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Radiometric dating of rocks, combined with fossil analysis, is the most powerful method of determining geological age. But in places where sedimentary rocks do not contain suitable igneous intrusions and few fossils are present, paleontologists turn to other dating methods.
One method, known as paleomagnetic dating, relates the ages of rocks to patterns in Earth’s magnetism, which change over time. Earth’s magnetic field is not constant. Changes in both the strength and the polarity of Earth’s magnetic field occur over time. Reversals of the poles occur periodically. Both sedimentary and igneous rocks preserve a record of Earth’s magnetic field at the time they were formed, and that record can be used to determine the ages of those rocks. Other dating methods use information about continental drift, information about sea level changes, and *molecular clocks.
*connect the concepts As Key Concept 21.3 explains, a molecular clock measures the average rate at which a given gene or protein product accumulates changes, and this rate of change can be used to deduce the geological time when two biological lineages diverged from one another.
Using all of these methods, geologists developed the geological time scale (see Table 24.1). They divided the broad history of life into four eons. The Hadean eon refers to the time on Earth before life evolved. The early history of life occurred in the Archean eon, which ended about the time that photosynthetic organisms first appeared on Earth. Prokaryotic life diversified rapidly in the Proterozoic eon, and the first eukaryotes in the fossil record date from this time. These three eons are sometimes referred to collectively as Precambrian time, or simply the Precambrian. The Precambrian lasted for approximately 4 billion years and thus accounts for the vast majority of geological time. It was in the Phanerozoic eon, however—