chapter 13 3 Clues to Earth s Past section 2 Relative Ages of Rocks Before You Read Think of two friends. You want to know who is older. What information do you need to figure out who s older? What You ll Learn how to tell the relative ages of rock layers how to interpret gaps in the rock record Read to Learn Superposition Imagine that you see an interesting car drive by. Then you remember seeing a picture of the car in the January edition of a magazine you have at home. In your room is a pile of magazines from the past year. As you dig down through the pile, you find magazines from March, then February. January must be next. How did you know that the January issue would be at the bottom? To find the older magazine under newer ones, you used the principle of superposition. How does this principle apply to rocks? The principle of superposition states that in layers of rock that have not been disturbed, the oldest rocks are on the bottom and the rocks become younger and younger toward the top. Why are rocks in layers? Sediments build up, forming layers of sedimentary rocks. The first layer to form is on the bottom. A new layer forms on top of the first one. A third layer forms on top of the second layer. The bottom layer is the oldest, because it was formed first. Sometimes, the layers of rock are disturbed. When layers have been turned upside down, other clues are needed to tell which rock layer is oldest. Study Coach Make Flash Cards As you read this section, make flash cards for each main topic. On one side of the card, write the topic. On the other side, write key information. B Compare Make a three-tab Foldable to compare the concept of relative age and the principle of superposition. Reading Essentials 193
1. Identify What term describes the age of something compared with the age of other things? 2. Interpret Highlight the layer of limestone in both figures. In which rock layer might the oldest fossils be found? Undisturbed Layers Relative Ages Remember the old magazine you were trying to find? What if you want to find another magazine? You don t know how old it is, but you know it came after the January issue. You can find it in the stack by using the principal of relative age. Relative age is the age of something compared with the ages of other things. Scientists figure out the relative ages of rocks by studying their places in a sequence. For example, if layers of sedimentary rock have been moved by a fault, or a break in Earth s surface, the rock layers had to be there before the fault cut through them. So, the relative age of the rocks is older than the relative age of the fault. Relative age doesn t tell you how old the rock is in actual years. The rock layer could be 10,000 years old or one million years old. The relative age only tells you that the rock layer is younger than the layers below it and older than the fault cutting through it. How do other clues help? It s easy to figure out relative age if the rocks haven t been moved. Look at the figure below on the left showing rock layers that haven t been disturbed. Which layer is the oldest? According to the principle of superposition, the bottom layer is oldest. Now look at the figure on the right where the rock layers have been disturbed. If a fossil is found in the top layer that is older than a fossil in the lower layer, it shows that the layers have been turned upside down. This could have been caused by folding during mountain building. Limestone Coal Sandstone Folded Layers Limestone Coal Sandstone 194 Clues to Earth s Past
Unconformities Layers of rock form a record of the past. But the record may not be whole. Layers or parts of layers might be missing. These gaps in the rock layers are called unconformities (un kun FOR muh teez). Unconformities develop when erosion removes rock layers by washing or scraping them away. There are three types of unconformities. What are angular unconformities? Forces below Earth s surface can lift and tilt layers of sedimentary rock as shown in the figure below. Over time, erosion and weathering wear down the tilted rock layers. Later, new layers of sedimentary rock are deposited on top of the tilted and eroded layers. The unconformity that results when new layers form on tilted layers is called an angular unconformity. 3. Identify What are gaps in the rock layers called? Sedimentary rocks are deposited originally as horizontal layers. The tilted layers erode. The horizontal rock layers are tilted as forces within Earth deform them. Angular unconformity An angular unconformity results when new layers are deposited on the tilted layers. Why would a layer of rock be missing? Now and then, a layer of rock is missing from a stack of sedimentary rock layers. Careful study reveals an old surface of erosion. At one time the rocks were exposed and eroded. Later, younger rocks formed above the erosion surface when sediments were deposited again. Even though all the layers are parallel, the rock record still has a gap. 4. Identify Highlight the angular unconformity in the last figure. Reading Essentials 195
Disconformity When a rock layer is missing, this type of unconformity is called a disconformity, shown in the figure below. A disconformity also forms when a long period of time passes without any new layers of rock forming. 5. Identify Highlight the surface where rocks were exposed and eroded before new sediments were deposited over them. Old surface of erosion disconformity Newer layers formed as sediments were deposited Parallel rock layers above and below What are nonconformities? Another type of unconformity is shown in the figure below. A nonconformity occurs when metamorphic or igneous rocks are uplifted and eroded. Sedimentary rocks are then deposited on top of the erosion surface. The surface between the two rock types is a nonconformity. 6. Determine In the figure color the rock being uplifted red. Color the sedimentary rock being deposited blue. 7. Explain What are two ways to correlate rock layers? Old surface of erosion nonconformity Sedimentary rock deposited on top of eroded surface Igneous or metamorphic rock underneath Matching Up Rock Layers Suppose scientists are studying a layer of sandstone. Later, at an area 250 km away, they observe a layer of sandstone that looks like the sandstone they studied in the first location. Above the sandstone is a layer of limestone and then another layer of sandstone. They return to the first area and find the same sequence sandstone, limestone, sandstone. Based on their observations, they theorize that the same layers of rock are in both locations. Often, layers of rocks that are far apart can be matched up, or correlated. What evidence can correlate rock layers? One way to correlate exposed rock layer from two places that are far apart is to walk along the layer from one place to the next. Walking along a layer can prove it is unbroken. Layers can also be matched using fossil evidence. If the same types of fossils are found in the same rock layer in both places, it shows that the rock layer in each place is the same age and also that it is from the same deposit. 196 Clues to Earth s Past
After You Read Mini Glossary principle of superposition: states that in undisturbed rock layers, the oldest rock is at the bottom and the rocks become younger and younger toward the top relative age: age of something compared to the age of other things unconformity: a gap in the rock layers due to erosion or a period without rock deposit 1. Review the terms and their definitions in the Mini Glossary. Choose one term and explain in your own words what it means. 2. Complete the table about unconformities. Unconformities Type Description Causes Angular unconformity Sedimentary rock layers over igneous or metamorphic rock Erosion of whole layers or no new deposition 3. As you read this section, you made flash cards to help you learn. How did the flash cards help you learn about how layers can be correlated? Visit earth.msscience.com to access your textbook, interactive games, and projects to help you learn more about the relative ages of rocks. End of Section Reading Essentials 197