Lecture Outlines Physical Geology, 10/e Plummer, McGeary & Carlson

Steve Kadel, Glendale Community College Mountain Belts and the Continental Crust Physical Geology 10/e, Chapter 20 Steve Kadel, Glendale Community College

Mountain Belts and Earth’s Systems Mountain belts are chains of mountain ranges that are 1000s of km long Commonly located at or near the edges of continental landmasses Composed of multiple mountain ranges Mountain belts are part of the geosphere Form and grow, by tectonic and volcanic processes, over tens of millions of years As mountains grow higher, erosion by running water and ice (hydrosphere) occur at higher rates Air (atmosphere) rising over mountain ranges directly results in precipitation and erosion

Characteristics of Mountain Belts Mountain belts are very long compared to their width The North American Cordillera runs from southwestern Alaska down to Panama Older mountain ranges (Appalachians) tend to be lower than younger ones (Himalayas) due to erosion over time Young mountain belts are tens of millions of years old, whereas older ones may be hundreds of millions of years old Even older mountain belts (billions of years) have eroded nearly flat and form the ancient stable cores (cratons) of the continents Shields - areas of cratons laid bare by erosion

Rock Patterns in Mountain Belts Mountain belts typically contain thick sequences of folded and faulted sedimentary rocks, often of marine origin May also contain great thicknesses of volcanic rock Fold and thrust belts (composed of many folds and reverse faults) are common, indicating large amounts of crustal shortening (and thickening) has taken place under compressional forces Mountain belts are common at convergent boundaries May contain large amounts of metamorphic rock Erosion-resistant batholiths may be left behind as mountain ranges after long periods of erosion

Rock Patterns in Mountain Belts Erosion-resistant batholiths may be left behind as mountain ranges after long periods of erosion Localized tension in uplifting mountain belts can result in normal faulting Horsts and grabens can produce mountains and valleys, respectively Earthquakes common along faults in mountain ranges

Evolution of Mountain Belts Rocks (sedimentary and volcanic) that will later be uplifted into mountains are deposited during accumulation stage Typically occurs in marine environment, such as an opening ocean basin or convergent plate boundary Mountains are uplifted at convergent boundaries during the orogenic stage May be the result of ocean-continent, arc-continent, or continent-continent convergence Subsequent gravitational collapse and spreading may allow deep-seated rocks to rise to the surface

Evolution of Mountain Belts After convergence stops, a long period of erosion, uplift and block-faulting occurs As erosion removes overlying rock, the crustal root of a mountain range rises by isostatic adjustment Tension in uplifting and spreading crust results in normal faulting and production of fault-block mountain ranges

Evolution of Mountain Belts Basin-and-Range province of western North America may be the result of delamination Overthickened mantle lithosphere beneath old orogenic mountain belt may break off and sink (founder) into asthenosphere Resulting inflow of hot asthenosphere can stretch and thin overlying crust, producing normal faults under tension

Growth of Continents Continents grow larger as mountain belts evolve along their margins Accumulation and igneous activity (e.g., when volcanic arcs plaster against continents during convergence) add new continental crust beyond old coastlines New accreted terranes can be added with each episode of convergence Western North America (especially Alaska) contains many such terranes Numerous terranes, of gradually decreasing age, surround older cratons that form the cores of the continents

End of Chapter 20