How do geologists date rocks? Radiometric dating!

Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and minerals contain tiny amounts of these radioactive elements.

Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay. Radioactive decay occurs at a constant rate, specific to each radioactive isotope. (Different forms of a single element that have the same number of protons but different numbers of neutrons in their nuclei. Some radioactive isotopes are unstable and shed nuclear particles over time until they become stable. For instance, unstable isotopes of uranium break down to become lead.).

Since the 1950s, geologists have used radioactive elements as natural “clocks” for determining numerical ages of certain types of rocks.

Radiometric clocks are “set” when each rock forms. “Forms” means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism (a changing that occurs due to heat, pressure, or the introduction of chemically active fluids) cools off. It’s this resetting process that gives us the ability to date rocks that formed at different times in earth’s history.

A commonly used radiometric (relating to the measurement of geologic time) dating technique relies on the breakdown of potassium-40 to argon-40. In igneous rocks, the potassium-argon “clock” is set the moment the rock first crystallizes from magma. Precise measurements of the amount of 40K relative to 40Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized. If an igneous (rock that has solidified from lava or magma) or other rock is metamorphism (changed), its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.

Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks. This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things. When living things die, they stop taking in carbon-14, and the radioactive clock is “set”! Any dead material incorporated with sedimentary deposits is a possible candidate for carbon-14 dating.

Radiometric dating has been used to determine the ages of the Earth, Moon, meteorites, ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth’s magnetic field, and many of other geological events and processes.

The Earth is a constantly changing planet. Its crust is continually being created, modified, and destroyed. As a result, rocks that record its earliest history have not been found and probably no longer exist. Nevertheless, there is substantial evidence that the Earth and the other bodies of the Solar System are 4.5-4.6 billion years old, and that the Milky Way Galaxy and the Universe are older still. The principal evidence for the antiquity of Earth and its cosmic surroundings is:

The oldest rocks on Earth, found in western Greenland, have been dated by four independent radiometric dating methods at 3.7-3.8 billion years. Rocks 3.4-3.6 billion years in age have been found in southern Africa, western Australia, and the Great Lakes region of North America. These oldest rocks are metamorphic rocks but they originated as lava flows and sedimentary rocks. The debris from which the sedimentary rocks formed must have come from even older crustal rocks. The oldest dated minerals (4.0-4.2 billion years) are tiny zircon crystals found in sedimentary rocks in western Australia.

The oldest Moon rocks are from the lunar highlands and were formed when the early lunar crust was partially or entirely molten. These rocks, of which only a few were returned by the Apollo missions, have been dated by two methods at between 4.4-4.5 billion years in age.

The majority of the 70 well-dated meteorites have ages of 4.4 to 4.6 billion years. These meteorites, which are fragments of asteroids and represent some of the most primitive material in the solar system, have been dated by 5 independent radiometric dating methods.

The “best” age for the Earth is based on the time required for the lead isotopes in four very old lead ores (galena) to have evolved from the composition of lead at the time the Solar System formed, as recorded in the Canyon Diablo iron meteorite. This “model lead age” is 4.54 billion years.

“Since the 1950s, geologists have used radioactive elements as natural “clocks” for determining numerical ages of certain types of rocks.”

The evidence for the antiquity of the Earth and Solar System is consistent with evidence for an even greater age for the Universe and Milky Way Galaxy. a) The age of the Universe can be estimated from the velocity and distance of galaxies as the universe expands. The estimates range from 7 to 20 billion years, depending on whether the expansion is constant or is slowing due to gravitational attraction. b) The age of the Galaxy is estimated to be 13.7 billion years from the rate of evolution of stars in globular clusters, which are thought to be the oldest stars in the Galaxy. The age of the elements in the Galaxy, based on the production ratios of osmium isotopes in supernovae and the change in that ratio over time due to radioactive decay, is about 12 billion years. Theoretical considerations indicate that the Galaxy formed within a billion years of the beginning of the Universe. c) Combining the data from a) and b), the “best, i.e., most consistent, age of the universe is estimated to be close to 14 billion years.

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 Godthab 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.

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.

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.

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.

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.

And lastly, if you know how long it takes for potassium-40 to become argon-40, and you measure the amounts of each in a sample, you can work out how old a material is.

Sometime in 1953 a fellow named Clair Patterson announced a definitive age of the Earth. He estimated the Earth was 4.5 billion years old. He was the first to come to this conclusion after much studying and after making many measurements.

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