Plate Tectonics

Designed to meet South Carolina

Department of Education
2005 Science Academic Standards
 Table of Contents
➢ Plate Tectonics: The Beginning (slides 3 and 4)
➢ Layers of the Earth (slides 5 and 6) Standard 8-3.1
➢ What are Tectonic Plates- movement? (slides 7 and 8) Standard 8-3.6
➢Tectonic Plate boundaries (slides 9-21)
1. Convergent boundary
Ocean-continent (slide 10)
Continent-continent (slide 11)
Oceanic-oceanic (slide 12)
Volcanism (slide 13)
2. Divergent boundary
Sea-floor spreading (slide 14)
The Mid-Atlantic Ridge (slide 15)
Sea-floor Exploration and Age Dating (slides 16-19)
3. Transform Boundary (San Andreas Fault, J. Tuzo Wilson) (slide 21)
➢ Faults (slides 22-24) and Folds (slide 25)
➢ Plate movement over Geologic Time (slides 26-29)
➢ Creation and change of Landforms (slides 30-33) Standard 8-3.7
Volcanic eruptions (Mt. St. Helens) (slide 32)
Mountain building events (Appalachian vs. Himalayas) (slide 33)
➢ Tectonics and the Ocean Floor (slide 34) Standard 5-3.2
Continental margins (slide 35)
Passive (slide 36)
Active (slide 37)
➢ South Carolina Standards (slides 39-40)

2
 Plate Tectonics: The Beginning
Background

❖ At the beginning of the 20th Century, scientists realized that that they could not explain many of
the Earth’s structures and processes with a single theory. Many scientific hypotheses were
developed to try and support the conflicting observations. One hypotheses was continental drift,
which was proposed by Alfred Wegener in a series of papers from 1910 to 1928.

❖ The principal thought of continental drift theory is that the continents are situated on slabs of
rock, or plates, and they have drifted across the surface of the Earth over time; however, originally,
they were all joined together as a huge super-continent at one time.

❖ In the 1960’s, the theory of

continental drift was combined with
the theory of sea-floor spreading to Alfred Lothar Wegener
create the theory of plate tectonics. (1880-1930)

(Photograph courtesy of the

Alfred Wegener Institute for
Polar and Marine Research,
Bremerhaven, Germany.) Table of Contents 3
 Plate Tectonics: The
Beginning
❖ The idea for Wegener's theory was sparked by
his observation of the nearly perfect “fit” of the
South American and African continents.

The “fit” of two continents.

Additional evidence supporting the continental
drift theory:

1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and South
America.

2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not
have swum across the existing Atlantic Ocean!

3. Glacial deposits found in current warm climates and warm climate plant fossils found in what
is now the Arctic.

4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe
4
and eastern South America and western Africa. Table of Contents
 What are Tectonic Plates?
Standard 8-3.1

The Earth is made up of three main layers:

1. The Core is at the center of the Earth. It is
divided into an inner and outer core.
2. The Mantle is the layer surrounding the core.
The upper mantle is partially molten and called
the asthenosphere.
3. The Crust, or lithosphere, is the rigid outer-most
layer. Thick continental crust underlies
continents, and thin, very dense oceanic crust
underlies oceans.
The layers of the earth.
Modified after Plummer/McGeary, 7th ed., pg. 14 Table of Contents 5

Inner Core Outer Core Mantle Crust

Thickness 1,216 km 2,270 km 2,900 km Continental 35-90 km

Oceanic 7-8 km
Physical Solid Iron; Molten Iron, Made mostly of silicates of Made of silicate rocks
Properties extremely very dense magnesium and iron; moderately and oxides; slightly
dense (17 (12 g/cm3) dense. Behaves like melted plastic dense; rigid. (2.67-3.3
g/cm3) in upper-most section (5.5 g/cm3) g/cm3)
Percentage of 30% 65% 5%
Earths’ Mass
Standard 8-3.1

What are Tectonic Plates? (continued)

Earth’s Sublayers
❖ Lithosphere: This layer combines the rigid crust plus the upper-most mantle. (Greek: Rock)

❖ Asthenosphere: Partially molten part of upper mantle (Greek: weak). Tectonic plates are able
to move about on top of the softer, partially molten asthenosphere.

