10 section 1 continental...

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Holt McDougal Earth Science 139 Plate Tectonics

Name Class Date

As you read this section, keep these questions in mind:

• What is Wegener’s hypothesis of continental drift?

• What is sea-floor spreading?

• How does paleomagnetism support the idea of sea-floor spreading?

• How does sea-floor spreading provide a mechanism for continental drift?

What Is Continental Drift?Alfred Wegener was a German scientist who studied

maps of the continents. He noticed that some of the continents seem to fit together like puzzle pieces. For example, he thought that South America and Africa look like they could fit together.

The shapes of the continents gave Wegener an idea. He thought that the continents looked like they could fit together because they were once joined together. He made a hypothesis that the continents were once joined together into a single large continent. He called this large continent a supercontinent.

Wegener thought that the supercontinent began to break apart about 200 million years ago. Over time, the pieces slowly moved apart. They became the continents that we see today. Wegener’s hypothesis is now known as continental drift.

Continental DriftSECTION

1CHAPTER 10 Plate Tectonics

KEY IDEAS

Wegener noticed that some of the continents looked like they could fi t together.

Describe Before you read this section, make a three-panel fl ip chart. Label the panels “Fossil evidence,” “Rock evidence,” and “Climate evidence.” As you read, fi ll in information from the section describing the three kinds of evidence for Wegener’s hypothesis.

READING TOOLBOX

LOOKING CLOSER1. Identify On the map, circle the parts of South America and Africa that look like they fi t together.

2. Describe What is continental drift?

READING CHECK

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIAATLANTICOCEAN

INDIANOCEANPACIFIC

OCEAN

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIA

hq10irna_tecs01.indd 139hq10irna_tecs01.indd 139 4/5/09 11:47:31 AM4/5/09 11:47:31 AM



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Continental Drift continued

What Is the Evidence for Continental Drift?The matching shapes of the continents support the

idea of continental drift. However, there are also other pieces of evidence that support the hypothesis. These pieces of evidence include the following:

• fossil evidence• rock evidence• climate evidence

FOSSIL EVIDENCE FOR CONTINENTAL DRIFT Some evidence that the continents were once joined

comes from fossils. Remember that fossils are signs that living things once existed in an area. Wegener learned that fossils of similar animals and plants exist on con-tinents that are far apart today. For example, fossils of Mesosaurus, an ancient animal, are found in South America and Africa. The fossils on both continents are identical.

The fossils show that Mesosaurus lived on both continents at about the same time. If the continents were always separate, like they are today, then Mesosaurus would have had to travel between them. It could not have swum such a long distance. There is no evidence that the continents were connected by land bridges that reached across the oceans. Therefore, Mesosaurus probably could not have traveled between separated continents.

However, if the continents were once joined, Mesosaurus could have lived on the superconti-nent. When the supercontinent broke apart, fossils remained on both of the new continents. Therefore, the Mesosaurus fossils and other similar fossils provide evi-dence for continental drift.

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIAATLANTICOCEAN

INDIANOCEANPACIFIC

OCEAN

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIA

Locations of Mesosaurus fossilsLocations of Glossopteris fossils

The locations of fossils of ancient life forms, such as Mesosaurus and Glossopteris, give evidence for continental drift.

Critical Thinking3. Explain Why was it important for Wegener to have evidence to support his hypothesis?

4. Explain Why is it unlikely that Mesosaurus traveled between the continents?

READING CHECK

LOOKING CLOSER5. Identify On which three continents are Glossopterisfossils found?




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ROCK EVIDENCE FOR CONTINENTAL DRIFTRocks also provide evidence for continental drift.

Similar rocks exist on continents that are far apart today. Some mountains that exist on one continent seem to con-tinue on another continent. If the continents were once joined together, these similar rocks would line up. They would form long mountain chains. Therefore, the rocks at different places on Earth’s surface give evidence that the continents were once joined.

CLIMATE EVIDENCE FOR CONTINENTAL DRIFTScientists think some of the continents had differ-

ent climates long ago than they have today. They have found signs that glaciers once existed in South America and Africa. A glacier is a huge, slow-moving river of ice. Today, most of South America and Africa are too hot for glaciers to form. If the continents were once closer to Earth’s poles, they might have been much colder.

Scientists have also found evidence that Antarctica once had a much warmer climate. They have found fos-sils of tropical plants in Antarctica. If Antarctica had been closer to the equator at one time, its climate would have been warmer. Therefore, the evidence for ancient cli-mates supports the idea that the continents have moved.

