Objectives

Objectives• Describe the elevation distribution of Earth’s surface.

Crust-Mantle Relationships

• Explain isostasy and how it pertains to Earth’s mountains.

• Describe how Earth’s crust responds to the addition and removal of mass.

– isostasy

– isostatic rebound

Vocabulary

Earth’s Topography• The change in elevation, or topography, of the

crust isn’t obvious from most maps and globes.


• These two modes reflect the basic differences in density and thickness between continental and oceanic crust.

Earth’s Topography• Most of Earth’s elevations

cluster around two modes: 0 to 1 km above sea level and 4 to 5 km below sea level.


– The slightly higher density of oceanic crust (basalt) causes it to displace more of the mantle than the same thickness of continental crust (granite) does.

– Continental crust extends deeper into the mantle because of its thickness, and it rises higher above Earth’s surface than oceanic crust because of its lower density.

Earth’s Topography• The different densities of basalt and granite

displace different amounts of the mantle, and these rock types thus float at different heights.


Earth’s Topography


Isostasy• Isostasy is a condition of equilibrium that

describes the displacement of the mantle by Earth’s continental and oceanic crust.


• In a state of isostatic equillibrium, the force of gravity on the mass of crust involved is balanced by the upward force of buoyancy.

• Mountains have thick roots that buoyantly support the overlaying material.

• According to the principle of isostasy, parts of the crust will rise or subside until these parts are buoyantly supported by their roots.

Isostasy

Isostasy and Erosion


– As mountains rise above Earth’s surface, deep roots form until isostatic equilibrium is achieved and the mountains are buoyantly supported.

– As peaks are eroded, mass decreases, and the roots become smaller.

– Isostatic rebound is the slow process of the crust’s rising as the result of the removal of overlying material.

Isostasy



Isostasy



– Seafloor structures, such as seamounts, must also be in isostatic equilibrium with the mantle.

– Elevation of Earth’s crust depends upon the thickness of the crust as well as its density.

– Mountain roots can be many times as deep as a mountain is high.

Section Assessment

1. What is isostatic rebound?


Isostatic rebound is the slow process of the crust’s rising as a result of the removal of overlying material.

Section Assessment

2. What two elevation ranges, or modes, dominate Earth’s topography?


Most of Earth’s elevations cluster around 0 to 1 km above sea level and 4 to 5 km below sea level.

______ Peridotite is less dense than basalt.

______ Mountain roots can extend far deeper than the height of the mountain.

______ Buoyancy and gravity are the basic two forces in isostasy.

______ Oceanic crust, because it is denser, extends deeper into the mantle than continental crust.

Section Assessment

3. Identify whether the following statements are true or false.


false

true

true

false

Objectives• Compare and contrast the different types of mountains

that form along convergent plate boundaries.

• Explain how the Appalachian Mountains formed.

– orogeny

Vocabulary

Convergent-Boundary Mountains

Orogeny• Orogeny is the process cycle that forms all

mountain ranges.


• Orogeny results in broad, linear regions of deformation known as orogenic belts, most of which are associated with plate boundaries.

• Convergent boundaries are the location of the greatest variety and the tallest orogenic belts.

• The compressive forces at these boundaries may cause the folding, faulting, metamorphism, and igneous intrusions that are characteristic of orogenic belts.

Orogeny


Orogeny

Oceanic-Oceanic Convergence


– When an oceanic plate converges with another oceanic plate, one plate descends into the mantle to create a subduction zone.

– As parts of the subducted plate melt, magma is forced upward to form a series of volcanic peaks called an island arc complex.

– Basaltic and andesitic magmas rise to the surface and erupt to form the island arc complex.

– Sediments around the complex can be uplifted, folded, faulted, and thrust against the island arc to form a mass of sedimentary and island-arc volcanic rocks.

Orogeny

Oceanic-Oceanic Convergence


Orogeny

Oceanic-Continental Convergence


– Convergence along Oceanic-continental boundaries creates subduction zones and trenches.

– The edge of the continental plate is forced upward, marking the beginning of orogeny.

– Compressive forces may cause the continental crust to fold and thicken, forming mountains with deep roots.

– As it melts, the subducting oceanic plate produces magma that gives rise to granitic intrusions and volcanoes fueled by andesitic magma.

– Sediments shoved against the continent form a jumble of folded, faulted, and metamorphosed rocks.

Orogeny



Orogeny

Continental-Continental Convergence


– Because of the relatively low density of continental crust, the energy associated with a continental-continental collision is transferred to the crust involved.

– Compressional forces break the crust into thick slabs that are thrust onto each other along low-angle faults, possibly doubling the thickness of the deformed crust.

