Geology of the Rocky Mountains

Last updated
Location of the Rocky Mountains in western North America RockyMountainsLocatorMap.png
Location of the Rocky Mountains in western North America

The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins. Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through central New Mexico and which is part of the great mountain system known as the North American Cordillera.

Contents

The rocky cores of the mountain ranges are, in most places, formed of pieces of continental crust that are over one billion years old. In the south, an older mountain range was formed 300 million years ago, then eroded away. The rocks of that older range were reformed into the Rocky Mountains.

The Rocky Mountains took shape during an intense period of plate tectonic activity that resulted in much of the rugged landscape of the western North America. The Laramide orogeny, about 80–55 million years ago, was the last of the three episodes and was responsible for raising the Rocky Mountains. [1] Subsequent erosion by glaciers has created the current form of the mountains.

Ancestral rock

The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock is Precambrian metamorphic rock that forms the core of the North American continent. There is also Precambrian sedimentary argillite, dating back to 1.7 billion years ago. During the Paleozoic, western North America lay underneath a shallow sea, which deposited many kilometers of limestone and dolomite. [2]

In the southern Rocky Mountains, near present-day Colorado and New Mexico, these ancestral rocks were disturbed by mountain building approximately 300 Ma, during the Pennsylvanian. This mountain building produced the Ancestral Rocky Mountains. [3] :1 The uplift created two large mountainous islands, known to geologists as Frontrangia and Uncompahgria, located roughly in the current locations of the Front Range and the San Juan Mountains. They consisted largely of Precambrian metamorphic rock, forced upward through layers of the limestone laid down in the shallow sea. [4] The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock. [3] :6

Western Interior Seaway 95 million years ago Western Interior Seaway - 95Ma.svg
Western Interior Seaway 95 million years ago

Mesozoic deposition in the Rockies occurred in a mix of marine, transitional, and continental environments as local relative sea levels changed. By the close of the Mesozoic, 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognized formations. The most extensive non-marine formations were deposited in the Cretaceous period when the western part of the Western Interior Seaway covered the region. [5]

Terranes and subduction

Terranes started to collide with the western edge of North America in the Mississippian age (approximately 350 million years ago), causing the Antler orogeny. [6] During the last half of the Mesozoic Era, much of today's California, British Columbia, Oregon, and Washington were added to North America. Western North America suffered the effects of repeated collision as the Kula and Farallon plates sank beneath the continental edge. Slivers of continental crust, carried along by subducting ocean plates, were swept into the subduction zone and scraped onto North America's western edge. [7]

These terranes represent a variety of tectonic environments. Some are ancient island arcs, similar to Japan, Indonesia and the Aleutians; others are fragments of oceanic crust obducted onto the continental margin while others represent small isolated mid-oceanic islands. [8]

Sketch of an oceanic plate subducting beneath a continental plate at a collisional plate boundary. The oceanic plate typically sinks at a high angle (exaggerated here). A volcanic arc grows above the subducting plate. Active Margin.svg
Sketch of an oceanic plate subducting beneath a continental plate at a collisional plate boundary. The oceanic plate typically sinks at a high angle (exaggerated here). A volcanic arc grows above the subducting plate.

Magma generated above the subducting slab rose into the North American continental crust about 200 to 300 miles (300 to 500 km) inland. Great arc-shaped volcanic mountain ranges, known as the Sierran Arc, grew as lava and ash spewed out of dozens of individual volcanoes. Beneath the surface, great masses of molten rock were injected and hardened in place. [7]

For 270 million years, the effects of plate collisions were focused very near the edge of the North American plate boundary, far to the west of the Rocky Mountain region. [6] It was not until 80 MA that these effects began to reach the Rockies. [1]

Raising the Rockies

The current Rocky Mountains were raised in the Laramide orogeny from between 80 and 55 Ma. [1] For the Canadian Rockies, the mountain building is analogous to a rug being pushed on a hardwood floor: [9] :78 the rug bunches up and forms wrinkles (mountains). In Canada, the subduction of the Kula plate and the terranes smashing into the continent are the feet pushing the rug, the ancestral rocks are the rug, and the Canadian Shield in the middle of the continent is the hardwood floor. [9] :78

Farther south, the growth of the Rocky Mountains in the United States is a geological puzzle. [1] Mountain building is normally focused between 200 to 400 miles (300 to 600 km) inland from a subduction zone boundary. Geologists continue to gather evidence to explain the rise of the Rockies so much farther inland; the answer most likely lies with the unusual subduction of the Farallon plate, [7] or possibly due to the subduction of an oceanic plateau. [1] [10]

