted and folded rocks making up the the main body of the Rocky Mountains occupy a relatively narrow strip passing through the central part of Colorado and New Mexico, and swinging northwest through most of Wyoming (the Wind River Mtns mark the shift that carries the Rockies into Idaho and Montana). This limited width of the orogenic belt comprising the Rockies may seem a surprise because once in these mountains, their lofty grandeur gives an impression that they extend well to the west. In fact, from western Colorado into Utah, mountains of a different type, found in the Colorado Plateau (next page), predominate. The San Juan Mountains look to the traveler much like the Rockies but they are actually a great pile of highly dissected volcanic flow units intermixed with sedimentary rocks and are considered a separate unit. The Front is well displayed by this perspective natural color image using Landsat-7 data merged with DEM topographic data.The foreground area shown is the western terminus of the High Plains around Greeley, Colorado; the reservoir in the foothills is near Fort Collins.
The Rocky Mountain National Park is in the background, near the snow-capped high mountains near Estes Park. This popular destination for tourists is shown in the image below, made from combining a Landsat TM scene with DEM data:
The Park is included in this false color composite of central Colorado acquired in early October, 1973 soon after the launch of Landsat 1. Red colors within Denver and around Boulder and Fort Collins signify trees and crops still bright in the near-IR in this October scene. Their distribution is affected by proximity to the Platte and other rivers. These rivers provide irrigation for the alfalfa, hay, oats, corn, beets, and barley grown in this part of the Plains. Uncultivated areas, such as the Pawnee Grasslands (brownish patch at the right center), contain buffalo and grama grasses. A January 1973 winter view of Denver places the region in a rather different perspective. One consequence is the "heat island" effect of towns and cities, in which melting and snow-plow removal cause the streets and roofs to appear very dark from the loss of the snow cover. 6-9: This winter scene brings out the infrastructure of much of Denver. Four large towns are now visible to the north; name them. Are they visible in the summer scene. Explain the brownish color associated with the Rockies. Is there snow above the treeline? What factor(s) help(s) to indicate drainage patterns. What happened to the lakes so visible in the summer image? ANSWER The Rocky Mountains form the eastern edge of the North American Cordillera, which is made of groups of diverse, usually complexly-folded and faulted blocks of crust. The blocks are uplifted or thrust against other blocks separated by structural basins. They were deformed and emplaced during periods of major orogenies that were often disconnected in time and place. Rock units in the Rockies were finally compressed and shoved upwards about 65-70 million years ago. The present topography of these mountains is the result of strong erosion thereafter that has lowered their original heights to under 4,420 meters (14,500 ft). Alpine glaciation has steepened and widened already deep valleys, creating the rugged vistas that make this region especially popular for tourists and skiers. The barren (whitish) area along the crest of the Front Range in the October image coincides with the Continental Divide (a high elevation line where rainwater on its eastward side drains into east-flowing rivers that empty into the Gulf of Mexico and on the west side into rivers that ultimately reach the Pacific Ocean) that runs largely above the treeline. The slopes below are forested with Douglas fir, spruce, pine, and aspen trees. Rocky Mountain National Park is in the left center of the image. The southern end of the Medicine Bow Range in Wyoming extends into the image at the top left. The photo below shows a typical landscape within this part of the Rocky Mountains.
Flying over the Colorado Rockies in a commercial flight establishes the distinct character of these mountains. Denver, CO, appears at the edge of the plains in the lower center in both the Fall and Winter scenes. This burgeoning city is shown first from space and then in an aerial oblique view, with the Rockies in the background.
Just west of downtown Denver is Invesco Stadium, home of the Denver Broncos. This IKONOS image shows the details in this part of the city. To the west of central Denver, the land slowly rises until it abuts against the foothills of the actual Rocky Mountains. This is shown here in a Landsat-7 ETM+ image. To Denver’s west, against the mountains, are narrow outcrops of red sandstone (Lyons Formation) that bend upwards at about 45° to heights up to about 50 meters (160 ft) or more; see labeled "Red Rocks". These landforms are "hogbacks," made up of steeply dipping sedimentary rocks inclined down to the east as the Front Range block of rocks was pushed upwards. The hogbacks are quite evident in this aerial oblique view taken along the Front south of Denver.
The structural configuration of the rocks at the Front is shown in this geologic cross-section, beneath which is an exposed section of the geologic units at a roadcut along I-70 (there is a sign at the cut which explains the local geology).
