Geology of the Rocky Mountains

The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock is Precambrian Wyoming craton that forms the core of the North American continent. The Wyoming Craton originated as a 100,000 km2 middle Archean craton that was modified by late Archean volcanic magmatism and plate movements and Proterozoic extension and rifting. The Wyoming Craton mainly consists of two rock units: granitoid plutons (2.8–2.55 Ga) and gneiss and migmatite. The granitoid rocks are mainly potassic granite and were derived principally from reworked older (3.1–2.8 Ga) gneiss. 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. In the Paleoproterozoic, terranes also accumulated on the west side of the Wyoming Craton, forming the Selway terrane in Idaho. 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. ==Ancestral Rocky Mountains==

During the Paleozoic, western North America lay underneath a shallow sea, which deposited many kilometers of limestone and dolomite. In the southern Rocky Mountains, near present-day Colorado and New Mexico, the Precambrian and Paleozoic rocks were disturbed by mountain building approximately 300 Ma, during the Pennsylvanian. This mountain building produced the Ancestral Rocky Mountains. The uplift formed 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. The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock. == Mesozoic 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. 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. 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. Magma generated above the subducting slab rose into the North American continental crust about 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. 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. It was not until 80 MA that these effects began to reach the Rockies. ==Raising the Rockies==

The current Rocky Mountains were raised in the Laramide orogeny from between 80 and 55 Ma. 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. At a typical subduction zone, an oceanic plate 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. 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. 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. ==Alternative theories for raising the 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 a terrane in the Pacific Ocean, possibly the Insular Superterrane. Today, geological remnants of this collision are represented by the Insular 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 the Laramide Orogeny. The merits of this theory are still being reviewed and debated in the scientific community. ==Current landscape==

Immediately after the Laramide orogeny, the Rockies were like Tibet: a high plateau, probably 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. Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation. 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. 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). ==See also==