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sx05_TE_(nc8-8)c04G.fm Page 338 Thursday, June 2, 2005 1:16 PM 5 Section 5 Early Earth Early Earth Reading Preview Objectives Key Concepts After this lesson, students will be able to G.4.5.1 State when Earth was formed. G.4.5.2 Explain how Earth’s physical features developed during Precambrian Time. G.4.5.3 Describe what early Precambrian organisms were like. • When did Earth form? • How did Earth’s physical features develop during Precambrian Time? • What were early Precambrian organisms like? Key Terms • comet • continental drift Target Reading Skill Comparing and Contrasting Explain that comparing and contrasting information shows how ideas, facts, and events are similar and different. The results of comparison can have importance. Answers Possible answers: Early Earth: Atmosphere—Hydrogen and helium Oceans—Earth’s surface is too hot. All water evaporates into water vapor. Continents—Less dense rock at surface forms continents. Later Precambrian Earth: Atmosphere—Carbon dioxide, nitrogen, and water vapor Oceans—Earth cools, water vapor condenses, and rain falls. Rain forms oceans. Continents—Old continents break apart and new continents form as a result of continental drift. Target Reading Skill Comparing and Contrasting As you read, compare and contrast early Earth with Earth later in Precambrian Time by completing a table like the one below. Precambrian Earth Feature Atmosphere Oceans Continents Think It Over Making Models If you repeated Steps 2 and 3, what would happen to the size of your “planet”? How is this model like the early Earth? How is it different? Your science class is going on a field trip, but this trip is a little out of the ordinary. You’re going to travel back billions of years to the earliest days on Earth. Then you will move forward through time to the present. Enter the time machine and strap yourself in. Take a deep breath—you’re off! A dial on the dashboard shows the number of years before the present. You stare at the dial—it reads 4.6 billion years. You peer out the window as the time machine flies above the planet. Earth looks a little strange. Where are the oceans? Where are the continents? How will Earth change over the next billions of years? You’ll answer these and other questions about Earth’s history as you take this extraordinary trip. Your journey starts at the beginning of Precambrian Time with the formation of planet Earth. Scientists hypothesize that Earth formed at the same time as the other planets and the sun, roughly 4.6 billion years ago. • Transparency G44 The Age of Earth How do scientists know the age of Earth? Using radioactive dating, scientists have determined that the oldest rocks ever found on Earth are about 4 billion years old. But scientists think Earth formed even earlier than that. Preteach 338 ◆ L1 Characteristics of Earth Show students a photo of Earth taken from outer space by astronauts. Ask: What are some of the features that you see on Earth? (A typical answer will include cloud formations, oceans, landmasses, or continents.) Do you think that Earth had any of these features when it first formed? What were they? (Accept all reasonable responses. Record them on the board to refer to as students read through the section.) 338 Later Precambrian Earth 1. Place a sheet of paper on top of a small magnet. The paper represents outer space and the magnet models gravity. 2. Sprinkle a half teaspoon of iron filings along one end of the paper to model the materials that formed Earth. 3. Gently blow through a straw for about 10 seconds from the end of the paper with the iron filings toward the magnet. CAUTION: Be sure the straw is pointed away from other students. 4. Observe what happens to the iron filings. The Planet Forms Teaching Resources Build Background Knowledge Early Earth How Could Planet Earth Form in Space? L1 Skills Focus making models Materials circular magnet, iron filings, paper, straw Time 10 minutes Tips Caution students to blow gently through the straw so as not to spread iron filings all over. Be sure students do not blow the iron filings in the direction of other students. Expected Outcome The iron filings will gather around the poles of the magnet. Think It Over The iron filings would continue to gather in clumps around the magnet. The model shows how early materials in space were gathered in a ball by the force of gravity to form Earth. In the magnet model, the force is the attraction of iron to the poles of the magnet. sx05_TE_(nc8-8)c04G.fm Page 339 Thursday, June 2, 2005 1:16 PM Instruct The Planet Forms For: Links on Precambrian Earth Visit: www.SciLinks.org Web Code: scn-0745 Download a worksheet that will guide students’ review of Internet resources on Precambrian Earth. Teach Key Concepts According to this hypothesis, Earth and the moon are about the same age. When Earth was very young, it collided with a large object. The collision threw a large amount of material from both bodies into orbit around Earth. This material combined to form the moon. Scientists have dated moon rocks that were brought to Earth by astronauts during the 1970s. Radioactive dating shows that the oldest moon rocks are about 4.6 billion years old. Scientists infer that Earth is also roughly 4.6 billion years old—only a little older than those moon rocks. Earth Takes Shape Scientists think that Earth began as a ball of dust, rock, and ice in space. Gravity pulled this mass together. As Earth grew larger, its gravity increased, pulling in dust, rock, and ice nearby. As objects made of these materials struck Earth at high speed, their kinetic