PROBLEMS WITH THE CONTINENTAL DRIFT HYPOTHESISWegener had a lot of evidence to support the idea that

the continents were once joined. However, he could not explain how they might have moved. He thought that the continents might push through the ocean crust. Other scientists showed that was impossible. Therefore, most scientists did not accept Wegener’s hypothesis.

LOOKING CLOSER6. Compare On the map, circle the continents that match up based on fossil evidence. Draw boxes around the continents that match up based on rock evidence.

7. Explain Why is it unlikely that glaciers existed in South America and Africa recently?

READING CHECK

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIAATLANTICOCEAN

INDIANOCEANPACIFIC

OCEAN

AFRICA

NORTHAMERICA

SOUTHAMERICA

EUROPE

ASIA

Mountains formed ~ 410 million years ago

Mountains formed ~ 250 million years ago

Locations of Mesosaurus fossils

Locations of Glossopteris fossils

Rocks give evidence that the continents were once joined together.




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What Is Sea-Floor Spreading?Wegener and other scientists living during the early

1900s thought that the ocean floor was smooth and flat. They thought it was all the same age and did not change very much over time. However, in the 1940s and 1950s, scientists began to learn that this idea is not true.

Scientists found long mountain ranges on the ocean floor. Many of these mountains run through the middle of the oceans. Therefore, scientists called them mid-ocean ridges. As scientists studied the mid-ocean ridges, they learned two surprising facts.

First, they learned that the sediment on the ocean floor is much thinner near a ridge than far from it. Sediment on the ocean floor is made of dirt, dust, and pieces of shells. The older the ocean floor is, the thicker the layer of sedi-ment there is. Rocks at the middle of the ridge have thin-ner layers of sediments than rocks farther from the ridge. Therefore, rocks at the middle of the ridge are probably younger.

Second, scientists found that the rock of the ocean floor is much younger than they thought. None of the rock on the ocean floor is older than 200 million years. (The oldest rocks on land are more than 4 billion years old.) Using radiometric dating, they also found that the rock near the ridge is younger than the rock farther away.

Mid-ocean ridge

Sediment close to the rift is ___________ than sediment far from the rift.

Rock close to the rift is ___________ than rock far from the rift.

Rift

Sediment

Oceanic crust

MagmaLithosphere

Asthenosphere

Mid-ocean ridges are found in most oceans.

Talk About ItDiscuss Look up the prefi x mid– in a dictionary. With a partner, discuss why mid–ocean ridge is a good name for a line of mountains in the center of an ocean.

8. Explain How did fi nding thin sediment near the ridges tell scientists that the rocks at the ridge are very young?

READING CHECK

LOOKING CLOSER9. Describe Fill in the missing terms to describe the sediment near the rift and the age of the rocks near the rift.




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MID-OCEAN RIDGES AND SEA-FLOOR SPREADING In the late 1950s, a scientist named Harry Hess made

a new hypothesis. He knew that there are rifts, or val-leys, running through the middle of mid-ocean ridges. He hypothesized that those rifts are cracks in Earth’s crust. He thought that magma, or melted rock, could form below the crust and push up through the rift. When the melted rock cools and hardens, it forms new ocean crust.

The new crust forming at the rift makes the ocean floor wider. In other words, new crust pushes apart, or spreads, the sea floor on each side of the rift. Therefore, the process is known as sea-floor spreading.

Sea-floor spreading makes the ocean floor on the two sides of a rift move apart. Hess thought that the moving sea floor might carry the continents. If that happened, then sea-floor spreading could explain how the conti-nents move. Sea-floor spreading therefore gave more evidence for the continental drift hypothesis.

As magma rises to the surface, the sides of the ridge spread apart.This process is called sea-fl oor spreading.

10. Explain Why is the process in which new crust forms called sea-fl oor spreading?

READING CHECK

LOOKING CLOSER11. Apply Concepts Where in the bottom picture would you probably fi nd the thick-est sediment? Label that place on the picture.

Newly formed oceanic lithosphere

Newest lithosphere

Newest lithosphere

Older lithosphere

Oldest lithosphere

Oldest lithosphere




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What Is the Other Evidence for Sea-Floor Spreading?

As scientists studied the ocean floor in the 1950s and 1960s, they found more evidence for sea-floor spreading. One strong piece of evidence for sea-floor spreading is based on paleomagnetism. Paleomagnetism is the study of the magnetic properties of ancient rocks.