The outer-
most layers
of the earth.
McGraw Hill/
Glencoe, 1st ed., pg.
142.

6
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Standard 8-3.6

What are Tectonic Plates? (continued)

❖ The Earth’s crust consists of about a dozen large slabs of rock, or PLATES, that the continents
and oceans rest on. These tectonic plates can move centimeters per year— about as fast as your
fingernails grow up to 15cm/yr in some places.
❖ Tectonic plates are also called lithospheric plates because the crust and the upper-most mantle
make up a sub-layer of the earth called the lithosphere. The plates can move about because the
uppermost mantle, or the asthenosphere, is partially molten and possesses a physical property
called plasticity, allowing the strong, rigid plates of the crust to move over the weaker, softer
asthenosphere.

Plates and relative

plate motion.
Modified after NOAA

South Carolina is
located on the North
American plate

❖ The word TECTONICS is of Greek origin and it means “to build.” The word “tectonism”
refers to the deformation of the lithosphere. This deformation most notably includes mountain
building. 7
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Standard 8-3.6

What are Tectonic Plates? (continued)

❖ Tectonic plates, or lithospheric plates, are constantly moving, being created, and consumed
simultaneously. The motion sometimes results in earthquakes, volcanoes, and mountain ranges at
the plate boundaries.
❖ Plate motion is driven by heat escaping from the mantle. The constant movement of heat in the
mantle leads to circular convection currents. These hot convective cells are similar to the rolling
boil that occurs when water is heated on a stovetop. The flowing mantle has also been compared to
a “conveyor belt,” moving the rigid plates in different directions.
❖ Fundamentally, convection occurs due to uneven heating and different densities within the
liquid.
Spreading ridge Subduction
zone
Upwelling

Downwelling
=
Core

Convection currents within the mantle.

Modified after Plummer/McGeary, 7th ed., pg. 15
8
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 Plate Boundaries
There are three basic ways that plates interact with one another. Each of these plate
boundaries has the potential to create different geological features.

1. When plates collide with each other = Convergent boundary

2. When plates separate from each other = Divergent boundary

3. When plates slide along side each other = Transform boundary

The tectonic plates and

plate boundaries.
McGraw Hill/Glencoe, 1st ed., pg 143

Table of Contents 9
 1. Convergent Boundary:
Ocean-Continent Collision
❖ Because the oceanic crust is more dense than continental crust, when these two collide, the
continental crust rides up over the oceanic crust and the oceanic crust is bent down and subducted
beneath the continental crust. This is called a subduction zone, where the old oceanic crust is
dragged downward and “recycled.”
❖ Deep-sea trenches are created at subduction zones. Trenches are narrow, deep troughs parallel
to the edge of a continent or island arc. They typically have slopes of 4-5 degrees, and they are
often 8-10 km deep. The deepest spots on earth are found in oceanic trenches. The Mariana
Trench is the deepest ocean depth at 11 km (35,798 ft) below sea level.

Figure depicting oceanic crust

subducting beneath continental
crust, creating volcanoes on the
land surface above, and a deep-
sea trench off of the coast.

Credit: U.S. Geological Survey

Department of the Interior/USGS
10
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 Convergent Boundary:
Continent-Continent Collision
❖ If two continental plates collide, mountain building usually takes place because they
are both relatively low in density.
❖ Earthquake activity at these boundaries is common; however, because igneous
activity is different from ocean-continent collisions, volcanoes are rare.
❖ Examples: The Himalayan
and the Appalachian mountain
chains.

