Chapter 18
Plate Tectonics
History of Earth’s interior
• Earth formed ~4.6 billion years ago in the solar
nebula
• Early Earth had molten surface from bombarding
materials
• Surface cooled and crystallized to igneous rocks
• Heat accumulated from radioactive decay led to
a second melting of the interior. This time
melting occurred in pockets, not throughout
interior.
Seismic Wave
• A seismic wave is a vibration that moves
through any part of the earth. They can be
used by geologists to learn about the
earth’s interior.
• Seismic waves radiate outward from an
earthquake, spreading in all directions
through the solid earth’s interior like sound
waves from an explosion.
Earth’s Internal Structure
• The 3 main zones of the earth’s interior,
determined through seismic studies are:
1. The crust, is the outer layer of rock that
forms a thin shell around the earth.
2. The mantle is below the crust and is a
much thicker shell than the crust.
3. The core is the center part. There is an
inner core and a liquid outer core.
The structure of the earth’s interior.
The Crust
• The crust is a thin skin that covers the
entire earth. It is below oceans and also
makes up continents, although the two are
different because they are made of
different types of rock:
1. The oceanic crust is much thinner and
denser, it is much younger and it is
composed of basalt.
2. The continental crust is much thicker and
less dense, it is much older and it is
composed of granite and other rocks.
Continental crust is less dense, granite-type rock, while the oceanic crust is more dense,
basaltic type rock. Both types of crust behave as if they were floating on the mantle,
which is more dense than either type of crust.
The Crust
• The boundary between the crust and the
mantle is called Mohorovicic discontinuity,
Moho for short.
The Mantle
• The mantle is a thick layer between the core and
the crust. It takes up about 80% of the total
volume of the earth and accounts for about 2/3
of the earth’s total mass.
• The mantle is composed of silicates, particularly
the ferromagnesian silicate olivine.
• The evidence for the composition of the mantle
comes from seismic studies, studies of the
composition of meteorites, and studies of
materials that have been ejected to the earth’s
surface by volcanoes.
The Core
• The core has been found to have 2 parts:
1. A liquid outer core
2. A solid inner core
• The core makes up about 15% of the earth’s total
volume and about 1/3 of its mass.
• The core has been found to be mostly iron. This is
believed because the magnetic field which is produced
indicates that it is composed of an electrical conductor, a
metal such as iron.
• Meteorites that fall to earth are either stony (made of
rock which are silicate materials), or iron meteorites
made up of iron or a nickel-iron alloy.
• Since the earth has a silicate-rich crust and mantle, by
analogy the earth’s core must consist of iron or a nickel
and iron alloy.
Earth’s Interior Structure
Iron meteorites – composed of
iron or nickel-iron alloy
Stony meteorites – composed of
silicates
What happens when both melted on early planet?
Earth’s Interior Structure
Differentiation –
separation of materials
while in molten stage
Silicates remained near
surface
Iron and nickel sank to
core
Asthenosphere
• Another way of looking at the earth’s structure is
by considering the following:
• There is also a part of the mantle called the
asthenosphere, after the Greek for “weak
shell”.
• It is weak because it is plastic like, mobile, and
yields to stresses. It is a hot, elastic, semi liquid
layer that extends around the entire earth.
• In some regions it is completely liquid,
containing pockets of magma.
The Lithosphere
• The rocks above and below the asthenosphere are rigid,
solid and brittle.
• The solid layer above the asthenosphere is called the
lithosphere, after the Greek for “stone shell”.
• The lithosphere is also known as the strong layer in
contrast to the asthenosphere, which is the weak layer.
• The lithosphere includes the entire crust, the Moho, and
the upper part of the mantle.
• The lithosphere is one of the important sources of
magma that reaches the earth’s surface.
• It is also an important part of the mechanism involved in
the movement of the crust. It is made up of
comparatively rigid plates that are moving, floating in the
upper mantle like giant ice sheets floating in the ocean.
