Transcript Plate Tectonics

Plate Tectonics
Section 1: Earth’s Interior
Section 2: Convection and the Mantle
Section 3: Drifting Continents
Section 4: Sea-Floor Spreading
Section 5: The Theory of Plate Tectonics
Earth’s Interior

How have geologists learned about
Earth’s inner structure?
• Geologists have used two main types of
evidence to learn about Earth’s interior:

Direct evidence
• from rock samples

Indirect evidence
• from seismic waves
Earth’s Interior

Geologists cannot
look inside Earth
• Use an indirect
method.

Seismic Waves
(Earthquake waves)
• Study how they
travel through
Earth.
 Data reveals
several layers
Earth’s Interior
Earth’s Interior
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
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
Crust
Mantle
Outer Core
Inner Core
Earth’s Interior

Crust
• Solid layer of rock that includes both dry land
and the ocean floor
• Between 5 and 40 km thick

Dry land
• Granite
 light in color
 less dense

Ocean floor
• Basalt
 dark in color
 more dense
Earth’s Interior

Mantle
• very hot, but solid rock
• about 3,000 km thick

Lithosphere
• Crust and Upper Mantle region
• “Lithos”, Greek for Stone

Asthenosphere
• Remainder of Mantle
• Very soft and hotter than Lithosphere
• “Asthenes”, Greek for Weak

*Lower Mantle
• Solid and extends to the Core region
Earth’s Interior
Earth’s Interior

Core
• Mostly Iron and Nickel

Outer Core
• molten metal (Fe and Ni) that surrounds the
Inner Core.
• Liquid
 creates the magnetic field of the Earth
 Bar-magnet with North and South Poles

Inner Core
• dense ball of metal (Fe and Ni)
 Intense pressure prevents it from changing
from solid to liquid.
Earth’s Interior

Pressure and Temperature increase
as you move towards the center of
the Earth
Earth’s Interior
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Core
Earth’s Interior
Assessment Questions
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Why is it difficult to determine Earth’s
inner structure?
How are seismic waves used to provide
evidence about Earth’s interior?
List Earth’s three main layers
What is the difference between the
lithosphere and the asthenosphere?
In which layer is each located?
Section 1.2
Convection in the Mantle

How is heat transferred?
• There are three types of heat transfer:
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Radiation
Conduction
Convection
Radiation

The transfer of energy through space.
• No direct contact between a heat source and
an object.

Sunlight warms the Earth
Conduction

Heat transfer within a material or
between materials that are touching.
• Holding a hot spoon.

Conduction is responsible for some of the
heat transfer inside Earth.
Convection

Heat transferred by the movements
of liquids and gases.
• Caused by:


Density differences
Gravity
• Density
 mass in a given volume
 More heat = expansion = less dense
 Less heat = contraction = more dense
Convection

What causes convection currents?
• Gravity
• Difference in Density of liquids or gases.

Gravity pulls denser materials downward
• More heat = expansion = less dense = moves upward
• Less heat = contraction = more dense = moves
downward
Convection
Convection Currents

What causes convection currents in
Earth’s mantle?
• Heat from the core and mantle causes
convection currents in the mantle.
Visual Summary of the Three
Types of Heat Transfer
1.2 Assessment Questions
1. How is heat transferred?
2. What causes convection currents?
3. What causes convection currents in
Earths mantle?
4. What is conduction?
1.2 Assessment Questions
5. What is the role of gravity in
creating convection currents?
6. What part of Earth’s interior is like
the soup in the pot? What part is
like the burner on the stove? (Fig 9)
7. How is heat transferred through
space?
8. What is a convection current?
1.2 Assessment Questions
9. In general, what happens to the density
of a fluid as it becomes hotter?
10. Describe how convection currents form.
11. Name two layers of Earth in which
convection currents take place.
12. What causes convection currents in the
mantle?
13. What will happen to the convection
currents in the mantle if Earth’s interior
eventually cools down? Explain.
Section 1.3
Drifting Continents
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What was Alfred Wegener’s
hypothesis about the continents?
What evidence supported Wegener’s
hypothesis?
Why was Wegener’s hypothesis
rejected by most scientists of his
day?
Continental Drift