EARTH’S MAGNETIC FIELDYou probably know that you can use a compass to find

which way is north. Most compass needles have one end painted red. The needle on the compass will spin until the red end points toward the north pole. A compass needle is a small magnet. It points north because Earth acts like a giant magnet. Like a magnet, Earth has a north mag-netic pole and a south magnetic pole. The red end of the needle is attracted to Earth’s northern magnetic pole.

Earth’s magnetic poles do not line up perfectly with the geographic poles. In other words, the needle of a compass does not point to the actual North Pole. It points to an area near the North Pole. The area a compass nee-dle points to is known as the north geomagnetic pole.

MAGNETIC REVERSALSEarth’s magnetic poles have not always been where

they are today. Many times in the past, they have changed places. The northern magnetic pole moved to the south, and the southern magnetic pole moved to the north. Scientists call the times when the poles were in opposite places magnetic reversals. During a magnetic reversal, a compass needle points south, not north.

North geographic pole

North geomagnetic

pole

A compass needle points to Earth’s north geomagnetic pole.

12. Explain Why does a compass needle point toward the north pole?

READING CHECK

LOOKING CLOSER13. Identify Circle the spot on Earth that a compass needle points toward.




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PALEOMAGNETISM AND SEA-FLOOR SPREADINGScientists have learned that some minerals are

magnetic. They act like tiny compasses. When rock melts, the minerals can line up with Earth’s magnetic field. When the melted rock cools and hardens, the minerals are stuck in place. If a magnetic reversal happens after the rock cools, the minerals cannot move to line up with the new magnetic field. Therefore, the rocks can show what Earth’s magnetic field was like when they formed.

Rocks that contain iron-rich minerals have magnetic properties. Scientists can study these properties to learn about Earth’s magnetic field in the past. The study of these properties is called paleomagnetism.

Paleomagnetism gives more evidence for sea-floor spreading. When scientists study the magnetic fields in the rocks on the sea floor, they see a pattern. The rocks close to a mid-ocean ridge have normal magnetic fields. Farther from the ridge, the rocks have reversed magnetic fields. Even farther out, the fields are normal again. The pattern of normal and reversed fields is the same on both sides of the ridge.

As new crust forms at a mid-ocean ridge, the mag-netic minerals line up with Earth’s magnetic field. The older crust moves away from the ridge as new crust forms. When a magnetic reversal happens, the minerals in the new crust line up with the reversed magnetic field. However, the minerals in the older crust cannot move. They are frozen in place. Over time, “stripes” of rock form with normal and reversed magnetic fields.

14. Explain How can magnetic minerals show the direction of Earth’s magnetic fi eld in the past?

READING CHECK

LOOKING CLOSER15. Describe What hap-pened to Earth’s magnetic fi eld from 1 million years ago to 2 million years ago?

The rock that forms at the ridge forms a magnetic fi eld that lines up with Earth’s magnetic fi eld. When that rock is pushed away from the ridge, new rock forms. If Earth’s magnetic fi eld is reversed, the new rock has a reversed magnetic fi eld.

Reversed magnetic field

Age of sea floor in millions of years14 4321023

Normal magnetic fieldRift




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Section 1 Review SECTION VOCABULARY

continental drift the hypothesis that a single large landmass broke up into smaller land-masses to form the continents, which then drifted to their present locations

mid-ocean ridge a long, undersea mountain chain that has a steep, narrow valley at its center, that forms as magma rises from the asthenosphere, and that creates new oceanic lithosphere (sea fl oor) as tectonic plates move apart

paleomagnetism the study of the alignment of magnetic minerals in rock, specifi cally as it re-lates to the reversal of Earth’s magnetic poles; also the magnetic properties that rock acquires during formation

sea-fl oor spreading the process by which new oceanic lithosphere (sea fl oor) forms when magma rises to Earth’s surface at mid-ocean ridges and solidifi es, as older, existing sea fl oor moves away from the ridge

1. Explain How does sea-floor spreading support the hypothesis of continental drift?

2. Infer Scientists once thought that all of the ocean floor was very old. Did they probably expect to find very thick sediment or very thin sediment on the ocean floor? Explain your answer.

3. Describe In which direction would the red end of a compass needle point during a magnetic reversal?

4. Explain How do fossils support the continental drift hypothesis?

5. Describe What did scientists observe when they studied the magnetic fields of rocks on the sides of mid-ocean ridges?


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