Dating With Radioactivity 7:Geologic Time

An organism dies with a small amount of potassium-40 in its tissues. The half-life of potassium-40 is 1.3 billion years. What fraction of the original potassium-40 is present after 3.9 billion years?

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Half-Life

A half-life is a common way of expressing the rate of radioactive decay. A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to its stable isotope. If the half-life of a radioactive isotope is known and the parent/daughter ratio can be measured, the age of the sample can be calculated. For example, if the half-life of an unstable isotope is 1 million years, and 1/16 of the parent isotope remains, this amount indicates that four half-lives have passed. The sample must be 4 million years old.

Simple but Complex

Although the basic principle of radiometric dating is simple, the actual procedure is complex. The analysis that determines the quantities of parent and daughter must be very precise. In addition, some radioactive materials do not decay directly into the stable daughter product. Uranium-238, for example, produces thirteen intermediate unstable daughter products before the fourteenth and final daughter product, the stable isotope lead-206, is produced

Accurate Radiometric Date

An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation. If the addition or loss of either parent or daughter isotopes occurs, then it is not possible to calculate a correct date. For example, an important limitation of the potassium-argon method stems from the fact that argon is a gas. Argon may leak from minerals and throw off measurements. Cross-checking of samples, using two different radiometric methods, is done whenever possible to ensure accuracy.

Apollo Missions

Between 1969 and 1972, six Apollo missions brought back rocks and soil from the moon. Through radiometric dating, scientists have determined the age of the moon rocks ranged from 3.3 to 4.6 billion years old. The oldest rocks were collected from the lunar highlands. These highlands are thought to have formed when the moon was still partially or entirely molten. Since 1970, over 35,000 meteorites have been found in Antarctica. These samples have been well preserved in the Antarctic ice. Using the rubidium-strontium method of dating, scientists have determined that these Antarctic meteorites all have ages between 4.48 and 4.56 billion years. If fact, nearly all of the meteorites found on Earth have yielded ages between 4.4 and 4.6 billion years old.

Introduction

Early geologists like William Smith could only determine the relative ages of rock layers. They could not find exact dates for events in Earth's past. Today, geologists can determine the absolute age of a rock. A rock's absolute age is the approximate number of years before the present that the rock formed. For example, we know that Earth is about 4.56 billion years old and that the dinosaurs became extinct about 65 million years ago. To understand the method geologists used to arrive at these dates, you need first to understand radioactivity.

Igneous rock

For igneous rock, radiometric dating establishes when minerals in the rock crystallized. For metamorphic rock, radiometric dating determines when heat and pressure caused new minerals to form in the rock or when daughter isotopes escaped from the minerals. One frequently used type of radiometric dating is the potassium-argon method. In this method, geologists measure the ratio of radioactive potassium-40 atoms to stable argon atoms in a sample. Given the long half-life of potassium-40, this method can be used to date rocks that are hundreds of millions of years old. For example, geologists used potassium-argon dating to date lava from the Hawaiian islands.

Radio active isotope

How can a radioactive isotope serve as a reliable "clock"? The rates of decay for many isotopes have been precisely measured and do not vary under the physical conditions that exist in Earth's outer layers. Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. The products of decay have also been accumulating at a constant rate. For example, when uranium is incorporated into a mineral that crystallizes from magma, lead isn't present from previous decay. The radiometric clock starts at this point. As the uranium decays, atoms of the daughter product (lead) are formed and begin to accumulate. Of the many radioactive isotopes that exist in nature, five have proved particularly useful in providing radiometric ages for ancient rocks.

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How do scientists use radiometric dating to measure the absolute ages of rocks?

In radiometric dating, scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample to be dated. The older the sample, the more daughter product it contains.

Radioactive Isotopes Frequently Used in Radiometric Dating

Radioactive Parent Stable Daughter Product Currently Accepted Half-Life Values Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 713.0 million years Thorium-232 Lead-208 14.1 billion years Rubidium-87 Strontium-87 47.0 billion years Potassium-40 Argon-40 1.3 billion years

Scientists use radiometric dating to determine the absolute age of a rock. Why is radiometric dating useful for this task?