– The magma that forms as a result of continental-continental mountain building forms granite batholiths.

– Another common characteristic of mountains that form when two continents collide is the presence of marine sedimentary rock near the mountains’ summits.

Orogeny



The Appalachian Mountains–A Case Study• The geology of the Appalachian mountain range,

which is located in the eastern United States, has been the subject of many studies.


• Geologists have divided the Appalachian Mountain Belt into several distinct regions, including the Valley and Ridge, the Blue Ridge, and the Piedmont Provinces.

• Each region is characterized by rocks that show different degrees of deformation.

The Appalachian Mountains–A Case Study

The Early Appalachians


– About 700 to 800 million years ago, ancestral North America separated from ancestral Africa along two divergent boundaries to form two oceans with a continental fragment between them.

– The ancestral Atlantic Ocean was located off the western coast of ancestral Africa and a shallow, marginal sea formed along the eastern coast of ancestral North America.




700–600 Million Years Before Present (M.Y.B.P.) Convergence causes the ancestral Atlantic Ocean to begin to close. An island arc develops east of ancestral North America.




500–400 M.Y.B.P. The continental fragment, which eventually becomes the Blue Ridge Province, becomes attached to ancestral North America.


The Final Stages of Formation


400-300 M.Y.B.P. The island arc becomes attached to ancestral North America and the continental fragment is thrust farther onto ancestral North America. The arc becomes the Piedmont Province.


The Final Stages of Formation


300-260 M.Y.B.P. Ancestral Africa collides with ancestral North America to close the ancestral Atlantic Ocean. Compression forces the Blue Ridge and Piedmont rocks farther west, and the folded Valley and Ridge Province forms.

Section Assessment

1. What is orogeny?


The processes that form all mountain ranges are called orogeny.

Section Assessment


2. What is an orogenic belt? Where are most orogenic belts located?

An orogenic belt is a broad, linear region of deformation associated with mountain building. Most orogenic belts are located along plate boundaries, particularly convergent boundaries.

Section Assessment



______ The Philippine islands are an example of an island arc complex.

______ A subduction zone forms during a continental-continental collision.

______ The Blue Ridge province is the composed of the remnants of a continental fragment.

______ The modern Atlantic Ocean formed about 200 million years ago.

true

false

true

true

– pillow basalt

– uplifted mountain

– fault-block mountain

Objectives• Describe the mountain ranges that form along

ocean ridges.

• Compare and contrast uplifted and fault-block mountains.

• Describe the mountains that form as a result of hot spots in Earth’s mantle.

Vocabulary

Other Types of Mountains

Divergent-Boundary Mountains• Ocean ridges are regions of very broad uplift that

seems to be related to the rising convection cells in the mantle.


• Magma is less dense than surrounding mantle material, and thus it is forced upward, where it warms the overlying lithosphere.

• The lithosphere along a divergent boundary bulges upward to form a gently sloping mountain range.

Divergent-Boundary Mountains



Ocean-Ridge Rocks


– Ocean ridges are composed mainly of igneous rocks.

– As tectonic plates separate along an ocean ridge, hot mantle material is forced upward and accumulates in a magma chamber beneath the ridge.

– From the chamber, the mixture intrudes into the overlying rock to form a series of vertical dikes that resemble a stack of index cards standing on edge.

– Pillow basalts are igneous rocks, resembling a pile of sandbags, that are formed when magma pushes through the dikes and erupts onto the seafloor.



Ocean-Ridge Rocks

Nonboundary Mountains• Some mountains and peaks form in places far

removed from tectonic boundaries.


• Three nonboundary types of mountains are uplifted mountains, fault-block mountains, and some volcanoes.

Nonboundary Mountains

Uplifted Mountains


– Uplifted mountains are mountains that form when large regions of Earth have been slowly forced upward as a unit.


Uplifted Mountains


– The cause of large-scale regional uplift is not well understood. • It is possible that warmer regions of the mantle heat portions of the lithosphere, causing the density of the crust to

decrease, which results in slow uplift.

• Another possible cause is upward movement in the mantle, which lifts regions of the crust without causing much deformation.

– The Adirondack Mountains in New York are an example of uplifted mountains.


Fault-Block Mountains


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

– The Basin and Range Province of the southwestern United States and northern Mexico, as well as the Grand Tetons in Wyoming, are examples of fault-block mountains.


Volcanic Peaks


– Volcanoes that form over hot spots are generally solitary peaks that form far from tectonic plate boundaries.

– The shield volcanoes that make up the state of Hawaii are volcanic peaks that formed as the Pacific Plate moved over a hot spot in the mantle.

