Geochronology

Geochronology is the science of determining the age of rocks, fossils, and sediments using clues found inside the rocks themselves. Scientists can find out exactly how old something is by looking at special types of atoms called radioactive isotopes. They can also learn about the order of events using tools like paleomagnetism and stable isotope ratios.

An artistic depiction of the major events in the history of Earth

Geochronology works together with another science called biostratigraphy. Biostratigraphy looks at fossils to place rocks into known time periods, but it does not tell the exact age. Instead, it shows that a rock existed during a certain time when those fossils were alive. Both sciences help us understand the order and timing of Earth's history.

Geochronology is very important in a field called chronostratigraphy. This field tries to find exact dates for all fossils and to understand the history of the Earth and even other planets like extraterrestrial bodies. By using geochronology, scientists can piece together the story of our planet and others in space.

Dating methods

Radiometric dating

Main article: Radiometric dating

Scientists can find the exact age of rocks and fossils by studying certain types of atoms that change over time. These atoms, called radioactive isotopes, have a known rate of change, called a half-life. By measuring how much of these atoms have changed, scientists can figure out how old a sample is. Some methods work best for very old samples, while others are good for more recent ones. One common method is the radiocarbon method, which works well for objects up to about 60,000 years old.

Some common ways to date samples include:

Radiocarbon dating. This looks at how much carbon-14 is left in organic materials.
Uranium–lead dating. This measures the ratio of lead isotopes to uranium in minerals like zircon.
Uranium–thorium dating. This dates things like cave formations, corals, and bones.
Potassium–argon dating and argon–argon dating. These date rocks and volcanic ash layers.
Electron spin resonance (ESR) dating

Fission-track dating

Main article: Fission track dating

Cosmogenic nuclide geochronology

Main article: Cosmogenic radionuclide dating

These methods help find out when a surface was formed or when materials were buried. They look at special atoms made by cosmic rays to figure out the age of surfaces like alluvial fans or buried sediments.

Luminescence dating

Luminescence dating looks at the light given off by certain materials like quartz and diamond. Techniques such as optically stimulated luminescence and thermoluminescence are used to date things like pottery or cooking stones, and can also help study how sand moves.

Tephra horizons in south-central Iceland. The thick and light-to-dark coloured layer at the height of the volcanologist's hands is a marker horizon of rhyolitic-to-basaltic tephra from Hekla.

Incremental dating

Main article: Incremental dating

Incremental dating can build year-by-year timelines, either linked to today’s calendar or not. Examples include:

Paleomagnetic dating

A sequence of known magnetic poles can help date rocks. By comparing the magnetic poles of unknown rocks to well-known sequences, scientists can estimate their age. There are two methods: one for rocks within the same continental block and another for folded areas where rotations might have happened.

Magnetostratigraphy

Main article: Magnetostratigraphy

Magnetostratigraphy finds the age of rocks by looking at patterns of magnetic polarity in layered sediments or volcanic rocks. These patterns are compared to a known timeline of magnetic changes.

Chemostratigraphy

Trends in certain chemicals, like carbon-13 and strontium isotopes, can help match rock layers from different places.

Correlation of marker horizons

Marker horizons are special layers of rock that look the same everywhere and can help tell if rocks are the same age. Fossils and volcanic ash layers are often used as markers. Tephrochronology matches unknown volcanic ash to well-dated ash layers, which is also useful in archaeology.

Interval hierarchy

Geochronological divisions, from the largest to the smallest, include:

Differences from chronostratigraphy

It is important not to mix up geochronological and chronostratigraphic units. Geochronological units refer to periods of time. For example, it is correct to say that Tyrannosaurus rex lived during the Late Cretaceous Epoch. Chronostratigraphic units refer to geological materials. So, it is also correct to say that fossils of the genus Tyrannosaurus have been found in the Upper Cretaceous Series. You can visit places like the Hell Creek deposit where Tyrannosaurus fossils were found, but you cannot visit a period of time like the Late Cretaceous Epoch because time cannot be visited.