The Geological Time Scale

The geological time scale is based on the rock record, including erosion, mountain formation, and other geological phenomena. Continents, oceans, and mountain ranges have migrated across tremendous distances vertically and horizontally, over hundreds to millions of years. For example, areas that were once deep oceans are now mountainous desert zones, dating back to hundreds of millions of years.

Having understood what the geological time scale is, it’s time to move ahead with the measurement.

How is geological time measured?

The earliest geological time scales relied solely on the order in which rocks were set in strata, with the oldest at the bottom. However, the fossilised remains of ancient creatures and plants found within the rock strata proved a more potent instrument. Geologists realised that specific fossils were constrained to specific rock layers after Charles Darwin published the book, “Origin of Species”, in 1859 (Darwin was also a geologist). This resulted in the creation of the first geological time scale.

Sequences could be matched from the faunal successions once formations and stratigraphic sequences were traced worldwide. These sequences apply from the beginning of the Cambrian epoch, when macro-fossils first appeared. Even though certain species may span numerous strata, fossil assemblages ‘fingerprint’ them. This feature enabled William Smith, an engineer and surveyor who worked in England’s coal mines in the late 1700s, to place an order for the fossils he began collecting in 1793 in south-eastern England.

He observed that different formations contained distinct fossils and that the differences in the fossils might be used to map one formation distinct from another. He drew up a stratigraphic succession of rocks as he mapped southern England.

Understanding the unit of time in geological time scale chart

Let’s understand the geological time scale chart in detail.

Aeons

Aeons, which last hundreds of millions of years, are the longest geological time intervals. On the time scale above, the Phanerozoic Aeon is the most recent aeon.

Eras

Aeons are separated into eras, which are smaller periods. The Phanerozoic is divided into three eras – Cenozoic, Mesozoic, and Paleozoic. The limits of the periods are determined by key events in Earth’s history.

Periods

Periods are separated from eras. The events that bind periods are more prevalent, but they are not as momentous as those that bind the eras.

Epochs

The Cenozoic periods are typically broken into epochs, allowing for finer chronological subdivisions. Periods can only be divided into epochs for the most recent segment of the geologic time scale. This is because older rocks have been extensively buried, highly deformed, and altered by long-term Earth processes. As a result, the history enshrined in these rocks is more difficult to decipher.

How are the division dates decided?

After knowing what a geological time scale chart is, it’s essential to understand that various event categories are used to divide the numerous aeons, eras, periods, epochs, and ages. These types of events include, but are not limited to, the following:

• The advent of a species for the first time: This is true for all living things, including plants, animals, bacteria, etc. Species that are the “first of first” are of particular importance (e.g. the first oxygen-breathing organism, the first seed-producing plant, and so on).
• Extinction of species – Major species extinction or mass extinction is a geological event in which many species go extinct in a short period. The Cretaceous-Paleogene extinction event, when most dinosaur species became extinct, is a well-known example of mass extinction.
• Major global climate shifts – when the climate changes drastically from the normal period, such as when ice ages occur
• Supercontinent creation and/or separation – Supercontinents are formed when all of Earth’s major landmasses merge as one landmass.
• Large-scale catastrophic events can induce or trigger other events like climate change and extinction. Massive volcano eruptions, meteorite impacts, and major floods are examples of catastrophic disasters.
• Global magnetic polarity shifts – The planet’s magnetic polarity “flips” on an irregular basis, with the North Pole becoming the South and vice versa.

Conclusion

Earth’s history has been classified into time intervals by geologists. These intervals do not have the same length as the hours in a day. Instead, the length of the periods is changeable. This is because geological time is separated by key events in Earth’s history. The geological time scale arose from a need to organise the vastness of geological time and correlate geological events on a global scale. The geological time scale that we use today was not proposed by a single person or an expert body. It grew by trial and error, thanks to the labour of several independent geologists.