At a typical subduction zone, an oceanic plate typically sinks at a fairly steep angle, and a volcanic arc grows above the subducting plate. During the growth of the Rocky Mountains, the angle of the subducting plate may have been significantly flattened, moving the focus of melting and mountain building much farther inland than is normally expected. [7] It is postulated that the shallow angle of the subducting plate greatly increased the friction and other interactions with the thick continental mass above it. Tremendous thrusts piled sheets of crust on top of each other, building the extraordinarily broad, high Rocky Mountain range. [7]

Tilted slabs of sedimentary rock in Colorado Roxborough.jpg
Tilted slabs of sedimentary rock in Colorado

The current southern Rockies were forced upwards through the layers of Pennsylvanian and Permian sedimentary remnants of the Ancestral Rocky Mountains. Such sedimentary remnants were often tilted at steep angles along the flanks of the modern range; they are now visible in many places throughout the Rockies, and are prominently shown along the Dakota Hogback, an early Cretaceous sandstone formation that runs along the eastern flank of the modern Rockies.

Current landscape

Immediately after the Laramide orogeny, the Rockies were like Tibet: a high plateau, probably 6,000 metres (20,000 ft) above sea level. In the last 60 million years, erosion stripped away the high rocks, revealing the ancestral rocks beneath, and forming the current landscape of the Rockies. [9] :80–81

Glaciers, such as Jackson Glacier as shown here, have dramatically shaped the Rocky Mountains. Jackson Glacier terminus.jpg
Glaciers, such as Jackson Glacier as shown here, have dramatically shaped the Rocky Mountains.

Periods of glaciation occurred from the Pleistocene Epoch (1.8 million–70,000 years ago) to the Holocene Epoch (fewer than 11,000 years ago). The ice ages left their mark on the Rockies, forming extensive glacial landforms, such as U-shaped valleys and cirques. Recent glacial episodes included the Bull Lake Glaciation that began about 150,000 years ago and the Pinedale Glaciation that probably remained at full glaciation until 15,000–20,000 years ago. [11] [12] Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation. [11] The little ice age was a period of glacial advance that lasted a few centuries from about 1550 to 1860. For example, the Agassiz and Jackson Glaciers in Glacier National Park reached their most forward positions about 1860 during the Little Ice Age. [11]

All of the geological processes, above, have left a complex set of rocks exposed at the surface. For example, in the Rockies of Colorado, there is extensive granite and gneiss dating back to the Ancestral Rockies. In the central Canadian Rockies, the main ranges are composed of the Precambrian mudstones, while the front ranges are composed of the Paleozoic limestones and dolomites. [13] Volcanic rock from the Cenozoic (66 million–1.8 million years ago) occurs in the San Juan Mountains and in other areas. Millennia of severe erosion in the Wyoming Basin transformed intermountain basins into a relatively flat terrain. The Tetons and other north-central ranges contain folded and faulted rocks of Paleozoic and Mesozoic age draped above cores of Proterozoic and Archean igneous and metamorphic rocks ranging in age from 1.2 billion (e.g., Tetons) to more than 3.3 billion years (Beartooth Mountains). [11]

See also

Related Research Articles

Rocky Mountains Major mountain range in western North America

The Rocky Mountains, also known as the Rockies, are a major mountain range in western North America. The Rocky Mountains stretch 3,000 mi (4,800 km) in straight-line distance from the northernmost part of British Columbia, in western Canada, to New Mexico in the Southwestern United States. The northern terminus is located in the Liard River area east of the Pacific Coast Ranges, while the southernmost point is near the Albuquerque area adjacent the Rio Grande Basin and north of the Sandia–Manzano Mountain Range. Located within the North American Cordillera, the Rockies are distinct from the Cascade Range and the Sierra Nevada, which all lie farther to the west.

Orogeny The formation of mountain ranges

An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis. A synorogenic event is one that occurs during an orogeny.

The Coast Mountains are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the Coast of British Columbia south to the Fraser River. The mountain range's name derives from its proximity to the sea coast, and it is often referred to as the Coast Range. The range includes volcanic and non-volcanic mountains and the extensive ice fields of the Pacific and Boundary Ranges, and the northern end of the volcanic system known as the Cascade Volcanoes. The Coast Mountains are part of a larger mountain system called the Pacific Coast Ranges or the Pacific Mountain System, which includes the Cascade Range, the Insular Mountains, the Olympic Mountains, the Oregon Coast Range, the California Coast Ranges, the Saint Elias Mountains and the Chugach Mountains. The Coast Mountains are also part of the American Cordillera—a Spanish term for an extensive chain of mountain ranges—that consists of an almost continuous sequence of mountain ranges that form the western backbone of North America, Central America, South America and Antarctica.

