The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Why can't we date sedimentary rocks directly? How do we assign numerical ages to intervals on the geologic column to produce a geologic time scale? (F)

Sedimentary rocks are formed from the accumulation and cementation of mineral and organic particles. These particles, or sediments, are derived from pre-existing rocks or biological debris. Therefore, sedimentary rocks represent a mixture of materials that may have significantly different ages. Consequently, directly dating the formation of a sedimentary rock is difficult because it does not have a unique age.

Unlike sedimentary rocks, igneous rocks are formed from the solidification of molten material, allowing for an accurate age determination of the rock. Radiometric dating methods, which rely on the decay of radioactive isotopes, are commonly used to date igneous rocks. These methods provide a specific numerical age for the rock, allowing geologists to determine the time of formation.

Though dating sedimentary rocks directly is challenging, they can be dated indirectly by considering the ages of nearby igneous rocks. Techniques such as bracketing, which involves using the ages of igneous rocks above and below a sedimentary layer, can provide a range of possible ages for the sedimentary rock. By narrowing down this range, geologists can infer the age of the sedimentary rock and the events that formed it.

The geologic column is a representation of Earth's rock layers and their ages, with the oldest at the bottom and the youngest at the top. Geologists use the information gathered from studying rock layers, fossils, and radiometric dating to build a geologic time scale. Intervals on this scale represent significant events in Earth's history, such as mass extinctions, major climate shifts, and the appearance of new life forms.

To assign numerical ages to intervals on the geologic column, geologists combine data from radiometric dating of igneous rocks and bracketing of sedimentary rocks, along with other dating techniques such as the study of magnetic reversals and fossil correlation. By systematically organizing this information, geologists create a detailed and precise geologic time scale that corresponds to the geologic column. In conclusion, sedimentary rocks cannot be dated directly due to their mixed composition and origins. Instead, geologists use radiometric dating of nearby igneous rocks and bracketing techniques to estimate sedimentary rock ages. This information is compiled into a geologic time scale, which assigns numerical ages to intervals on the geologic column and helps us understand Earth's history.