EARTH SCIENCE geology, the environment, and the universe 2008 (21)

Mountain Building BIG Idea Mountains form through dynamic processes which crumple, fold, and cre-ate faults in Earth’s crust.. These two ranges dominate Earth’s topography and reflect

Mountain Building

BIG Idea Mountains form

through dynamic processes

which crumple, fold, and

cre-ate faults in Earth’s crust.

20.1 Crust-Mantle

Relationships

MAIN Idea The height of

mountains is controlled primarily

by the density and thickness

of the crust.

20.2 Orogeny

MAIN Idea Convergence

causes the crust to thicken and

form mountain belts.

20.3 Other Types of

Mountain Building

MAIN Idea Mountains on the

ocean floor and some mountains

on continents form through

pro-cesses other than convergence.

GeoFacts

• The layers of a mountain record

the vast geologic history of the

region

• Fossils of marine organisms

have been found at the top of

Mount Everest.

• The Himalayas are geologically

young mountains — they are

Section 1 • XXXXXXXXXXXXXXXXXX 561

Start-Up Activities

L

LA AU UNCH NCH Lab

Chapter 20 • Mountain Building 561

How does crust displace the

mantle?

Continental and oceanic crust have different

densi-ties Each displaces the mantle

Procedure

1 Read and complete the lab safety form.

2 Obtain 3 wood blocks from your teacher

Determine the mass, volume, and density of each block Record all of these values in a data table.

3 Half fill a clear plastic container with

water Place both of the 2-cm-thick blocks

in the container.

4 Using a ruler, measure and record how

much of each block is above the water surface

5 Replace the 2-cm-thick blocks with the

4-cm-thick softwood block.

6 Measure and record how much of the block

is above the water surface.

Analysis

1 Describe How do density and thickness

affect the height of flotation?

2 Infer Which block represents oceanic crust?

Continental crust?

Mountain Building Processes

Make this Foldable to compare the processes that form plate boundary and non-plate bound- ary mountains.

STEP 1 Fold a sheet of paper in half lengthwise.

STEP 2 Fold the top down about 4 cm.

STEP 3 Unfold and draw lines along the fold lines

Label the columns Plate Boundary Mountains and Non-Plate Boundary Mountains.

F OLDABLES Use this Foldable with Sections 20.2 and 20.3 As you read, record the different types of mountains and the processes that form them Include examples and their locations

review content with the Interactive Tutor and take Self-Check Quizzes.

Plate Boundary Mountains

Non-Plate Boundary s

Earth’s Topography

When you look at a globe or a map of Earth’s surface, you ately notice the oceans and continents From these representations of Earth, you can estimate that about 71 percent of Earth’s surface is below sea level, and about 29 percent lies above sea level What is not obvious from most maps and globes, however, is the variation in

immedi-elevations of the crust, which is referred to as its topography Recall

from Chapter 2 that topographic maps show an area’s hills and leys When a very large map scale is used, such as the one in

val-Figure 20.1, the topography of Earth’s entire crust can be shown

When Earth’s topography is plotted on a graph such as Figure 20.2,

a pattern in the distribution of elevations emerges Note that most of Earth’s elevations cluster around two main ranges of elevation

Above sea level, elevation averages around 0 to 1 km Below sea level, elevations range between –4 and –5 km These two ranges dominate Earth’s topography and reflect the basic differences in den-sity and thickness between continental and oceanic crust

■ Figure 20.1 Topographic maps show

differences in elevation on Earth’s surface.

Interpret the map to determine

Earth’s highest and lowest elevations

Where are they?

Section 2 20 0.1 1

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.

Review Vocabulary

equilibrium: a state of balance

between opposing forces

0.5 4.5

4.8 8.5

20.9 1.1

1.0

2.2

3.0 13.9 23.2 16.4

0 +2

–2 –4 –6 –8 –10

+4 +6 +8 +10

0 10 20 30 40

Percentage of the Earth’s surface

Elevations and Depths Relative to Sea Level

Mount Everest

Average elevation of exposed land

841 m Average depth of oceans

3865 m

Marianas Trench

■ Figure 20.2 About 29 percent of Earth

is land and 71 percent is water

Interpret At what elevation does most

of Earth’s surface lie? At what depth?

