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442 CHAPTER 25 Earth’s Internal Processes
Look carefully at the shapes of the continents on a globe or a map of the world. What do the shapes suggest to you?
What You’ll Learn■ evidence for the
theory of continentaldrift
■ failings of the theory of continentaldrift
Before You Read
Read to LearnContinental Drift
At the start of the twentieth century, geologists studied onlythe land that was close to them. They developed theoriesabout erosion and mountain-building processes. There wasno theory to explain that the geologic processes on Earthwere related.
What was Pangaea? In 1915, Alfred Wegener (VAY guh nur) proposed a theory
that suggested Earth’s continents once were part of a super-continent called Pangaea (pan GEE uh). He thought thatPangaea broke into pieces that drifted over Earth’s surface totheir current locations.
Wegener theorized that Pangaea probably started to breakapart around 200 million years ago. Unfortunately, he wasunable to identify a force strong enough to move continents.
Puzzle Pieces Compare the eastern coastline of SouthAmerica and the western coastline of Africa in the figure atthe top of the next page. Do they match? When SouthAmerica and Africa are joined together, their southern tipsfit very well into the Weddell Sea of Antarctica. The coast-lines of these continents are like puzzle pieces that fittogether.
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chapter Earth’s Internal Processes
section ● Evolution of Earth’s Crust25
1
Discussion After reading this section, use an index card to write down the two mostimportant things you learned.Put one idea on each side of thecard. Form a group of four students to discuss the thingsyou learned.
Study Coach
Reading Essentials 443
How are continents like a torn newspaper?Wegener needed to prove that the continents once actually
were joined. He compared the continents to a torn newspaper.To repair a torn newspaper, you need to make sure the wordsconnect, as well as the edges. In other words, the lines of printhave to match in terms of their content, not just their shapes.
Wegener argued that rock types, fossils, erosion features,and mountain ranges on different continents could bematched. He said that continents could have been joined ifthere were similar structures and formations on them.
How are fossils evidence of Pangaea?Wegener thought that he could prove Pangaea existed by
using fossils. If fossils of large land animals that could notswim or fly across oceans were found on different continents,it would suggest that the continents once had been joined.
The fossils of large land animals, such as Lystrosaurus andCynognathus, supported Wegener’s hypothesis. The fossils of Glossopteris, a large fern with large, heavy spores, also supported the hypothesis.
How are mountains evidence of Pangaea?When Pangaea broke apart, some mountain ranges were
split. For decades, geologists had studied these mountainranges as if they were separate ranges that had no connections. Wegener showed that they were once joinedbecause they shared specific rocks and minerals.
Why didn’t people believe Wegener?Other scientists did not accept Wegener’s hypothesis
because he could not describe a force strong enough to movethe continents apart. He thought that Earth’s rotation, thegravitational pull of the Sun and the Moon, and centrifugalforce could move continents. Physicists quickly proved thateven when these forces were combined, they weren’t strongenough to move continents.
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2. Explain What was the main reason Wegener’shypothesis was notaccepted?
1. Highlight the coastlines of South America and Africawhere they meet.
Picture This
444 CHAPTER 25 Earth’s Internal Processes
Seafloor Spreading HypothesisAfter World War II, Dr. Harry Hess studied Wegener’s ideas.
Hess used sonar to map the seafloor. Three-dimensionalmodels of the seafloor were created from sonar data. Look atthe figure below. The mid-ocean ridge (MOR) system is a con-tinuous underwater system of mountain ranges and valleys onthe seafloor that wraps around Earth.
What is seafloor spreading?After studying the MOR, Hess proposed a hypothesis of
seafloor spreading. He said that liquid rock, or magma, fromEarth’s mantle is forced upward through the MOR becausemagma has a lower density than the surrounding rock. Thiscauses the crust to crack (fault) and move apart. Twin mountain ranges are formed with a valley in between. A riftvalley is a down-dropped valley between twin mountainranges caused by faulting.
What are the ages of seafloor sediments androcks?
In the 1960s, scientists drilled into the ocean floor and tookout samples, or cores, of the rock layers. Sediments near thecontinents are thick, but thin near the MOR. Continentalrocks are billions of years old, but seafloor rocks are less than200 million years old. Rocks of the oceanic crust increase inage as their location extends farther from the MOR.
What is magnetic polarity of rocks?As the seafloor spreads, magma rises through the rift valley
and erupts as lava to form new oceanic crust. When lavacools, iron-rich minerals behave like compass needles andbecome oriented along Earth's magnetic field. When the rockhardens, this magnetic orientation known as polarity, islocked in place. Studies of the seafloor have found parallelbands of reversed polarity on each side of the MOR.
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3. Identify Using a highlighter, trace the MOR.
Picture This
4. Explain What allowsmagma to travel upwardthrough the MOR?
Reading Essentials 445
The Plate Tectonics TheoryThe theory of plate tectonics originated in the 1960s. It
describes how moving, crustal plates cover Earth’s surface. Seafloor spreading showed that Earth’s crust movessideways. Scientists wanted to understand the motion of allEarth’s plates.
