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What did Hutton observe about the forces that shaped the land on Determining Relative Age 1. How old is Earth estimated to be? Who originated the idea that Earth is billions of years old? On what did the 18th-century Scottish physician and farmer base his conclusions? What did James Hutton theorize? What is the principle of uniformitarianism? In what way is the principle of uniformitarianism important to the science of geology? Before Hutton, what two things did people believe about the age and geology of Earth?

What did Hutton observe about the forces that shaped the land on his farm? Layers of rock are called a. The order of rock layers reveals a. Relative age indicates a. Although various types of rock form layers, what type of rock is commonly used by scientists to determine the relative age of rocks?

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Sedimentary rocks form when a. Layers of compressed and hardened sediments are called a. What is a bedding plane? The law of superposition helps scientists determine the a. In what kinds of layers does sedimentary rock generally form? What can scientists assume when sedimentary rock layers are not horizontal?

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The rock has been tilted or deformed. The rock is not actually sedimentary. The rock has been eroded. The law of superposition is wrong. What causes sedimentary rock layers to be tilted or deformed? When sedimentary rock is tilted or deformed, scientists know that crustal movements occurred a. In what cases is it difficult to apply the law of superposition? When sedimentary rock layers have been tilted or deformed, what must scientists do before they can apply the law of superposition? What is graded bedding?

What can scientists assume if large particles are in the top of a layer of sedimentary rock? When sandy sediments form curved beds at an angle to the bedding plane, what are the sedimentary layers called? Why do the layers in cross-beds appear to be curved at the bottom and cut off at the top? Why do scientists study the shapes of cross-beds?

What are ripple marks, and how are they formed? What can scientists assume if ripple marks in sedimentary rock point up?

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How do scientists use ripple marks to determine the relative ages of rocks? How are buried rock layers exposed to erosion? They are lifted up by changes in weather. Buried layers are never exposed to erosion. An unconformity shows that a. According to the law of superposition, what is the age relationship of rocks on either side of an unconformity?

All the rocks beneath an unconformity are younger than the rocks above it. All the rocks at the boundary of an unconformity are the same age. All the rocks beneath an unconformity are older than the rocks above it. The age relationship between rocks at an unconformity cannot be determined. Which of the following is NOT a type of unconformity? How does a nonconformity form? Unstratified igneous or metamorphic rock is folded and tilted and then eroded.

Unstratified igneous or metamorphic rock is uplifted, erodes, and then covered by new igneous or metamorphic rock. Stratified rock is buried beneath unstratified igneous or metamorphic rock when a volcano erupts. Unstratified igneous or metamorphic rock us uplifted, erodes, and then sediments are deposited on the eroded surface.

In the space provided, write the letter of the description that best matches the term or phrase. What can happen when rock layers have been disturbed by faults or intrusions? What is a fault? Explain how an intrusion forms. What law do scientists apply to determine relative ages of rock when they find faults or intrusions? Explain the law of crosscutting relationships. What is the relative age of a fault or igneous intrusion that cuts through an unconformity?

Determining Absolute Age 1. What does relative age indicate? What is absolute age? A method scientists use to determine absolute age is a. Another method of determining absolute age is a. The age of a stream can be measured using rates of erosion a. Over what time period can rates of erosion help scientists determine absolute age?

Which geologic feature can be given an absolute age using rates of erosion? Rate of erosion is not a dependable way of determining the absolute age of the Grand Canyon because a. In what way can the rate of deposition be used to estimate absolute age? In general, at about what rate is sedimentary rock such as limestone, shale, or sandstone deposited?

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What are two reasons a sedimentary layer might not be deposited at the average rate? How are varves similar to the rings of a tree? What do varves look like? Where and how do varves generally form? How many layers make up a single varve? Because of radioactive decay, the amounts of parent and daughter isotopes in a rock are always changing. However, they change at a constant, known rate. Therefore, scientists can learn the age of a rock by studying the amounts of parent and daughter isotopes in it. Radiometric dating is the process of determining the absolute age of a sample based on the ratio of parent isotope to daughter isotope.

In order to use radiometric dating, you need to know the half-life of the parent isotope. The half-life of a radioactive isotope is how long it takes for half of a sample of the isotope to decay. For example, imagine that a parent isotope has a half-life of 10, years. A sample of this isotope has a mass of 12 mg. After 10, years, only one-half, or 6 mg, of the sample will be left. The half-lives of different isotopes can be very different.

Some parent isotopes have half-lives of more than 4 billion years. Others have half-lives of only about 6, years. Very old rocks can be dated only if isotopes with long half-lives are used. Very young rocks can be dated only if isotopes with short half-lives are used. How do scientists know which isotope to use to find the age of a rock?

They use information about the relative age of the rock to guess about how old the rock is. Then, they find its age, using an isotope that is useful for dating rocks of that age. Remember that different parent isotopes have different half-lives. Each parent isotope can be used to date rocks of different ages.