lesson 2 development of a theory -...

Chapter Introduction

Lesson 1 The Continental Drift Hypothesis

Lesson 2 Developmentof a Theory

Lesson 3 The Theory ofPlate Tectonics

Chapter Wrap-UpArctic_Images/Getty Images

What is the scientific theory of plate tectonics?

What do you think?

Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements.

1. India has always been north of the equator.

2. All the continents once formed one supercontinent.

3. The seafloor is flat.

Do you agree or disagree?

4. Volcanic activity occurs only on the seafloor.

5. Continents drift across a molten mantle.

6. Mountain ranges can form when continents collide.

Do you agree or disagree?

Lesson 1 Reading Guide - KC

• What evidence supports continental drift?

• Why did scientists question the continental drift hypothesis?

The Continental Drift Hypothesis

Lesson 1 Reading Guide - Vocab

• Pangaea

• continental drift

The Continental Drift Hypothesis

Lesson 1-1

• Nearly 100 years ago, Alfred Wegener proposed that all the continents were once part of a supercontinent called Pangaea.

• Over time, Pangaea began breaking apart and the continents slowly moved to their present position.

Pangaea

Lesson 1-1

• Wegener proposed the hypothesis of continental drift, which suggested that continents are in constant motion on the surface of Earth.

• Wegener observed the similarities of continental coastlines now separated by oceans and how they could fit together like pieces of a puzzle.

Pangaea (cont.)

Lesson 1-2

The eastern coast of South America mirrors the shape of the west coast of Africa.

Lesson 1-2

• Evidence to support Wegener’s hypothesis is found in

• climate clues;

• fossil clues;

• rock clues.

Evidence That Continents Move

• When Wegener pieced Pangaea together, he proposed that the continents were located closer to the South Pole 250 million years ago.

Lesson 1-2

• Wegener suggested that a large sheet of ice covered the continents.

• Wegener studied the sediments left behind and the glacial grooves that formed when the ice sheets melted and Pangaea spread apart.

• This provided climate evidence for continental drift.

Evidence That Continents Move (cont.)

Lesson 1-2

The presence of an ice sheet covering Pangea could explain glacial features found on some continents today.

Lesson 1-2

• Animals and plants that live on separate continents can be unique to that continent alone.

• Fossils of similar organisms have been found on several continents separated by oceans.

• Fossils of a plant called Glossopteris have been found on continents that are now separated by oceans.

Evidence That Continents Move (cont.)

Lesson 1-2

• The orange area shows where fossils of Glossopteris have been found.

• Fossils provide evidence for continental drift.

Lesson 1-2

• Wegener observed that mountain ranges and rock formations on different continents had common origins, providing rock evidence for continental drift.

• Volcanic rock that is identical in chemistry and age has been found on both the western coast of Africa and the eastern coast of South America.

Evidence That Continents Move (cont.)

Lesson 1-2

The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type.

Lesson 1-2

Evidence That Continents Move (cont.)

How were similar rock types used to support the continental drift hypothesis?

Lesson 1-3

• Wegener’s ideas were not widely accepted until nearly four decades later.

• Scientists questioned continental drift because it was a slow process and Wegener could not measure how fast continents moved or how they moved.

• Scientists could not understand how continents could push their way through the solid rock of the mantle and the seafloor.

What was missing?

Lesson 1-3

What was missing? (cont.)

mantle

Science Use the middle layer of Earth, situated between the crust above and the core below

Common Use a loose, sleeveless garment worn over other clothes

Lesson 1-3

What was missing? (cont.)

Why did scientists argue against Wegener’s continental drift hypothesis?

Lesson 1 - VS

• All continents were once part of a supercontinent called Pangaea.

Lesson 1 - VS

• Alfred Wegener proposed that continents move around on Earth’s surface.

Lesson 1 – LR1

A. Pangaea

B. continental drag

C. continental movement

D. continental drift

What term did Wegener use to describe the constant motion of continents on the surface of Earth?

Lesson 1 – LR2

A. fossils

B. ice sheets

C. plates

D. continental coastlines

In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle?

Lesson 1 – LR3

A. equator

B. South Pole

C. North Pole

D. Pacific ocean

Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what?

Lesson 1 - Now

1. India has always been north of the

equator.