Courtesy: Carolina Map Distributors
Within the hogbacks just north of Interstate 70 is the famed Red Rocks Amphitheatre, first opened in 1941. There each summer are classical concerts given by the Colorado Symphony and concerts by rock bands and other popular music groups. With hogbacks on either side, the acoustics are surprisingly good in these outdoor conditions. Here is a ground photo:
One of the most handsome campuses in North America is that of the University of Colorado, in Boulder, CO. It has the reputation of being a "party school" but there is also quite good science being done there, including a center for Remote Sensing applications, under the direction of Dr. Alexander Goetz. Here is an aerial oblique view of the campus and surrounding homes;
The second largest city in Colorado is Colorado Springs, about 170 km (100 miles) south of Denver, which also is situated at the Rocky Mountain Front. It is the home of the Air Force Academy. Look first at a Landsat-DEM perspective view that shows Pikes Peak in the high Rockies. Just north of the city is the beautiful Garden of the Gods, another series of red sandstone hogbacks (also called "flatirons" from their shape) such as we saw west of Denver.
Moving southward from Colorado Springs, note this perspective view (Landsat + DEM) of the Rocky Mountain Front near Walsenburg, CO :
Although a part of the Colorado Rockies, the San Juan mountains in the southwestern part of the state are different geologically. Some igneous/metamorphic rocks, and sedimentary rocks, are present but the main rock types are volcanic. Here is a satellite overview of these mountains, and beneath it is an ASTER image that shows the Creede caldera, a volcanic complex:
Before leaving Colorado, we point out that on the west side of the Sangre de Cristos above the New Mexico border lies one of America's largest sand dunes fields - The Great Sand Dunes National Monument. Although small in area, windblown sand from western winds piles up against the mountains to produce dunes as high as 220 meters (700 ft), as seen in this Landsat-7 subscene.
Compare this vertical scene with an astronaut photo taken from the ISS.
Our last look near the south end of the Southern Rocky Mountains is in New Mexico in this Landsat image that includes Albuquerque, the Sandia Mountains to its east, and the Rio Grande to the west:
To the northeast is Santa Fe, New Mexico's state capital, seen in this perspective made from a Landsat image. The Glorieta Mountains are in the background to the east:
To the northwest of Santa Fe, are the Jemez Mountains, seen in this satellite view. At their center is the extinct volcano known as the Valles Caldera. On the east side is the town of Los Alamos, built from scratch starting in 1942 to house the thousands of scientists and support personnel who together developed the first atomic bomb. After World War II, Los Alamos has remained as a research center concerned with atomic energy and its uses, and more recently basic energy research.
Here is a space image of the setting for Los Alamos, and an aerial view of the present day Los Alamos National Laboratory:
East of the main surface expression of the Rocky Mountains in New Mexico are several mountain masses that can be considered as uplifted basement rocks and folded sedimentary flank rocks. Below is a Landsat scene that has a true western flavor in which the Rocky Mountain blocks lie between broad lowlands.
The most conspicuous feature in the scene is the White Sands National Monument. The sand is composed not of quartz, as is usual, but of Gypsum (CaSO4.2H2O). It was there that the U.S. for nearly a decade launched captured German V-2 rockets and some early U.S. rockets. The program was under the direction of Dr. Wernher von Braun, the brilliant scientist-engineer who led the German rocket program during WWII. Also shown are the Sacramento Mountains (dark red), covered mostly with evergreens (Ponderosa Pine); these mountains are the southernmost extention of the Rockies. Mountains of the Basin and Range province's easternmost extension (the San Andres in the center) occur in the center and to the west of the scene (two small segments of the Rio Grande River appear at the left of the image). The towns of Alamagordo and Tucumcari lies at the western base of the Sacramento Mts; not far to the northwest, in the Tularosa Basin is the site of Trinity, where the first atomic bomb was detonated in July 1945. The dark patch is the Malpais basaltic lava field, a young volcanic extrusion of late Pleistocene age.
Lets now head back north. In eastern Utah, just south of the Wyoming border, is an unusual subdivision of the Rocky Mountains - the Uinta Mountains. This is one of the few East-West-trending ranges in North America. We examine this first in a satellite image, then from an aerial oblique photo:
The Middle Rocky Mountains lie mostly in the state of Wyoming. The Rockies there are exposed as a group of separated mountain ranges with deep intermontane basins (filled with erosional sediments from the mountains) in between. This map (from E. Raisz; Landforms of the United States) shows the distribution of these uplifts and basins; read the caption to identify individual ranges:
These self-same ranges appear in the mosaics of Wyoming that we will show on page 7-1.