Section Assessment

1. Match the following mountain types with an example.

___ divergent-boundary

___ uplifted

___ fault-block

___ volcanic peaks


A. ocean ridges

B. Grand Tetons in Wyoming

C. Mauna Kea in Hawaii

D. Adirondacks in New York

A

D

B

C

Section Assessment

2. What are pillow basalts?


Pillow basalts are igneous rocks, resembling a pile of sandbags, that are formed when magma pushes through the dikes and erupts onto the seafloor.

Section Assessment


3. How do fault-block mountains form?

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

Chapter Resources Menu

Chapter Resources Menu

Study Guide

Section 20.1

Section 20.2

Section 20.3

Chapter Assessment

Image Bank

Section 20.1 Main Ideas• Earth’s elevations cluster around two intervals: 0 to 1 km

above sea level and 4 to 5 km below sea level. These modes reflect the differences in density and thickness of the crust.

• Isostasy is a condition of equilibrium. According to this principle, the mass of a mountain above Earth’s surface is supported by a root that projects into the mantle. The root provides buoyancy for the massive mountain.

• The addition of mass to Earth’s crust depresses the crust; the removal of mass from the crust causes the crust to rebound in a process called isostatic rebound.

Section 20.1 Study Guide

Section 20.2 Main Ideas• Orogeny is the cycle of processes that form mountain belts.

Most mountain belts are associated with plate boundaries.

• Island arc complexes are volcanic mountains that form as a result of the convergence of two oceanic plates.

• Highly deformed mountains with deep roots may form as a result of the convergence of an oceanic plate and a continental plate.

• Earth’s tallest mountains form along continental-continental plate boundaries, where the energy of the collision causes extensive deformation of the rocks involved.

• The Appalachian Mountains, which are located in the eastern United States, formed millions of years ago mainly as the result of convergence between two tectonic plates.


Section 20.3 Main Ideas• At a divergent boundary, newly formed lithosphere

moves away from the central rift, cools, contracts, and becomes more dense to create a broad, gently sloping mountain range called an ocean ridge. Rocks that make up ocean ridges include dikes and pillow basalts.

• Regional uplift can result in the formation of uplifted mountains that are made of nearly horizontal, undeformed layers of rock.

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

• Most solitary volcanic peaks form as a tectonic plate moves over a hot spot in Earth’s mantle.


1. A mountain’s root ____ as mass is removed from the mountain through erosion.

a. expands c. rises

b. sinks d. melts

Multiple Choice

Chapter Assessment

Buoyant force will cause the root of the mountain to rise, maintaining isostatic equilibrium.

2. When did the tectonic history of the Appalachian Mountains begin?

a. 700–600 M.Y.B.P. c. 400–300 M.Y.B.P.

b. 500–400 M.Y.B.P. d. 300–260 M.Y.B.P.The Appalachians are one of the oldest surviving mountain chains on Earth and have been the subject of numerous studies.

Multiple Choice

Chapter Assessment

Multiple Choice

3. Which type of convergence could create a mountain that has marine sedimentary rock near its summit?

a. oceanic-oceanic c. continental-continental

b. oceanic-continental d. none of the above

Chapter Assessment

Continental-continental convergence thrusts thick slabs onto each other to form mountains. Some of the crust that is thrust upward may be made of marine sedimentary rock, such as the case of K2 in the western Himalayas.

Multiple Choice

4. What is the average elevation of exposed land in relation to sea level?

a. 364 m c. 841 m

b. 562 m d. 1257 m

Chapter Assessment

Two elevations dominate Earth’s surface: 0 to 1 km above sea level and 4 to 5 km below sea level. The average elevation above sea level is 841 m and the average depth of Earth’s oceans if 3865 m.

Multiple Choice

5. The processes that form all mountain ranges are called ____.

a. convergence c. uplift

b. divergence d. orogeny

Chapter Assessment

Convergence, divergence, and uplift are all types of orogeny.

Short Answer

6. How large is a mountain’s root in relation to the mountain?

Chapter Assessment

A mountain’s root can be many times larger than the mountain itself. It is estimated that the root under Mount Everest extends over 80 km into the mantle.

Short Answer

7. Isostasy involves the equalization of what two forces?

Chapter Assessment

The two forces at work in isostasy are buoyancy and gravity.

True or False


______ As a mountain erodes, it rises.

______ Warmer regions of the mantle may be responsible for uplifted mountains.

______ The Hawaiian islands formed along a divergent boundary.

______ The piedmont is the remnants of an ancient island arc.

______ Continental crust extends deeper into the mantle than oceanic crust.

Chapter Assessment

true

false

false

true

true

Image Bank

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Chapter 20 Images

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Chapter 20 Images

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