Chapter 20 Mountain Building

Section 20.1 Crust-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

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

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

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 rising results 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

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

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  continues for 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

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

Section 20.2 Orogeny Objectives: Define: 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

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

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

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

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

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

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

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

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

Section 20.3 Other 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

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

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

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