Lecture Outlines Physical Geology, 10/e

1. Lecture OutlinesPhysical Geology, 10/e Plummer, McGeary & Carlson

2. Mountain Belts and the Continental CrustPhysical Geology 10/e, Chapter 20 Steve Kadel, Glendale Community College

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. End of Chapter 20