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Earth. Dividing the total thickness by the rate of deposition others, such as carbon-14, are only useful for dating things to
resulted in estimates of an Earth age that ranged from about 20 perhaps 50,000 years. Carbon-14 is not used to age rocks, but is
to 1,500 million years. The wide differences occurred because very useful in aging materials that are of relatively recent biologi-
there are gaps in many sedimentary rock sequences, periods cal origin, since carbon is an important part of all living things.
when sedimentary rocks were being eroded away to be depos- Also, a slightly different method is used to determine carbon-14
ited elsewhere as sediments again. There were just too many dating. (See “A Closer Look: Carbon Dating?” on p. 333).
unknowns for this technique to be considered acceptable. A recently developed geologic clock is based on the magnetic
The idea of measuring the rate that Earth is cooling for use orientation of magnetic minerals. These minerals become aligned
as a geologic clock assumed that Earth was initially a molten with Earth’s magnetic field when the igneous rock crystallizes,
mass that has been cooling ever since. Calculations estimating making a record of the magnetic field at that time. Earth’s mag-
the temperature that Earth must have been to be molten were netic field is global and has undergone a number of reversals in
compared to Earth’s present rate of cooling. This resulted in an the past. A geomagnetic time scale has been established from the
estimated age of 20 to 40 million years. These calculations were number and duration of magnetic field reversals occurring dur-
made back in the nineteenth century before it was understood that ing the past 6 million years. Combined with radiometric age dat-
natural radioactivity is adding heat to Earth’s interior, so it has re- ing, the geomagnetic time scale is making possible a worldwide
quired much longer to cool down to its present temperature. geologic clock that can be used to determine local chronologies.

MODERN TECHNIQUES EXAMPLE 21.1
Soon after the beginning of the twentieth century, the discovery Potassium-argon analysis of feldspar crystals in an igneous rock finds
of the radioactive decay process in the elements of minerals 25 percent of the parent potassium-40 isotope present in the specimen.
and rocks led to the development of a new, accurate geologic What is the age of the rock? (Refer to Table 21.2 for the half-life of the
potassium-40/argon-40 isotope pair.)
clock. This clock finds the radiometric age of rocks in years by
meas uring the radioactive decay of unstable elements within
the crystals of certain minerals. Since radioactive decay occurs SOLUTION
at a constant, known rate, the ratio of the remaining amount of
After the first half-life, 50 percent of the potassium would remain in
an unstable element to the amount of decay products present
the feldspar crystal. After the second half-life, 25 percent would remain.
can be used to calculate the time that the unstable element has Therefore, 2 half-lives have elapsed.
been a part of that crystal (see chapter 13). Certain radioactive
number of half-lives = 2
isotopes of potassium, uranium, and thorium are often included 9
in the minerals of rocks, so they are often used as “radioactive time of half-life = 1.25 × 10 yr
clocks.” By using radiometric aging techniques along with other age = ?
information, we arrive at a generally accepted age for Earth of
age = (number of half-lives)(time of half-life)
about 4.5 billion years. It should be noted that radiometric aging 9
= (2)(1.25 × 10 yr)
is only useful in aging igneous rocks, since sedimentary rocks are 9
the result of weathering and deposition of other rocky materials. = 2.50 × 10 yr
Table 21.2 lists several radioactive isotopes, their decay prod-
ucts, and their half-lives. Often two or more isotopes are used
to determine an age for a rock. Agreement between them in- EXAMPLE 21.2
creases the scientist’s confidence in the estimates of the age of the Laboratory analysis of a wood specimen from a glacial deposit indicated
rock. Because there are great differences in the half-lives, some one-sixteenth of the normal proportion of carbon-14 to carbon-12.
are useful for dating things back to several billion years, while How old is the wood specimen? (Refer to Table 21.2 for the half-life
of carbon-14 and to “A Closer Look” in chapter 13 for a discussion of
4
carbon-14 dating.) (Answer: 2.292 × 10 yr.)
TABLE 21.2
Radioactive isotopes and half-lives
For an additional worked example on this material, see
Radioactive Stable Daughter the chapter 21 resources on www.mhhe.com/tillery.
Isotope Product Half-Life
Samarium-147 Neodymium-143 106 billion years
THE GEOLOGIC TIME SCALE
Rubidium-87 Strontium-87 48.8 billion years
A yearly calendar helps you keep track of events over periods of
Thorium-232 Lead-208 14.0 billion years
time by dividing the year into months, weeks, and days. In a simi-
Uranium-238 Lead-206 4.5 billion years
lar way, the geologic time scale helps you keep track of events that
Potassium-40 Argon-40 1.25 billion years have occurred in Earth’s geologic history. The first development
Uranium-235 Lead-207 704 million years of this scale came from the work of William “Strata” Smith, the
Carbon-14 Nitrogen-14 5,730 years English surveyor described in the section on fossils earlier in this
chapter. Recall that Smith discovered that certain rock layers in

530 CHAPTER 21 Geologic Time 21-10