Chapter 20

Mountain Building

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

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 20.2Orogeny

• 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

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

II. 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 20.3Other 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

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