Earth s History. Geologic Time

Transcription

1 CHAPTER 3 Earth s History LESSON 1 Geologic Time What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. All rocks contain fossils. 2. Humans produce all radioactive materials. Key Concepts What evidence supports the idea that Earth is very old? What evidence did scientists use to develop the geologic time scale? How does the geologic time scale compare to the human time scale? Evidence That Earth Has Changed You have changed in many ways throughout your life. What kinds of evidence show these changes? Maybe you have some old photographs, some clothing that no longer fits, or toys that you played with when you were younger. These items show that you have changed. Similarly, scientists search for evidence that Earth has changed over time. Scientists study Earth s past. They develop ideas and theories about how Earth formed and how it has changed. Sometimes when scientists make new discoveries, they must change their theories. What kinds of evidence show that Earth has changed? Some evidence is under your feet. Rock layers above and below Earth s surface hold the clues to Earth s past. Rock Layers and Fossils Weathering breaks rocks exposed at Earth s surface into smaller pieces called sediment. Over time, gravity, water, and wind carry sediment downhill and deposit it in low areas called basins. Eventually, layers of sediment form. As more sediment is deposited, the layers become heavier. The increasing weight of the sediment slowly causes the layers to compress, or pack tightly together. Layers of rock form. Rock layers form over millions of years. Therefore, scientists know that Earth must be very old. Identify the Main Ideas Work with a partner. Read a paragraph together. Then discuss what you learned in the paragraph. Continue until you and your partner understand the main ideas of the lesson. 1. Explain How are rocks evidence of Earth s age? Reading Essentials Earth s History 33

2 Math Skills You can use percentages to calculate how much of the original element is left after each half-life. In a sample of a radioactive element, 50 percent of the atoms decay after each half-life. If you start with 1,000 atoms of a radioactive element, how many atoms of that element will be left after one half-life? a. Change the percentage to a decimal by moving the decimal point two places to the left. 50% = 0.50 b. For each half-life, multiply the number of starting atoms by the decimal number. 1,000 atoms 0.50 = 500 atoms 2. Use Percentages If you start with 1,000 atoms, how many atoms would remain after 2 half-lives? Visual Check 3. Describe What percentage of parent atoms remains after 3 half-lives? Principle of Superposition New sediment layers collect on top of older layers. The oldest layer is usually on the bottom. Geologists use this observation to organize rock layers according to their ages. The principle of superposition states that in rock layers that have not been folded or deformed, the oldest rock layers are on the bottom. This principle cannot give the actual ages of rock layers in years, or absolute age. Instead, it gives the relative ages of rock layers. Relative age tells whether layers are younger or older than other rock layers. Fossils Sometimes as sediment builds up, it buries organisms within the layers. Under certain conditions, the organisms become rock along with the layers. Fossils are the preserved remains or evidence of past living organisms. Many fossils represent species that are no longer alive. Radioactivity Tiny particles called atoms make up all matter on Earth. An element is a substance that contains only one type of atom. Most elements are stable. Stable elements remain unchanged under normal conditions. But some elements are unstable. Over time they decay, or break down, and form different elements. Radioactive Decay The process by which one element naturally changes into another is called radioactive decay. The decay occurs when the nucleus of the atom throws off particles. The original element is called the parent element. The new element that forms is called the daughter element. A radioactive element decays at a rate that is constant, or always the same, for that element. The graph below shows how the percentage of parent atoms decreases as parent atoms decay and form daughter atoms. Parent Atoms (%) Half-Life Parent atom Daughter atom Number of Half-Lives 34 Earth s History Reading Essentials

3 Half-Life Scientists have calculated the rates of decay of radioactive elements. The half-life of an element is the time required for half of the amount of a radioactive parent element to decay into a stable daughter element. With each half-life, the percentage of parent atoms decreases by half. Because radioactive elements decay at a constant rate, scientists can use them as clocks to measure time. First, they must know the element s half-life. Then, scientists compare the amount of parent element to the amount of daughter element in the sample. From this comparison, scientists can calculate the age of the sample. Analyzing radioactive elements shows that some rocks are billions of years old. The Geologic Time Scale In the 1800s, geologists organized the rocks and rock layers. They based their organization on the principle of superposition and the fossils present in the layers. They saw that older rock layers usually lie below younger ones. So, they put the oldest layers at the bottom of their charts. Geologists also named the rock layers. Each name represented a time in Earth s history. Eventually, they drew a chart that represented all the time periods in Earth s history. The geologic time scale is a visual record of Earth s history, with the individual units based on changes in the rocks and fossils. The time scale is always drawn with the oldest rocks at the bottom and the youngest rocks at the top. Eons, Eras, Periods, and Epochs You can divide the history of your life into years, months, weeks, and days. You might use events, such as birthdays or your first day of school, to mark divisions in your life. The geologic time scale shows the units used to describe Earth s history. Geologists divide Earth s history into eons, eras, periods, and epochs. They use the fossil record to mark geologic divisions. The divisions in the geologic time scale are not all the same length. The divisions mark places in the rock record where major changes occur in the types of fossils present in the rocks. For example, the sudden appearance of complex life-forms marks the beginning of the Cambrian period. A widespread die-off of organisms marks the end of the Permian period. 4. Identify What evidence supports the idea that Earth is very old? 5. Identify What evidence did scientists use to develop the geologic time scale? Make a vertical four-tab Foldable to show the relationships among eons, eras, periods, and epochs. Eons Eras Periods Epochs Reading Essentials Earth s History 35

4 6. Compare How does the geologic time scale compare to the human time scale? Comparing Time Scales Earth is 4.6 billion years old, as shown on the geologic time scale below. It s probably difficult for you to imagine that much time. Comparing the geologic time scale to something familiar, such as a year, can help. Imagine all of Earth s history occurring within one year, starting January 1. The first tiny organisms float in the ocean on February 21. The earliest animals crawl onto land on November 20. The last of the dinosaurs becomes extinct on December 25. And human ancestors appear on the afternoon of December 31. Humans have been only a small part of Earth s history. Observing Slow and Rapid Changes According to the geologic time scale, Earth has changed over billions of years. Most of these changes occurred slowly. It takes millions of years for a mountain range to erode. Other changes, such as volcanoes, occur rapidly. And an earthquake can change Earth s surface in just seconds. The Geologic Time Scale Eons Eras Periods Phanerozoic Mesozoic Cenozoic Tertiary Paleozoic Cretaceous Present Quaternary 65.5 mya Visual Check 7. Interpret Circle the name of the geologic era in which we live today. Precambrian time Proterozoic Archean Hadean 2,500 mya 3,800 mya 4,560 mya Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian 251 mya 542 mya 36 Earth s History Reading Essentials

5 Mini Glossary fossil: the preserved remains or evidence of past living organisms geologic time scale: a visual record of Earth s history, with the individual units based on changes in the rocks and fossils half-life: the time required for half of the amount of a radioactive parent element to decay into a stable daughter element principle of superposition: principle which states that in rock layers that have not been folded or deformed, the oldest rock layers are on the bottom radioactive decay: the process by which one element naturally changes into another 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes the process of radioactive decay. 2. The diagram below represents the positions of rock layers according to the principle of superposition. Complete the diagram by writing oldest and youngest in the correct boxes. Rock Layer A Rock Layer B Rock Layer C 3. Explain one main idea that you learned from discussing a paragraph with your partner. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Earth s History 37