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Chapter 4: Plate Tectonics
Section 1: Continental Drift
Vocabulary
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Continental Drift
Alfred Wegener
Pangaea
Evidence for Continental Drift (5)
Continental Drift
• Continental Drift: Hypothesis that
continents have moved slowly to their
current locations
– Alfred Wegener
• 1880-1930
• German meteorologist
• Proposed The Theory of Continental Drift in 1912
• Wegener believed all continents were
once connected as one large landmass
that broke apart 200 mya
– He called this landmass Pangaea
which means “all land”
• Wegener’s ideas about continental drift
were controversial
• Why wasn’t it accepted?
– Unable to explain how the continents drifted
apart
• He proposed the continents were driven through
the ocean floor by the spin of the Earth
• Physicists and Geologists strongly disagreed with
this explanation
• It wasn’t until after Wegener’s death in
1930 that has hypothesis was accepted
Actual Puzzle Company Logo
• What’s wrong with it?
Evidence for Continental Drift
1. Puzzle-like fit of the continents
– Shape of the continents fit together like
puzzle pieces
2. Fossil Clues: Mesosaurus
– Fossils of the Mesosaurus have
been found in South America and Africa
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Mesosaurus lived on land and in freshwater
How could fossils be found on land areas
separated by a large salt water ocean?
 Wegener hypothesized that this reptile must
have lived on both continents when they
were joined
Other fossils that support the
Theory of Continental Drift:
Lystrosaurus and Cynognathus
3. Plant Clues: Glossopteris
– Fossils of this plant have been found in
Africa, Australia, India, South America, and
Antarctica
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The presence of this plant in so many areas
supports the hypothesis of Pangaea
4. Climate Clues
– Wegener used continental drift as evidence
of changing climates
– Fossils of warm-weather plants were found
on the island of Spitsbergen in the Arctic
Ocean
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Wegener hypothesized that Spitsbergen drifted
from tropical regions to the arctic
This also explains evidence of glaciers found in
temperate and tropical areas like South America,
Africa, India, and Australia
– These areas must have been connected and partly
covered with ice near the South Pole long ago
5. Rock Clues
– Similar rocks/rock structures have been
found on different continents that match
rocks/rock structures on the continents they
were joined to
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Parts of the Appalachian Mountains are similar to
those of Greenland and western Europe
Rocks from eastern South America are similar to
those of western Africa
How Could the Continents Drift?
• Wegener provided evidence to support his
theory but couldn’t explain:
– How?
– When?
– Why?
• Because other scientists couldn’t explain it
either, Wegener’s idea was forgotten about
• His idea was also so radically different that most
people closed their minds to it
• After Wegener’s death more clues were found
due to advances in technology and new ideas
(like seafloor spreading) were developed
Section 2: Seafloor Spreading
Vocabulary
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Mid-ocean ridges
Theory of Seafloor Spreading
Harry Hess
Evidence for Seafloor Spreading (2)
Mapping the Ocean Floor
• During the 1940s and 1950s scientists began
using sound waves to map large areas of the
ocean floor in detail
– Sound waves echo off the ocean floor and the longer
the sound waves take to return to the ship the deeper
the water is
– Researchers discovered an underwater system of
ridges (or mountains) and valleys like those found on
land
• This system of underwater mountains is called the mid-ocean
ridges
The Seafloor Moves
• Harry Hess
• 1906-1969
• Princeton University Scientist
• Proposed the Theory of Seafloor Spreading in 1960
• Seafloor Spreading: Theory that as hot, less
dense material below the Earth’s crust rises
towards the surface at the mid-ocean ridges
It then flows sideways, carrying the seafloor away in both
directions
As the seafloor spreads apart, magma moves upward and
flows from the cracks
This magma becomes solid as it cools and forms new
seafloor.
This denser, colder seafloor sinks, helping to form the ridge
• Seafloor Spreading: Theory that as hot, less
dense material below the Earth’s crust rises
towards the surface at the mid-ocean ridges
It then flows sideways, carrying the seafloor away
in both directions
As the seafloor spreads apart, magma moves
upward and flows from the cracks
This magma becomes solid as it cools and forms
new seafloor.