As an example of the Middle Rocky Mountains, this Landsat-1 scene shows the Wind River Mountains (left) and the Owl Creek Mountain (near top). The low central area is the Wind River Basin; note Ocean Lake and the Boysen Reservoir:
Nestled within the broad stretch of the Middle Rocky Mountains as they pass through northwestern Wyoming is America's first National Park: Yellowstone. This next image shows Yellowstone Park in context with the surrounding mountains:
Here is a Landsat view that zeros in on the Park itself; for reference note Yellowstone Lake:
Yellowstone Lake is surrounded by a darker area (evergreens) near the image center. Look in this image about due west (towards the top) of Yellowstone Lake - this is how it appears in a Landsat-7 ETM+ subscene below: grazing and clearcut forests are to the left (west); fully preserved forestlands are on the right; the sharp boundary is related to a fenceline (this effect is similar to the U.S.-Canadian boundary shown on the previous page):
In the mid-1990s, Yellowstone Park and some surrounding areas were plagued with wildfires that extended over huge areas. The next two images - one Landsat, the other SIR-C radar - show some of the burn scars (in dark purple):
As we shall shortly see, Yellowstone owes its special character mainly to recent volcanic activity. This interesting perspective of Yellowstone Park and surrounding mountains was made from satellite imagery.
The next two pictures show typical terrain in the Park and Yellowstone Falls (with the yellowish pyroclastic ash deposits that give the park its name):
Of course, Yellowstone Park is most famed because of its geysers (fountains of water spewed out after groundwater has reached its near boiling point after being heated by residual heat from subterranean volcanic activity). Here are the Lion Geyser in eruption and a view of the Norris geyser basin:
So, how do geysers work? Groundwater collects in fissures and pockets below the surface. The water is heated (from deeper magma; see below) to the boiling point. The water expands slightly, relieving the pressure enough to cause some of it to flash into steam. The steam drives the water upward through openings to spill over or erupt vigorously at the surface as hot pools and geysers.
The obvious question you have in mind: What causes the varied phenomena that make Yellowstone almost unique? The answer lies in its special geologic setting. For millions of years this part of the northwest U.S. has been the site of widespread volcanism. This diagram summarizes the relevant activity.
The ultimate cause of the volcanism is a stationary hot spot coming from the Earth's mantle which heats crustal rock to produce large pockets of magma. The North American tectonic plate has been moving to the southwest for millions of years. The path of motion passes over the hot spot which causes this subterranean heating to reach a stage where violent eruption must ensue. The diagram shows a series of previous eruptions progressing back in time along what is today the eastern Snake River Plains of Idaho. The most recent eruption took place within the terrain making up Yellowstone Park.
That eruption took place about 640,000 years ago. Like its predecessors, it was huge (one estimate: 80 times larger than Krakatoa in Indonesia, which was the most violent in the last 500 years). This supervolcano's explosion blew away a large upbent part of the crust (mainly, the volcanic edifice that had been building up) creating a caldera some 48 x 72 km (30 by 45 miles) that then was filled by subsequent lava and ash deposits (so that the present day topography offers little direct evidence of the scar that was left behind). The photo below shows some of the deposits (ash and flows that are dominated by rhyolite volcanics) laid down by the eruption itself and later activity.
The question one naturally poses: If there have been previous major eruptions several times in the last few million years, what can be predicted about the next one? Since there is strong evidence (from earthquake studies) that there is still a large pocket of magma beneath Yellowstone, the likelihood of another super-eruption is fairly high. Just when cannot yet be forecast but it seems probable that it will happen sometime in the next half million years.
The next scene to the south of Yellowstone contains some of the most spectacular scenery in America. See caption for identification of natural features:
The Tetons are the up-down range at top center. Southeast of the Tetons are the E-W Gros Ventre Mountains (those who know French should try to translate these names, given by French trappers in the 18th century). The two curving ranges below are the Wyoming and Hoback ranges. The beginning of the Snake River Plains (page 6-8) appears in the upper left.
The Tetons are dramatically photogenic (backdrop in the movie "Shane"). Here they are in a panoramic view from the alluvial plains through which the Snake River flows":
Fit these images, which show the beauty of the Tetons close-up, with this perspective view of the entire range, the Snake River Plains, and the Hoback Range to the south as constructed from a Landsat-7 natural color composite and STRM elevation data:
This perspective view discloses the Tetons to be a block fault mountain range in which the east side is thrust upwards as a swinging motion. Thus, the west slope is gently inclined, so much so that bicyclists have reached the crest whereas those climbing from the east side must use ropes and other such advanced scaling equipment.