Geology of the Appalachians

The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor – strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians and neighboring Little Atlas near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

Laramide orogeny A period of mountain building in western North America, which started in the Late Cretaceous

The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.

Geology of the United States national geology

The richly textured landscape of the United States is a product of the dueling forces of plate tectonics, weathering and erosion. Over the 4.5 billion-year history of our Earth, tectonic upheavals and colliding plates have raised great mountain ranges while the forces of erosion and weathering worked to tear them down. Even after many millions of years, records of Earth's great upheavals remain imprinted as textural variations and surface patterns that define distinctive landscapes or provinces.

Geology of the Grand Teton area

The geology of the Grand Teton area consists of some of the oldest rocks and one of the youngest mountain ranges in North America. The Teton Range, partly located in Grand Teton National Park, started to grow some 9 million years ago. An older feature, Jackson Hole, is a basin that sits aside the range.

Basement (geology) Metamorphic or igneous rocks below a sedimentary platform or cover

In geology, basement and crystalline basement are the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In the same way, the sediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".

This is a list of articles related to plate tectonics and tectonic plates.

Geology of North America Overview of the geology of North America

The geology of North America is a subject of regional geology and covers the North American continent, the third-largest in the world. Geologic units and processes are investigated on a large scale to reach a synthesized picture of the geological development of the continent.

Geological history of the Chiricahua Mountains

The Geologic history of the Chiricahua Mountains concerns the Chiricahua Mountains, an inactive volcanic range located in Coronado National Forest of southeastern Arizona, in the United States. They are part of an "archipelago" of mountain ranges known as the sky islands that connect the Sierra Madre Occidental in Mexico with the Rocky Mountains. The Chiricahua Mountains are home to a number of unusual geologic features associated with the Turkey Creek Caldera, some of which are protected by Chiricahua National Monument. The landscape has been dominantly shaped by faulting due to Basin and Range extension during the Miocene, volcanic activity, and erosion.

Geology of Iran

The main points that are discussed in the geology of Iran include the study of the geological and structural units or zones; stratigraphy; magmatism and igneous rocks; ophiolite series and ultramafic rocks; and orogenic events in Iran.

Lhasa terrane A fragment of crustal material, sutured to the Eurasian Plate during the Cretaceous that forms present-day southern Tibet

The Lhasa terrane is a terrane, or fragment of crustal material, sutured to the Eurasian Plate during the Cretaceous that forms present-day southern Tibet. It takes its name from the city of Lhasa in the Tibet Autonomous Region, China. The northern part may have originated in the East African Orogeny, while the southern part appears to have once been part of Australia. The two parts joined, were later attached to Asia, and then were impacted by the collision of the Indian Plate that formed the Himalayas.

Tectonic evolution of Patagonia

Patagonia comprises the southernmost region of South America, portions of which lie either side of the Chile–Argentina border. It has traditionally been described as the region south of the Rio Colorado, although the physiographic border has more recently been moved southward to the Huincul fault. The region's geologic border to the north is composed of the Rio de la Plata craton and several accreted terranes comprising the La Pampa province. The underlying basement rocks of the Patagonian region can be subdivided into two large massifs: the North Patagonian Massif and the Deseado Massif. These massifs are surrounded by sedimentary basins formed in the Mesozoic that underwent subsequent deformation during the Andean orogeny. Patagonia is known for their vast earthquakes and the damage.

Geology of Sudan

The geology of Sudan formed primarily in the Precambrian, as igneous and metamorphic crystalline basement rock. Ancient terranes and inliers were intruded with granites, granitoids as well as volcanic rocks. Units of all types were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny. Dramatic sheet flow erosion prevented almost any sedimentary rocks from forming during the Paleozoic and Mesozoic. From the Mesozoic into the Cenozoic the formation of the Red Sea depression and complex faulting led to massive sediment deposition in some locations and regional volcanism. Sudan has petroleum, chromite, salt, gold, limestone and other natural resources.

