Chapter 12 Guided Reading Strategies 12.1 South America Answers

Chapter 12 Answer Key

Study Guide

Summary Answers

1. Answers will vary, but should include 1 main idea from each Reading (3) or the summary.
2. Answers will vary, but should include a sentence using three vocabulary terms.
3. The United States is found on the North American plate.

Guided Reading

12.1 Continental Drift

1. surface
2. supercontinent
3. Pangaea
4. hypothesis

12.2 Sea-Floor Spreading

5. Hess
6. ridges
7. spreading

12.3 Moving Tectonic Plates

8. divergent
9. convergent
10. transform fault

Let's Review

1. c
2. b

Check Your Understanding

Reading 12.1

1. Plate tectonics is a relatively new field of science. The idea that Earth has tectonic plates that move on Earth's surface became established in the 1960s.
2. Alfred Wegener, a German climatologist and arctic explorer, hypothesized that Earth's continents were once connected as a large supercontinent called Pangaea. He collected a lot of evidence to support this idea and published this information in his 1915 book called Origins of the Continents and Oceans.
3. A list of Wegener's observations that provided evidence for continental drift included: (1) Coal beds in the eastern US match coal beds in southern Europe; (2) Matching rock types and mountain belts occur in North America and the British Isles, and Africa and South America; (3) Plant fossils in South America match plant fossils in Africa, India, Australia, and Antarctica; (4) Reptile fossils in South American match reptile fossils in Africa; (5) Early mammal fossils in South America match those in Africa; (6) Fossils in South America are the same age and type as those in Africa; and (7) There is evidence of a warm, dry climate that is buried under evidence of glaciers which indicates that continents that are now in Antarctica were once located at Earth's equator. Students should write about one of these and explain why that observation supports the idea that continents were once connected. For example, they could say that if fossils in South America are the same type and age as those in Africa, then this evidence indicates that these continents were once close together. The animals that once lived on these two landmasses were able to go back and forth from South America to Africa. But today that would be impossible since the continents are separated by the Atlantic Ocean.
4. Wegener could not explain how the continents could have pushed through the ocean floor. He could not identify a force strong enough to cause this to happen. Nor could he describe a mechanism by which this could happen.
5. Answers are:
1. Wegener followed the scientific process by making observations and then stating a hypothesis.
2. Scientists were using the scientific process when they rejected Wegener's hypothesis. They could not prove the hypothesis; therefore they had to conclude that it was not correct. The next step is to develop a new hypothesis. The new hypothesis was put forth by Harry Hess—the idea of sea-floor spreading.
6. No because there was no evidence to support this idea yet.
7. and 8. Sample answers:

1. Observations: Plants grow best in my front yard but not my backyard. My front yard is sunny, but my backyard is shady
   Hypothesis: If plants receive enough sunlight, then they grow better.
2. Observations: The soil in my yard is sandy without a lot of humus. Water disappears quickly after a rainstorm.
   Hypothesis: If soil is sandy, then rain water is able to flow quickly into the soil.
3. Observations: My pet cat seems to spend most of its time sleeping. It also eats, seeks affection, walks around the house, and plays. Hypothesis: If cats spend most of the time sleeping during the day, then they are likely to be nocturnal animals.

12.2 Reading

1. Magnetic minerals in newly formed rock align with Earth's magnetic field. Magnetic patterns match on both sides of mid-ocean ridges. Also, oldest formed minerals are found further from the ridges. This shows that sea floor spreading is occurring.
2. Mid-ocean ridges were discovered during World War II when the US Navy mapped the ocean floor for the first time.
3. Harry Hess's hypothesis was that new sea floor is made at the mid-ocean ridges. As this new floor is made, the continents are pushed away from each other. The name of this hypothesis is sea-floor spreading. (Because Harry Hess did not have enough evidence to support his idea initially, he called seafloor spreading "geo-poetry.")
4. The discoveries that supported Hess's hypothesis were: (1) the matching magnetic patterns on the sea-floor on either side of the mid-ocean ridges, and (2) scientists discovered that the rock that is far from the mid-ocean ridges is older than rock that is closer to the ridges.
5. d
6. The two types of plates are the continental plates and the oceanic plates. The continental plates are granitic and andesitic. The oceanic plates are basaltic. The oceanic plates are denser than continental plates. Note: Plates often have both oceanic parts and continental parts.
7. Sample questions: Why do earthquakes and volcanoes occur? How do mountains form?
8. Slab pull is the process of the pulling movement that a plate experience when one part of it sinks into the mantle. Ridge push is a plate being pushed from where it is being newly formed at a mid-ocean ridge. These forces work together to cause plates to move on Earth's surface.
9. There was no evidence to explain how the continents moved.
10. Subduction occurs when a heavier, colder, and denser part of a plate (usually an oceanic plate) sinks into the mantle and goes under the plate it is colliding with which is lighter and less dense (a continental plate or a younger oceanic plate). The plate entering the mantle creates a cold spot in the mantle that may contribute to how convection cells form in the mantle.
11. The Hawaiian Islands are an example of an island chain formed by a mantle plume hot spot. As a plate moves over this hot spot, magma squirts up through the plate. The result over millions of years is layers and layers of lava rock that are eventually higher than sea level and form an island. As the plate moves, the island grows in size or a completely new island forms. Note: Diagram not shown.
12. The two large land masses that formed after Pangaea split are called: (1) Laurasia: named for the two continents it divided into around 200 million years ago—Laurentia (now called North America) and Eurasia (excluding India and Arabia). (2) Gondwana: Eduard Suess (1831–1914), a geologist who discovered this supercontinent (proposed 1861) named it after a region in eastern India called Gondwana. The geology of Gondwana was determined in this region of India.

