Welcome to the history of the earth. This blog looks at the geological time scale in order to explore the history of the earth. The aim of the blog is to highlight our small place in this timeline and to also better understand the journey that the earth and life has gone through.
What Is The Geological Time Scale
The Geological Time Scale was developed to organise the vast history of earth into manageable chunks of time based on rock and fossil evidence. Early geologists like Nicolas Sterno laid the groundwork for this scale in the 17th century, with the understanding that deeper layers of rocks are older than the younger top layers of rock.
The chunks of time are divided according to geological events that occurred in earth’s history. These events range from mass extinctions, to changes in the global temperatures or the atmosphere, it could be because of a massive volcanic eruption or the changes caused by ice ages.
How Did The Scale Come About
Based on the work of geologists in the 17th and 18th centuries, the geologists of the 19th century correlated fossil discoveries with layers of rock to define the ages of rock layers. This led to the identification of ‘eons’ and ‘eras’ of time which were based on significant fossil and rock changes.
In the late 19th century, radioactivity was discovered which enabled accurate dating methods.
In the 20th century, radiometric dating refined the timeline further allowing for further divisions of time.
Today we have advanced radiometric dating techniques and other modern methods of obtaining data which are being used to define the timeline further.
The Geological Time Scale - History of the Earth
For this blog, we will be using the timeline above. The large pink dots represents the beginning and end of an eon of time, and the small gold dots represent 500 million year points in history.
As the key also demonstrates, abbreviations have been used to represent billions of years ago (bya) and millions of years ago (mya).
What Are Eons
As mentioned above, the history of the earth is divided into chunks of time depending on geological events. The chunks of time are as follows:
Eons – The largest chunks of time ranging from the high millions to billions of years
Era – Eons can be divided into eras of time which are measured in hundreds of millions of years
Periods – Eras can be divided into periods of time which are usually tens of millions of years
Epochs – Periods can be divided into epochs which are thousands to low millions of years
Ages – Epochs can be divided into ages which is usually thousands of years
Hadean Eon – 4.5 to 4 Billion Years Ago
We begin with the first eon of time, the Hadean, named after the Greek god of the underworld Hades. This is represented by the first pink dot of the timeline, as the formation of earth begins this timeline and this eon.
This eon lasted around 500 million years and was a chaotic, hellish eon of earth’s history. There was extreme heat, volcanic activity and many asteroid impacts as earth was forming towards its current size and position.
This was also the time when the moon formed. This happened due to a collision with another planetary body, almost the size of Mars.
After a while, the heat cooled slightly and a primitive crust began to form. Also during this time, an early atmosphere developed.
Subdivisions of the Eon
This eon is divided into three periods of time however they haven’t been formally recognised and so are referred to as ‘informal’. They are the early, middle and late Hadean. They are categorised by the following attributes:
Early – Formation of earth and the moon
Middle – Cooling begins and first crust forms
Late – Crust stabilises and possibly oceans form
Archean Eon – 4 to 2.5 Billion Years Ago
The next eon of time is the Archean, named by geologists in the 19th century from the Greek word for ‘beginning’ or ‘origin’. It was named this because this was when the first stable crust formed and early life evolved.
The eon lasted from 4 to 2.5 billion years ago, a solid 1.5 billion years and the second longest eon of time in Earth’s history.
The key features of this eon includes a stable crust and early continents, formation of oceans, frequent asteroid impacts, the emergence of early life forms and ended with a dramatic change in the composition of the atmosphere.
Subdivisions of the Eon
This eon has formalised eras of time based on geological and biological developments. This demonstrates two things. Firstly that events in deep history began to leave their mark, enough for us to dig in and discover what happened billions of years later. Secondly, the formalised eras demonstrates confirmed geological events and changes, things began to happen.
