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Unit 3: Earth’s Dynamic Geosphere
Section 1: Plate Tectonics
Plate tectonics is a scientific theory. Remember that a scientific theory is a well-tested
concept that is supported by experimental or observational evidence. It explains a wide
range of observations. The theory of plate tectonics explains the formations, movement,
and changes of the outer, rigid layer of the Earth, called the lithosphere.
The Interior Structure of the Earth (G68)
The thin, outermost layer of the Earth is called the crust. There are two kinds of crust:
1. Continental – Forms the Earth’s continents, between 30-50 km thick, and very
old in geological time. (~ 4 billion years old)
2. Oceanic – Forms under the Earth’s bodies of water, only 5-10 km thick, and
young in geological time. (~ 200 million years old)
*Notice the vertical and
horizontal scales are
different. This was
done to exaggerate the
vertical view so you
could see the
separation better. If
drawn to scale, the
layers would look much
thinner.
Beneath the Earth’s crust is the mantle. The rocks of the mantle are very different is
composition from the crust. The division between the crust and the mantle is well defined.
The upper part of the mantle is a cool, rigid block and the crust rests on top. They move
as one and together they are called the lithosphere. The lower part of the mantle is hot
enough that it can flow very slowly like a very stiff liquid. That part is called the
asthenosphere.
Measuring Plate Motions with GPS
The Global Positioning System (GPS) consists of 24 satellites that orbit the Earth.
Receivers at stations on Earth use the signals from satellites to calculate the location of
the station. Geoscientists have set up a network of targets all over the world in order to
monitor the movement of the lithospheric plates. Changes in horizontal and vertical
positions can be detected within several millimeters.
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GPS data collected at stations all over the world confirm that the surface of the Earth is
moving. However, data has only been collected for the past decade. In order to answer
the question whether or not the Earth has been moving in the past, we must look
elsewhere for evidence.
Sea-Floor Spreading (G70)
All magnets and materials that have
magnetism have a north and south
direction, or magnetic polarity. Rocks
with normal magnetic polarity match
that of the Earth’s magnetic field and
point toward the north pole of the
Earth. The other group has magnetic
minerals with reversed polarity and
point toward the south pole.
It was known that as lava cools to
form basalt, its iron minerals become
magnetized and “lock in” the polarity
of the Earth’s magnetic field. Beginning in the 1950’s, scientists began to noting patterns in
the magnetism of rocks on the ocean floor. The alternating belts were of higher (normal
polarity) and lower (reverse polarity) than the average magnetic field strength.
In 1963, two scientists, Vine and Matthews, proposed the sea-floor spreading theory to
explain this pattern. According to their theory, the matching patterns on either side of the
mid-ocean ridge could be explained by new ocean crust forming at the ridge and
spreading away from it. As the ocean crust forms, it obtains the polarity of the Earth’s
magnetic field at the time. Since the Earth’s polarity is always changing, it explains the
magnetic “stripes”.
Since the theory was proposed, samples of volcanic rock have been taken in order to
measure their age. The results supported the theory, with the rocks becoming older as
you moved to the left and right of the ridge. The mid-ocean ridge zig-zags between the
continents, winding its way across the globe like the seams on a baseball. (G71)
Define the following terms using your textbook.
Lithosphere – the outermost layer of the Earth, consisting of the Earth’s crust and part of
the upper mantle. The lithosphere is a rigid layer.
Crust – the thin outermost layer of the Earth. There are two types: continental and oceanic
crust.
Mantle – the zone of the Earth below the crust and above the core. It is divided into the
upper mantle and lower mantle with a transition zone between.
Asthenosphere – the part of the mantle beneath the lithosphere. The asthenosphere
behaves like a thick liquid that undergoes slow flow.
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Global Positioning System – a satellite-based system for accurate location of points on the
Earth.
Basalt – a kind of volcanic igneous rock, usually dark colored, with a high content of iron.
Mid-Ocean Ridge – a chain of undersea ridges extending throughout all of the Earth’s
ocean basins, and formed by sea-floor spreading.
Review Questions
1. Using the map on page G63 of your text, which named GPS station is closest to us?
2. How does GPS provide evidence that the surface of the Earth moves over time?
3. Describe at least one advantage of using GPS technology to gather evidence of plate
motion.
4. How do studies of the magnetism of rocks on the sea floor provide evidence that the
surface of the Earth moves over time?
5. Predict: How might the difference in motion of these plates affect the Earth’s
lithosphere?
6. Using the map of the plates on page G66, answer the following questions:
a. Look at the names of the plates. On what basis does it appear that the plates
were named?
b. Write down the name of the plate we live on.
c. Describe the motion of your plate relative to all the plates that border it.
7. On the map provided on the following page, use colored pencils to draw and label the
major lithospheric plates. Then add their direction of movement with arrows.
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New plates are created at certain places on Earth, and existing plates are consumed at
certain other places. The total surface area of the planet stays the same, so the creation of
new plates has to be exactly equal to the consumption of existing plates.
Types of Plate Boundaries
Geologists use three descriptive terms to classify the boundaries between plates:
1. divergent boundaries – two plates move away from each other
2. convergent boundaries – two plates move toward each other
3. transform boundaries – two plates slide parallel to each other
1. Divergent Plate Boundaries
a. Characterized by basaltic volcanoes and shallow
focus earthquakes.
b. As the plates move apart, the mantle
asthenosphere rises slowly upward, and a rift
valley is created.
c. Rift valleys are usually found atop mid-ocean
ridges such as the Mid-Atlantic Ridge.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
2. Convergent Plate Boundaries
There are three types of convergent plate
boundaries:
a. Ocean – Ocean Convergence
 Occurs when two oceanic plates collide.
 One plate stays at the surface and the other
gets pushed underneath. This process is
called subduction.
 The very deepest ocean depths are at
subduction zones.
 Forms volcanic islands called a volcanic
island arc.
b. Ocean – Continent Convergence
 Occurs when an oceanic plate collides with a
continental plate.
 The ocean plate always gets subducted
because it is less dense than the continental
plate.
 Forms a chain of volcanoes on the continent called a continental
volcanic arc. (Example: The Andes Mountains in western South
America)
c. Continent – Continent Convergence
 Occurs when two continental plates collide.
 Continental plates cannot be subducted because they are too
dense.
 Creates a suture zone which is a place where two continents
have met and become welded into a single continent.
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3. Transform Plate Boundaries
 The place where two plates slide past each other is called a transform fault.
 The boundary between the North American and Pacific plates is a transform fault
called the San Andreas Fault. California has many earthquakes because of this
fault line.
Define the following terms using your textbook.
Divergent Plate Boundary – a plate boundary where two plates move away from one
another.
Rift Valley – a large, long valley on a continent, formed where the continent is pulled apart
by forces produced when mantle material rises up beneath the continent.
Convergent Plate Boundary – a plate boundary where two plates move toward each other.
Subduction – the movement of one plate downward into the mantle beneath the edge of
the other plate at a convergent plate boundary. Only oceanic plates subduct.
Suture Zone – the zone on the Earth’s surface where two continents have collided and
have been welded together to form a single continent.
Transform Plate Boundary – a plate boundary where two plates slide parallel to one
another.
Transform Fault – occurs when two lithospheric plates slide parallel to each other and
there is an offset between two segments of a separating ridge.
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Review Questions
1. Name the three types of boundaries between lithospheric plates.
2. How and where are rift valleys formed?
3. Convergent plate boundaries can be in three different settings. What are they?
a.
b.
c.
4. Why does the oceanic plate always get subducted?
5. Question 5 has been removed
6. What happens when two continents collide along a convergent plate boundary?
Evidence of Earth’s Layered Structure
1. The density of a substance refers to how concentrated the mass of the material is. Less
dense materials tend to rise upward and float on more dense material. Rocks in the
Earth’s crust are less dense than the rocks of the underlying mantle. The crust floats on
the more dense interior material.
Density = Mass / Volume



