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Geology of the Pacific Northwest

Basics -- Absolute Ages of Geologic Materials

Introduction

In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years. Most absolute age determinations in geology rely on radiometric methods.

Radiometric dating methods

The earth is billions of years old. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials.

An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus. By definition, all atoms of a given element have the same number of protons. However, they do not all have the same number of neutrons. The different numbers of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element.

For example, all carbon atoms have 6 protons. Carbon-12 is the isotope of carbon that has 6 neutrons. Carbon-13 is the isotope of carbon that has 7 neutrons. Carbon-14 has 8 neutrons in its nucleus, along with its 6 protons, which is not a stable combination. That is why carbon-14 is a radioactive isotope-it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product - a different isotope, which is not radioactive.

Each type of radioactive isotope has a half-life, a length of time that it will take for half of the atoms in a sample of that isotope to decay into the stable daughter product. Physicists have measured the half-lives of most radioactive isotopes to a high level of precision.

The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle. If a granite crystallizes with minerals containing radioactive isotopes, it is as though the rock crystallizes with a built-in batch of stopwatches that begin ticking away as soon as the granite has cooled.

Radiometric age determination

Radiometric age determinations are expensive and time-consuming. A geologist has to be sure that an age of a rock will help answer an important research question before he or she devotes time and money to making a radiometric age measurement.

Before determining the age of the granite, it must be analyzed under a powerful microscope, and with an electron microprobe, to make sure that its original minerals have not been cracked and altered by metamorphism since the rock first formed. Separating the minerals from the granite is the next step in determining its age. High-precision laboratory analyses are then used to measure the amounts of radioactive parent isotope and stable daughter product in the minerals. Once these quantities have been measured, the half-life of the radioactive isotope is used to calculate absolute age of the granite.

Parent Isotopes, Daughter Isotopes, and Half-Lives

The following table lists a selection of isotope pairs that are used in making radiometric age determination. Note that carbon-14 has a relatively short half-life, which makes it useful only for young, carbon-rich geologic materials, less than about 70,000 years old.

Parent Daughter Half Life (years) Dating Range (years) Minerals/materials
238U
Uranium-238
206Pb
Lead-206
4.5 billion 10 million -
4.6 billion
Minerals include zircon, uraninite. Igneous or metamorphic rocks.
40K
Potassium-40
40Ar
Argon-40
1.3 billion 0.05 million -
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Volcanic rocks.
87Rb
Rubidium-87
87Sr
Strontium-87
47 billion 10 million -
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Igneous or metamorphic rocks.
14C
Carbon-14
14N
Nitrogen-14
5,730 years 100 - 70,000 years Not used for dating rocks, except carbonates from earth's surface such as recent coral reefs. Used for young organic materials, or surface-water samples: Wood, charcoal, peat, bone, tissue, carbonate minerals from surficial environments, water containing dissolved carbon.

Web Links

To learn more about the geologic time scale, go to the University of California Museum of Paleontology at
http://www.ucmp.berkeley.edu/exhibit/histgeoscale.html
or try out their interactive geologic time scale at
http://www.ucmp.berkeley.edu/help/timeform.html.

Glossary terms that appear on this page: absolute age; parent isotope; half-life; daughter product; granite


Geology of the Pacific Northwest
Basics--Absolute Ages
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 9/11/13