Geology of the Rocky Mountains
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.
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
Precambrian
The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock is
During the Paleoproterozoic, island-arc terrane associated with the Colorado orogeny accreted to the Wyoming Craton along the Cheyenne belt, a 500-km-wide belt of Proterozoic rocks named for Cheyenne, 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 the Laramie Mountains.[3] The Colorado orogeny was likely part of the larger Yavapai orogeny, which extended across North America and probably to other continents that were joined to North America as part of the supercontinent, Columbia.[4]
In the Paleoproterozoic, terranes also accumulated on the west side of the Wyoming Craton, forming the Selway terrane in Idaho.[5]
Mesoproterozoic (~1.4 Ga) anorthosite and syenites of the Laramie Anorthosite Complex and granite intrude into rocks of the Colorado orogen in the Laramie and adjacent Medicine 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 the Rodinia supercontinent produced rifts between 900 million and 600 million years ago in the Neoproterozoic. These deep extensional basement faults filled with sediments, such as the Uinta rift basin and were reactivated more recently in Earth history by orogenies. The Uinta Formation and Uncompahgre 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 the Great Unconformity, from 1.1 billion to 510 million years ago. Twelve to 24 kilometers of basement rock eroded away.[6]
Ancestral Rocky Mountains
During the Paleozoic, western North America lay underneath a shallow sea, which deposited many kilometers of limestone and dolomite.[7]
In the southern Rocky Mountains, near present-day
Mesozoic deposition in the Rockies occurred in a mix of marine, transitional, and
Mesozoic terranes and subduction
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.[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 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.[12]
For 270 million years, the effects of plate collisions were focused very near the edge of the
Raising the Rockies
The current Rocky Mountains were raised in the
Farther south, the growth of the Rocky Mountains in the United States is a geological puzzle.
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.[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. Tremendous thrusts 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 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.[14]: 80–81
Multiple periods of glaciation occurred during the
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
See also
References
- ^ S2CID 129901811.
- doi:10.1139/e03-054.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - ^ a b c Sims, PK; Finn, CA; Rystrom, VL (2001). "Preliminary Precambrian Basement Map Showing Geologic-Geophysical Domains, Wyoming". Open-File Report 01-199. United States Geological Survey.
- .
- doi:10.1139/e06-052.
- ^ Willis, Grant (2021-09-13). "Utah's Great Unconformity". Survey Notes. Utah Geological Survey.
- ISBN 9780969263111.
- ^ ISBN 1-55591-554-X.
- ISBN 978-0-87842-105-3.
- ISBN 978-0-7872-5353-0.
- ^ a b Blakey, Ron. "Geologic History of Western US". Archived from the original on 2010-06-22.
- ^ a b c d e This article incorporates public domain material from "Rocky Mountains". Geologic Provinces of the United States. United States Geological Survey. Archived from the original on 2006-09-22. Retrieved 2006-12-10.
- S2CID 120813880.
- ^ ISBN 9780969263128.
- S2CID 27153755.
- ^ a b c d This article incorporates public domain material from Stohlgren, TJ. "Rocky Mountains". Status and Trends in the Nation's Biological Resources. United States Geological Survey. Archived from the original on 2006-09-27.
- ^ 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.
- ^ Gadd, Ben (2008). "Geology of the Rocky Mountains and Columbias" (PDF). Archived from the original (PDF) on 2012-04-02. Retrieved 2010-01-01.
External links
- Southern Rockies Geology - J.S. and S.W. Aber, Emporia State University