Example: D = 100 g / 25 cm3
D = 4.0 g/cm3
Heating materials decreases their density, thus magma rises to the surface.
Earth’s average density is 5.5 g/cm3 with its surface being 2.8 g/cm3
Earth’s core is much more dense than the crust because it’s under great pressure and
made mostly of iron.
2. Seismic waves are waves of motion generated within the Earth by earthquakes. Waves
travel faster the deeper they are within the Earth due to increased density. By studying the
changes in wave speed, scientists concluded that the Earth’s interior structure is layered.
3. The last piece of evidence has to do with the major layers of the Earth. From the inside
moving out they are: inner core, outer core, mantle, and crust. Thermal convection is
movement in a fluid caused by heating from below and cooling from above.
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Mantle material is heated up deep inside the Earth and then rises to the crust. When the
crust cools it becomes more dense and therefore sinks back down into the mantle.
Building Features
on Earth’s Surface
The building of the
features on Earth’s
surface due to
deformation caused
by plate movements
is called plate
tectonics. Plate
movements create
mountain ranges,
trenches, rift valleys,
and cause earthquakes and volcanoes.

Subduction at plate boundaries forms trenches that can be 10,000 meters deep.

Magma rises at subduction zones forming volcanic arcs which together are called the
Ring of Fire. However, not all “hot spots” are located near the edges of plates but
sometimes occur elsewhere like Yellowstone National Park where magma rises up
close to the surface.

Friction caused by plates subducting or sliding past each other cause earthquakes.

Mountain ranges are formed when two plates are pushed together and instead of one
subducting, they both thrust upward.
Define the following terms using your textbook.
Density – the mass per unit volume of a material or substance.
Core – the solid, innermost part of the Earth consisting mainly of iron.
Thermal Convection – a pattern of movement in a fluid caused by heating from below and
cooling from above.
Plate tectonics – the field of study of plate motion.
Continental Accretion – the growth of a continent along its edges.
Pangea – Earth’s most recent Supercontinent which was rifted apart about 200 million
years ago.
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Supercontinent – a large continent consisting of all of the Earth’s continental lithosphere.
Supercontinents are assembled by plate-tectonic processes of subduction and continentcontinent collision.
Paleomagnetism – the record of the past orientation and polarity of the Earth’s magnetic
field recorded in rocks containing the mineral magnetite.
Review Questions
1. How is the density of the Earth calculated?
2. Name the three major layers of the Earth from the exterior to the interior.
3. Why is the inner core of the Earth solid even though it is hot?
4. How do convection currents work?
5. What part of the Earth’s interior layers are in motion due to the density difference?
6. What geographic features would you expect to see at plate boundaries? (3 features)
7. In your own words, explain the process of continental accretion. (G103)
Section 1 Vocabulary
_____________________________________________________________________
Lithosphere
Basalt
Asthenosphere
Crust
Mantle
Mid-Ocean Ridge
Global Positioning System (GPS)
_____________________________________________________________________
Divergent Plate Boundary
Rift Valley
Subduction
Convergent Plate Boundary
Suture Zone
Transform Plate Boundary
Transform Fault
______________________________________________________________________
Thermal convection
Plate tectonics
Continental accretion
Density
Pangea
Paleomagnetism
Core
Supercontinent
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