Constructive mountain building

during continent-continent
collision.
The Himalaya
McGraw Hill/Glencoe, 1st ed., pg 149
mountains are still
forming today as the
Ind-Australian Plate
collides with the
Eurasian Plate 11
Table of Contents
 Convergent Boundary:
Ocean-Ocean Collision
❖ If 2 oceanic plates collide, the older, denser one is subducted downward into the mantle and
a chain of volcanic islands can form, called a volcanic arc.
❖ Example: Mariana Islands (Mariana Trench). It is deeper than the earth’s tallest mountain is
tall. Mariana Trench: 11,000 meters deep. Mt. Everest: 8850 meters high.
❖ The interaction of the descending oceanic plate causes incredible amounts of stress
between the plates. This usually causes frequent earthquakes along the top of the
descending plate known as the “Benioff Zone.” The focii of Benioff earthquakes can
be as deep as 700 km below sea level.

Oceanic/oceanic collision
resulting in a chain of island arcs.
Credit: U.S. Geological Survey
Department of the Interior/USGS

Benioff Zone

12
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 Convergent Boundary: Volcanism
❖ Most volcanoes form above subduction zones because as one slab is subducted
beneath the other, the interaction of fluids and geothermal heat form new magma.
The new magma then rises upward through the overlying plate to create volcanoes
at the surface.
❖ The Andes Mountains are home to many volcanoes that were formed at the
convergent boundary of the Nazca and South American Plates.
Left: Image of the
❖ Nazca Plate
subducting
beneath the South
American Plate.
Modified after McGraw
Hill/Glencoe, 1st ed., pg.
143
Right: Red dots
indicate general
locations of
volcanoes along
western coast of
South America.
13
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 2. Divergent Boundary: Sea-floor Spreading
❖ At a divergent boundary, two oceanic plates pull apart from each other through a
process called sea-floor spreading.
❖ Sea-floor spreading was proposed by Harry Hess in the early 1960’s. Hess proposed
that hot magma rises from the asthenosphere and up into existing ocean crust through
fractures. The crust spreads apart making room for new magma to flow up through it.
The magma cools, forming new sea floor and resulting in a build-up of basaltic rock
around the crack, which is called a mid-ocean ridge.

Sea-floor spreading at an
oceanic divergent boundary.
Modified after McGraw Hill/ Glencoe, 1st ed., pg.
138 (with permission)

❖ New material is constantly being created. This is the opposite of a convergent

boundary, where material is constantly being destroyed.
Table of Contents 14
 Divergent Boundary: Mid-Atlantic Ridge
❖ The world’s longest mountain chain is underwater. It is 56,000 km long and is
called the Mid-Atlantic Ridge.
❖ The Mid-Atlantic Ridge is considered a slow-spreading ridge, spreading at about 1-2
centimeters per year. An example of a fast-spreading ridge is the East Pacific Rise,
which spreads at about 6-8 centimeters per year.

Satellite bathymetry of the East Modified after http://www.ocean.udel.edu/deepsea/level-

2/geology/ridge.html
Pacific Rise spreading ridge. Credit:
U.S. Geological Survey 15
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Department of the Interior/USGS
 Sea-floor Exploration
❖ The Deep Sea Drilling Project (DSDP) began in 1968 aboard the research vessel
Glomar Challenger. This ship was outfitted with a drill rig capable of drilling into the
ocean floor beneath many kilometers of water.
❖ Before this type of research was available, scientists had to rely on dredging or
grabbing single rock samples from line weights on boats.
❖ Scientists quickly determined that continental crust is thicker than oceanic crust,
that continental crust is less dense than oceanic crust, and that the youngest seafloor is
located at mid-ocean ridges and increases in age with distance from the ridge.

❖ Before technology like this, most

people thought the ocean floor was flat
and smooth. This is understandable as
2/3 of the Earth’s surface lies under
oceans.

