Chapter 20

1. Chapter 20 Mountain Building

2. Section 20.1Crust-Mantle Relationships • Objectives: • Describe the elevation distribution of earth’s surface • Explain isotasy and how it pertains to Earth’s mountains • Describe how Earth’s crust responds to the addition and removal of mass • Define: • Topography • Isotasy • Root • Isostatic rebound

3. I. Earth’s Topography • 71% of Earth’s surface is below sea level • Topography – variation in elevations of the crust • Pattern: most of Earth’s elevations cluster around 2 main ranges of elevation • Above sea level – elevation avg = 0-1km • Below sea level – elevation range = -4 - -5km • Reflects basic differences in density and thickness b/w continental and oceanic crust

4. A. Continental Crust • Oceanic crust slightly higher density than continental crust • Causes it to displace more of mantle than same thickness of continental crust • Differences in elevation not caused by density alone • Continental crust (thicker & less dense) extends deeper into mantle b/c of thickness & rises higher above Earth’s surface b/c of lower density

5. II. Isostasy • Isostasy – displacement of mantle by Earth’s continental and oceanic crust (condition of equilibrium) • Crust and mantle in equilibrium when downward force of gravity on crust is balanced by upward force of buoyancy result from displacement of mantle by crust • Sinking and risingresults from addition and removal of mass w/in crust (people on and off a small boat) • Roots – thickened areas of continental material extending into mantle below mountain ranges

6. A. Mountain Roots • Mountain range requires large roots • Counters enormous mass above surface • Parts of crust rise or subside until parts are buoyantly supported by roots • Continents and mountains float on mantle b/c they are less dense than underlying mantle • Project into mantle to provide necessary buoyant support • If erosion continues, mountain will eventually disappear, exposing roots

7. III. Isostasy and Erosion • Rates of erosion on land are such that mountains should have been completely eroded millions of years ago • As mountains rose above Earth’s surface, deep roots formed until isostatic equilibrium was achieved and mountains buoyantly supported • As peaks eroded, mass decreased  allowed roots to rise and erode • Balance b/w erosion and decrease in size of root  continuesfor hundreds of millions of years until mountains disappear and roots are exposed at surface • Isostatic rebound – process of crust’s rising as result of removal of overlying material • Erosion and rebound allows metamorphic rocks formed at great depths to rise to top of mountain ranges

8. A. Seamounts • Hot spots create mountains underwater = seamounts • Form very quickly • Add mass to oceanic crust crust around peaks displaces underlying mantle until equilibrium is achieved (as result of isostasy) • Elevation of earth’s crust depends on thickness of crust and density • Mountain peak is countered by root • Roots can be many times deeper than height • Himalayas – 9km above & 70km roots – combined = 868 football fields lined up

9. Section 20.2Orogeny • Objectives: • Identify orogenic processes • Compare and contrast the different types of mountains that form along convergent plate boundaries • Explain how the Appalachian Mountains formed • Define: • Orogeny • Compressive force

10. I. Mountain Building at Convergent Boundaries • Orogeny – all processes that form mountain ranges • Metamorphism – rocks squeezed and folded • Igneous intrusions – rising magma • Movement along faults • Results in broad, linear regions of deformation = mountain ranges  orogenic belts • Compressive forces – squeeze crust & cause intense deformation • Folding, faulting, metamorphism, igneous intrusions • Tallest & most varied orogenic belts form at convergent boundaries • Interactions at each type of convergent boundary create different types of mountain ranges

11. A. Oceanic-oceanic convergence • 1 plate subducts into mantle = subduction zone  melt, rise = volcanic island arc • Jumbled mixture of rock types • Basaltic and andesitic • Some contain sedimentary • b/w island arc & trench = basin • Basin fills w/ sediments eroded from island arc • Subduction for 10s of millions of years  sediments uplifted, folded, faulted, thrusted against existing island arc • Forms complex of new masses of sedimentary and volcanic rocks • Japan

12. B. Oceanic-continental convergence • Subduction zones and trenches • Mountain belts = much bigger and more complicated than island arc • descending oceanic plate forces edge of continental plate upward (beginning of orogeny) • Compressive forces cause continental crust to fold and thicken = higher mountains • Deep roots develop to support rock • Volcanic mountains form over subducting plate • Sediments eroded from volcanic mountains fill low areas b/w trench and coast • Sediments + ocean sediments + material scraped off descending plate = shoved against edge of continent to form jumble of highly folded, faulted, metamorphosed rocks • United Kingdom

