mountains, mountain building, & growth of continents

mountains and mountain building mountain belts • are chains of mountains 1,000’s of km’s long • sit at or near edges of continents • form from tectonic or volcanic processes over millions of years--geosphere • erode as they grow higher and steeper--hydrosphere • cause precipitation as air rises above them--atmosphere

mountains and mountain building mountain belts are very long compared to their width North American Cordillera extends from Alaska to Panama

mountains and mountain building mountain belts height is related to age: old mountains (100’s of millions of years) (Appalachians) have lower elevations (due to erosion) than young mountains (a few million years) (Himalayas)

mountains and mountain building ancient mountain belts eroded flat: form stable, interior of continents: craton -- oldest parts are shields -- e.g. Canadian shield

mountains and mountain building mountain belts • thick sequences of folded and faulted rocks --typically marine (formed in ocean) sedimentary rocks • metamorphic rocks locally common

mountains and mountain building mountain belts • fold and thrust belts --crust shortened and thickened (remember: thrust faults indicate shortening) • common at convergent boundaries (compression)

green arrow shows Lewis Thrust, Glacier National Park mountains and mountain building mountain belts fold-thrust mountains Himalayas, Alps, Urals, northern and Canadian Rockies green arrow shows Lewis Thrust, Glacier National Park

mountains and mountain building evolution of mountain belts accumulation stage rocks (sedimentary) that will later be uplifted, faulted, and folded into mountains are deposited in opening ocean (sea floor spreading) orogenic stage mountain building episode from plate convergence (provides compression necessary for thrusting)

Mt. Everest Himalayan foothills Swiss Alps

mountains and mountain building evolution of mountain belts when mountains get very tall (e.g. Himalayas), gravitational collapse and spreading may occur --normal faulting and extension/thinning of crust --uplift of metamorphic rocks from depth as crust thins and spreads

mountains and mountain building evolution of mountain belts after convergence stops, erosion and uplift occur --isostatic adjustment-- to thin continental root

mountains and mountain building evolution of mountain belts uplifting crust spreads and results in tension (extension) that produces normal faulting and creates fault-block mountains (horsts and grabens from normal faulting)

mountains and mountain building evolution of mountain belts topography fault-block mountains Basin and Range, western US

Basin and Range

mountains and mountain building evolution of Basin and Range crustal extension dominates today --accompanied by high heat flow-- older period of thrusting and formation of mountains when continental root developed uplift and erosion yield extension but must explain high heat flow delamination of mantle? mantle lithosphere detaches and sinks into asthenosphere warm asthenosphere fills space and results in stretching of crust

extension at surface; upwelling asthenosphere at depth mountains and mountain building modern Basin and Range extension at surface; upwelling asthenosphere at depth

continent along its edge mountains and mountain building continents grow as mountain belts evolve at active continental margins igneous activity adds new crust sedimentary rocks originally deposited in ocean are uplifted, folded, and faulted to form new terranes that are “accreted” or added to continent along its edge accreted terranes in western US

continental growth continents decrease in age toward their margins

(Ouachitas, Marathons) growth of continents: US example geologic map of the United States on-going subduction (Cascadia) Basin and Range (rifting) Canadian shield transform boundary (San Andreas) Paleozoic orogenic belts (Appalachians) craton Paleozoic to Recent active margin Mesozoic to Recent passive margin Paleozoic to Recent orogenic belts Paleozoic orogenic belts (Ouachitas, Marathons) from: http://pubs.usgs.gov/publications/text