Geological time and dating methods

Geological Time

Earth’s history is divided into a series of time intervals - this makes up the geological time scale.
The geologic time scale is divided into eons, eras, periods, epochs, and ages. Each represents a specific duration of geological time; eons last billions of years where as ages last for millions of years.
The eons are divided into the Hadean, Archean, Proterozoic, and the current one, Phanerozoic. All periods of the Phanerozoic, from oldest to youngest, are the Palaeozoic, Mesozoic, and Cenozoic.
The division of the geologic time scale is based on fossil evidence and significant geological or paleontological events, such as mass extinctions.

Methods of Dating

Understanding geological time also involves understanding the methods used for dating rocks and fossils. There are two main methods: relative dating and absolute dating.

Relative Dating

The Principle of Superposition states that in an undisturbed succession of rock layers, the oldest rocks are at the bottom.
The Principle of Original Horizontality suggests that layers of sediment are originally deposited horizontally and then get deformed.
Lateral Continuity indicates sediment layers extend laterally in all directions until they thin out or meet a barrier.
The Principle of Cross-Cutting Relationships states that geological features such as faults and intrusions are younger than the rocks they cut through.
Fossil Succession allows geologists to identify and match sediment layers across wide geographic distances, using characteristic fossils found in each layer.

Absolute Dating

Radiometric dating is the most common method of obtaining absolute ages. This method measures the abundance of a naturally occurring radioactive isotope and its decay products to determine an age.
The specific ages of rocks and minerals can be determined from the half-lives of radioactive isotopes that they contain, such as uranium-lead, potassium-argon and carbon-14 dating methods.
Carbon-14 dating, or radiocarbon dating, can date organic material up to about 60,000 years old. Older geological materials are dated using isotopes with much longer half-lives, such as potassium-40 or uranium-238.
Dendrochronology or tree-ring dating, provides annual dating precision to studies of climate change, ecology, and archaeology.
Varve analysis counts annual sediment layers in lakes to provide a chronological sequence going back up to 20,000 years in some cases.
Ice core dating works similarly to varve analysis, counting layers of ice to date events and environments of the past.

Understanding these fundamental concepts and dating methods gives geologists the tools to unravel the history of our planet, and to understand the processes that have occurred over its vast geological timescales.