Lesson 4 Analyzing Rocks and Fossils 6 Science Lesson...Not all fossils record the shapes of the bodies of plants and animals. A trace fossil records something about an animal’s

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48 Unit 2 • Changes in Earth Over Time

Lesson 4

Analyzing Rocks and Fossils

Study the photo. Describe the soil and rock formation. What patterns do you see?

Imagine standing in a field with a shovel. If you dig a hole and spread the material out on a tray, what kinds of things might you find? What could you learn from the things you find? Explain your ideas.

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Lesson 4 • Analyzing Rocks and Fossils 49

Earth has been changing ever since it formed about 4.6 billion years ago. Volcanoes erupt, seas lower and rise, islands form, and mountains rise and wash away. Entire continents move slowly across the globe.

Living organisms first appeared on Earth about 3.8 billion years ago. They, too, have changed Earth. Together with their physical environment, plants and animals create ecosystems such as forests, plains, wetlands, and ocean reefs. Plants and animals help make the soil that covers most of Earth’s land. And they fundamentally alter Earth’s atmosphere.

A scientist says that a forest replaced a lake thousands of years ago. What kind of evidence would convince you of the change? Record your ideas below.

Look AheadTo study Earth’s history, or geology, we generally have to look beneath our feet. Rock formations underground provide important clues about the planet’s past. What are rock layers, and what can we learn from them about Earth’s history? When plants and animals die, what clues do they leave behind, and how do these clues help us understand Earth’s history? Read the rest of this lesson to find answers to these and other questions.

Kilauea, HawaiiKilauea, Hawaii

Explore!

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Layers of TimeMaterials• Jar with lid • Four different colors of pencil• Four different colors of sand

Steps

Study the jar of sand prepared by your teacher. Talk with your group about how your teacher might have filled the jar with the four layers of sand. Draw a diagram of the jar. Mark each layer of sand in order from 1 to 4, with 1 being the first layer poured.

Pour four layers of sand into your jar so that it matches your teacher’s jar. With the last layer, pour enough sand to fill the jar. Put the lid on. Do not turn your jar upside down yet. Draw a diagram of your jar. Mark each layer of sand in order from 1 to 4, with 1 being the first layer poured.

1

2

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Now turn your jar upside down and compare it with your teacher’s jar. Do the four layers of sand in your jar match the layers in your teacher’s jar? Describe any differences, and explain how you would pour your layers of sand differently to match your teacher’s jar.

Think About It

3

The activity models the process of rock formation. What do you think the layers of sand represent?

Talk with your group about Step 2. Which layer of sand represents the oldest kind of rock? Which layer of sand represents the youngest kind of rock? Explain your ideas.

Talk with your group about Step 3. What happened to the layer representing the oldest kind of rock? What happened to the layer representing the youngest kind of rock?


Understand

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An Earth process is something that changes features on the planet’s surface. A mountain range rises into the sky. A desert replaces an inland sea. Ocean levels drop, exposing miles of new land. Even rivers can change the face of the planet in important ways. The photo below shows the Rio Grande, between the United States and Mexico.

Earth processes that occur today are similar to those that happened in the past. An earthquake today is like an earthquake ten thousand years ago—or ten million years ago. It is caused by the same geologic forces, and it leaves the same kind of evidence. It changes Earth’s landscape in the same way.

Scientists use what they know about today’s Earth processes to draw conclusions about the planet’s history. They can observe, for example, that rivers carry small pieces of rock and sand in their currents. Where rivers slow down, these pieces of rock and sand build up, creating new landforms. Scientists know that rivers have been doing this for millions of years.

How do rivers change features on Earth’s surface?


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Much of Earth’s surface is covered with sedimentary rock. Sediment is small grains of rock or other material. Water and wind wear sediment away from older rocks. Currents in rivers, lakes, and oceans deposit sediment in certain places. Wind deposits sediment in other places. Over time, new sediment builds up on top of the older sediment beneath it. If the source of sediment changes, a new layer forms above the layer that formed earlier. Upper layers of sediment compress the layers below. Gradually, the grains are pressed into solid rock—sedimentary rock.

Scientists study sedimentary rock to determine the relative ages of events in Earth’s history. They know that older sediment was deposited before younger sediment could be deposited on top of it. So, older layers of rock lie beneath younger layers.

Youngest

Oldest

The photo above shows sedimentary rock. Describe the layers. What can you conclude about the rocks’ formation?

The diagram shows layers of sedimentary rock. Compare the diagram with your model of rock formation in the Explore activity. Which step of the activity modeled the formation shown in the diagram? Explain your ideas.

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The order of sedimentary layers does not reveal the rocks’ absolute ages. A rock’s absolute age is its actual age, or how many years ago it was formed. To learn a rock’s absolute age, scientists need other kinds of information.

