Finding the age of rocks based on the presence of naturally occurring long-lived radioactive isotopes of several elements in certain minerals and rocks. Measurements of rock ages have enabled geologists to reconstruct the geologic history of the Earth from the time of its formation 4.6 × 109 years ago to the present. Age determinations of rocks from the Moon have also contributed to knowledge of the history of the Moon, and may someday be used to study the history of Mars and of other bodies within the solar system. See also Radioactivity.
Many rocks and minerals contain radioactive atoms that decay spontaneously to form stable atoms of other elements. Under certain conditions these radiogenic daughter atoms accumulate within the mineral crystals so that the ratio of the daughter atoms divided by the parent atoms increases with time. This ratio can be measured very accurately with a mass spectrometer, and is then used to calculate the age of the rock by means of an equation based on the law of radioactivity. The radioactive atoms used for dating rocks and minerals have very long half-lives, measured in billions of years. They occur in nature only because they decay very slowly. The pairs of parents and daughters used for dating are listed in the table. See also Dating methods.
Parent | Daughter | Half-life, 109 years |
|---|---|---|
Potassium-40 | Argon-40 | 11.8 |
Potassium-40 | Calcium-40 | 1.47 |
Rubidium-87 | Strontium-87 | 48.8 |
Samarium-147 | Neodymium-143 | 107 |
Rhenium-187 | Osmium-187 | 43 |
Thorium-232 | Lead-208 | 14.008 |
Uranium-235 | Lead-207 | 0.7038 |
Uranium-238 | Lead-206 | 4.468 |
The rubidium-strontium method is based on rubidium-87, which decays to stable strontium-87 (87Sr) by emitting a beta particle from its nucleus. The abundance of the radiogenic strontium-87 therefore increases with time at a rate that is proportional to the Rb/Sr ratio of the rock or mineral. The method is particularly well suited to the dating of very old rocks such as the ancient gneisses near Godthaab in Greenland, which are almost 3.8 × 109 years old. This method has also been used to date rocks from the Moon and to determine the age of the Earth by analyses of stony meteorites.
The potassium-argon method is based on the assumption that all of the atoms of radiogenic argon-40 that form within a potassium-bearing mineral accumulate within it. This assumption is satisfied only by a few kinds of minerals and rocks, because argon is an inert gas that does not readily form bonds with other atoms. The K-Ar method of dating has been used to establish a chronology of mountain building events in North America beginning about 2.8 × 109 years ago and continuing to the present. In addition, the method has been used to date reversals of the polarity of the Earth's magnetic field during the past 1.3 × 107 years. See also Orogeny; Paleomagnetism.
The uranium, thorium-lead method is based on uranium and thorium atoms which are radioactive and decay through a series of radioactive daughters to stable atoms of lead (Pb). Minerals that contain both elements can be dated by three separate methods based on the decay of uranium-238 to lead-206, uranium-235 to lead-207, and thorium-232 to lead-208. The three dates agree with each other only when no atoms of uranium, thorium, lead, and of the intermediate daughters have escaped. Only a few minerals satisfy this condition. The most commonly used mineral is zircon (ZrSiO4), in which atoms of uranium and thorium occur by replacing zirconium. See also
The common-lead method is based on the common ore mineral galena (PbS) which consists of primordial lead that dates from the time of formation of the Earth and varying amounts of radiogenic lead that formed by decay of uranium and thorium in the Earth. The theoretical models required for the interpretation of common lead have provided insight into the early history of the solar system and into the relationship between meteorites and the Earth.
The fission-track method is based on uranium-238 which can decay both by emitting an alpha particle from its nucleus and by spontaneous fission. The number of spontaneous fission tracks per square centimeter is proportional to the concentration of uranium and to the age of the sample. When the uranium content is known, the age of the sample can be calculated. This method is suitable for dating a variety of minerals and both natural and manufactured glass. Its range extends from less than 100 years to hundreds of millions of years. See also Fission track dating.
The samarium-neodymium method of dating separated minerals or whole-rock specimens is similar to the Rb-Sr method. The Sm-Nd method is even more reliable than the Rb-Sr method of dating rocks and minerals, because samarium and neodymium are less mobile than rubidium and strontium. The isotopic evolution of neodymium in the Earth is described by comparison with stony meteorites. See also Meteorite.
The rhenium-osmium method is based on the beta decay of naturally occurring rhenium-187 to stable osmium-187. It has been used to date iron meteorites and sulfide ore deposits containing molybdenite.