The earth’s interior, showing the weak,
plastic layer called the asthenosphere.
The rigid, solid layer above the
asthenosphere is called the lithosphere.
The lithosphere is broken into plates that
move on the upper mantle like giant ice
sheets floating on water.
This arrangement is the foundation for
plate tectonics, which explains many
Changes that occur on the earth’s surface
such as earthquakes, volcanoes, and
mountain-building.
Theory of Plate Tectonics
• Continental shapes suggest that the continents were at
one time together, breaking apart and moving to their
present position sometime in the past. For example, the
Americas and Europe and Africa.
• A geologist named Wegener in the early 1900’s believed
that at one time there was a single land mass called
Pangaea, from the Greek meaning “all lands”. He
pointed out that similar fossils found in landmasses on
both sides of the Atlantic Ocean today must be from
animals and plants that lived in Pangaea, which later
broke up into smaller continents.
• Wegener’s concept became known as continental drift,
the idea that individual continents could shift positions on
earth’s surface.
• In the 1950’s this idea resurfaced.
Theory of plate tectonics
Land masses appeared to
belong to one giant land
mass, Pangaea.
Plate Tectonics-Evidence from the
Ocean
• Ocean basins are the bottom of the ocean floor.
They are covered by 3-4 miles of water.
• There are valleys, hills, mountains, and
mountain ranges. Oceanic ridges are long,
high, and continuous chains of mountains that
seem to run clear around the earth.
• The Mid-Atlantic ridge is an oceanic ridge that is
located at the center of the Atlantic Ocean basin.
It divides the Atlantic Ocean into two nearly
equal parts. Where it is high enough to reach
sea level it makes oceanic islands such as
Iceland.
The Mid-Atlantic Ridge divides the Atlantic Ocean into two nearly equal parts. Where the
Ridge reaches above sea level, it makes oceanic islands, such as Iceland.
Plate Tectonics-Evidence from the Ocean
• The following has been observed in
relation to the Mid-Atlantic Ridge:
1. Submarine earthquakes which occur in a
narrow band under the crest of the MidAtlantic ridge.
2. A rift (a long, continuous, and deep
valley) which runs along the crest of the
Mid-Atlantic Ridge for its length.
3. Large amounts of heat found to be
escaping from the rift.
Plate Tectonics-Evidence from the Ocean
• It was concluded that the rift may be a
crack in the earth’s crust, a fracture
through which basaltic lava flowed to build
up the ridge.
• This led to the hypothesis of seafloor
spreading, that hot, molten rock moved
up from the interior of the earth to emerge
along the ridge, flowing out in both
directions to create new rocks along the
ridge.
• The creation of new rock would tend to
spread the seafloor in both directions.
Plate Tectonics-Evidence from the Ocean
• Fossil samples taken by drilling into the ocean’s
floor has indicated that the layers of sediments
near the ridge were thin and became
progressively thicker toward the continents.
• This is what indicated that the seafloor was
spreading, since the older layers would have
more time to accumulate greater depths of
sediments. The oldest layers of sediments
proved to be the ones near the continents, about
150 million years old.
• This indicates that the Atlantic Ocean did not
exist until 150 million years ago. At that time a
fissure formed between Africa and South
America.
The pattern of seafloor ages on both sides of the Mid-Atlantic Ridge reflects seafloor
spreading activity. Younger rocks are found closer to the ridge.
Plate Tectonics
• According to plate tectonics, the lithosphere is
broken into a number of fairly rigid plates that
move on the asthenosphere.
• Some plates contain continents and part of an
ocean basin, while other plates contain only
ocean basins.
• The plates move and the movement is helping to
explain why mountains form where they do, the
occurrence of earthquakes and volcanoes, and
in general, the entire changing surface of the
earth.
• Earthquakes, volcanoes, and most rapid
changes in the earth’s crust occur at the edge of
a plate, which is called a plate boundary.