Continental crust You observed in the Launch Lab that

blocks of wood with different densities displaced different

amounts of water, and thus floated at various heights above the

surface of the water You observed that blocks of higher density

displaced more water than blocks of lower density Recall from

Chapter 1 that oceanic crust is composed mainly of basalt, which

has an average density of about 2.9 g/cm3 Continental crust is

composed of more granitic rock, which has an average density of

about 2.8 g/cm3 The slightly higher density of oceanic crust

causes it to displace more of the mantle — which has a density of

about 3.3 g/cm3 — than the same thickness of continental crust

Differences in elevation, however, are not caused by density

differences alone Also recall from the Launch Lab that when the

thicker wood block was placed in the water, it displaced more

water than the other two blocks However, because of its density,

it floated higher in the water than the hardwood block

Conti-nental crust, which is thicker and less dense than oceanic crust,

behaves similarly It 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, as shown in Figure 20.3

Isostasy

The displacement of the mantle by Earth’s continental and

oceanic crust is a condition of equilibrium called isostasy

(i SAHS tuh see) The crust and mantle are in equilibrium when

the downward force of gravity on the mass of crust is balanced by

the upward force of buoyancy that results from displacement of

the mantle by the crust This balance might be familiar to you if

you have ever watched people get in and out of a small boat As

the people boarded the boat, it sank deeper into the water

Conversely, as the people got out of the boat, it displaced less

water and floated higher in the water A similar sinking and rising

that results from the addition and removal of mass occurs within

Earth’s crust Gravitational and seismic studies have detected

thickened areas of continental material, called roots, that extend

into the mantle below Earth’s mountain ranges

Section 1 • Crust-Mantle Relationships 563

Model Isostatic Rebound

How can isostatic rebound be measured? Isostatic rebound is the process through which the

underlying material rises when the overlying mass is removed.

Procedure

1 Read and complete the lab safety form.

2 Working in groups, fill a 1000-mL beaker with corn syrup.

3 Using a pencil, push a paper or plastic cup (open side up) down into the syrup far enough so the cup

is three-fourths of the way to the bottom of the syrup Record the depth of the bottom of the cup relative to the surface, then let go of the cup.

4 At 5-s intervals, record the new depth of the bottom of the cup.

Analysis

1 Describe In which direction did the cup move? Why?

2 Explain why the speed of the cup changes as it moves.

3 Infer If enough time passes, the cup stops moving Why?

Mountain roots A mountain range requires large roots to counter the enormous mass of the range above Earth’s surface

Figure 20.4 illustrates how, according to the principle of isostasy,

parts of the crust rise or subside until these parts are buoyantly supported by their roots 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 What do you think happens when mass is removed from a mountain or mountain range? If erosion contin-ues, the mountain will eventually disappear, exposing the roots

■ Figure 20.4 According to the principle of isostasy,

parts of Earth’s crust rise or subside until they are buoyantly

supported by their roots

Force of gravity Force of gravity

Continental crust

Continental crust Roots

Mantle

Buoyant force Buoyant force

Original height

Original depth of roots

Massive roots underlie mountains As erosion takes place, the mountain loses

mass The root rises in response to this decrease in mass.

When the mountain erodes to the average continental thickness, both root and moun- tain are gone.

564 Chapter 20 • Mountain Building

Interactive Figure To see an animation

of isostasy, visit glencoe.com.

Isostasy and Erosion

The Appalachian Mountains, shown in Figure 20.5, in the eastern

United States formed hundreds of millions of years ago when the

North American continent collided with Gondwana Rates of

ero-sion on land are such that these mountains should have been

com-pletely eroded millions of years ago Why, then, do these

mountains still exist? As the mountains rose 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 erode

A balance between erosion and the decrease in the size of the

root will continue for hundreds of millions of years until the

mountains disappear and the roots are exposed at the surface This

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

Section 1 • Crust-Mantle Relationships 565

■ Figure 20.5 Before erosion, the Appalachian Mountains were thousands of meters taller than they are now Because of isostatic rebound, as the mountains eroded, the deep root also rose thousands of meters closer to the surface The mountains visible today are only the roots of an ancient mountain range They too are being eroded and will someday resemble the craton in northern Canada.

Analysis

1 Plot a graph with Years before present on the x-axis and Total amount of rebound on the y-axis.

2 Describe how the rate of isostatic rebound decreases with time by studying your graph.

Data and Observations

Isostatic Rebound Data Years before

Self-Check Quiz glencoe.com

■ Figure 20.6 Mount Everest, a peak in Asia, is

cur-rently the highest mountain on Earth A deep root

sup-ports its mass Scientists have determined that Mount

Everest has a root that is nearly 70 km thick.