The Glomar Challenger (1968)

Credit: U.S. Geological Survey
Department of the Interior/USGS
16
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 Sea-floor Exploration
❖ In the mid-1960’s, magnetometer surveys at sea indicated that alternating magnetic
anomalies existed within marine rock. These anomalies were aligned parallel to the
Mid-Atlantic Ridge forming stripe-like patterns on the sea floor, and they were
symmetrically distributed on either side of the Mid-Atlantic Ridge.
❖ Geologists Fred Vine and Drummond Matthews first noticed these symmetric
patterns of magnetic “stripes” and concluded that this pattern of magnetic anomalies
at sea matched the pattern of magnetic reversals over time.

❖ The Earth’s magnetic field flows from a

southerly direction to northerly direction. This is
what makes the arrow on our compasses point
towards north.

❖ The Earth’s magnetic field has changed over the past 100 million years
approximately once every 250,000 years. When the magnetic field “reverses” from
today’s “normal” N-S direction it becomes a period of magnetic reversal. The normal
magnetic field is considered a positive anomaly, and, when the magnetic field is
reversed, it is considered a negative anomaly.
17
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 Sea-floor Exploration: Age-dating

❖ The relative age of the sea floor T1

can be determined by changes in T2
magnetic polarities of the earth.
Oldest
T3
❖ These periods of normal and Youngest
reversed polarity are recorded in the
magnetic minerals within the newly
formed sea floor at mid-ocean
ridges. Scientists can see a clear
pattern of normal and reversed
magnetization in the rock record Above: Figure depicting magma flowing out
that shows up as “magnetic stripes” from a spreading ridge, cooling and spreading
on either side of mid-ocean ridges, out symmetrically about the ridge to produce
which gives them a relative time of successively older rock as you travel in either
direction from the ridge. The magnetized
how old the sea floor is and lets
minerals within the rock “tell” how old it is.
them compare ocean basins to each Credit: U.S. Geological Survey
other. Department of the Interior/USGS

Table of Contents 18
 Sea-floor Exploration: Age-dating
❖ The oldest oceanic crust found is ~ 180 million years old.
❖ Because the age of the earth is ~4,600 million years, we know that oceanic crust is
continually being formed at spreading ridges and being destroyed at subduction
zones. The ocean floor is constantly changing shape and size through the processes of
sea-floor spreading and subduction.

❖ Because no older
ocean crust has been
found, recycling of the
ocean crust takes place
about every 180 million
years.

The relative ages of

the ocean floor.
Credit: Nova.

Table of Contents 19
 3. Transform Boundary
❖ When two plates slide past each other moving in different directions or the same
direction, it is termed a transform boundary and is characterized by a transform fault
and earthquake activity.
❖ An example of a transform fault is the San Andreas Fault in California. Here the
North American Plate joins the Pacific Plate. The difference in plate motion along the
contact (fault) leads to a buildup of strain energy that sometimes slips releasing a huge
amount of energy and causing an earthquake.