Earth processes can disturb layers of sedimentary rock. Earthquakes can fold rock layers, or push them upward or downward. Floods can wash layers away, creating gaps. Then new layers can fill the gaps. Volcanoes can disrupt or destroy layers. In each of these cases, the order of layers is altered. Newer layers may be next to older layers—or even below them. The diagram below shows two beds of sedimentary rocks. One is undisturbed, and the other has been overturned.

Youngest

Oldest

Oldest

Youngest

Undisturbed Overturned

Think back to the Explore activity. How did you model a disturbance to your sedimentary rocks? What happened to the order of your sedimentary rocks?

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Magma is hot, molten rock below Earth’s surface. When magma pushes upward—through cracks in rock formations or at volcanoes—it forms igneous rock. These intrusions of igneous rock are younger than the rock they cut through.

Igneous Rock

Because sedimentary rocks can be disturbed, scientists have to study a range of rock formations in any region. They map the different formations and compare them. This allows scientists to determine the original order of layers and how those layers may have been altered. Then the scientists can interpret the relative ages of the rocks they find.

Imagine that you are digging through the rock formation in the diagram above. Describe how the intrusion of igneous rock would change the way you interpret relative ages.


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Go Further


The remains of living organisms also help scientists learn about Earth’s history. Plants and animals evolve, or change, over time. New species arise, and when environmental conditions change, species may die out. So the types of organisms alive today are not the same as the organisms that lived in the past. Fossils are the preserved remains of organisms that lived in the distant past. Most fossils are found in rock—sedimentary rock.

A fossil is formed when a plant or animal dies and is covered by sediment. The remains of the plant or animal decay, leaving a mold that shows the organism’s body shape or skeleton. Deposits of minerals or sediment fill the mold. As more layers of sediment are deposited, the layer around the mold compresses and gradually hardens into rock.

The photo below shows a fossil of a species of trilobite. Trilobites were marine animals that lived hundreds of millions of years ago.

Since a fossil forms in a layer of sedimentary rock, it has the same age as the rock. So scientists can determine the relative ages of fossils by examining the rock formations in which the fossils are found.


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Mucrospirifer mucronatus

Tetragraptus fructicosus

Monotis subcircularis

The opposite reasoning is also true. Today, scientists know the relative ages of many kinds of fossils. They can use this fossil information to help determine the relative ages of sedimentary rocks. Imagine that you have found an older fossil in a rock layer above a younger fossil. That would be an important clue about the rocks’ relative ages. The rock layers must have been disturbed—the older layer is above the younger layer. This could help you interpret the rest of the rock formation.

The diagram shows three marine fossils and a model of the rock layers in which they were found. Use what you know about sedimentary rock to determine the relative ages of the fossils.


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Use soil and water to make a layer of mud on a plastic tray. Use three or four objects to make imprints in the mud. Let the mud dry overnight. Exchange imprints with a partner. Can you identify the objects that your partner used to make the imprints? How do the imprints relate to fossils?

Try This

Not all fossils record the shapes of the bodies of plants and animals. A trace fossil records something about an animal’s behavior. Footprints, for example, provide evidence about an animal’s size and gait. Footprints may fill in with sediment and eventually become trace fossils. Trails, burrows, and nests can also become trace fossils.

Recall that Earth was formed about 4.6 billion years ago. Ever since then, Earth’s surface has been changing. Mountain chains have risen and worn away. Seas have formed and dried up. Millions of species of plants and animals have flourished and gone extinct. Through all of this, layers of sedimentary rock—and the fossils in them—have been creating a record. This record makes it possible to organize Earth’s history into time periods. The principle is simple. If a rock formation is undisturbed, each sedimentary layer is older than the layer on top of it. This simple principle gives scientists an important tool for understanding Earth’s history—how to determine relative ages.


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Check YourUnderstanding


1 Are older layers of sedimentary rock always located beneath younger layers? Explain your ideas.

2 Describe how scientists can use the fossil record to help determine the relative ages of rocks.

3 Draw a diagram that shows layers of undisturbed sedimentary rock. Number the layers from oldest to youngest, starting with 1.

4 Scientists have found fossils of mats formed by bacteria in ancient layers of sedimentary rock. Bacteria are single-celled organisms. Scientists have not found fossils of organisms below those layers of rock. How would you interpret this break in the fossil record? Explain your ideas.

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5 How would you interpret the fossil record shown in the diagram? Explain your ideas.

Leaf Fossils

Shark Tooth Fossils

6 Study the diagram of sedimentary rock below. Fossils of a particular species of dinosaur have been found in layer A. Layer B contains sediment created by the impact of a massive asteroid. Layers C and D do not contain fossils of the dinosaur species that is in layer A. What conclusion can you draw from the fossil record? Explain your ideas.

A

B

C

D

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Lesson 4 Analyzing Rocks and Fossils 6 Science Lesson...Not all fossils record the shapes of the bodies of plants and animals. A trace fossil records something about an animal’s