Seamounts Crustal movements resulting from tasy are not restricted to Earth’s continents For exam-ple, recall from Chapter 18 that hot spots under the ocean floor can produce individual volcanic mountains

isos-When these mountains are underwater, they are called seamounts On the geologic time scale, these mountains form very quickly What do you think happens to the seafloor after these seamounts form? The seamounts are added mass As a result of isostasy, the oceanic crust around these peaks displaces the underlying mantle until equilibrium is achieved

You have just learned that the elevation of Earth’s crust depends on the thickness of the crust as well as its density You also learned that a mountain peak is countered by a root Mountain roots can be many times as deep as a mountain is high Mount Everest, shown in Figure 20.6,towers nearly 9 km above sea level and is the tallest peak in the Himalayas Some parts of the Himalayas are underlain by roots that are nearly 70 km thick As India continues to push north-ward into Asia, the Himalayas, including Mount Ever-est, continue to grow in height Currently, the

combined thickness is approximately equal to 868 football fields lined up end-to-end Where do the immense forces required to produce such crustal thickening originate? You will read about these forces

in Section 20.2

Section 2 20 0.1 1 Assessment

Section Summary

◗◗ The majority of Earth’s elevations

are either 0 to 1 km above sea level

or 4 to 5 km below sea level.

◗

◗ The mass of a mountain above

Earth’s surface is supported by a root

that projects into the mantle.

◗

◗ The addition of mass to Earth’s

crust depresses the crust, while the

removal of mass from the crust

causes the crust to rebound in a

process called isostatic rebound.

Understand Main Ideas

1 MAIN Idea Relate density and crustal thickness to mountain building.

2 Describe the pattern in Earth’s elevations, and explain what causes the pattern

in distribution.

3 Explain why isostatic rebound slows down over time.

4 Infer why the crust is thicker beneath continental mountain ranges than it is

under flat-lying stretches of landscape.

Think Critically

5 Apply the principle of isostasy to explain how the melting of the ice sheets that

once covered the Great Lakes has affected the land around the lakes.

6 Consider how the term root applies differently to mountains than it does

to plants.

Earth Science MATH in

7 Suppose a mountain is being uplifted at a rate of 1 m every 1000 y It is also being eroded at a rate of 1 cm/y Is this mountain getting larger or smaller? Explain

566 Chapter 20 • Mountain Building

Section 2 • Orogeny 567

Section 2 20 0 2 2

Objectives

◗ Identify orogenic processes.

◗ Compare and contrast the

dif-ferent types of mountains that form

along convergent plate boundaries.

◗ Explain how the Appalachian

Mountains formed.

Review Vocabulary

island arc: a line of islands that

forms over a subducting oceanic plate

Mountain Building at Convergent Boundaries

Orogeny (oh RAH jun nee) refers to all processes that form mountain ranges In earlier chapters you read about many of these processes

Recall what you read in Chapter 6 about metamorphism and how rocks can be squeezed and folded In Chapter 18 you read about rising magma and igneous intrusions, and in Chapter 19 you read about movement along faults The result of all these processes can be broad, linear regions of deformation that you know as mountain ranges, but

in geology are also known as orogenic belts Look at Figure 20.7and recall from Chapter 17 what you read about the interaction of converging tectonic plates at their boundaries Most orogenic belts are

associated with convergent plate boundaries Here, compressive forces squeeze the crust and cause intense deformation in the form of folding, faulting, metamorphism, and igneous intrusions In general, the tallest and most varied orogenic belts form at convergent boundar-ies However, interactions at each type of convergent boundary create different types of mountain ranges

Philippine Plate

Pacific Plate

Indian-Australian Plate

Antarctic Plate

South American Plate

North American Plate

Nazca Plate

Cocos Plate

Juan

de Fuca Plate Caribbean

Plate

African Plate

Arabian Plate

Eurasian Plate

Eurasian Plate

■ Figure 20.7 Most of

Earth’s mountain ranges (blue

and red peaks on the map)

formed along plate boundaries

Identify the mountain

ranges that lie along the

South American Plate by

comparing a world map

with the one shown here.

Careers In Earth Science

Petrologist A petrologist studies

the composition and formation of

rocks Generally, petrologists

specialize in a particular type of rock:

igneous, sedimentary, or

metamor-phic To learn more about Earth

science careers, visit glencoe.com.