An aerial photo of the San Andreas Movement between the 2 plates at the San Andreas
fault line. McGraw Hill/Glencoe, 1st ed., pg. 146 Transform Fault. McGraw Hill/Glencoe, 1st ed., pg. 146 (with
(with permission) permission). Table of Contents 20
 Transform Boundary (continued)
❖ J. Tuzo Wilson was a geophysicist who was fascinated by Wegener's theory of
continental drifting. He was also inspired by Harry Hess, whom he studied under at
Princeton University in the 1930’s.
❖ Wilson is recognized today for advancing plate-tectonic theory by introducing three
major concepts:
1. Wilson (1963) introduced the concept of a “stationary hotspot”, where the
heat from the mantle could affect the thin crust, forming volcanic islands such
as Hawaii.
2. Wilson (1965) proposed the “transform” boundary as the third type of plate
boundary. They commonly offset ocean ridges and trenches, and transform
the motion between the offset. Unlike ridges and trenches, transform faults
offset the crust horizontally, without creating or destroying crust.
3. Wilson proposed what is known today as the Wilson Cycle. This concept
explains the origin for the Appalachian Mountains. The “cycle” goes through
the sequence: 1. The splitting of a supercontinent,
2. the opening of an ocean basin,
3. the closing of the ocean basin,
4. the collision of continents and formation of mountains.
21
Table of Contents
 Mountain building processes, faults and folds
❖ Plate tectonics cause many of the physical features that we see on earth today like volcanoes and
earthquakes, but also many other geological features like faults. Faults are planar rock fractures
along which movement has occurred.
❖ A transform fault occurs at a transform plate
boundary like the San Andreas Fault in California. It
connects two of the other plate boundaries.
❖ Similar in movement, a strike-slip fault occurs
through shearing when two blocks move in
horizontal but opposite directions of each other.
Depending on the direction of offset, it can be a Right-lateral offset
“right-lateral offset” or a “left-lateral offset.”

In the example above, it is obvious that the fence The photograph above displays a light-colored
has been offset to the right, therefore it is called a pegmatite vein offset to the right in a schistose
right lateral strike-slip fault (Credit: U.S. Geological matrix. Photo courtesy of K. McCarney-Castle. 22
Survey Department of the Interior/USGS) Table of Contents
 Faults: Normal Faults
❖ Faults caused by blocks of crust pulling apart under the forces of tension are called normal
faults. Entire mountain ranges can form through these processes and are known as fault block
mountains (examples: Basin and Range Province, Tetons).
❖ In a normal fault, the hanging-wall block moves down relative to the foot-wall block.
❖ The footwall is the underlying surface of an inclined fault plane.
❖ The hanging wall is the overlying surface of an inclined fault plane.

Hanging
wall block
Footwall
block
Hanging
Wall
Foot Wall

Relative movement of two blocks Diagrammatic sketch of the two types of

blocks used in identifying normal faults.
indicating a normal fault. (Credit: Modified after 23
U.S. Geological Survey Department of the Interior/USGS) Table of Contents
 Faults: Reverse Faults
❖ Faults caused by blocks of crust colliding under the forces of compression are called
reverse faults.
❖ Reverse faults form during continent-continent collision. Usually, there is also
accompanying folding of rocks.
❖ During reverse faulting, the hanging wall block moves upward (and over) relative to
the footwall block.

Hanging
wall block
Footwall
block
Hanging
Wall
Foot Wall

Diagrammatic sketch of the two

Relative movement of two blocks types of blocks used in identifying
indicating a reverse fault. (Credit: U.S. Geological reverse faults.
Survey Department of the Interior/USGS)
24
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 Folding
❖ During mountain building processes, rocks can undergo folding as well as faulting.
❖ Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. At
elevated temperature and pressure within the crust, folds can form from compressional forces.
❖ Entire mountain rages, like the Appalachians, have extensive fold systems.

Z-fold in schist with white felsic dike Large fold in outcrop (geologists for scale).
(hammer for scale). Near Lake Murray, Near Oakridge, Tennessee, Appalachian
South Carolina. Mtns. Photo courtesy of K. McCarney-Castle.
25
Photo courtesy of K. McCarney-Castle
Table of Contents
 Plate Movement Over Geologic Time
❖ Alfred Wegener proposed that all of the continents once formed a “supercontinent”
called Pangaea.
❖ From the Greek language, ‘pan’ meaning ALL and ‘gaea’ meaning EARTH. It was
thought to have come together and formed approximately 200 million years ago.
❖ Evidence for a supercontinent included:

1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica, and South
America.

2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not
have swum across the existing Atlantic Ocean!

3. Glacial deposits found in current warm climates and warm-climate plant fossils found in what
is now the Arctic.

4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe
and on eastern South America and western Africa.

