Grade 8 Relative-Age Dating and Absolute-Age Dating New Vocabulary Term Relative Age Definition the age of rocks and geologic features with respect to other rocks and features nearby Superposition principle that in undisturbed rock layers, the oldest rocks are on the bottom inclusion piece of an older rock that becomes part of a new rock unconformity surface where rock has eroded away, producing a break in the rock record correlation process of matching rocks and fossils from separate locations Index fossil species that existed on Earth for a short length of time, was abundant, and inhabited many locations Absolute age numerical age of a rock or other object Isotope atoms of the same element that have different numbers of neutrons Radioactive decay process by which an unstable element naturally changes into another stable element Half-Life time required for half of the parent isotopes to decay into daughter isotopes Relative ages of rocks In the rock formation shown the oldest rocks are in the bottom layer and the youngest rocks are in the top layer. Geologists have developed a set of principles to compare the ages of rock layers. Relative age is the age of rocks and geologic features compared with other rocks and features nearby. Inclusions Occasionally when rocks form, they contain pieces of other rocks. This can happen when part of an existing rock breaks off and falls into soft sediment or flowing magma. A piece of an older rock that becomes part of a new rock is called an inclusion. According to the principle of inclusions, if one rock contains pieces of another rock, the rock containing the pieces is younger than the pieces. Unconformities An unconformity is a surface where rock has eroded away, producing a break, or gap, in the rock record. An unconformity is not a hollow gap in the rock. It is a surface on a layer of eroded rocks where younger rocks have been deposited. However, an unconformity does represent a gap in time. It could represent a few hundred years, a million years, or even billions of years. Correlation Rock layers contain clues about Earth. Geologists use these clues to build a record of Earth’s history. Many times the rock record is incomplete, such as happens in an unconformity. Geologists fill in gaps in the rock record by matching rock layers or fossils from separate locations. Matching rocks and fossils from separate locations is called correlation. Geologists correlate rocks by matching exposed rock layers in different locations. Index Fossils The rock formations above are correlated based on similarities in rock type, structure, and fossil evidence. If scientists want to learn the relative ages of rock formations that are very far apart or on different continents, they often use fossils. If two or more rock formations contain fossils of about the same age, scientists can infer that the formations are also about the same age. Index fossils represent species that existed on Earth for a short length of time, were abundant, and inhabited many locations. When an index fossil is found in rock layers at different locations, geologists can infer that the layers are of similar age. Absolute ages of rocks Scientists use the term absolute age to mean the numerical age, in years, of a rock or object. By measuring the absolute ages of rocks, geologists have developed accurate historical records for many geologic formations. Scientists have been able to determine the absolute ages of rocks only since the beginning of the twentieth century. That is when radioactivity was discovered. Radioactivity is the release of energy from unstable atoms. Atoms An atom is the smallest part of an element. Each atom contains smaller particles called protons, neutrons, and electrons. Protons and neutrons are in an atom’s nucleus. Electrons surround the nucleus. Isotopes Isotopes are atoms of the same element that have different numbers of neutrons. Radioactive Decay Some isotopes are unstable. These isotopes are known as radioactive isotopes. Radioactive isotopes decay, or change, over time. As they decay, they release energy and form new, stable atoms. Radioactive decay is the process by which an unstable element naturally changes into another element that is stable. Half-Life The rate of radioactive decay is constant for a given element. This rate is measured in time units called half-lives. An isotope’s half-life is the time required for half of the parent isotopes to decay into daughter isotopes. Half-lives of radioactive isotopes range from a few microseconds to billions of years. When half the parent isotopes have decayed into daughter isotopes, the isotope has reached one half-life. At this point, 50 percent of the isotopes are parents and 50 percent of the isotopes are daughters. After two half-lives, one-half of the remaining parent isotopes have decayed so that only one-quarter as much parent remains as at the start. At this point, 25 percent of the isotopes are parent and 75 percent of the isotopes are daughter. This process continues until nearly all parent isotopes have decayed into daughter isotopes. Radiometric Ages Because radioactive isotopes decay at a constant rate, they can be used like clocks to measure the age of the material that contains them. Radiocarbon Dating One important radioactive isotope used for dating is an isotope of carbon called radiocarbon. Radiocarbon is also known as carbon-14, or C-14, because there are 14 particles in its nucleus—six protons and eight neutrons. As long as an organism is alive, the ratio of C-14 to C-12 in its tissues remain constant. However, when an organism dies the C14 present in the organism starts to decay to nitrogen-14 (N-14). As the dead organism’s C14 decays, the ratio of C-14 to C-12 changes. Scientists measure the ratio of C-14 to C-12 in the remains of the dead organism to determine how much time has passed since the organism died. The half-life of carbon-14 is 5,730 years and is therefore not very useful to date any organism older than 60 000 years. Dating Rocks Radiocarbon dating is useful only for dating organic material—material from once-living organisms. Most rocks do not contain organic material. For dating rocks, geologists use different kinds of radioactive isotopes. Dating Igneous Rock One of the most common isotopes used in radiometric dating is uranium-235, or U-235. U235 is often trapped in the minerals of igneous rocks that crystallize from hot, molten magma. As soon as U-235 is trapped in a mineral, it begins to decay to lead-207, or Pb-207, Different Types of Isotopes The half-life of uranium-235 is 704 million years. This makes it useful for dating rocks that are very old. Radioactive isotopes with short half-lives cannot be used for dating old rocks. They do not contain enough parent isotope to measure. Geologists often use a combination of radioactive isotopes to measure the age of a rock. Radiometric dating of rocks from the Moon and meteorites indicate that Earth is 4.54 billion years old. Scientists accept this age because evidence suggests that Earth, the Moon, and meteorites all formed at about the same time. Radiometric dating, the relative order of rock layers, and fossils all help scientists understand Earth’s long history.
