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Absolute Dating

Radiometric Dating
Radiometric dating provides science with a powerful tool for reconstructing our planet’s history. The idea that radioactivity could be used as a measure of the age of geologic formations was first suggested in 1905 by a British physicist, Lord Rutherford. In 1907 Professor B. B. Boltwood, a radiochemist at Yale University, made the first attempt to establish a geologic time scale. The invention of the mass spectrometer after World War I led to the discovery of isotopes (see below) and the calculation of accurate decay rates. Not until the 1950s, however, was precise dating achieved and accepted by the scientific community. The methodologies and instruments for radiometric dating have been expanded and fine-tuned in the half-century since, and very accurate dating is now possible.

Atoms are composed of a nucleus orbited by negatively charged electrons. The nucleus is made up of protons, particles with a positive charge, and neutrons, particles with no charge. Every atom of a given element has the same number of protons in the nucleus. Each element may have one or more isotopes. Different isotopes of a given element have the same number of protons but a different number of neutrons.

Radioactive elements are unstable atoms that give off particles. Emitting these particles transforms the unstable atoms into different, more stable elements. This is called radioactive decay, and it occurs at a constant rate specific to each isotope of each element. The original radioactive material is called the parent; the stable product is called the daughter. The rate of decay is described by the half-life of the isotope—the average time an atom of a radioactive element remains in the parent state. When the half-life has elapsed, half the parent element will have decayed into the daughter element.

Potassium-40, for example, decays into Argon-40 with a half-life of 1.25 billion years, so that after 1.25 billion years half of the Potassium-40 in a rock will have become Argon-40. This means that if a rock sample contained equal amounts of Potassium-40 and Argon-40, it would be 1.25 billion years old. If the sample contained three atoms of Potassium-40 for every one atom of Argon-40, it would be 625 million years old. And if it contained one atom of Potassium-40 for every three atoms of Argon-40 it would be 1.875 billion years old.

Most radioactive isotopes decay too rapidly to be useful in determining age on a geologic scale. Carbon-14 dating is probably one of the best-known dating methods, but the half-life of Carbon-14 is approximately 5730 years, plus or minus 40 years. That makes the half-life far too short for dating material that is millions of years old. A few isotopes, however, do decay extremely slowly and can be used as geologic clocks. These isotopes are:

PARENT ISOTOPE
HALF-LIFE STABLE DAUGHTER
Uranium-235 704 Million Years Lead-207
Potassium-40 1.25 Billion Years Argon-40
Uranium-238 4.5 Billion Years Lead-206
Thorium-232 14.0 Billion Years Lead-208
Lutetium-176 35.9 Billion Years Hafnium-176
Rubidium-87 48.8 Billion Years Strontium-87
Samarium-147 106 Billion Years Neodymium-143

Depending on the kind of rock studied, radiometric data can give different kinds of information. Igneous rock is formed from cooling magma or lava, and it contains small amounts of radioactive elements. By determining the ratio of the parent material to the daughter material in the igneous rock, it’s possible to calculate the rock’s age. As igneous rock erodes, the eroded particles are deposited to become sedimentary rock. Dating sedimentary rock by using radiometric techniques will tell the age of the original igneous rock, not the time since the sedimentary rock formed. (Although sometimes the two ages are very similar, for example when a volcanic explosion deposits ash on a surface and that ash is quickly incorporated into sediments. The age of the ash and the age of the sedimentary rock would then be very similar.) Metamorphic rock, by contrast, is formed from earlier rock through intense heat and pressure. Metamorphism can reset some radiometric clocks (Potassium-Argon is particularly susceptible), so that radiometric dates record the time of alteration rather than the date when the earlier rock first solidified from magma or was deposited as sediment. Other parent-daughter pairs are less susceptible to alteration.

The oldest dated rocks on Earth come from northern Canada and are about 4 billion years old. Rocks older than 3 billion years have been found in many places around the planet. Moon rocks have been dated at 4.4 to 4.5 billion years. Meteorites that are left over from the earliest time of the solar system have been dated at 4.4 to 4.6 billion years.


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