energy was changed into thermal energy. The energy from these collisions caused Earth’s temperature to rise until the planet was very hot. Scientists think that Earth may have become so hot that it melted. Denser materials sank toward the center, forming Earth’s dense, iron core. At the same time, Earth continuously lost heat to the cold of space. Less dense, molten material hardened to form Earth’s outer layers—the solid crust and mantle. As the growing Earth traveled around the sun, its gravity also captured gases such as hydrogen and helium. But this first atmosphere was lost when the sun released a strong burst of particles. These particles blew away Earth’s first atmosphere. What force caused the materials that formed Earth to come together? FIGURE 16 Early Earth This artist’s illustration shows Earth shortly after the moon formed. Notice the rocky objects from space striking Earth, and the molten rock flowing over the surface. When Earth Formed Focus Refer students to Figure 16. Teach Ask: How was early Earth different from planet Earth today? (Earth today is made up of oceans and landmasses, and early Earth was initially a mass of dust, rock, and ice that eventually turned into a molten mass.) When did Earth form? (About 4.6 billion years ago) How do scientists know? (The oldest moon rocks are 4.6 billion years old, and scientists think that the moon formed after Earth.) Apply Explain that though Earth is older than the moon, moon rocks are older than Earth rocks. The oldest rocks on Earth have been destroyed by processes such as plate tectonics and erosion. These processes do not occur on the moon. learning modality: logical/mathematical Independent Practice L2 Teaching Resources • Guided Reading and Study Worksheet: Early Earth For: Links on Precambrian Earth Visit: www.SciLinks.org Web Code: scn-0745 Chapter 8 ◆ Student Edition on Audio CD Help Students Read 339 Differentiated Instruction L1 English Learners/Beginning Comprehension: Modified Cloze Distribute a simplified paragraph about the age of Earth and how it formed. For example, “Scientists infer that Earth is about 4.6 _____ years old. Earth began as a mass of dust, rock, and ice in _____. The mass was pulled together by _____.” Provide students with a list of correct answers to fill in the blanks. learning L2 L2 English Learners/Intermediate Comprehension: Key Concepts Ask students to complete the Beginning activity and then use the statements to write a paragraph describing the formation of the atmosphere, oceans, and continents. learning modality: verbal L1 Identifying Supporting Evidence See the Content Refresher for guidelines on identifying supporting evidence. Have students create a graphic organizer for evidence that supports the hypothesis about how Earth formed. Monitor Progress L2 Writing Have students write a description of how Earth was formed. Answer Gravity modality: verbal 339 sx05_TE_(nc8-8)c04G.fm Page 340 Thursday, June 2, 2005 1:16 PM Earth’s Surface Forms Teach Key Concepts L2 Development of Physical Features Focus Ask volunteers to read the captions in Figure 17 that describe the atmosphere. Teach Ask: What factors contributed to Earth’s second atmosphere? (Volcanic eruptions and comets) How did oceans form? (Water vapor condensed to form rain and began to accumulate.) How were the continents formed? (Earth’s rock cooled and hardened to form large landmasses.) Apply Ask: Could humans or other animals have survived on early Earth after oceans and continents formed? (No. They could not have breathed the atmosphere because it did not contain the oxygen they need.) FIGURE 17 Development of the Atmosphere Earth soon lost its first atmosphere (left) of hydrogen and helium. Earth’s second atmosphere (right) slowly developed the mixture of gases—nitrogen, oxygen, carbon dioxide, water vapor, and argon— of the atmosphere today. As oxygen levels increased, the ozone layer also developed. Comparing and Contrasting Compare and contrast Earth’s first and second atmospheres. Earth’s Surface Forms The Atmosphere After Earth lost its first atmosphere, a second atmosphere formed. This new atmosphere was made up mostly of carbon dioxide, water vapor, and nitrogen. Volcanic eruptions released carbon dioxide, water vapor, and other gases from Earth’s interior. Collisions with comets added other gases to the atmosphere. A comet is a ball of dust and ice that orbits the sun. The ice in a comet consists of water and frozen gases, including carbon dioxide. Life Develops L2 Early Precambrian Organisms Focus Remind students that oxygen is a product of photosynthesis. Teach Ask: What did the first life forms look like? (Single-celled; resembled presentday bacteria) How did they get energy? (They used energy from the sun to make their own food.) What was the effect of the release of oxygen in the atmosphere? (The amount of oxygen in the atmosphere slowly increased.) Apply Ask: If photosynthetic organisms had not appeared during Precambrian Time, what do you think Earth would be like today? (Possible answers: The atmosphere would contain little or no oxygen; life that depends on oxygen as we know it would probably not exist.) learning modality: ind rw la So Watching early Earth from your time machine, you can see the planet change as the years speed by. During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form. learning modality: visual Teach Key Concepts First Atmosphere Hydrogen and helium blasted back into space by impact of space debris and particles from the sun Calculating Precambrian Time lasted about 4 billion years. What percentage is this of Earth’s entire history of 