13. C. Continental-continental convergence • Tallest mountain ranges • Relatively low density  cannot be subducted  becomes highly folded, faulted, thickened • Compressional forces break crust into thick slabs thrust onto each other along low-angle faults • Can double thickness of defomed crust • Deformation can extend laterally for 100s of km into continents • Southern Tibet – original edge of Asia has been pushed approx. 2000km eastward since collision of Indian and Eurasian plates • Magma below convergence  solidifies beneath surface = granite batholiths

14. 1. Marine sedimentary rock • Located near mountains’ summits • Forms from sediments deposited in ocean basin that existed b/w continents before their collision • Mount Godwin Austen – 1000s m marine limestone that sits upon granite base • Limestone = northern portions of old continental margin of India pushed up and over rest of continent when India began to collide w/ Asia 50 mya

15. The Appalachian Mountains – A case study • Wegener – matching rocks and geologic structures in Appalachian & mountains in Greenland/N. Europe • Divided into several distinct regions • Each regions characterized by rocks that show different degrees of deformation • Valley & Ridge Province = highly folded sedimentary rocks • Piedmont Province = older, deformed metamorphic and igneous rocks overlain by relatively undeformed sedimentary layers

17. A. The Early Appalachians • Tectonic history = 800-700 mya – N. america separated from africa • Ancestral atlantic ocean located off west coast of ancestral africa • Shallow, marginal sea formed along eastern coast of ancestral north america • Continental fragment located b/w 2 divergent boundaries • 700-600mya directions of plate motions reversed • Ancestral atlantic ocean began to close as plates converged • Resulted information of a volcanic island arc east of ancestral n america • 200my passed before continental fragment became attached to ancestral n. america • Highly metamorphosed rocks – thrust over younger rocks to become Blue Ridge Province

19. B. The Final Stages of Formation • 400-300mya – island arc became attached to n. america • Evidence preserved in Piedmont Province as group of metamorphic and igneous rocks • Faulted over continent  pushing Blue Ridge rocks farther west • 300-200mya – ancestral Atlantic ocean closed as ancestral africa, europe, and south america collided w/ ancestral n. america to form pangaea • Collision resulted in extensive folding and faulting to form Valley and ridge Province • Rifting caused pangaea to break apart ~200mya  modern atlantic ocean formed & continents moved to present positions

20. Section 20.3Other Types of Mountain Building • Objectives: • Identify the processes associated with non-boundary mountains • Describe the mountain ranges that form along ocean ridges • Compare and contrast uplifted and fault-block mountains • Define: • Uplifted mountain • Plateau • Fault-block mountain

21. I. Divergent-Boundary Mountains • Ocean ridges – underwater volcanic mountains = continuous chain that snakes along Earths ocean floor (>65,000km) • Longer than continental mountain ranges • Regions of broad uplift that form when new oceanic crust is created by seafloor spreading • Newly formed crust and underlying mantle at ocean ridge = hot • When rocks are hot  expand  decr. Density  ridge bulges upward • Newly formed crust and mantle cool and contract  surface of crust subsides • Results: crust stands highest where ocean crust is youngest underwater mountain chains have gently sloping sides

23. II. Uplifted Mountains • Form when large regions of Earth have been slowly forced upward as a unit = uplifted mountains • Adirondack Mountains (NY) • Rocks undergo less deformation than plate-boundary orogeny • Cause of uplift is not well understood • Hypothesis: part of lithosphere made of mantle rocks becomes cold and dnse enough that it sinks into underlying mantle  mantle lithosphere replaced by hotter and less dense mantle  lower density of new mantle provides buoyancy which vertically lifts overlying crust • When whole region is uplifted = plateau – relatively flat-topped area • Erosion eventually carves relatively undeformed, uplifted masses to form peaks, valleys and canyons

25. III. Fault-Block Mountains • Movement at faults lifts land on one side of a fault and drops it on the other • Fault-block mountains – form b/w large faults when pieces of crust are tilted, uplifted, or dropped downward • Basin and Ridge Province of SW US and N. Mexico = 100s of nearly parallel mountains separated by normal faults • Grand Tetons WY