This denser, colder seafloor sinks, helping to form
the ridge
– NOTE: This process happens VERY SLOWLY. Only
about 25 millimeters (1 inch) is added per year, or
about 25 km (16 miles) every million years
http://education.sdsc.edu/optip
uter/flash/seafloorspread.htm
Evidence for Seafloor Spreading
1. Rock Samples from the Glomar Challenger
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1968
Glomar Challenger was a research ship equipped
with a drilling rig that allowed scientists to collect rock
samples from the ocean floor
Scientists found that the youngest rocks are located
at the mid-ocean ridges
The ages of the rocks become increasingly older in
samples gathered farther from the ridges
Newest Rock
Oldest Rock
Core Samples from the Glomar Challenger
2. Magnetic Clues
– Earth’s magnetic field has reversed itself
many times in the past
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Magnetic lines of force leave Earth near the
south pole and enter near the north pole
N
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S
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During a reversal the lines of magnetic force
run the opposite way
N
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S
– By looking at rocks forming along mid-ocean
ridges, Scientists have found proof of these
reversals in these rocks
– Magnetic Time Scale
• Iron-bearing minerals such as magnetite are found
in the rocks of the seafloor
• These rocks record Earth’s magnetic field direction
whenever they form
• Using a sensing device called a magnetometer
that detects magnetic fields, scientists found many
magnetic reversals in the ocean floor that run
parallel to mid-ocean ridges
• This proves that new rock was being formed at
mid-ocean ridges
Magnetic Poles
Magnetic Reversals
• Normal = North to South
• Reverse = South to North
• Brunhes normal - present time to 730,000
years ago
• Matuyama reverse - 0.73 to 2.48 mya
• Gauss normal - 2.48 to 3.40 mya
• Gilbert reverse - 3.40 to 5.3 mya
Section 3: Plate Tectonics
Vocabulary
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Plate tectonics
Tectonic plate
Lithosphere
Asthenosphere
Divergent boundary
Convergent boundary
Subduction zone
Transform Boundary
Convection current
Plate Movements
• In the 1960s scientists came up with a
theory that combined continental drift and
seafloor spreading
• Plate tectonics: Theory that Earth’s crust
and upper mantle are broken into sections
– These sections are called tectonic plates and
move on a plastic-like layer of the mantle
Composition of the Earth’s Plates
• Lithosphere:
– Made up of the crust and a part of the upper
mantle
– This layer is about 100km thick and generally
less dense than the material underneath
(asthenosphere) which they float around on
• Asthenosphere:
– Plastic-like layer below the lithosphere
Yum!
Plate Boundaries
• When plates move they can interact in
several ways
• The result of their movement is seen at the
plate boundaries
Plates Moving Apart
• Divergent boundary: Boundary between
two plates that are moving apart.
– Under water can cause mountains (ex. MidAtlantic ridge)
– On continents can cause rift valleys (ex. East
African Rift Valley)
Mid-Atlantic Ridge
East African Rift Valley
Plates Moving Together
• Convergent Boundary: Boundary where
two plates come together
– When two continental plates collide they push
up the crust to form mountain ranges (ex.
Himalaya)
Himalayas
• Subduction: Boundary where two plates
come together and one plate sinks under
the other
– Between an oceanic and continental plate
– The colder, more dense oceanic plate sinks
under (subducts) the continental plate
– The point at which the oceanic plate goes
down into the mantle is called a subduction
zone
– This is why the Earth doesn’t get any larger, it
recycles itself
– Forms volcanoes (Ring of Fire) and
mountains (ex. Andes)
Ring of Fire
Andes
• Transform Boundary: Boundary that
occurs when two plates slide past each
other
– May move in opposite directions or in the
same direction at different rates
– When one plate slips past another suddenly it
causes an earthquake
• The San Andreas Fault is a transform boundary
and the site of many earthquakes
The San Andreas Fault
Causes of Plate Tectonics
1. Convection Inside the Earth
– Hot, less dense mantle is forced upwards by
the surrounding cool, more dense mantle
– The hot mantle reaches the surface, cools,
and sinks back down
– This cycle of heating, rising, cooling, and
sinking is called a convection current
2. Moving Mantle Material
– Movement of the plates is directly related to
the movement of the convection currents
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Convection currents cause the movement of the
plates by providing energy to move the plates
Testing for Plate Tectonics
• Scientists have had support for their
theories but not proof
• New technologies have allowed the
movement to be measured to provide
proof.
– Using satellites and lasers scientists can
measure the exact movements of the plates
of as little as 1cm per year
– Satellite data shows that not all plates are
moving at the same rate.
• Plates have been observed moving from 1cm to
12cm per year.