26
Table of Contents
 1. About 1,100 million years ago, a super-
continent called Rodinia existed (pre-
Cambrian).

2. Rodinia broke apart, and about 400

million years ago, the oceans began to close
up to form a pre-Pangea (early Devonian).

Table of Contents 27

3. Pangea formed around 250 million years

ago and animals could migrate from the north
to the south pole (Early Triassic).
PaleoMaps used with permission from Christopher
Scotese and are under copyright of C.R. Scotese, 2002
 4. Pangaea began to break apart into 2
halves approximately 200 million years ago
(Early Jurassic). The northern half is called
Laurasia and the southern half is called
Gondwanaland. These two huge continents
were separated by a body of water called the
Tethys Sea.

5. Gondwananland split to form Africa,

South America, Antarctica, Australia and
India. Laurasia split to form North America,
Eurasia (minus India) and Greenland.

6. Around 15 million years ago, the continents

finally looked like they do today
PaleoMaps used with permission from Christopher Scotese and
are under copyright of C.R. Scotese, 2002
Table of Contents
28
 Continents in the future?

In 50 million years, it is possible that

the Mediterranean could close due to
the collision of Africa with Europe.
Australia may eventually join Asia.

Table of Contents 29

It is though that in another 250 million

years, another Pangea will form.

PaleoMaps used with permission from Christopher 29

Scotese and are under copyright of C.R. Scotese, 2002
Standard 8-3.7

Creation and Change of Landforms

Volcanoes
❖ Most volcanoes form above subduction zones
because as one slab is subducted beneath the other, it
causes melting, forming new magma, which then rises
upward. This is why most volcanoes are found near
plate boundaries.
❖ Volcanoes are constructive because they add new
rock, form new islands, and create new land masses.
However, they are also destructive when they erupt and
change the landscape (possibly even the climate).
Above: Diagrammatic sketch
of a volcanic arc located above
a subduction zone

Left: Active volcanoes found

along plate boundaries

(Credit: U.S. Geological Survey

Department of the Interior/USGS)

30
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 Volcanic Eruptions:
❖ Most volcanoes form above subduction zones because as one slab is subducted beneath the
other, melting occurs, forming new magma, which then rises upward along the plate boundary.
❖ Being hot, magma is less dense than the solid rock surrounding it. This enables the liquid
magma to rise up through cracks in the solid rock and collect in pockets within the earth, called
magma chambers. Eventually some of the magma pushes through vents and fissures in Earth's
surface, causing an eruption that may be violent or quiet. Once the magma reaches the earth’s
surface, it is called lava.
❖ The explosiveness of an eruption depends on the viscosity of the magma. Viscosity is the
resistance of a liquid to flow. If the magma is very thick and viscous, gases can build up within the
magma. Finally, when threshold is reached, there is a violent eruption from the built up pressure
of the gases in the magma chamber.
❖ If magma is less viscous, or more fluid, gases Vent
Volcano
can escape easily from it. When this type of
magma erupts, it flows out of the volcano and Magma chamber
violent explosions are rare. Land
❖ Sometimes, huge clouds of ash race down surface
mountainsides destroying almost everything in
Crust
their path. These are pyroclastic clouds. They
Upper mantle
can travel faster than a high-speed train. The ash
produced from an eruption will fall back to the Lower mantle
ground and suffocate humans, animals, and
Core
plants.
Cross-sectional view looking through 31
the side of a volcano Table of Contents
 Volcanoes: Mount St. Helens
❖ A famous volcano that erupted in the U.S. nearly 30 years ago was Mt. St. Helens in
the Cascade Range of Washington.
❖ The volcanic activity of Mt. St. Helens is caused by the ongoing subduction of the
Juan de Fuca plate under the North American plate. It is part of the Cascade Volcanic
arc, which includes some 160 active volcanoes along the west coast.
❖ On May 18, 1980 Mount St. Helens erupted in what was the most economically
destructive volcanic event in U.S. history. Nearly 60 people lost their lives and 250
homes were destroyed.
❖ Before the volcano exploded, it was 2,950 m tall. Afterwards, it was 2,550 m tall,
and instead of a sharp peak at the summit, a mile-wide horseshoe shaped crater was
left. The debris avalanche associated with the eruption was nearly 3 cubic kilometers
in volume.