4.6 billion years? If the first continents formed about 500 million years after Earth itself formed, what percentage of Precambrian Time had elapsed? (Hint: To review percentages, see the Math Review section in the Skills Handbook.) The Oceans At first, Earth’s surface was too hot for water to remain as a liquid. All water evaporated into water vapor. However, as Earth’s surface cooled, the water vapor began to condense to form rain. Gradually, rainwater began to accumulate to form an ocean. Rain also began to erode Earth’s rocky surface. Over time, the oceans affected the composition of the atmosphere by absorbing much of the carbon dioxide. The Continents During early Precambrian Time, more and more of Earth’s rock cooled and hardened. Less than 500 million years after Earth’s formation, the less dense rock at the surface formed large landmasses called continents. Scientists have found that the continents move very slowly over Earth’s surface because of forces inside Earth. This process is called continental drift. The movement is very slow—only a few centimeters per year. Over billions of years, Earth’s landmasses have repeatedly formed, broken apart, and then crashed together again, forming new continents. What is continental drift? 340 ◆ visual Teaching Resources • Transparency G45 L2 Skills Focus calculating Materials calculators Time 10 minutes Tips Tell students to write out the zeroes in the large numbers and then cancel them before entering the numbers in their calculators. Expected Outcome Precambrian Time lasted for 86% of Earth’s history. About 11% of Precambrian Time elapsed before the development of the continents. Extend Have students refer to the geologic time scale and calculate the percentage of Earth’s history for which each era lasted. learning modality: logical/mathematical 340 sx05_TE_(nc8-8)c04G.fm Page 341 Thursday, June 2, 2005 1:16 PM Second Atmosphere L2 Carbon dioxide, water vapor, and nitrogen from volcanic eruptions and comet impacts Comparing and Contrasting Atmospheric Gases Oxygen from bacteria in the oceans Ul Ozone layer Materials paper, colored pencils Time 15 minutes Focus Review early Earth’s atmosphere. Teach Tell students that Earth’s early atmosphere was about 92% carbon dioxide, 5% nitrogen, 0% oxygen, and 3% other gases. Today’s atmosphere is about 78% nitrogen, 21% oxygen, and 1% other gases, including carbon dioxide. Have students make circle graphs of each atmosphere and discuss the differences. Apply Ask: Why did the percentage of oxygen change? (Early life forms gave off oxygen as a waste product.) learning Ozone layer gradually forms as amount of oxygen increases tra vio l e t l ig ht Life Develops Scientists cannot pinpoint when or where life began on Earth. But scientists have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present-day bacteria. Scientists hypothesize that all other forms of life on Earth arose from these simple organisms. About 2.5 billion years ago, many organisms began using energy from the sun to make their own food. This process is called photosynthesis. One waste product of photosynthesis is oxygen. As organisms released oxygen into the air, the amount of oxygen in the atmosphere slowly increased. Processes in the atmosphere changed some of this oxygen into a form called ozone. The atmosphere developed a layer rich in ozone that blocked out the deadly ultraviolet rays of the sun. Shielded from the sun’s ultraviolet rays, organisms could live on land. modality: logical/mathematical Monitor Progress 5 Section 5 Assessment Target Reading Skill Comparing and Contrasting Use the information in your table 3. a. Identifying What do scientists think were the first organisms to evolve on Earth? about early Earth to answer the questions below. b. Predicting How would Earth’s atmosphere Reviewing Key Concepts 1. a. Reviewing How long ago did Earth form? b. Summarizing Summarize the process by which scientists determined the age of Earth. 2. a. Listing What physical features formed during Earth’s first several hundred million years? b. Explaining How did volcanic eruptions and comets change early Earth? c. Relating Cause and Effect What caused water erosion to begin on Earth’s surface? be different if organisms capable of photosynthesis had not evolved? Explain. Web Site Plan a Web site for early Earth. To plan your Web site, make a list of the topics you will include. Make sketches of the screens that visitors to the Web site will see. Then write short descriptions for each topic. Chapter 8 Writing Mode Description Scoring Rubric 4 Exceeds criteria; plan includes information about early Earth’s atmosphere, oceans, continents, and life forms; sketches and descriptions are accurate and informative 3 Meets criteria but sketches and descriptions are not imaginative 2 Includes only some criteria 1 Is inaccurate and incomplete ◆ 341 L2 Answers Figure 17 The first atmosphere contained hydrogen and helium. The second atmosphere contained nitrogen, oxygen, carbon dioxide, water vapor, and argon. The process by which the continents move very slowly over Earth’s surface because of forces inside Earth Assess Reviewing Key Concepts 1. a. About 4.6 billion years ago b. Radioactive dating of rocks from Earth and the moon 2. a. Oceans, the atmosphere, and continents b. They released carbon dioxide and water vapor into the atmosphere. c. Rain 3. a. Bacteria-like organisms b. It would have little or no oxygen. Reteach L1 Summarize how oceans, the atmosphere, and continents were formed. Teaching Resources • Section Summary: Early Earth • Review and Reinforce: Early Earth • Enrich: Early Earth 341