Time is the foundation of geology
The geologic time scale of Earth’s creation is almost unimaginable to us.
This is because humans lifespans are so short in comparison. We work in hours, days, months and years.
But the Earth works in decades, centuries and millions of years.
Like a geologic calendar, geologists divide time. From longest to shortest, they are eons, eras, periods, epochs and ages.
The concept of time is very long in geology
First, you have to realize that Earth’s history stretches a length of time approximately 4.5 billion years long.
For time markers in geologic time, we typically use abbreviations like ‘Ga’ (giga-annum), ‘Ma’ (mega-annum), and ‘Ka’ (kilo-annum).
- ‘Ga’ or ‘Gya’ (billion) is 1,000,000,000 years ago
- ‘Ma’ or ‘Mya’ (million) is 1,000,000 years ago
- ‘Ka’ or ‘Kya’ (thousand) 1,000 years ago
For example, 2.5 Ga refers to 2.5 billion years ago. Because Earth is about 4.5 billion years old, Earth would be about 2 billion years old at this time.
EONS are the longest division of time
Eons are the longest division of geologic time. Generally, we measure eons as billions of years ago (Ga) and millions of years ago (Ma). Each unit is half a billion years or more.
Geologists divide the lifespan of Earth into a total of 4 eons. From origin to now, Earth’s 4 eons are the Hadean, Archean, Proterozoic and Phanerozoic Eon. The Hadean, Archean and Proterozoic eons are sometimes grouped as the Precambrian Eon.
Since 542 mya, the Earth (including humans) has been in the Phanerozoic Eon. This division marks the appearance of animal life in the record.
ERAS span several hundred millions of years
Eras are divisions of geologic time shorter than eons but longer than periods. In terms of geochronological units, there are 10 defined eras which generally span several hundred million years.
For example, the Paleozoic, Mesozoic, and Cenozoic eras are within the Phanerozoic Eon. Each of these era boundaries are separated because each one ended in catastrophic extinction.
PERIODS divide eras
Periods are divisions of geologic time longer than epochs but shorter than an era. There are 22 defined periods. Each period spans a length of tens to one hundred million years. For example, the Mesozoic Era which contains the Cretaceous Period lasted about 79 million years.
A notable division in periods is between the Cretaceous/Tertiary Periods (~65 Ma) which was the extinction of the dinosaurs. Also notable is the Carboniferous period which marked a time of vast swamp forests when coal was produced.
EPOCHS break down periods
The geologic time scale conceptually consists of eons that are broken down into smaller eras. These eras are then divided into periods, which in turn are broken down into epochs.
Epochs are an extended period of time usually characterized by a distinctive development or by a memorable series of events. There are 34 defined epochs which generally last for tens of millions of years.
AGES are one of the shortest geochronologic units
Finally, we can divide epochs even further into ages. Ages are one of the shortest division of geologic time which contain minor differences between epochs.
In terms of the number of geochronological units, there are 99 defined which can stretch over millions of years. If you divide ages even further, the geochronological unit are known as chrons.
International Commission of Stratigraphy (ICS)
The role of the International Commission of Stratigraphy (ICS) is to define global units as it relates to a geological time scale. ICS chronologically orders Earth’s history into eons, eras, periods, epochs and ages as an International Chronostratigraphic Graph.
From signatures inherent in rocks, geologists can realize the true age of the earth. By tracing back on embedded fossilized organisms in rock strata, we can achieve a context of time.
Because we can date rocks through stratigraphy, we can achieve a better understanding of geological events in Earth’s history. Based on our geological time scale, we can better understand the theory of evolution and origin of life on our planet.
Geologic Time Scale as a Geological Calendar
Instead of working in days, months and years, geologists work in millions and billions of years.
And they subdivide these long chunks of time into eons, eras, periods, epochs and ages.
Like a geologic calendar, they chronologically order units of time into a geologic time scale.
Finally, each division of time identifies a prominent event or characteristic feature based on their record.