1. Objectives Describe the elevation distribution of Earth’s surface.
Section 20.1
Crust-Mantle Relationships
Objectives
Describe the elevation distribution of Earth’s surface.
Explain isostasy and how it pertains to Earth’s mountains.
Describe how Earth’s crust responds to the addition and removal of mass.

2. Section 20.1
Crust-Mantle Relationships
Earth’s Topography
Topography is the variation in elevations of Earth’s crust.
Topographic maps show differences in elevation on Earth’s surface.

3. Section 20.1
Crust-Mantle Relationships
Earth’s Topography
When Earth’s topography is plotted on a graph, a pattern in the distribution of elevations emerges. Most of Earth’s elevations cluster around two main ranges of elevation—0 to 1 km above sea level and 4 to 5 km below sea level.

4. Earth’s Topography Continental crust
Section 20.1
Crust-Mantle Relationships
Earth’s Topography
Continental crust
Continental crust is thicker and less dense than oceanic crust, so it extends higher above Earth’s surface and deeper into the mantle than oceanic crust.

5. Section 20.1
Crust-Mantle Relationships
Isostasy
The displacement of the mantle by Earth’s continental and oceanic crust is a condition of equilibrium called isostasy.

6. Section 20.1
Crust-Mantle Relationships
Isostasy
Gravitational and seismic studies have detected thickened areas of continental material, called roots, that extend into the mantle below Earth’s mountain ranges.

7. Isostasy Mountain roots
Section 20.1
Crust-Mantle Relationships
Isostasy
Mountain roots
A mountain range requires large roots to counter the enormous mass of the range above Earth’s surface.- like a tree.
Continents and mountains are said to float on the mantle because they are less dense than the underlying mantle. They project into the mantle to provide the necessary buoyant support.

8. Section 20.1
Crust-Mantle Relationships
Isostasy and Erosion
The Appalachian Mountains in the eastern United States formed hundreds of millions of years ago when the North American continent collided with Europe and Africa.

9. Section 20.1
Crust-Mantle Relationships
Isostasy and Erosion
As the Appalachian Mountains rose(was forming) above Earth’s surface, deep roots formed until isostatic equilibrium was achieved and the mountains were buoyantly supported. As peaks eroded, the mass decreased. This allowed the roots themselves to rise and eventually erode.

10. Section 20.1
Crust-Mantle Relationships
Isostasy and Erosion
A balance between erosion and the decrease in the size of the roots will continue for hundreds of millions of years until the mountains disappear and the roots are exposed at the surface.

11. Isostasy and Erosion (like ice floating in water)
Section 20.1
Crust-Mantle Relationships
Isostasy and Erosion (like ice floating in water)
The slow process of the crust’s rising as the result of the removal of overlying material is called isostatic rebound.
Erosion and rebound allows metamorphic rocks formed at great depths to rise to the top of mountain ranges such as the Appalachians.

12. Isostasy and Erosion Seamounts
Section 20.1
Crust-Mantle Relationships
Isostasy and Erosion
Seamounts
Individual volcanic mountains produced by hot spots under the ocean floor are called seamounts. As a result of isostasy, the oceanic crust around these peaks displaces the underlying mantle until equilibrium is achieved.

13. Objectives Identify orogenic processes.
Section 20.2
Orogeny
Objectives
Identify orogenic processes.
Compare and contrast the different types of mountains that form along convergent plate boundaries.
Explain how the Appalachian Mountains formed.

14. Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
Mountain Building at Convergent Boundaries
Orogeny refers to all processes that form mountain ranges.
Broad, linear regions of deformation commonly known as mountain ranges are also known in geology as orogenic belts.

15. Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
Mountain Building at Convergent Boundaries
Most of Earth’s mountain ranges formed along plate boundaries.

16. 1.Divergent-Boundary Mountains- ocean ridges.
Section 20.3
Other Types of Mountain Building
1.Divergent-Boundary Mountains- ocean ridges.
Underwater volcanic mountains known as ocean ridges form a continuous chain that snakes along Earth’s ocean floor for over 65,000 km.

17. Divergent-Boundary Mountains
Section 20.3
Other Types of Mountain Building
Divergent-Boundary Mountains
An ocean ridge is a broad, topographic high that forms as lithosphere bulges upward due to an increase in temperature along a divergent boundary.

18. 2.Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
2.Mountain Building at Convergent Boundaries
At convergent plate boundaries, compressive forces squeeze the crust and cause intense deformation in the form of folding, faulting, metamorphism, and igneous intrusions.
Interactions at each type of convergent boundary create different types of mountain ranges.

19. Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
Mountain Building at Convergent Boundaries
a.Oceanic-oceanic convergence
Convergence between two oceanic plates results in the formation of individual volcanic peaks that make up an island arc complex.

20. Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
Mountain Building at Convergent Boundaries
b.Oceanic-continental convergence- volcanic mountain ranges.
At an oceanic-continental boundary, compression causes continental crust to fold and thicken. Igneous activity and metamorphism are also common along such boundaries.

21. Mountain Building at Convergent Boundaries
Section 20.2
Orogeny
Mountain Building at Convergent Boundaries
3.Continental-continental convergence
Intense folding and faulting along continental-continental boundaries produce folded mountains - highest mountain ranges on Earth.
presence of marine sedimentary rock near the mountains’ summits.

22. The Appalachian Mountains—A Case Study
Section 20.2
Orogeny
The Appalachian Mountains—A Case Study
Geologists have divided the Appalachians into several distinct regions. Each region is characterized by rocks that show different degrees of deformation.

23. The Appalachian Mountains—A Case Study
Section 20.2
Orogeny
The Appalachian Mountains—A Case Study
The early Appalachians (P.572, Fig 20.13)
The tectonic history of the Appalachians began about 800 to 700 mya when ancestral
1. North America separated from ancestral Africa along two divergent boundaries to form two oceans— the ancestral Atlantic Ocean and a shallow, marginal sea. A continental fragment was located between the two divergent boundaries.

24. 2. 700-600 mya Atlantic ocean began to close , an island arc formed(Piedmont province)
mya continental fragment (Blue Ridge province)attached to N.America.
mya Island arc(Piedmont province) attached to N.America.
mya Pangea forms, further compression results in the folded Valley and Ridge province.

25. 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.

26. Section 20.3
Other Types of Mountain Building
Mountains on the ocean floor and some mountains on continents form through processes other than convergence.
Review Vocabulary
normal fault: a crack in Earth where the rock above the fault plane has dropped down

27. Section 20.3
Other Types of Mountain Building
1.Uplifted Mountains
Uplifted mountains form when large sections of Earth’s crust are forced upward without much structural deformation.

28. Section 20.3
Other Types of Mountain Building
Uplifted Mountains
When a whole region is uplifted, a relatively flat-topped area called a plateau can form.
Erosion eventually carves these relatively undeformed, uplifted masses to form peaks, valleys, and canyons.

29. 2.Fault-Block Mountains
Section 20.3
Other Types of Mountain Building
2.Fault-Block Mountains
Fault-block mountains form between large faults when pieces of crust are tilted, uplifted, or dropped downward.