2. All the continents once formed one

supercontinent.

Do you agree or disagree?

Lesson 2 Reading Guide - KC

• What is seafloor spreading?

• What evidence is used to support seafloor spreading?

Development of a Theory

Lesson 2 Reading Guide - Vocab

• mid-ocean ridge

• seafloor spreading

• normal polarity

• magnetic reversal

• reversed polarity

Development of a Theory

Lesson 2-1

• During the late 1940s scientists were able to determine the depth of the ocean using a device called an echo sounder.

• Once ocean depths were determined, scientists used these data to create a topographic map of the sea floor that revealed vast mountain ranges, called mid-ocean ridges, that stretch for many miles deep below the ocean’s surface.

Mapping the Ocean Floor

Lesson 2-1

Seafloor Topography

Lesson 2-2

• By the 1960s, scientists discovered the process of seafloor spreading.

• Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.

• When the seafloor spreads, the mantle below melts and forms magma.

Seafloor Spreading

Lesson 2-2

• Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock.

• Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas.

• As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge.

Seafloor Spreading (cont.)

Lesson 2-2

Seafloor Spreading (cont.)

Lesson 2-2

Seafloor Spreading (cont.)

What is seafloor spreading?

Scientists argued that if the seafloor spreads, the continents must also be moving.

Lesson 2-2

• The rugged mountains that make up the mid-ocean ridge system can form in two different ways.

• Large amounts of lava can erupt from the center of the ridge, cool, and build up around the ridge.

Seafloor Spreading (cont.)

Lesson 2-2

• Or, as the lava cools and forms new crust, it cracks and the rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.

• The abyssal plain, the smooth part of the seafloor, is made when the layer of sediment that accumulates far from the mid-ocean ridge becomes thick enough.

Seafloor Spreading (cont.)

Lesson 2-2

Continents move as the seafloor spreads along a mid-ocean ridge.

Lesson 2-3

• The first evidence used to support seafloor spreading was discovered in rocks on the seafloor.

• Scientists studied the magnetic signature of minerals in these rocks.

• Earth’s magnetic field today is described as having normal polarity—a state in which magnetized objects, such as compass needles, will orient themselves to point north.

Development of a Theory

Lesson 2-3

• Sometimes a magnetic reversaloccurs and the magnetic field reverses direction.

• The opposite of normal polarity is reversed polarity: a state in which magnetized objects reverse direction and orient themselves to point south.

Development of a Theory (cont.)

Lesson 2-3

Lesson 2-3

• Volcanic rock on the seafloor contains iron-rich minerals that are magnetic.

• Magnetic minerals in cooling lava from the mid-ocean ridge record the direction of Earth’s magnetic field.

• Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid-ocean ridge.

Development of a Theory (cont.)

Lesson 2-3

Minerals in fresh lava record Earth’s magnetic signature.

Lesson 2-3

• Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to measure and record the magnetic signature.

• They discovered parallel magnetic stripes on either side of the mid-ocean ridge.

Development of a Theory (cont.)

Lesson 2-3

• Each pair of stripes has a similar composition, age, and magnetic character.

• The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridges in opposite directions.

Development of a Theory (cont.)

Lesson 2-3

Seafloor Spreading Theory

Lesson 2-3

• Other measurements made on the seafloor confirm seafloor spreading.

• Measuring the amount of thermal energy leaving the Earth shows that more thermal energy leaves Earth near mid-ocean ridges than is released from beneath the abyssal plains.

Development of a Theory (cont.)

Lesson 2-3

Sediment collected from the seafloor can be dated to show that the sediment closest to the mid-ocean ridge is younger than the sediment farther away from the ridge.

Development of a Theory (cont.)

Lesson 2 - VS

• New ocean crust forms along mid-ocean ridges.

• Mid-ocean ridges are large mountain ranges that extend throughout Earth’s oceans.

Lesson 2 - VS

• A magnetic reversal occurs when Earth’s magnetic field changes direction.

Lesson 2 – LR1

A. toward it

B. above it

C. away from it

D. under it

As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge?

Lesson 2 – LR2

A. reversed polarity

B. normal polarity

C. magnetic reversal

D. no polarity

Today’s magnetic field is described as having which of these?