The major plates of the lithosphere that move on the asthenosphere.
Plate Boundaries
• There are 3 types of plate boundaries that
describe how one plate moves relative to
another: divergent, convergent, and
transform.
• Divergent boundaries occur between two
plates moving away from each other.
Magma forms as the plates separate. New
crust is then added to the edges of the
separating plates. The divergent boundary
is a new crust zone.
A diverging boundary at a mid-oceanic ridge. Hot asthenosphere wells upward
beneath the ridge crest. Magma forms and squirts into fissures. Solid material
that does not melt remains as mantle in the lower part of the lithosphere. As the
lithosphere moves away from the spreading axis, it cools, becomes denser, and
sinks to a lower level.
Divergent Boundaries
• The Mid-Atlantic Ridge is a divergent boundary
between the South American and African Plates,
extending north between the North American
and Eurasian plates.
• The results of divergent plate movement can be
seen in Iceland, where the Mid-Atlantic Ridge
runs as it separates the North American and
Eurasian Plates. In the northeastern part of
Iceland, ground cracks are widening, often
accompanied by volcanic activity. Between 1975
and 1984 displacements caused a total
separation of about 23 feet.
At some point along the mid-ocean ridge, it
comes above sea-level.
The major plates of the lithosphere that move on the asthenosphere.
Divergent Boundaries
• The measured rate of spreading along the
Mid-Atlantic Ridge ranges from 1-6 cm per
year.
• Another major ocean may be in the
making in East Africa, where a divergent
boundary has already moved Saudi Arabia
away from the African continent, forming
the Red Sea.
This section moving away from mainland
Africa and could be a large island.
Lithospheric plates and boundaries
Divergent boundaries – two plates moving away from each other.
Convergent Boundaries
• Convergent Boundaries occur between two
plates moving towards each other.
• The creation of new crust at a divergent
boundary means that old crust must be
destroyed somewhere else or else the earth
would have an ever expanding diameter. Old
crust is destroyed by returning to the
asthenosphere at convergent boundaries.
• The collision produces an elongated belt of
down-bending called a subduction zone.
Convergent Boundaries
• The lithosphere of one plate, which
contains the crust, is subducted beneath
the second plate and partially melts, then
becoming part of the mantle.
• The oceanic lithosphere is being recycled,
which explains why ancient seafloor rocks
do not exist.
• Earthquakes, volcanoes, and mountain
ranges result from convergent boundaries.
Lithospheric plates and boundaries
Convergent boundaries – two plates moving toward each other.
Lithospheric plates and boundaries
Density differences
force one plate to
go below another.
Oceanic crust goes
under continental
crust
Since our Earth is not growing in size, what is created at
divergent boundaries goes away somewhere else.
Lithospheric plates and boundaries
Trenches created at
subduction zones.
Subduction zones where old crust is sent back to
asthenosphere for recycling.
3 types of convergent boundaries:
Continental-Oceanic, OceanicOceanic, Continental-Continental
Continental-oceanic
Gives rise to the
Andes Mountains
and deep-seated
earthquakes.
Lithospheric plates and boundaries
Peru-Chile trench
Continent-ocean plate convergence. This type of
Plate boundary accounts for shallow and deep-seated
earthquakes, an oceanic trench, volcanic activity,
And mountains along the coast.
Andes mountains
Oceanic-oceanic. Gives rise to curving chains
of volcanic islands called island arcs that occur
over the belt of deep-seated earthquakes.
Aleutian Islands
Japanese, Marianas,
and Indonesians.
Lithospheric plates and boundaries
Ocean-ocean plate
convergence. This type of
plate convergence accounts for shallow
and deep-focused earthquakes,
an oceanic trench, and a volcanic arc
Above the subducted plate.
