1. Chapter 20
Mountain Building

2. 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

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 = km
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 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

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  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

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 20.2 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

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 = 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
mya 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
mya – 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
mya – 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 20.3 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

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