Lesson 2 – LR3

A. reversed polarity

B. normal polarity

C. seafloor spreading

D. magnetic polarity

In which state do magnetized objects reverse themselves to point south?

Lesson 2 - Now

3. The seafloor is flat.

4. Volcanic activity occurs only on the

seafloor.

Do you agree or disagree?

Lesson 3 Reading Guide - KC

• What is the theory of plate tectonics?

• What are the three types of plate boundaries?

• Why do tectonic plates move?

The Theory of Plate Tectonics

Lesson 3 Reading Guide - Vocab

• plate tectonics

• lithosphere

• divergent plate boundary

• transform plate boundary

• convergent plate boundary

The Theory of Plate Tectonics

Lesson 3 Reading Guide - Vocab

• subduction

• convection

• ridge push

• slab pull

The Theory of Plate Tectonics

• The theory of plate tectonics, proposed in the late 1960s, states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other.

• Plate tectonics suggests that Earth’s surface is divided into large plates of rigid rock and each plate moves over Earth’s hot and semiplastic mantle.

The Plate Tectonics Theory

The Plate Tectonics Theory (cont.)

What is plate tectonics?

• Geologists use the word tectonic to describe the forces that shape Earth’s surface and the rock structures that form as a result.

• The cold and rigid outermost rock layer of the Earth is called the lithosphere.

• The lithosphere is made up of the crust and the solid, uppermost mantle.

The Plate Tectonics Theory (cont.)

Earth’s Tectonic Plates

• The lithosphere is thin below mid-ocean ridges and thick below continents.

• Earth’s tectonic plates are large pieces of the lithosphere that fit together like the pieces of a giant jigsaw puzzle.

• The layer of Earth below the lithosphere, called the asthenosphere, is so hot that it behaves like a plastic material and enables Earth’s plates to move.

The Plate Tectonics Theory (cont.)

plastic

Science Use capable of being molded or changing shape without breaking

Common Use any of numerous organic, synthetic, or processed materials made into objects

The Plate Tectonics Theory (cont.)

• A divergent plate boundary forms where two plates separate.

• When the seafloor spreads at a mid-ocean ridge, lava erupts, cools, and forms new oceanic crust.

• Divergent plate boundaries can also exist in the middle of a continent, where they pull continents apart and form rift valleys.

Plate Boundaries

• A transform plate boundary forms where two plates slide past each other.

• As they move past one another, the plates can get stuck and stop moving.

• Stress builds up where the plates are stuck until they eventually break and suddenly move apart, resulting in a rapid release of energy as earthquakes.

Plate Boundaries (cont.)

• Convergent plate boundaries form where two plates collide.

• The denser plate sinks below the more buoyant plate in a process called subduction.

• The area where a denser plate descends into Earth along a convergent plate boundary is called a subduction zone.

Plate Boundaries (cont.)

Plate Boundaries (cont.)

subduction

from Latin subductus, means “to lead under, removal”

• When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent, creating a deep ocean trench and a line of volcanoes above the subducting plate on the edge of the continent.

• When two continental plates collide, neither plate is subducted, and mountains form from uplifted rock.

Plate Boundaries (cont.)

Plate Boundaries (cont.)

What are the three types of plate boundaries?

• Scientists can measure how fast continents move using a network of satellites called the Global Positioning System.

• Because plates are rigid, tectonic activity occurs where plates meet.

Evidence for Plate Tectonics

• Volcanoes form where plates separate along a mid-ocean ridge or continental rift or collide along a subduction zone.

• Mountains can form where two continents collide.

Evidence for Plate Tectonics (cont.)

Notice the relationship between earthquake epicenters, volcanoes, and plate boundaries.

Evidence for Plate Tectonics (cont.)

How are earthquakes and volcanoes related to the theory of plate tectonics?

• Earth’s plates move because the asthenosphere moves underneath the lithosphere.

• Convection is the circulation of material caused by differences in density.

• Hot mantle material rises upward and comes in contact with Earth’s crust.

Plate Motion

• As the mantle cools, it becomes denser and then sinks, forming a convection current.

• Convection currents in the asthenosphere act like a conveyor belt moving the lithosphere above it.

• There are three forces that interact to cause plate motion: basal drag, ridge push, and slab pull.

Plate Motion (cont.)

Plate Motion (cont.)

What causes convection?