12.3 Reading

1. The three types of plate boundaries are: (1) convergent, (2) divergent, and (3) transform fault. Plates collide (or come together) at convergent boundaries, plates separate at divergent boundaries. Plates slide past each other at transform boundaries.
2. A mid-ocean ridge is a divergent boundary.
3. Rifts are caused by forces that pull tectonic plates apart.
4. A rift on a continent or island will eventually become an ocean basin filled with ocean water. Once that happens, the ridge will be a mid-ocean ridge.
5. The oceanic plate sinks into the mantle subducts under a less-dense plate. The resulting surface feature is a deep-ocean trench.
6. An ocean basin is formed when a rift vally forms from plates being pulled apart, then fills with water.
7. d
8. A continental plate is more buoyant. The oceanic plate would subduct under the continental plate because the oceanic plate is denser.
9. When two continental plates collide the land crumples and forms mountains. The Himalaya Mountains are an example of mountains formed by two colliding continental plates. These plates are still colliding and the mountains are growing.
10. Any two of these clues are correct: (1) If a straight-line object like a creek or a road becomes offset, and (2) the occurrence of earthquakes. Another clue that indicates the location of a transform fault boundary is (3) a small valley or pond in an area that seems to have transform fault boundary movement.
11. The formation of ocean basins where plates began to separate (divergent boundaries)
12. Sample answer: The Nazca Plate gets its name from the Nazca region in Peru. This plate is subducting under the South American Plate. The results of this subduction include the formation of the Andes Mountains and the Peru-Chile Trench. The other sides of the Nazca Plate are divergent boundaries. Where the Nazca, Antarctic, and South American plates meet is called a triple junction. Plate movement at this junction is thought to have been responsible for the 1960 Chilean earthquake (9.5 magnitude).

Solve It Answers

p. 286 -

1,000 years x 5 centimeters/year = 5,000 centimeters or 50 meters

The plate would have moved 50 meters over 1,000 years at a rate of 5 centimeters per year.

Challenge! Answers

p. 287 -

Magnetic patterns on the sea floor occur when lava erupts at a mid-ocean ridge. Magnetic particles in the liquid lava align themselves with Earth's magnetic north pole. The lava solidifies, creating new sea floor. Later more lava erupts and the old lava rock is pushed away from the mid-ocean ridge. Magnetic particles in the new lava again align themselves with Earth's magnetic north pole. However, in the years between eruptions, Earth's magnetic north pole may shift from the northern to the southern hemisphere. If there is a shift, the magnetic pattern of the two bands of lava will be different. Scientists who studied these patterns realized that the magnetic patterns on either side of a mid-ocean ridges matched. This provided strong evidence for sea-floor spreading.

p. 295 -

1. Oceanic plates subduct under continental plates, so sea floor gets "recycled" back into the mantle more readily than continental plates.
2. Older sea floor is subducted and recycled back into the mantle.
3. The oldest rocks on Earth would be more likely to be found in continental crust, not oceanic crust.
4. Erosion and subduction/recycling of rock make it very unlikely that we would find rock as old as Earth itself.

Connection - Robert T. Hill Texas Geologist

1. He worked for the US Geological Survey and published a paper about the geology of Texas. He led a month-long expedition through the Big Bend region to study its geology among other expeditions.
2. Fossils of sea animals such as snails and clams were found by Hill in Big Bend National Park.
3. Pangea formed before the Cretaceous period.

Activity - Make a Plate Tectonics Book

1. The title of my group's book was: The Continental Travels of Cynognathus.
2. A very long time ago, Cynognathus lived on an ancient supercontinent. Today, fossils of Cynognathus are found on two different continents-Africa and South America. This story describes how these continents separated.
3. My favorite thing about making the book with my group was figuring out how to put the book together. We decided to use paper bag paper for the cover and back of the book. We used regular paper for the inside pages. We held the book together using tape, staples, and string.
4. Our whole group read our book to a group of first graders. Each member of my group took turns reading a page from our book. It was fun to do this and I learned a lot about the history of Earth's surface.

