Thegeology of the Rocky Mountains is that of a discontinuous series ofmountain ranges with distinct geological origins. Collectively these make up theRocky Mountains, a mountain system that stretches from Northern British Columbia through centralNew Mexico and which is part of the great mountain system known as theNorth American Cordillera.
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 ofplate tectonic activity that resulted in much of the rugged landscape of westernNorth America. TheLaramide 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 produced the current form of the mountains.
The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock isPrecambrianWyoming craton that forms the core of the North American continent. The Wyoming Craton originated as a 100,000 km2 middleArchean craton that was modified by late Archean volcanic magmatism and plate movements andProterozoic extension andrifting.[2] The Wyoming Craton mainly consists of two rock units: granitoidplutons (2.8–2.55 Ga) andgneiss andmigmatite. The granitoid rocks are mainly potassic granite and were derived principally from reworked older (3.1–2.8 Ga) gneiss.[3]
During thePaleoproterozoic, island-arc terrane associated with theColorado orogeny accreted to the Wyoming Craton along theCheyenne belt, a 500-km-wide belt of Proterozoic rocks named forCheyenne, Wyoming. As a result of the collision, older, Archean rocks of the Wyoming craton were intensely deformed and metamorphosed for at least 75 km inboard from the suture, which is marked today by theLaramie Mountains.[3] The Colorado orogeny was likely part of the largerYavapai orogeny, which extended across North America and probably to other continents that were joined to North America as part of thesupercontinent,Columbia.[4]
In the Paleoproterozoic, terranes also accumulated on the west side of the Wyoming Craton, forming theSelway terrane in Idaho.[5]
Mesoproterozoic (~1.4 Ga)anorthosite andsyenites of the Laramie Anorthosite Complex and granite intrude into rocks of the Colorado orogen in the Laramie and adjacentMedicine Bow Mountains. Both the anorthosite and granite transect the Cheyenne belt in the Laramide Mountains, and intrude crystalline rocks of the Wyoming province. These intrusions comprise the northernmost segment of a wide belt of 1.4 Ga granitic intrusions that occur throughout the Colorado orogen.[3]
The breakup of theRodinia supercontinent produced rifts between 900 million and 600 million years ago in theNeoproterozoic. These deep extensional basement faults filled with sediments, such as the Uinta rift basin and were reactivated more recently in Earth history by orogenies. TheUinta Formation andUncompahgre Formation are both examples of remnant Precambrian rift basin sediments.[citation needed] The end of the Neoproterozoic is not known from the rock record, indicating a period of long-running terrestrial erosion which produced by theGreat Unconformity, from 1.1 billion to 510 million years ago. Twelve to 24 kilometers of basement rock eroded away.[6]
During thePaleozoic, western North America lay underneath a shallow sea, which deposited many kilometers oflimestone anddolomite.[7]
In the southern Rocky Mountains, near present-dayColorado andNew Mexico, the Precambrian and Paleozoic rocks were disturbed by mountain building approximately 300 Ma, during thePennsylvanian. This mountain building produced theAncestral Rocky Mountains.[8]: 1 The uplift formed two large mountainous islands, known togeologists asFrontrangia andUncompahgria, located roughly in the current locations of theFront Range and theSan Juan Mountains. They consisted largely ofPrecambrianmetamorphic rock, forced upward through layers of thelimestone laid down in the shallow sea.[9] The mountains eroded throughout the late Paleozoic and earlyMesozoic, leaving extensive deposits ofsedimentary rock.[8]: 6
Mesozoic deposition in the Rockies occurred in a mix of marine, transitional, andcontinentalenvironments as local relative sea levels changed. By the close of theMesozoic, 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognizedformations. The most extensive non-marine formations were deposited in theCretaceous period when the western part of theWestern Interior Seaway covered the region.[10]
Terranes started to collide with the western edge of North America in theMississippian age (approximately 350 million years ago), causing theAntler orogeny.[11] During the last half of theMesozoic Era, much of today'sCalifornia,British Columbia,Oregon, andWashington were added toNorth America. Western North America suffered the effects of repeated collision as theKula andFarallon 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.[12]
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 crustobducted onto thecontinental margin while others represent small isolated mid-oceanic islands.[13]
