Chapter 7: Earth Materials as Time Keepers How Does Earth Work? 2e Chapter 7: Earth Materials as Time Keepers

7.4 How Was the Geologic Time Scale Constructed? The names of periods (the fundamental time interval) derive from places where rocks of that age, based on their fossils, were first described. Periods are grouped into eras, which are grouped into eons. Periods include shorter intervals called epochs, and the periods in the Cenozoic era as well as the Carboniferous of the Paleozoic are subdivided once prior to the epoch.

Fig 7.9 The Geologic Time Scale

7.4 How Was the Geologic Time Scale Constructed? Fig 7.10 top

7. 7 How Is the Absolute Age of a Rock Determined 7.7 How Is the Absolute Age of a Rock Determined? Rate of Radioactive Decay Radioactivity Refers to the energy and subatomic particles released as one element transforms into another due to changes in the number of protons and neutrons Isotopes are atoms of the same element with the same number of protons, but a different number of neutrons. Radioactive decay—a change in the number of protons, neutrons, or both that transforms an unstable isotope towards a stable one.

7.7 How Is the Absolute Age of a Rock Determined? Fig. 7.17 Measure the isotope abundances An unstable “parent” isotope decays towards a stable “daughter” isotope. Assuming each daughter comes from a parent isotope, and that we know the average time it takes to convert, we can use the ratio of the two to calculate the age of the rock they are in. Half-life is the time interval during which the number of parent isotope atoms decreases by half, turning them into the daughter element.

7.7 How Is the Absolute Age of a Rock Determined? The four major isotopic dating techniques and the time spans they can be used to measure; the longer the half-life, the older the material that can be measured Table 7.1

7.7 How Is the Absolute Age of a Rock Determined? Applying the potassium-argon dating method Fig. 7.20

7.7 How Is the Absolute Age of a Rock Determined? How to estimate absolute ages of sedimentary rocks Fig. 7.21

7.7 How Is the Absolute Age of a Rock Determined? Combining absolute and relative age dating Fig. 7.22