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.
Vocabulary
– isostasy
– isostatic rebound
Crust-Mantle Relationships
Earth’s Topography
• The change in elevation, or topography, of the
crust isn’t obvious from most maps and globes.
Crust-Mantle Relationships
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.
• These two modes
reflect the basic
differences in density
and thickness between
continental and
oceanic crust.
Crust-Mantle Relationships
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.
– 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.
Crust-Mantle Relationships
Earth’s Topography
Crust-Mantle Relationships
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.
Crust-Mantle Relationships
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.
Crust-Mantle Relationships
Isostasy
Isostasy and Erosion
Crust-Mantle Relationships
Isostasy
Isostasy and Erosion
– 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.
Crust-Mantle Relationships
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.
Crust-Mantle Relationships
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.
Crust-Mantle Relationships
Section Assessment
3. Identify whether the following statements are
true or false.
______
false Peridotite is less dense than basalt.
______
true Mountain roots can extend far deeper than the
height of the mountain.
______
true Buoyancy and gravity are the basic two forces
in isostasy.
______
false Oceanic crust, because it is denser, extends
deeper into the mantle than continental crust.
Convergent-Boundary Mountains
Objectives
• Compare and contrast the different types of mountains
that form along convergent plate boundaries.
• Explain how the Appalachian Mountains formed.
Vocabulary
– orogeny
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.
Convergent-Boundary Mountains
Orogeny
Convergent-Boundary Mountains
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.
Convergent-Boundary Mountains
Orogeny
Oceanic-Oceanic Convergence
Convergent-Boundary Mountains
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.
Convergent-Boundary Mountains
Orogeny
Oceanic-Continental Convergence
Convergent-Boundary Mountains
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 continentalcontinental 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.
Convergent-Boundary Mountains
Orogeny
Oceanic-Continental Convergence
Convergent-Boundary Mountains
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.
Convergent-Boundary Mountains
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.
Convergent-Boundary Mountains
The Appalachian Mountains–A Case Study
The Early Appalachians
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.
Convergent-Boundary Mountains
The Appalachian Mountains–A Case Study
The Early Appalachians
500–400 M.Y.B.P. The continental fragment, which
eventually becomes the Blue Ridge Province, becomes
attached to ancestral North America.
Convergent-Boundary Mountains
The Appalachian Mountains–A Case Study
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.
Convergent-Boundary Mountains
The Appalachian Mountains–A Case Study
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.
Convergent-Boundary Mountains
Section Assessment
1. What is orogeny?
The processes that form all mountain ranges
are called orogeny.
Convergent-Boundary Mountains
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.
Convergent-Boundary Mountains
Section Assessment
3. Identify whether the following statements are
true or false.
______
true The Philippine islands are an example of an
island arc complex.
______
false A subduction zone forms during a continentalcontinental collision.
______
true The Blue Ridge province is the composed of
the remnants of a continental fragment.
______
true The modern Atlantic Ocean formed about 200
million years ago.
Other Types of Mountains
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
– pillow basalt
– uplifted mountain
– fault-block mountain
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.
Other Types of Mountains
Divergent-Boundary Mountains
Other Types of Mountains
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.
Other Types of Mountains
Divergent-Boundary Mountains
Ocean-Ridge Rocks
Other Types of Mountains
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.
Other Types of Mountains
Nonboundary Mountains
Uplifted Mountains
– Uplifted mountains are mountains that form when
large regions of Earth have been slowly forced
upward as a unit.
Other Types of Mountains
Nonboundary Mountains
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.
Other Types of Mountains
Nonboundary 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 faultblock mountains.
Other Types of Mountains
Nonboundary 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.
Other Types of Mountains
Section Assessment
1. Match the following mountain types with
an example.
___
A divergent-boundary
A. ocean ridges
___
D uplifted
B. Grand Tetons in Wyoming
___
B fault-block
C. Mauna Kea in Hawaii
___
C volcanic peaks
D. Adirondacks in New York
Other Types of Mountains
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.
Other Types of Mountains
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
Study Guide
Section 20.1
Section 20.2
Section 20.3
Chapter Assessment
Image Bank
Section 20.1 Study Guide
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 20.2 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 Study Guide
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.
Chapter Assessment
Multiple Choice
1. A mountain’s root ____ as mass is removed from
the mountain through erosion.
a. expands
c. rises
b. sinks
d. melts
Buoyant force will cause the root of the mountain to rise,
maintaining isostatic equilibrium.
Chapter Assessment
Multiple Choice
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.
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. continentalcontinental
b. oceanic-continental
d. none of the above
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.
Chapter Assessment
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
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.
Chapter Assessment
Multiple Choice
5. The processes that form all mountain ranges are
called ____.
a. convergence
c. uplift
b. divergence
d. orogeny
Convergence, divergence, and uplift are all types
of orogeny.
Chapter Assessment
Short Answer
6. How large is a mountain’s root in relation to
the mountain?
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.
Chapter Assessment
Short Answer
7. Isostasy involves the equalization of what
two forces?
The two forces at work in isostasy are
buoyancy and gravity.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
true As a mountain erodes, it rises.
______
false Warmer regions of the mantle may be
responsible for uplifted mountains.
______
false The Hawaiian islands formed along a
divergent boundary.
______
true The piedmont is the remnants of an ancient
island arc.
______
true Continental crust extends deeper into the
mantle than oceanic crust.
Image Bank
Chapter 20 Images
Image Bank
Chapter 20 Images
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