• Basal drag refers to how convection currents in the asthenosphere circulate and drag the lithosphere like a conveyor belt.

• Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push.

Plate Motion (cont.)

As a slab, or dense plate, sinks, it pulls on the rest of the plate with a force called slab pull.

• Plate tectonics has become the unifying theory of geology, but several unanswered questions remain.

• Why is Earth the only planet in the solar system that has plate tectonic activity?

• Why do some earthquakes and volcanoes occur far away from plate boundaries?

A Theory in Progress

• What forces dominate plate motion?

• What will scientists study next? This is a 3-D image of seismic wave velocities from a new technique called anisotropy.

Lesson 3 - VS

• Tectonic plates are made of cold and rigid slabs of rock.

• Mantle convection—the circulation of mantle material due to density differences—drives plate motion.

Lesson 3 - VS

• The three types of plate boundaries are divergent, convergent, and transform boundaries.

Lesson 3 – LR1

A. semiplastic

B. lithosphere

C. tectonic

D. mantle

What word do geologists use to describe the forces that shape Earth’s surface?

Lesson 3 – LR2

A. seafloor

B. lithosphere

C. biosphere

D. hydrosphere

Which of these is made up of the crust and the solid, uppermost mantle?

Lesson 3 – LR3

A. convergent plate boundary

B. transform plate boundary

C. new plate boundary

D. divergent plate boundary

What type of boundary forms where two plates separate?

Lesson 3 - Now

5. Continents drift across a molten

mantle.

6. Mountain ranges can form when

continents collide.

Do you agree or disagree?

Key Concept Summary

Interactive Concept Map

Chapter Review

Standardized Test Practice

The scientific theory of plate tectonics states that Earth’s lithosphere is broken up into rigid plates that move over Earth’s surface.

• The puzzle piece fit of continents, fossil evidence, climate, rocks, and mountain ranges supports the hypothesis of continental drift.

• Scientists were skeptical of continental drift because Wegener could not explain the mechanism for movement.

Lesson 1: The Continental Drift

Hypothesis

Lesson 2: Development of a Theory

• Seafloor spreading provides a mechanism for continental drift.

• Seafloor spreading occurs at mid-ocean ridges.

• Evidence of magnetic reversal in rock, thermal energy trends, and the discovery of seafloor spreading all contributed to the development of the theory of plate tectonics.

• Types of plate boundaries, the location of earthquakes, volcanoes, and mountain ranges, and satellite measurement of plate motion support the theory of plate tectonics.

• Mantle convection, ridge push, and slab pull are the forces that cause plate motion. Radioactivity in the mantle and thermal energy from the core produce the energy for convection.

Lesson 3: The Theory of Plate

Tectonics

A. animals

B. ice sheets

C. plant fossils

D. plate boundaries

Which of these have been found on several continents separated by oceans, suggesting that they were once geographically closer together?

A. continental drift

B. plate tectonics

C. seafloor spreading

D. magnetic reversal

Which of these refers to the process by which new oceanic crust forms along a mid-ocean ridge as older oceanic crust moves away from the ridge?

A. mantle

B. lithosphere

C. plate

D. transform plate boundary

What is the rigid outermost layer of Earth called?

A. transform plate boundary

B. divergent plate boundary

C. convergent plate boundary

D. semiplastic mantle

What forms where two plates slide past each other?

A. seafloor spreading

B. convergence

C. convection

D. subduction

What is the process in which a denser plate sinks below a more buoyant plate?

A. Wegener

B. Pangaea

C. mantle

D. seafloor

What is the name of the supercontinent that all continents were once part of?

A. Antarctica

B. plate boundaries

C. mountain ranges

D. Africa

Where did scientists find glacial features that are usually associated with cooler climates?

A. volcanoes

B. oceanic crust

C. pillow lava

D. mid-ocean ridges

Which term refers to vast mountain ranges on the seafloor?

A. the theory of plate tectonics

B. the theory of mid-ocean ridges

C. the theory of divergent plate boundaries

D. the theory of transform plate boundaries

Which of these states that Earth’s surface is made of rigid slabs of rock that move with respect to one another?

A. ridge push

B. basal drag

C. slab pull

D. plate tectonics

What is the name for the force that pulls on the rest of a plate as part of a dense plate sinks?