568 Chapter 20 • Mountain Building

Oceanic-oceanic convergence Recall from Chapter 17 that 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 where

it can form a series of individual volcanic peaks that together are called an island arc complex The Aleutian Islands off the coast of Alaska and the Lesser Antilles in the Caribbean are examples of island arc complexes The tectonic relationships and processes associated with oceanic-oceanic convergence are detailed in

Figure 20.8

What kinds of rocks make up island arc complexes? Often, they are a jumbled mixture of rock types They are partly com-posed of the basaltic and andesitic magmas that you read about in Chapter 18 In addition to these volcanic rocks, some large island arc complexes contain sedimentary rocks How do these sedimen-tary rocks eventually become part of a mountain? Recall from Chapter 17 that between an island arc and a trench is a depres-sion, called a basin This basin fills with sediments that have been eroded from the island arc If subduction continues for tens of millions of years, some of these sediments can be uplifted, folded, faulted, and thrust against the existing island arc This ultimately forms complex new masses of sedimentary and volcanic rocks

Parts of Japan formed in this way

■ Figure 20.8 Convergence between two oceanic plates results in the

forma-tion of individual volcanic peaks that make up an island arc complex Mount

Mazinga is one of several volcanic peaks that make up the island arc complex in

the southern Caribbean known as the Lesser Antilles.

Trench

Oceanic crust

Oceanic plate

Basin sediments

Island arc

Basaltic and andesitic magmas

Mantle

Subducting ocean

ic plate

Mount Mazinga in the Lesser Antilles

Lesser Antilles island arc

Interactive Figure To see an animation of island formation, visit glencoe.com.

Section 2 • Orogeny 569

Oceanic-continental convergence Oceanic-continental

boundaries are similar to oceanic-oceanic boundaries in that

con-vergence along both creates subduction zones and trenches Unlike

convergence at oceanic-oceanic boundaries, convergence between

oceanic and continental plates produces mountain belts that are

much bigger and more complicated than island arc complexes

When an oceanic plate converges with a continental plate, the

descending oceanic plate forces the edge of the continental plate

upward This uplift marks the beginning of orogeny In addition to

uplift, compressive forces can cause the continental crust to fold

and thicken As the crust thickens, higher mountains form Deep

roots develop to support these enormous masses of rocks

Recall from Chapter 18 that volcanic mountains can form over

the subducting plate As illustrated inFigure 20.9,sediments eroded

from such volcanic mountains can fill the low areas between the

trench and the coast These sediments, along with ocean sediments

and material scraped off the descending plate, are shoved against

the edge of the continent to form a jumble of highly folded, faulted,

and metamorphosed rocks The metamorphosed rocks shown in

Figure 20.9 are from Cwm Tydu, Cardigan Bay, Wales They formed

when the landmass that is now the United Kingdom collided with the

North American Plate millions of years ago

Reading Check Compare convergence at oceanic-continental

bound-aries with convergence at oceanic-oceanic boundbound-aries.

■ Figure 20.9 At an oceanic-continental boundary,

compres-sion causes continental crust to fold and thicken Igneous activity

and metamorphism are also common along such boundaries This

sample of metamorphosed rock formed as the result of

conver-gence of an oceanic plate with a continental plate.

Trench

Oceanic crust

Continental plate

Continental crust Sediments

Highly folded metamorphic rock

Volcanic mountain belt

rising from subducted plate

VOCABULARY

S CIENCE USAGE V C OMMON USAGE

Uplift

Science usage: to cause a portion of

Earth’s surface to rise above adjacent areas

Common usage: to improve the

spiritual, social, or intellectual condition

Sinclair Stammer/Photo Researchers

570 Chapter 20 • Mountain Building

Continental-continental convergence Earth’s tallest mountain ranges, including the Himalayas, are formed at continental-continental plate boundaries Because of its relatively low density, continental crust cannot be subducted into the mantle when two continental plates converge Instead, the low-density continental crust becomes highly folded, faulted, and thickened as shown in

Figure 20.10 Compressional forces break the crust into thick slabs

that are thrust onto each other along low-angle faults This process can double the thickness of the deformed crust Deformation can also extend laterally for hundreds of kilometers into the continents involved For example, studies of rocks in southern Tibet suggest that the original edge of Asia has been pushed approximately 2000 km eastward since the collision of Indian and Eurasian plates The magma that forms as a result of continental-continental mountain building solidifies beneath Earth’s surface to form granite batholiths

Reading Check Explain why continental crust does not subduct.