The eras of time are as follows:
Eoarchean Era – 4.0 to 3.6 billion years ago (400 million years long)
Paleoarchean Era – 3.6 to 3.2 billion years ago (400 million years long)
Mesoarchean Era – 3.2 to 2.8 billion years ago (400 million years long)
Neoarchean Era – 2.8 to 2.5 billion years ago (300 million years long)
Proterozoic Eon – 2.5 Billion Years Ago to 538 Million Years Ago
We come to the Proterozoic Eon which is the longest eon of time in Earth’s history. It lasted from 2.5 billion years ago to 538 million years ago, a solid 2 billion years. The name Proterozoic comes from Greek meaning ‘earlier life’.
This is an incredible amount of time for us to comprehend and the amount of things that happened in this eon, were immense. The events of this eon and the previous, Archean, fundamentally laid the groundwork for complex life to evolve and thrive, taking over every corner of the Earth.
The eon begins with the Great Oxidation Event. It includes the emergence of complex cells with nuclei known as eukaryotes, the forerunners of complex life. This led to multi-cellular organisms towards the end of the eon. This eon also saw confirmed supercontinents and major ice age periods leading to Snowball Earth.
Subdivisions of the Eon
We have confirmed subdivisions for this eon in the form of eras and periods. The increase in global activity both geologically and biologically calls for many more divisions of time.
The subdivisions of eras are as follows:
Paleoproterozoic Era – 2.5 to 1.6 billion years ago (900 million years long)
This is era is subdivided further into four periods.
Mesoproterozoic Era – 1.6 to 1 billion years ago (600 million years long)
This era is further subdivided into three periods of time.
Neoproterozoic Era – 1 billion to 538 million years ago (462 million years long)
This era is further divided into three periods.
Phanerozoic Eon - 538 Million Years Ago to Today
The last pink dot on the timeline brings us to the current eon of time, the Phanerozoic. This is named after the Greek for ‘visible life’. The date for the beginning of this eon varies from source to source, but it’s around 541-538 million years ago.
The main distinguishing feature of this eon is that we have complex multi-cellular life, diversifying and inhabiting the waters, land and the skies. There are major tectonic shifts, supercontinents, and mass extinctions. There are ice ages, global atmospheric changes and different species reigning dominance over the land at a given time.
Subdivisions of the Eon
This eon sees many more subdivisions of time which we will explore in future blogs. As a summary, this eon is divided into three eras of time.
Paleozoic Era – 538 – 251 million years ago (287 million years long)
This era is divided into six periods of time.
Mesozoic Era – 251 – 66 million years ago (185 million years long)
The era is divided into three periods of time.
Cenozoic Era – 66 million years ago to today.
This era is subdivided into three periods of time and several more smaller epochs of time.
Future of Earth
While it isn’t really possible to determine what will happen in the near future or even the few thousand years, scientists do have a pretty good idea of Earth’s distant future as it relates to the Sun’ activities.
Consider the following image as the potential timeline of Earth, from its beginnings to its end.
1 billion years from now – the Sun’s luminosity will increase by 10% causing a ‘moist greenhouse effect’, effectively evaporating the oceans and ending the carbon cycle.
2-3 billion years from now – Earth’s magnetic field may collapse, surface temperatures will rise creating Venus like conditions
4 billion years from now – the Sun enters its dying phase
5 billion years from now – the Sun will expand as it goes through it’s the phases of a dying star, making the Earth uninhabitable at the least
7 billion years from now – as the Sun goes deeper into its dying phases, it will expand and absorb the Earth
Further Exploration
In this blog we have explored the Earth’s timeline, from its beginnings to its likely future end, billions of years from now.
We have seen the Earth has been through four geological eons of time, each one shifting and shaping the Earth in different ways. We will go through each eon of time in future blogs, to expand our knowledge on what’s happened in the past and to further appreciate the small amount of time that we take up in its history.
Further topics to explore for your own knowledge outside of the Evolution of Stuff would be to go into each eon in more depth, to look more at ice ages and what events caused them, consider the evolution of life in more detail and consider the causes of mass extinctions and what we can learn from them.
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