How do we know whether one rock is older than another? We can determine the relative ages of rocks by studying the stratigraphy, fossils, and cross-cutting relationships of rock formations observed at outcrops. According to Steno’s principles, an undeformed sequence of sedimentary beds will be horizontal, with each bed younger than the beds beneath it and older than the beds above it. In addition, the fossils found in each bed reflect the organisms that were living when that bed was deposited. Knowing the faunal succession makes it easier to spot unconformities, which indicate time gaps in the stratigraphic record where no rock was deposited or where existing rock was eroded away before the next strata were laid down.
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How was a global geologic time scale created? By using faunal successions to match rocks in outcrops around the world, geologists compiled composite stratigraphic successions, from which they developed a relative time scale. The use of isotopic dating allowed them to assign absolute ages to the eons, eras, periods, and epochs that constitute the geologic time scale. Isotopic dating is based on the decay of radioactive isotopes, in which unstable parent atoms are transformed into stable daughter atoms at a constant rate. By measuring the amounts of parent and daughter atoms in a sample, geologists can calculate the absolute ages of rocks. The isotopic clock starts ticking when radioactive isotopes are locked into minerals as igneous rocks crystallize or metamorphic rocks recrystallize.
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What are the principal divisions of the geologic time scale? The geologic time scale is divided into four eons: the Hadean (4.56 billion to 3.9 billion years ago), Archean (3.9 billion to 2.5 billion years ago), Proterozoic (2.5 billion to 542 million years ago), and Phanerozoic (542 million years ago to the present). The Phanerozoic eon is divided into three eras, the Paleozoic, Mesozoic, and Cenozoic, each of which is divided into shorter periods. The boundaries of the eras and periods are marked by abrupt changes in the fossil record; many correspond to mass extinctions.
What other methods are now being used to date the geologic record? The cyclical rise and fall of sea level produces complex sedimentary sequences on continental margins around the world that can be mapped using seismic imaging techniques and dated using fossils. Chemical fingerprints and magnetic reversals provide additional information about the ages of sedimentary sequences. Glacial cycles recorded in sediments can be dated using ice cores taken from the Antarctic and Greenland ice caps.