Why do the coasts of several
continents match so neatly?
• West Africa and Eastern South America
seem to fit together like puzzle pieces.
Continental Drift
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Alfred Wegener
• Hypothesized that
Earth’s continents
has moved over
time.
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All part of one large,
ancient landmass
Drifted apart over
time
• His idea was called
“Continental Drift”
Pangaea
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Ancient landmass
• “All Lands”
• All continents connected together
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300 million years ago
Pangaea
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Slowly broke apart
to where
continents are
located today.
• Supported by
studying:
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land features
fossils
evidence of climate
change
Pangaea
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Supporting Evidence
• Land Features
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mountain ranges on Africa and South
America lined-up.
coal fields in Europe and North America
match-up
Supporting Pangaea:
Land Features
Pangaea
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Supporting Evidence
• Fossils
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Glossopteris
• fernlike plant
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Mesosaurus and Lystrosaurus
• land-dwelling dinosaurs
 Fossils of these organisms are found on many
different continents separated by great
oceans!!
 How could that be possible?
Supporting Pangaea: Fossils
Pangaea
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Climate
• Spitsbergen
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Present location: Arctic Ocean (Cold!!)
• Tropical plant fossils are found there!
• South Africa
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Present climate: Mild/Warm
• Deep scratches, caused by glaciers are found in
the crust.
• Glaciers exist in very cold environments!!
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*Climates changed because landmasses
moved.
Hypothesis Rejected!!
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Wegener was not able to provide a
satisfactory explanation for the force
that pushes or pulls the continents.
• More evidence would be needed.
Mountain Formation
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Wegener hypothesized that when
continents collide, their edges
crumple and fold.
• Huge mountains form.
1.3 Assessment Questions
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1. What was Alfred Wegener’s hypothesis
about the continents?
2. What evidence supported Wegener’s
hypothesis?
3. Why was Wegener’s hypothesis
rejected by most scientists of his day?
1.3 Assessment Questions
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4. What do the matching mountain ranges
in Africa and South America show,
according to Wegener’s hypothesis?
5. How would continental drift affect the
continent’s climate?
6. According to Wegener, how do
mountains form?
1.3 Assessment Questions
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7. Who proposed the concept of
continental drift?
8. According to the hypothesis of
continental drift, how would a world map
have changed over the last 250 million
years?
9. What evidence supported the
hypothesis of continental drift?
10. How did fossils provide evidence for
continental drift?
1.3 Assessment Questions
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11. Deposits of coal have been found
beneath the ice of Antarctica. But coal
only forms in warm swamps. Use
Wegener’s hypothesis to explain how coal
could be found so near to the South Pole.
12. Why did most scientists reject
Wegener’s hypothesis of continental drift?
13. Do you think the scientists of
Wegener’s time should have accepted his
hypothesis? Why or why not?
Chapter 1.4
Sea-Floor Spreading
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Mid-Ocean Ridges
• an undersea maintain chain where new
ocean floor is produced.
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divergent (moving apart) plate boundary
Chapter 1.4
Sea-Floor Spreading
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What device is used to map the
ocean floor?
• Mapped in the mid-1900’s by using
SONAR.
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Underwater soundwaves
• How long does the echo take to travel? = distance
to the bottom.
Mid-Ocean Ridge
Chapter 1.4
Sea-Floor Spreading
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What is the process of Sea-Floor
Spreading?
• Sea floor spreads apart along both sides
of a mid-ocean ridge as new crust is
added from below.
Evidence of Sea-Floor Spreading
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What is the evidence of Sea-Floor
Spreading?
• 1. Molten Material
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Bubble-shaped rocks found on the ocean
floor.
• caused only by cooling magma
Evidence of Sea-Floor Spreading
• 2. Magnetic Stripes
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Iron in molten rock aligns to magnetic poles
Poles “Flip” over many thousands of years
• Repeated pattern of north and south orientation.
 pattern is the same on both sides of the MidOcean Ridge.
Magnetic Reversal Stripes
Evidence of Sea-Floor Spreading
• 3. Drilling Samples
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Oldest rocks
• farther from Mid-Ocean
Ridge
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Youngest rocks
• closer to Mid-Ocean Ridge
Subduction at Trenches
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What happens at Deep-Ocean
Trenches?
• Trenches
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Opposite of Sea-Floor Spreading
• Oceanic rock is forced downward, or subducts,
and eventually melts into magma.
Deep Ocean Trenches
Subduction at Trenches
• Together, Subduction and Sea-Floor
Spreading act as a conveyor belt
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cooling magma into new rock (Mid-Ocean
Ridges).
melting old rock into magma (Trenches)
1.4 Assessment Questions
1.Along what feature of the ocean floor
does sea-floor spreading begin?
2.What are the steps in the process of sea-floor
spreading?
3.What three types of evidence provided support
for the theory of sea-floor spreading?
4.How do rocks along the central valley of the midocean ridge provide evidence of sea-floor
spreading?
1.4 Assessment Questions
5.Where would you expect to find the oldest rock
on the ocean floor?
6.What is a deep-ocean trench?
7.What happens to oceanic crust at a deep–ocean
trench?