Geologic Time

Geologists first unraveled the geologic time scale by relative age-dating, discussed in the last chapter. Once geologists sorted out the order of rock units, subsequent advances in methodologies, detailed in this chapter, by chronometric and numerical means based on radioisotopes, other atomic measures, and quantitative techniques, were employed to measure time. Many minerals and rocks have “clocks” within them that can be used to pin down the actual age of the particular geologic sample or the age of boundaries between formal units of the geologic time scale. This chapter explains how geologists decipher those clocks and determine the ages of rocks by numerical age-dating. The history of radioisotopes is tracked, starting with Ernest Rutherford and Pierre and Marie Curie. The modern geologic time scale is depicted and expanded upon, along with why it is essential for geologic maps and how the time scale can help with people-sized problems and challenges faced on the Earth.