Radioactive isotopes decay at a constant rate.

Radioactivity

Radioactivity is a process that involves the nucleus of the atom. Each atom has a nucleus made up of protons and neutrons. The forces that bind protons and neutrons together in the nucleus are usually strong. However, in some atoms, the forces binding the protons and neutrons together are not strong enough, and the atoms are unstable. During radioactive decay, unstable atomic nuclei spontaneously break apart, or decay, releasing energy. The term for the process by which atoms decay is radioactivity.

Radiometric Dating of Sedimentary Rock

Radiometric dating can rarely be used to date sedimentary rocks directly. Sedimentary rocks may contain particles that can be dated. But these particles are not the same age as the rocks in which they occur. The sediment from which the rock formed probably weathered from older rocks. Radiometric dating would not be accurate because the sedimentary rock is made up of so many older rock particles. Geologists have developed an indirect method of dating sedimentary rocks. To determine the age of sedimentary rock, geologists must relate the sedimentary rock to datable masses of igneous rock. Geologists identify two igneous rock masses. One rock mass must be relatively older than the sedimentary rock. The other rock mass must be younger. Then they use radiometric methods to date the two igneous rock masses. The age of the sedimentary rock must lie between the ages of the igneous rocks.

Radiometric Dating

Radiometric dating, also called radioactive decay dating, is a way of calculating the absolute ages of rocks and minerals that contain certain radioactive isotopes. In radiometric dating, scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample to be dated. The older the sample, the more daughter product it contains.

Scientists date igneous rocks to indirectly date nearby sedimentary rocks. Why doesn't radiometric dating typically work on sedimentary rocks?

Sedimentary rocks contain bits of sediment, each much older than the rocks they form.

How does an atom change when it undergoes radioactive decay?

The nucleus breaks apart and releases energy.

Radioactive Isotopes

The number of neutrons in the atoms of a given element can vary. These different forms of an element are called isotopes. An unstable, or radioactive, an isotope of an element is called the parent. The isotopes that result from the decay of the parent are called the daughter products. What happens when unstable nuclei break apart? Radioactive decay continues until a stable or nonradioactive isotope is formed. For example, uranium-238 decays over time to form the stable isotope lead-206.

Dating with Carbon-14

To date organic materials, carbon-14 is used in a method called radiocarbon dating. Organic material is a substance that contains carbon and comes from a living thing. Carbon-14 is the radioactive isotope of carbon. Carbon-14 is continuously produced in the upper atmosphere. It quickly becomes incorporated into carbon dioxide, which circulates in the atmosphere and is absorbed by living matter. As a result, all organisms—including you—contain a small amount of carbon-14.

Carbon 14-12

While an organism is alive, the decaying radiocarbon is continually replaced. Thus, the ratio of carbon-14 to carbon-12—the stable isotope of carbon—remains constant. When an organism dies, the amount of carbon-14 gradually decreases as it decays. By comparing the ratio of carbon-14 with carbon-12 in a sample, radiocarbon dates can be determined. Because the half-life of carbon-14 is only 5730 years, it can be used to date recent geologic events up to about 75,000 years ago. Carbon-14 has become a valuable tool for anthropologists, archaeologists, and historians, as well as for geologists who study recent Earth history.

Which artifact could best be dated with carbon-14—a method called radiocarbon dating?

a fragment of a turtle shell that likely is 10,000 to 30,000 years old

Thorium-232 undergoes a series of radioactive decay by emitting energy and small particles from the atomic nucleus. Which could be the stable daughter product?

lead-208, which is not radioactive

Which dating method enables scientists to measure or estimate the absolute age of sedimentary rock?

radiometric dating of igneous rocks that relate to the sedimentary rock

Potassium-40 undergoes radioactive decay to become argon-40. The half-life is 1.3 billion years. Which measurement would be most useful to find the absolute age of a rock that contains potassium?

the ratio of potassium-40 to argon-40

What limits the usefulness of radiocarbon dating in studying Earth's history?

the relatively short half-life of carbon-14, which is 5,730 years