Objectives Describe the elevation distribution of Earth’s surface. Explain isostasy and how it pertains to Earth’s mountains. Describe how Earth’s crust

ObjectivesDescribe 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.

Crust-Mantle RelationshipsSection 20.1

Topography is the variation in elevations of Earth’s crust.

Topographic maps show differences in elevation on Earth’s surface.

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.



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.

Continental crust


Isostasy

The displacement of the mantle by Earth’s continental and oceanic crust is a condition of equilibrium called isostasy.


Isostasy

Gravitational and seismic studies have detected thickened areas of continental material, called roots, that extend into the mantle below Earth’s mountain ranges.


Isostasy

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.

Mountain roots


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.



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.



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.


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.



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.

Seamounts

ObjectivesIdentify orogenic processes.

Compare and contrast the different types of mountains that form along convergent plate boundaries.

Explain how the Appalachian Mountains formed.

OrogenySection 20.2

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.

OrogenySection 20.2


Most of Earth’s mountain ranges formed along plate boundaries.

OrogenySection 20.2

1.Divergent-Boundary Mountains- ocean ridges.

Other Types of Mountain BuildingSection 20.3

Underwater volcanic mountains known as ocean ridges form a continuous chain that snakes along Earth’s ocean floor for over 65,000 km.

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.

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.

OrogenySection 20.2


Convergence between two oceanic plates results in the formation of individual volcanic peaks that make up an island arc complex.

OrogenySection 20.2

a.Oceanic-oceanic convergence


At an oceanic-continental boundary, compression causes continental crust to fold and thicken. Igneous activity and metamorphism are also common along such boundaries.

OrogenySection 20.2

b.Oceanic-continental convergence- volcanic mountain ranges.


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.

OrogenySection 20.2

3.Continental-continental convergence

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.

OrogenySection 20.2

The Appalachian Mountains—A Case Study

OrogenySection 20.2

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.

The early Appalachians (P.572, Fig 20.13)

• 2. 700-600 mya Atlantic ocean began to close , an island arc formed(Piedmont province)

• 3. 500-400 mya continental fragment (Blue Ridge province)attached to N.America.

• 4. 400-300 mya Island arc(Piedmont province) attached to N.America.

5. 300-260 mya Pangea forms, further compression results in the folded Valley and Ridge province.

ObjectivesIdentify the processes associated with non-boundary mountains.

Describe the mountain ranges that form along ocean ridges.

Compare and contrast uplifted and fault-block mountains.



Mountains on the ocean floor and some mountains on continents form through processes other than convergence.

normal fault: a crack in Earth where the rock above the fault plane has dropped down

Review Vocabulary

1.Uplifted MountainsUplifted mountains form when large sections of Earth’s crust are forced upward without much structural deformation.


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.

2.Fault-Block Mountains


Fault-block mountains form between large faults when pieces of crust are tilted, uplifted, or dropped downward.

Documents

Objectives Describe the elevation distribution of Earth’s surface. Explain isostasy and how it pertains to Earth’s mountains. Describe how Earth’s crust