TheThe RockRock andand FossilFossil RecordRecord Earth’s Story and Those Who First Listened . 426 Apply . 427 Internet Connect . 428 When on Earth? . 429 Activity . 430 MathBreak . 434 Internet Connect 432, 435 Looking at Fossils . 436 QuickLab . 438 Internet Connect . 440 Time Marches On . 441 QuickLab . 443 Internet Connect . 445 Chapter Lab . 446 Chapter Review . 449 TEKS/TAKS Practice Tests . 451, 452 Feature Article . 453 Time Stands Still Pre-Reading Questions Sealed in darkness for 49 million years, this beetle still shimmers with the same metallic hues that once helped it hide among ancient plants. This rare fossil 1. How do scientists study was found in Messel, Germany. In the same rock formation, the Earth’s history? scientists have found fossilized crocodiles, bats, birds, and 2. How can you tell the age frogs. A living stag beetle (above) has a similar form and of rocks and fossils? color. Do you think that these two beetles would live in 3. What natural or human similar environments? What do you think Messel, Germany, events have caused mass was like 49 million years ago? In this chapter, you will extinctions in Earth’s learn how scientists answer these kinds of questions. history? 424 Chapter 16 Copyright © by Holt, Rinehart and Winston. All rights reserved. MAKING FOSSILS Procedure 1. You and three or four of your classmates will be given several pieces of modeling clay and a paper sack containing a few small objects. 2. Press each object firmly into a piece of clay. Try to leave an imprint showing as much detail as possible. 3. After you have made an imprint of each object, exchange your model fossils with another group. 4. In your ScienceLog, describe the fossils you have received. List as many details as possible. What patterns and textures do you observe? 5. Work as a group to identify each fossil, and check your results. Were you right? Analysis 6. What kinds of details were important in identifying your fossil? What kinds of details were not preserved in the imprints? For example, can you tell the colors of the objects? 7. Explain how Earth scientists follow similar methods when studying fossils. The Rock and Fossil Record 425 Copyright © by Holt, Rinehart and Winston. All rights reserved. Earth’s Story and Those Who First Listened How do mountains form? How is new rock created? How old is the Earth? Have you ever asked these questions? Nearly 250 years ago, a Scottish farmer and amateur scientist named James Hutton did. Terms to Learn Searching for answers to his questions, Hutton spent more uniformitarianism than 30 years studying rock formations in Scotland and Great catastrophism Britain. As you can see, geology, the scientific study of the Earth’s history, got a pretty late start. It wasn’t until the nineteenth What You’ll Do century that we began to understand the incredible history of G Contrast uniformitarianism the planet right beneath our feet. with catastrophism. G Predict land features that result from gradual geologic The Present Is the Key to the Past change. In 1795, two years before he died, James Hutton collected his G Describe how the science of notes and wrote Theory of the Earth. In that series of books, geology has developed over he stated that the key to understanding Earth’s history was all the past 200 years. around us. In other words, processes that we observe today— such as erosion and deposition—remain uniform, or do not change, over time. This assumption is now called uniformi- tarianism. Uniformitarianism is the idea that the same geologic processes shaping the Earth today have been at work through- Figure 1 Hutton observed out Earth’s history. Figure 1 shows how Hutton developed the gradual, uniform geologic idea of uniformitarianism. change. Judging by the rate of change, he concluded that the Earth must be incredibly old. 1 Hutton observed that rock is broken down into smaller particles. 2 He watched as rivers carried these particles downstream. 4 Hutton thought that in time the new rock would be raised, forming new landforms, and that the cycle would 3 begin again. He knew that rock particles formed layers of sediment. He predicted that these deposits would form new rock over time. 426 Chapter 16 Copyright © by Holt, Rinehart and Winston. All rights reserved. Uniformitarianism Versus Catastrophism Hutton’s theories sparked a scientific debate because he sug- gested that the Earth was much older than previously thought. Biology In Hutton’s time, most people thought that the Earth was only a few thousand years old. To explain the Earth’s history, most CONNECTION scientists supported catastrophism, the idea that all geologic It’s no coincidence that the theory of change happens suddenly. Supporters of this idea claimed that evolution was developed soon after all of the Earth’s features, such as mountains, canyons, and