chapter 39 Clues to Earth s Past section 3 Absolute Ages of Rocks What You ll Learn how absolute age differs from relative age how the half-lives of isotopes are used to tell a rock s age Before You Read How old are you? How do you know what your exact age is? On the lines below, tell different ways you could verify your exact age. Highlight As you read this section, highlight each of the vocabulary terms and their definitions. C Explain Use quarter sheets of notebook paper to explain absolute age, radioactive decay, half-life, and radiometric dating. Absolute Age Half-life Radioactive Decay Radiometric Dating Read to Learn Absolute Ages After you sort through your stack of magazines looking for that article about the car you saw, you decide that you need to get your magazines back into a neat pile. By now, they are all in a jumble. They are no longer in order according to their relative age. How can you stack them so the oldest are on the bottom and the newest are on the top? Luckily, all the magazines have dates on their covers. The dates make your job easy. By using the dates as your guide, you can put the magazines back in order easily. What is absolute age? Rocks don t have dates stamped on them. Or do they? Absolute age is the age, in years, of a rock or other object. Scientists who study rocks, or geologists, are able to figure out the absolute age of rocks. Geologists use the properties of atoms in rock material to determine absolute age. Knowing the absolute age of rocks leads to a better understanding of events in Earth s history. 30 Clues to Earth s Past
Radioactive Decay Each atom has a dense center called the nucleus, which is surrounded by particles with a negative charge called electrons. Inside the nucleus are protons, which have a positive charge, and neutrons, which have no electric charge. The number of protons determines the identity of the element. The number of neutrons determines the form of the element, or isotope. For example, every atom with just one proton is a hydrogen atom. Hydrogen atoms can have no neutrons, one neutron, or two neutrons. This means that there are three isotopes of hydrogen. Some isotopes break down into other isotopes, giving off a lot of energy. Radioactive decay is the process in which the nucleus of an atom breaks down. What are alpha and beta decay? In some isotopes, a neutron breaks down into a proton and an electron. This type of radioactive decay is called beta decay, because the electron leaves as a beta particle. The nucleus loses a neutron but gains a proton. Other isotopes give off two protons and two neutrons in the form of an alpha particle. This is called alpha decay. Alpha and beta decay are shown in the figure below.. Identify What is the process in which the nucleus of an atom breaks down called? Unstable parent isotope Unstable parent isotope Neutron Neutron Proton Beta decay Alpha decay Daughter product Daughter product Proton Beta particle (electron) Alpha particle Picture This 2. Determine the beta particle that is given off during beta decay and the alpha particle given off during alpha decay. Reading Essentials 3
3. Explain What has to happen to the parent isotope before the daughter product can form? What is a half-life? In radioactive decay, the parent isotope breaks down. The daughter product is formed. Each parent isotope decays to its daughter product at a certain rate. Based on its decay rate, it takes a certain period of time for one half of the parent isotope to decay to its daughter product. The half-life of an isotope is the time it takes for half of the atoms in the isotope to decay. The figure below shows how during each half-life, one half of the parent material decays to the daughter product. For example, the half life of carbon-4 is 5,730 years. So, it will take 5,730 years for half of the carbon-4 atoms to change into nitrogen-4 atoms. You might think that in another 5,730 years, all the remaining carbon-4 atoms will decay into nitrogen-4 atoms. But they don t. Only half the remaining atoms will decay during the next 5,730 years. So, after two half-lives, one fourth of the original carbon-4 atoms will remain. After many half-lives, such a small amount of isotope remains that it is not measurable. Picture This 4. Determine the fraction that shows what remains of the parent material after 4 half-lives. Write the fraction below. Parent material 2 2 halflife 3 4 4 2 halflives 7 5 8 6 8 3 halflives 4 halflives Radiometric Ages Decay of radioactive isotopes is like a clock keeping track of time that has passed since rocks have formed. As time passes, the amount of parent isotope in a rock decreases and the amount of daughter product increases. Scientists can use this information to figure out the absolute age of the rock. Radiometric dating is the process used to calculate the absolute age of rock by measuring the ratio of parent isotope to daughter product in a mineral and knowing the half-life of the parent. % 32 Clues to Earth s Past
What does radiocarbon dating show? Carbon-4 is useful for dating bones, wood, and charcoal up to 75,000 years old. Living organisms take in carbon from the environment to build their bodies. Most of the carbon is carbon-2, but some is carbon-4. The ratio of these two isotopes in the environment is always the same. After the organism dies, the carbon-4 slowly decays. Scientists can compare the isotope ratio in the sample to the isotope ratio in the environment. Once scientists know the amount of carbon-4 in a sample, they can determine the age of bones, wood, or charcoal. Can radiometric dating be used on all rocks? Aside from carbon-4 dating, rocks that can be radiometrically dated are usually igneous and metamorphic rocks. Most sedimentary rocks can t be dated this way. Why? Many sedimentary rocks are made up of particles that eroded from older rocks. Dating these pieces only gives the age of the original rocks they came from. What are the oldest known rocks? Radiometric dating has been used to date the oldest rocks on Earth. These rocks are about 3.96 billion years old. Scientists estimate Earth is about 4.5 billion years old. Rocks older than 3.96 billion years probably were eroded or changed by heat and pressure. Uniformitarianism Before radiometric dating was used, many people thought Earth was only a few thousand years old. But in the 700s, Scottish scientist James Hutton estimated the Earth to be much older. He used the principle of uniformitarianism. Uniformitarianism states that Earth processes occurring today are similar to those that occurred in the past. Hutton observed that the processes that changed the landscape around him were slow. He inferred that they were just as slow all through Earth s history. Hutton hypothesized that it took much longer than a few thousand years to form rock layers and erode mountains. Today, scientists agree that Earth has been shaped by two types of change. There are slow, everyday processes that take place over millions of years. There are also sudden, violent events such as the collision of a comet that might have caused the dinosaurs to become extinct. 5. Explain Why doesn t radiometric dating work on sedimentary rock? 6. Describe What are the two types of change that have changed Earth? Reading Essentials 33
After You Read Mini Glossary absolute age: age, in years, of a rock or other object half-life: time it takes for half of the atoms in an isotope to decay radioactive decay: process in which the nucleus of an atom breaks down radiometric dating: process used to calculate the absolute age of rock by measuring the ratio of parent isotope to daughter product in a mineral and knowing the half-life of the parent uniformitarianism: principle stating that Earth processes occurring today are similar to those that occurred in the past. Review the terms and their definitions in the Mini Glossary. Then explain the difference between absolute age and relative age. 2. Fill in the half-life chart to show the decay of carbon-4 over time. Half-Life of Carbon-4 Percent Years Carbon-4 Passed 00 0 2.5 6.25 3.25 3. In this section you highlighted vocabulary terms. Was this strategy helpful? Explain why or why not. End of Section 34 Clues to Earth s Past Visit blue.msscience.com to access your textbook, interactive games, and projects to help you learn more about the absolute ages of rocks.