❖ When Mount St. Helens

exploded, it had not
erupted for 123 years. Most
people thought Mount St.
Helens was a beautiful,
peaceful mountain and not
a dangerous volcano.
Before eruption… After eruption… 32
Table of Contents
Credit: Wikimedia Commons
 Mountain-building forces
❖ When two continental plates collide at a convergent boundary, the process produces a
mountain range. Compressional forces drive the mountain building process.
❖ The Appalachians, the Alps, and the Himalayas were formed through compression.

❖ The Himalayan mountain chain was formed

Continent/ approximately 150 million years ago. When we
continent
convergence think of the Himalayas, we think of very high,
steep mountains, cliffs, and, of course, Mt.
Everest.

The
Matterhorn,
Alps

❖ In contrast, when we consider our own Appalachians,

which formed about 400 million years ago, we see more
subdued topography than in the Himalayas. This is
because the process of wind and water erosion have
eroded hundreds of vertical feet of land surface from the
area and reduced high jagged mountains into the rolling The Appalachian Mountains. Photo
courtesy of K. McCarney-Castle
hills present today. 33
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Standard 5-3.2

Continental margins and deep-sea landforms

❖ Even though approximately 70% of the earth’s surface is covered by water, it is the study of sea-
floor rocks, sediment, and topography that provide most of the information used to validate the
theory of plate tectonics. Ongoing research on the sea floor continues to provide clues to the
Earth’s dynamic structural processes.
❖ Research and technology have granted us access to study both the shallow continental shelves
as well as the deep abyssal plains of the ocean basins. These include rock dredging, coring,
drilling, geophysical studies (seismic and magnetic), and age dating.
❖ One advancement in technology is the use echo sounders, which are used to draw profiles of
submarine topography. Seismic profilers also use acoustic echoes but can penetrate the bottom of
the seafloor. These profilers allow us to see a “picture” of the layered structure under the sea floor.

Africa, west
USA, west USA, east coast
coast coast
Cross-sectional view of
continents grading into
Continent deep ocean basins. Note
change in topography.
Ocean Basin

34
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Continental Margins: Continental Shelves and Slopes
❖ All continents are surrounded by a shallow, relatively flat platform called a continental shelf
and a sloping surface called a continental slope that gently descends down to the deep ocean floor.
❖ Continental shelves vary in width depending on the type of continental margin. On the U.S.
Pacific coast, it is only a few km wide, but off of the Atlantic Coast it is up to 500 km wide.
❖ The continental shelf area has thick accumulations of young sediment and has water depths less
than 200 meters.
❖ The continental slope has a relatively steep slope (4-5 degrees) and it joins the edge of the shelf
to the deep ocean floor. Relatively little is known about the slopes, as it is difficult to drill on the
steep surfaces.
❖ Beyond the continental shelf is the continental break where flat shelf ends and the steep
continental slope begins. Although typical continental slopes have an incline of around 4
degrees, some active margins, like the Gulf of California, slope at about 20 degrees.
❖ The continental slope may be marked by channels called submarine canyons that transport
sediment from the shelf to the sea floor, sometimes in violent events called turbidity currents.
The continental rise has been built up by these thick deposits of sediment. California
Continental
Margin looking
east towards Los
Angeles.
Shelf
(Credit: U.S.
Geological Survey
Slope Department of the
Interior/USGS)