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 theSierran Arc, grew aslava andash spewed out of dozens of individualvolcanoes. Beneath the surface, great masses ofmolten rock were injected and hardened in place.[12]
For 270 million years, the effects of plate collisions were focused very near the edge of theNorth American Plate boundary, far to the west of the Rocky Mountain region.[11] It was not until 80MA that these effects began to reach the Rockies.[1]
The current Rocky Mountains were raised in theLaramide 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:[14]: 78 the rug bunches up and forms wrinkles (mountains). In Canada, the subduction of theKula plate and the terranes smashing into the continent are the feet pushing the rug, the ancestral rocks are the rug, and theCanadian Shield in the middle of the continent is the hardwood floor.[14]: 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 and 400 miles (300 and 600 km) inland from asubduction 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 theFarallon plate,[12] or possibly due to the subduction of anoceanic plateau.[1][15]
At a typical subduction zone, an oceanic plate typically sinks at a fairly steep angle, and avolcanic arc grows above the subducting plate. During the growth of the Rocky Mountains, the angle of the subducting plate may have beensignificantly flattened, moving the focus of melting and mountain building much farther inland than is normally expected.[12] 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. Tremendousthrusts piled sheets of crust on top of each other, building the extraordinarily broad, high Rocky Mountain range.[12]
The current southern Rockies were forced upwards through the layers ofPennsylvanian andPermian 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 theDakota Hogback, an early Cretaceous sandstone formation that runs along the eastern flank of the modern Rockies.
An alternative theory emerged in 2023 for how the Rockies formed. Starting around 90 million years ago, what is now present-day northern California northward to British Columbia experienced a collision with aterrane in the Pacific Ocean, possibly theInsular Superterrane.[16] Today, geological remnants of this collision are represented by theInsular Belt from Washington state northward to Alaska. A secondary collision possibly occurred in what is present-day southern California around 70 million years ago. As a result of these one or more collisions, geological processes may have kicked off theLaramide Orogeny. The merits of this theory are still being reviewed and debated in the scientific community.[17]
Immediately after the Laramide orogeny, the Rockies were likeTibet: 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.[14]: 80–81
Multiple periods of glaciation occurred during thePleistocene Epoch (1.8 million–12,000 years ago), finally receding in theHolocene Epoch (fewer than 11,000 years ago). Theice ages left their mark on the Rockies, forming extensiveglacial landforms, such as U-shaped valleys andcirques. Recent glacial episodes included theBull Lake Glaciation that began about 150,000 years ago and thePinedale Glaciation that probably remained at full glaciation until 15,000–20,000 years ago.[18][19] Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation.[18] Thelittle ice age was a period of glacial advance that lasted a few centuries from about 1550 to 1860. For example, theAgassiz andJackson Glaciers inGlacier National Park reached their most forward positions about 1860 during theLittle Ice Age.[18]
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 extensivegranite andgneiss dating back to the Ancestral Rockies. In the central Canadian Rockies, the main ranges are composed of the Precambrianmudstones, while the front ranges are composed of thePaleozoic limestones and dolomites.[20] Volcanic rock from theCenozoic (66 million–1.8 million years ago) occurs in theSan Juan Mountains and in other areas. Millennia of severe erosion in theWyoming Basin transformed intermountain basins into a relatively flat terrain. TheTetons and other north-central ranges contain folded and faulted rocks ofPaleozoic andMesozoic age draped above cores ofProterozoic andArchean igneous and metamorphic rocks ranging in age from 1.2 billion (e.g., Tetons) to more than 3.3 billion years (Beartooth Mountains).[18]
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This article incorporatespublic domain material from"Rocky Mountains".Geologic Provinces of the United States.United States Geological Survey. Archived fromthe original on 2006-09-22. Retrieved2006-12-10. This article incorporatespublic domain material fromStohlgren, TJ."Rocky Mountains".Status and Trends in the Nation's Biological Resources.United States Geological Survey. Archived fromthe original on 2006-09-27.