Chapter 12 Review

Vocabulary

Reading 12.1

1. Pangaea
2. continental drift
3. plate tectonics

Reading 12.2

4. continental plates
5. subduction
6. tectonic plates
7. mid-ocean ridge
8. sea-floor spreading
9. oceanic plates
10. mantle plume

Reading 12.3

11. convergent boundary
12. divergent boundary
13. transform fault boundary
14. ocean basin

Concepts

Reading 12.1

1. Fossils formed on continents are records of the plants and animals that lived on the continents in the past. Therefore, the fossils indicate what kinds of environmental conditions occurred on the continents in the past. Fossils are today in cold locations (like Antarctica) that must have once been located in tropical places. Additionally, the locations of fossils on now separated continents indicate that at one time the animals were able to cross from one continent to the other. The only way that could have happened is if the continents were once connected.
2. Wegener could not identify the force that would have caused the continents to slide over, or push through, the rocky bottom of the ocean. Without any evidence for the source of the force involved in moving continents, continental drift could not be accepted by other scientists.

Reading 12.2

3. The Atlantic Ocean (and accompanying sea floor) is getting wider as a result of sea-floor spreading occurring at the Mid-Atlantic Ridge.
4. The oldest rocks of the sea floor are found at the farthest distance from the mid-ocean ridges. The youngest rocks are found very close to the mid-ocean ridges, where new rock (as new ocean floor) is being formed.
5. At a mid-ocean ridge, magma pushes up the edges of newly forming plates. Consequently, the less-dense new plate material slides away from the ridge creating a force called ridge push. The older edges of a plate (far from the ridge) are cooler and therefore, denser. As a result, they begin sinking in the mantle. This is called subduction. As the plate continues to subduct, the whole plate is affected by pulling force called slab pull. Scientists believe these two forces play important roles in plate movement. Convection currents in the mantle seem to either work to speed up or slow down plate movement.
6. To form an island on a plate, a single mantle plume must cause a strong and long-lasting volcanic eruption in the plate above it. Since plates move more quickly than the underlying mantle plumes, the island eventually moves away from the plume hot spot. Then, the plume forms another island on the plate in line behind the first. This process repeats over and over and forms a chain of islands. The rate at which and direction the plate is moving can be assessed by studying the distances between the islands and their ages.

Reading 12.3

7. The three kinds of plate boundaries are convergent, divergent, and transform fault boundaries. The San Andreas Fault in California is a very famous transform fault boundary.
8. Mountains form when two continental plates come together at a convergent plate boundary.
9. The East African Rift Valley represents a divergent boundary. It starts near the southern end of the Red Sea and moves southward toward Mozambique. Volcanoes and countries within this rift valley include Mount Kilimanjaro in Tanzania, Mount Nyiragongo in the Democratic Republic of Congo, and Mount Elgon between Uganda and Kenya.

Math and Writing Skills

Reading 12.1

1. A sample dialogue between Alfred Wegener and Harry Hess could involve Wegener discussing his problem of getting continental drift accepted by the scientific community. As the person who proposed the hypothesis of sea-floor spreading, Harry Hess would have just the right solution for Wegener. A dialog might go something like this.

Hess: How's it going Wegener? You look down.

Wegener: Yes, I'm frustrated that I cannot come up with a force that is strong enough to explain how continents could move and push through the ocean floor.

Hess: Well, Al, I think I may have discovered something that could solve your problem! My hypothesis is called sea-floor spreading. I believe new oceanic crust is made a huge undersea mountain range. Since this new oceanic crust is made at either side of the range, the sea-floor spreads. The continents don't push through ocean floor at all!

Wegener: Wow, that's great news! I think your hypothesis is brilliant!

Hess: Yes, but we need to wait until scientists gather evidence to support my idea!

Reading 12.2

2. Answers are:
1. 600 kilometers/10 million years = 60 kilometers/1 million years
2. 200 kilometers/8 million years = 25 kilometers/1 million years

Reading 12.3

3. Diagrams of each plate below.
   
   Sample captions: A divergent boundary is one where two plates are separating. Examples of geologic features at a divergent boundary are rift valleys and mid-ocean ridges. A convergent plate boundary is one where two plates are coming together. Examples of geologic features at a convergent boundary are a mountain range or a deep-ocean trench. A transform fault boundary is one where two plates slide past one another. The San Andreas Fault is an example of a transform fault boundary.

Test Practice

Reading 12.1

1. b

Reading 12.2

2. b

Reading 12.3

3. d
4. a

Project—Modeling Sea-Floor Spreading

Note about the magnetic stripes: As new ocean floor is made, the rock takes on the magnetic conditions of Earth at that time. Over millions of years, Earth's magnetic poles have switched positions. This means that the magnetic polarity of the rock made at the ridges has also changed. The result is magnetic patterns.

1. Mid-ocean ridges are represented by the box top.
2. The magnetic patterns match on either side of a mid-ocean ridge and the patterns close to the ridge are younger than the patterns that are far away from the ridge. These pieces of evidence show that new sea floor is made at the ridges and that the ocean is getting bigger (forcing the continents to move apart). The fact that the continents were once closer together is an important part of the theory of plate tectonics.
3. Magnetic reversal patterns are caused by reversals in Earth's magnetic field that occur every few to ten thousand years.

Chapter 12 Guided Reading Strategies 12.1 South America Answers

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