The geology of Arizona began to form in the Precambrian. Igneous and metamorphic crystalline basement rock may have been much older, but was overwritten during the Yavapai and Mazatzal orogenies in the Proterozoic. The Grenville orogeny to the east caused Arizona to fill with sediments, shedding into a shallow sea. Limestone formed in the sea was metamorphosed by mafic intrusions. The Great Unconformity is a famous gap in the stratigraphic record, as Arizona experienced 900 million years of terrestrial conditions, except in isolated basins. The region oscillated between terrestrial and shallow ocean conditions during the Paleozoic as multi-cellular life became common and three major orogenies to the east shed sediments before North America became part of the supercontinent Pangaea. The breakup of Pangaea was accompanied by the subduction of the Farallon Plate, which drove volcanism during the Nevadan orogeny and the Sevier orogeny in the Mesozoic, which covered much of Arizona in volcanic debris and sediments. The Mid-Tertiary ignimbrite flare-up created smaller mountain ranges with extensive ash and lava in the Cenozoic, followed by the sinking of the Farallon slab in the mantle throughout the past 14 million years, which has created the Basin and Range Province. Arizona has extensive mineralization in veins, due to hydrothermal fluids and is notable for copper-gold porphyry, lead, zinc, rare minerals formed from copper enrichment and evaporites among other resources.

The geology of New Mexico includes bedrock exposures of four physiographic provinces, with ages ranging from almost 1800 million years (Ma) to nearly the present day. Here the Great Plains, southern Rocky Mountains, Colorado Plateau, and Basin and Range Provinces meet, giving the state great geologically diversity.

The geology of Colorado was assembled from island arcs accreted onto the edge of the ancient Wyoming Craton. The Sonoma orogeny uplifted the ancestral Rocky Mountains in parallel with the diversification of multicellular life. Shallow seas covered the regions, followed by the uplift current Rocky Mountains and intense volcanic activity. Colorado has thick sedimentary sequences with oil, gas and coal deposits, as well as base metals and other minerals.

The geology of Nevada began to form in the Proterozoic at the western margin of North America. Terranes accreted to the continent as a marine environment dominated the area through the Paleozoic and Mesozoic periods. Intense volcanism, the horst and graben landscape of the Basin and Range Province originating from the Farallon Plate, and both glaciers and valley lakes have played important roles in the region throughout the past 66 million years.

The geology of Yukon includes sections of ancient Precambrian Proterozoic rock from the western edge of the proto-North American continent Laurentia, with several different island arc terranes added through the Paleozoic, Mesozoic and Cenozoic, driving volcanism, pluton formation and sedimentation.

References

  1. 1 2 3 4 5 English, Joseph M.; Johnston, Stephen T. (2004). "The Laramide Orogeny: What Were the Driving Forces?" (PDF). International Geology Review . 46 (9): 833–838. Bibcode:2004IGRv...46..833E. doi:10.2747/0020-6814.46.9.833. S2CID   129901811.
  2. Gadd, Ben (1995). Handbook of the Canadian Rockies. Corax Press. pp. 76–93. ISBN   9780969263111.
  3. 1 2 Kirk R. Johnson; Robert G. Raynolds (2006). Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range. Fulcrum Publishing for Denver Museum of Nature and Science. ISBN   1-55591-554-X.
  4. Chronic, Halka (1980). Roadside Geology of Colorado. ISBN   978-0-87842-105-3.
  5. Harris, Ann G.; Tuttle, Esther; Tuttle, Sherwood D (1997). Geology of National Parks (Fifth ed.). Iowa: Kendall/Hunt Publishing. pp. 566–567. ISBN   978-0-7872-5353-0.
  6. 1 2 Blakely, Ron. "Geologic History of Western US".
  7. 1 2 3 4 5 PD-icon.svg This article incorporates public domain material  from the  United States Geological Survey document:  "Geologic Provinces of the United States: Rocky Mountains". Archived from the original on 2006-09-22. Retrieved 2006-12-10.
  8. Jones, DL (1990). "Synopsis of late Palaeozoic and Mesozoic terrane accretion within the Cordillera of western North America". Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 331 (1620): 479–486. Bibcode:1990RSPTA.331..479J. doi:10.1098/rsta.1990.0084. S2CID   120813880.
  9. 1 2 3 Gadd, Ben (2008). Canadian Rockies Geology Road Tours. Corax Press. ISBN   9780969263128.
  10. Livaccari, RF; Burke, K; Sengor, AMC (1981). "Was the Laramide orogeny related to subduction of an oceanic plateau?". Nature. 289 (5795): 276–278. Bibcode:1981Natur.289..276L. doi:10.1038/289276a0. S2CID   27153755.
  11. 1 2 3 4 PD-icon.svg This article incorporates public domain material  from the  United States Geological Survey document: T.J. Stohlgren. "Rocky Mountains". Archived from the original on 2006-09-27.
  12. Pierce, K. L. (1979). History and dynamics of glaciation in the northern Yellowstone National Park area. Washington, D.C: U.S. Geological Survey. pp. 1–90. Professional Paper 729-F.
  13. Gadd, Ben (2008). "Geology of the Rocky Mountains and Columbias" (PDF). Archived from the original (PDF) on 2012-04-20. Retrieved 2010-01-01.