Plates are made of a rigid layer of uppermost mantle and alayer of either oceanic or continental crust above. Some platesare made only of oceanic crust. Others are made of partoceanic and part continental crust. There are about 12 majorplates and many minor ones.
There are three main kinds of plate motions. At theirboundaries, plates can move apart, move together, or slidepast each other.
What are divergent plate boundaries?Divergent boundaries are places where plates are pulling
apart. You learned that magma is pushed up through faults ina rift valley at a mid-ocean ridge. The magma spreads, cools,and hardens to form new oceanic crust at this divergentboundary.
What are convergent plate boundaries?Convergent boundaries are areas where plates collide.
Several things can happen at these boundaries, depending onthe types of plates that collide.
Continental lithosphere is less dense and thicker thanoceanic lithosphere. When a continental plate meets anoceanic plate, the oceanic plate bends and moves under thecontinental plate. Subduction is the process of one platemoving under another plate.
When heat along a subduction zone partially melts rock,magma forms and rises to the surface. It forms a volcanic arcthat runs parallel to the subduction zone. A deep sea trenchalso can form at a subduction zone. The Andes mountainrange in South America is in a subduction zone.
●A Organize InformationMake the following Foldable tohelp you organize informationabout the types of plateboundaries.
Divergent
Convergent
Ocean - ocean
Continental
Transform
ContinentalContinentalcrustcrust
Continental crust
Lithosphere Lithosphere
Asthenosphere Asthenosphere
Lithosphere
Asthenosphere
Oceanic crust
Oceanic crust
Oceanic crust
Volcanic arc
Ocean-continent
Deep-sea trench
Lithosphere Lithosphere
Oceanic crust Oceanic crust
Lithosphere
Oceanic crust
Asthenosphere Asthenosphere Asthenosphere
Oceanic crust
Oceanic crust
Oceanic crust
Island arc Deep-sea trench
Ocean-ocean
ContinentalContinentalcrustcrust
Continental crust
Lithosphere Lithosphere
Asthenosphere Asthenosphere
Lithosphere
Asthenosphere
Oceanic crust
Oceanic crust
Oceanic crust
Mountain range
Continent-continent
5. Identify At plate boundaries, what kind ofmotion can occur?
Picture This
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446 CHAPTER 25 Earth’s Internal Processes
6. Explain What are transform boundaries?
7. Determine What causes the movement of all plates?
Ocean-Ocean Convergent Boundaries Convergent plateboundaries also exist between two plates of oceaniclithosphere. The colder, denser plate subducts. Magma thaterupts there creates chains of volcanic islands called islandarcs. Japan is an example of an ocean-ocean convergentboundary.
Continental Convergent Boundaries Along some convergent plate boundaries, two continental slabs of lowdensity collide but do not subduct. Since both are low indensity, they both buckle upward to form a high range offolded mountains. The Himalaya of Asia are an example.
What are transform plate boundaries?At transform boundaries, two plates slide by each other.
No new lithosphere is created. No old lithosphere is beingdestroyed, or recycled. The main result of transform boundaries is horizontal plate movement.
Plates in MotionPlate motion is caused by a combination of forces. Thermal
energy from Earth’s interior drives plate motion in a processcalled convection. As plates move and interact along plateboundaries, forces like slab pull, ridge push, and friction alsocontribute to the process.
What is convection?Radioactive elements in Earth's core and mantle decay and
give off heat. As parts of the mantle heat up, they becomebuoyant and rise to the base of the crust. When the mantlecools it then sinks. This is a convection current and convectioncurrents can move plates by friction.
How do ridge push, slab pull, and friction act?The other forces that act on plates are ridge push, slab pull,
and friction. At the MOR, ridge push is in action. The liftingaction of mountain formation, gives the new end of a plate aslight push. At the opposite end of the plate, slab pull occurs.Slab pull is similar to a table cloth being tugged on by a kitten.If enough of the cloth is pulled over the edge of the table,the whole thing slides off on the floor. Slab pull is similar tothe table cloth as the plate is being pulled by gravity into themantle. Friction is the drag that occurs when the plate and themantle are in contact. As the mantle moves in a convectioncurrent, it drags the plate along with it. Continental plateshave more friction because of their deep roots.
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Reading Essentials 447
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explainshow subduction occurs.
2. Complete the chart below. List three arguments Wegener used to prove that the continentswere once joined:
3. Review the ideas your group wrote on the index cards. Write one idea thatyou all agreed was important. How did this idea help you to understand the evolution ofEarth’s crust?
Study Coach
convergent boundary: where tectonic plates collidedivergent boundary: where tectonic plates are pulling
apartmid-ocean ridge: an underwater mountain range on the
seafloor of the Atlantic Ocean that wraps around Earth
rift valley: a down-dropped valley between twin mountainranges caused by faulting.
subduction: when one tectonic plate moves under anothertectonic plate
transform boundary: where two tectonic plates slide byeach other
After You ReadMini Glossary
Pangaea1.
2.
3.
End ofSection
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