Table of Contents 35
 1. Passive Continental Margins
❖ A passive continental margin includes a continental shelf, slope, and rise and these margins
gently grade into a deep abyssal plain.
❖ The ocean floor and continent usually belong to the same continental plate.
❖ Passive margins form on “geologically quiet” coasts, also called trailing margins, where there is
no tectonic or volcanic activity, such as the eastern seaboard of the U.S.
❖ The continental rise is a wedge of sediment that extends from the lower part of the slope to the
deep sea floor sloping at about 0.5 degrees. It grades into a flat abyssal plain at around 5km in
depth.
❖ Abyssal plains are found at the base of the continental rise and are the flattest features on earth.
Abyssal plains form where turbidity currents carry amounts of sediment large enough to bury and
obscure the rugged relief normal found on the sea floor.

Continental break

Sketch of an abyssal fan forming on the

sea floor. Sediment for the fan is
carried from the shelf through the
canyon in violent turbidity currents and
are deposited, forming a fan .
Continental
Slope

Modified after Plummer/McGeary, 7th ed., pg. 400.

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 Active continental Passive continental
margin with trench margin
Cross-sectional sketch
showing the trench off
the active continental
margin and the rise off of
the passive continental
margin.

2. Active continental margin:

❖ An active continental margin includes a continental shelf, slope, and an ocean trench. It is the
result of an ocean plate colliding with a continental plate.
❖ Active margins are tectonically active, characterized by earthquakes, volcanoes, and mountain
belts. Tectonically active margins may have a deep trench instead of a continental rise where the
oceanic plate is subducted beneath the continental plate.
❖ The west coast of the U.S. and South America are examples of active margins.
❖ Ocean trenches are elongate features parallel to the edge of the continent and are the deepest
regions on earth at over 11 km below sea level.
❖ Ocean trenches are associated with seismic Benioff Zones, which begin at the trench and dip
underneath the continent as part of the subduction zone. Earthquakes are prevalent along the
Benioff Zone, which is the site of one tectonic plate descending underneath another.

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 Satellite
bathymetry of the
East Pacific Rise
spreading ridge.
Credit: U.S. Geological
Survey
Sea floor spreading at the mid-ocean ridge Department of the
Interior/USGS
and rift valley. Modified after McGraw Hill/
Glencoe, 1st ed., pg. 138 (with permission)

❖ The Mid-Atlantic Ridge is an example of a divergent boundary found extending

around the world dividing the world’s ocean basins. It spreads at about 1-2 centimeters
per year and is made of basalt. It is 56,000 km long and 2,500 km wide, and it rises 2 –3
km above the sea floor.
❖ The spreading of the ridge creates a rift valley running down the crest of the ridge.
The valley is about 1-2 km deep and several kilometers wide- similar to the dimensions
of the Grand Canyon!
❖ Shallow earthquakes are frequent along these ridges and long deep fractures run
perpendicular to the ridge.
❖ Seamounts, guyots, and black smokers are other geological features that can be
found on the deep sea floor.
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 South Carolina Science Academic
Standards: Grade 5
1) Landforms and Oceans
Standard 5-3: The student will demonstrate an understanding of features, processes, and changes in
Earth’s land and oceans. (Earth Science)

Indicators:
5-3.2: Illustrate the geologic landforms of the ocean floor (including the continental shelf and slope, the
mid-ocean ridge, rift zone, trench, and the ocean basin).

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 South Carolina Science Academic
Standards: Grade 8
1) Earth’s Structure and Processes:
Standard 8-3: The student will demonstrate an understanding of materials that determine the structure of
the Earth and the processes that have altered this structure.

Indicators:
8-3.1: Summarize the three layers of Earth—crust, mantle, and core—on the basis of relative position,
density, and composition.
8-3.6: Explain how the theory of plate tectonics accounts for the motion of the lithospheric plates, the
geologic activities at the plate boundaries, and the changes in landform areas over geologic time.
8-3.7: Illustrate the creation and changing of landforms that have occurred through geologic processes
(including volcanic eruptions and mountain-building forces).

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