Lyell introduced his ideas. Lyell and oceans, formed during rare, sudden events called catastrophes. Charles Darwin were good friends, These unpredictable events caused rapid geologic change over and their talks greatly influenced large areas—even across the entire Earth. Darwin’s theories. Similar to uniformitarianism, Darwin’s theory Slow and Steady Wins the Race! of evolution proposes that changes Despite Hutton’s work, people still thought that the Earth’s in species happen gradually over history could be explained by catastrophism. It took the work long periods of time. of Charles Lyell, another Scottish scientist, for people to seri- ously consider uniformitarianism. Armed with Hutton’s notes and new evidence of his own, Lyell successfully challenged catastrophism in the 1830s. Lyell’s victory inspired generations of geologists to interpret rock formations and unlock the Earth’s long and sometimes bizarre history. For example, fossil evidence showed that there were once great oceans where deserts now exist. Geologists learned that rocks formed by the gradual deposition of sediment on the ocean floor could be a twisted and deformed by incredible pressures and temperatures inside the c Earth. Scientists began to see layers of rock as if they were pages in the book of Earth’s history. Every stone told a tale. d Making Assumptions b Examine these photographs. List them in the order you think they were taken. Now write down all of the assumptions that you made to infer that order. Were your assumptions similar to those of your classmates? Briefly explain the importance of making assumptions in science. How does this apply to uniformitarianism? The Rock and Fossil Record 427 Copyright © by Holt, Rinehart and Winston. All rights reserved. Modern Geology—Rethinking the Past Today geologists understand that sudden catastrophes are also a part of Earth’s history. Although most geologic change is slow and uniform, rare catastrophes have left their mark in the rock and fossil record. For example, enormous craters have been found where asteroids and comets struck the Earth in Did you know that dinosaur the past. Some of these impacts may have been catastrophic. bones were not identified until 1841? Hutton and Lyell devel- Such an asteroid impact, illustrated in Figure 2, may have led oped their ideas unaware that to the extinction of the dinosaurs. The impact would have these giants dominated 180 spread dust and smoke into the Earth’s atmosphere, blocking million years of Earth’s history. the sun’s rays and causing major changes in the global cli- mate. Unable to adapt to the changes, the dinosaurs died out. Figure 2 Today scientists think that sudden events are responsible for some changes in Earth’s past. For example, an asteroid impact may have led to the extinction of the dinosaurs 65 million years ago. Why did Hutton’s theories suggest that the Earth is much older than previously thought? How has the role of catastrophism in Earth science TOPIC: Earth’s Story changed? sciLINKS NUMBER: HSTE130 Making Predictions Apply the principle of uniformi- tarianism to predict how land features such as river channels, rock layers, and mountains form. 428 Chapter 16 Copyright © by Holt, Rinehart and Winston. All rights reserved. When on Earth? Imagine that you are a detective investigating a crime scene. What is the first thing you would do? You might begin by dusting for fingerprints or by searching for witnesses. As a detective, your goal is to figure out the order of events that happened before you reached the crime scene. Terms to Learn You could think of the Earth as an enormous geologic crime relative dating scene. Like detectives, geologists try to figure out the order of superposition events in the Earth’s history. But instead of fingerprints and geologic column witnesses, geologists study rocks and fossils, and they use a unconformity method called relative dating. Relative dating is the process of absolute dating determining the order in which events happened. Relative dat- isotopes ing also compares the ages of objects such as rocks or fossils. radiometric dating half-life Relative Dating: Which Came First? What You’ll Do Suppose you have an older brother who takes a lot of photo- graphs of your family and piles them in a box. Over the years, G Describe relative dating. he keeps adding new pictures to the top of the stack. Think G Demonstrate an understand- about the family history recorded in those pictures. Where are ing of the geologic column and superposition. the oldest pictures—the ones taken when you were a baby? G Identify four events that alter Where are the most recent pictures—those taken last week? rock layers. Rock layers, such as those shown in Figure 3, are like stacked G Explain radioactive decay, pictures. As you move from top to bottom, the layers are older.