Problem - Solving Activity
Interpreting Rock Layers
The law of superposition states that in undisturbed beds, younger sedimentary rocks lie on top of older sedimentary layers. But over time, layers of sedimentary rock can change. Deformation can produce faults and folds. Erosion can remove some layers of rock entirely. Igneous dikes may cut across the layers. As you determine the relative ages of rock layers, remember these rules:
Sedimentary rock layers are horizontal before they deform.
A fault or dike did not exist when the sedimentary layers formed, so it is younger than the layers it cuts across.
Study the rock layers in the diagram, and then answer the following questions.
Inferring Which rock layer is older, B or E? J or G?
Analyzing Data Would a fossil in layer F be older or younger than fossils from layer J?
Inferring If the dike is 430 million years old, what can you say about the age of layer D?
Drawing Conclusions Make a table showing the relative ages of all the layers in the diagram, from oldest to youngest.
Figure 34 The geologic time scale shows the major intervals in Earth's history.
Interpreting Diagrams What three periods make up the Mesozoic Era?
Radioactive Dating
Geologists use radioactive dating to determine the absolute ages of rocks. A rock's absolute age is the time that has passed since the rock formed. When a rock forms, it has a known ratio of radioactive and stable isotopes. Because a radioisotope decays into a stable isotope at a steady rate as the rock ages, scientists can measure this ratio to find the rock's absolute age. Recall from chemistry that the time for half of a radioisotope to decay is called its half-life. Many igneous rocks are very old, so radioisotopes with a very long half-life are used to find their absolute age. A common radioisotope for dating older rocks is potassium-40, which has a half-life of 1.3 billion years.
A Brief History of Earth
Geologists have used information about the relative and absolute ages of rocks to develop a time line for the history of Earth. The geologic time scale is based on the relative ages of rock layers and the use of radioactive dating to find the absolute ages of rocks. The geologic time scale, shown in Figure 34, is a way of dividing Earth's history.
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