Aleutian islands, Japanese Islands
Continent-continent
Lithospheric plates and boundaries
Indian-Australian plate
Eurasian plate
Continent-continent plate convergence. Rocks are deformed,
and some lithosphere thickening occurs, but neither plate is
subducted to any great extent. Gave rise to the Tibetan Plateau and the
Himalayan Mountains.
Transform Boundaries
• Transform boundaries occur between two
plates sliding by each other.
• Crust is neither created nor destroyed at
transform boundaries as one plate slides
horizontally past another along a long, vertical
fault.
• The Pacific Plate is moving slowly to the
northwest, sliding past the North American Plate.
• The San Andreas Fault is one boundary along
the California coastline. Vibrations from plate
movement along this boundary are the famous
California earthquakes.
Lithospheric plates and boundaries
Transform boundaries – two plates next to each other.
Aleutian Islands
Japan
The Alps
The Andes
The Himalayas
The New Zealand “Alps”
Present-Day Understanding
• Today what is generally accepted about the
plate tectonic theory is the understanding that
the solid materials of the earth are engaged in a
continual cycle of change.
• Oceanic crust is subducted, melted, then partly
returned to the crust in volcanic igneous rocks in
island arcs and along continental plate
boundaries.
• Other parts of the subducted crust become
mixed with the upper mantle, returning as new
crust at diverging boundaries.
• The materials of the crust and the mantle are
thus cycled back and forth in a mixing that may
include the deep mantle and the core as well.
Exercises
• Applying the concepts.
p 458 # 1, 2, 4, 5, 7, 8, 9, 10
New Book: p. 504-506
# 1, 2, 4, 7, 8, 9, 10, 11, 13, 14, 16, 17, 18,
19, 20, 21, 27, 28, 30, 31, 32, 33, 36, 38,
41, 42, 43, 45, 46, 47, 48, 49, 50, 52
Review
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The Earth was molten at one time.
Seismic waves and seismic studies.
The 3 parts of the earth: crust (thinner, on top), mantle
(thicker, middle), core (solid inner and liquid outer,
innermost layer).
Oceanic crust is thinner, younger, denser, made up of
basalt. Continental crust is thicker, less dense, older, and
composed of granite.
The Moho is the boundary between the crust and the
mantle.
The mantle is made up largely of the ferromagnetic
silicate called olivine.
The core is made up largely of iron (evidence from
meteorites, and the magnetic field of the Earth.
The asthenosphere is a part of the mantle which is a
weak, plastic layer.
The lithosphere includes the entire crust, the Moho, and
the upper part of the mantle. It is made up of rock. It is
the “strong layer”.
The lithosphere is comparatively rigid plates that are
moving, floating in the upper mantle like giant ice sheets
floating in the ocean.
The continental drift.
Earthquakes, volcanoes, and most rapid changes in the
earth’s crust occur at the edge of a plate, which is called
a plate boundary.
Oceanic ridges are long, high, and continuous chains of
mountains that seem to run clear around the earth. The
Mid-Atlantic ridge is an oceanic ridge that is located at
the center of the Atlantic Ocean basin.
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There are 3 types of plate boundaries:
divergent, convergent and transform.
Divergent boundaries-Plates moving away
from each other-new crust is formed from
magma. Ocean crust near continents is
older. e.g. the Midatlantic Ridge.
Convergent boundaries-Plates moving
towards each other. Result in earthquakes,
volcanoes, mountains.
The collision produces an elongated belt of
down-bending called a subduction zone.
The oceanic lithosphere is being recycled,
which explains why ancient seafloor rocks
do not exist.
Continent-Ocean convergence leads to
earthquakes and mountains along the coast,
like the Andes in South America.
Ocean-ocean convergence leads to volcanic
islands like Japan and the Aleutian islands.
These are called island arcs.
Continent-continent convergence leads to
tall mountain ranges like the Himalayas.
There is not much subduction.
Transform boundaries-2 plates sliding past
each other-No land is recycled but
earthquakes result, e.g. San Andreas fault.