Marine sedimentary rock Another common characteristic of the mountains that form when two continents collide is the pres-ence of marine sedimentary rock near the mountains’ summits

Such rock forms from the sediments deposited in the ocean basin that existed between the continents before their collision For example, Mount Godwin Austen (also known as K2) in the west-ern Himalayas is composed of thousands of meters of marine lime-stone that sits upon a granite base The limestone represents the northern portions of the old continental margin of India that were pushed up and over the rest of the continent when India began to collide with Asia about 50 mya

Deformed ocean sediments

Faults

Continental

crust

Continental crust

■ Figure 20.10 Intense folding and faulting along

continental-continental boundaries produce some of the highest mountain ranges

on Earth The Himalayas are the result of the convergence between

the Indian and Eurasian plates.

Interactive Figure To see an animation of convergence,

visit glencoe.com.

Section 2 • Orogeny 571

The Appalachian Mountains—

A Case Study

Recall from Chapter 17 that Alfred Wegener used

the matching rocks and geologic structures in the

Appalachians and mountains in Greenland and

north-ern Europe to support his hypothesis of continental

drift In addition to Wegener, many other scientists

have studied the Appalachians Based on these

stud-ies, geologists have divided the Appalachians into

sev-eral distinct regions, as illustrated in Figure 20.11

Each region is characterized by rocks that show

differ-ent degrees of deformation For example, rocks of

the Valley and Ridge Province are highly folded

sedi-mentary rocks In contrast, the rocks of the Piedmont

Province consist of older, deformed metamorphic

and igneous rocks that are overlain by relatively

unde-formed sedimentary layers These regions, pictured

in Figure 20.12, are different because they formed

in different ways

The early Appalachians The tectonic history

of the Appalachians is illustrated in Figure 20.13

It began about 800 to 700 mya when ancestral North

America separated from ancestral Africa along two

divergent boundaries to form two oceans The

ances-tral Atlantic Ocean was located off the western coast

of ancestral Africa A shallow, marginal sea formed

along the eastern coast of ancestral North America

A continental fragment was located between the two

Sedimentary Appalachians (the Appalachian Basin) Northern

Appalachians

Central Appalachians (Atlantic states)

Appalachian plateau Valley and Ridge Blue Ridge Mtns.

Piedmont

■ Figure 20.12 The Valley and Ridge Province of the Appalachians has highly

folded rocks Rocks from the Piedmont Province are relatively undeformed.

■ Figure 20.11 The Appalachian Mountain Range is made up of more than one type of mountain It has several distinct regions, each with their own orogenic history.

Folded rock from the Valley and Ridge Province Undeformed rock from the Piedmont Province

Interactive Figure To see an animation of folding rocks, visit glencoe.com.

(l)E R Degginger/Photo Researchers , (r)Scott Camazine/Alamy Images

Master Page used: NGS

To explore more about other mountain ranges that formed along convergent

boundaries, visit glencoe.com.

572 Chapter 20 • Mountain Building

Figure 20.13 The Appalachians formed hundreds of millions of years ago as a result of convergence.

Visualizing the Rise and Fall

of the Appalachians

Continental fragment

Ancestral

North America

Ancestral Atlantic Ocean Island arc

Continental fragment

Valley and Ridge

Ancestral Atlantic Ocean Island arc Ancestral

Africa

Valley and Ridge

Blue Ridge

Ancestral Atlantic Ocean

Blue Ridge Valley and Ridge Piedmont

Africa

Continental crust Continental crust

Blue Ridge Valley

and Ridge

Piedmont Continental

shelf

Mid-Atlantic rift Africa

700–600 mya Convergence causes the

ances-tral Atlantic Ocean to begin to close An island arc develops east of ancestral North America.

500–400 mya The continental

fragment, which eventually becomes the Blue Ridge Province, becomes attached to ancestral North America.

400–300 mya The island arc

becomes attached to ancestral North America and the continen- tal fragment is thrust farther onto ancestral North America The arc becomes the Piedmont Province.

300–260 mya Pangaea forms 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.

Present After the breakup of Pangaea,

tension forces open the modern Atlantic Ocean and separates the continents North America and Africa continue to move apart as the Atlantic Ocean widens.