Top 10 Defining Moments in Earth Formation

12:00 pm 16 May, 2013

It is believed that the Earth was formed around 4.6 billion years ago through the process of planetary accretion. From initially being in a molten state due to extreme volcanic activity to the point where it is today, a lot of events have occurred in Earth’s geological time scale. By collisions within a giant disc-shaped cloud of matter aided by gravity, asteroids and planetesimals were formed. These, in turn, collided to form the solar system including our Earth. There have been many defining moments in the making of the planet we call home. For the sake of brevity, we pick out the top 10 such moments in Earth formation.

10. Cambrian explosion:

It was the period in Earth’s history, about 542 million years ago, when most major animal phyla appeared on Earth. It was rapid and the evolution of life accelerated in this period as evidenced by fossils. The development of hard body parts such as shells, skeletons or exoskeletons happened in this period. It was during this period that the first vertebrates appeared on Earth, among them the first fishes.

Cambrian explosion

9. Formation of the Moon:

It is the only natural satellite of the Earth which is responsible for quite a few things like the tides generated by the moon’s tidal forces and the Earth’s rotation due to the gravitational coupling between the two. According to a theory, the moon was formed around 4.5 billion years ago as a result of a giant impact between a Mars-sized body called Theia and the newly formed proto-Earth. The collision resulted in material being blasted into orbit which is now called the Moon.

Formation of the Moon

8. Big Bang:

The Big Bang theory explains the universe’s early development and since our planet is an integral part of the universe this surely fits the bill to be included in this list. Big Bang is a cosmological theory for the event that led to the formation of the universe. According to the theory, the Big Bang occurred approximately 13.798 billion years ago. The universe was extremely hot at the time and kept expanding till it cooled down sufficiently to allow the formation of stars and galaxies due to conversion of energies and gravity. The Earth formation story begins here.

Big Bang

7. Formation of atmosphere:

Can you imagine our planet without its atmosphere? During the time called the Hadean Eon – the first geologic eon of Earth – the planet was in a molten state with extremely hellish conditions. The outer layer eventually cooled down to form a solid crust. It was the outgassing and the volcanic activity in the process that formed the atmosphere in which we breathe.

Formation of atmosphere

6. Formation of oceans:

One of the most defining moments in Earth formation is linked to the formation of oceans. Surely, the importance of water in making the planet habitable is something that has been acknowledged by even the most ancient of texts. It is therefore natural that ocean formation is an important event in Earth’s history. It was during the very first geological eon of the Earth when the planet’s outer crust cooled that condensing water vapor, augmented by ice delivered from comets, produced the oceans.

Formation of oceans

5. Formation of rocks and continental plates:

If it weren’t for the continents, where would you be living? It was in the Archean Eon that the Earth cooled down substantially to aid in the formation of rocks and continental plates. Of course, the Earth was exceptionally hotter even at that time due to which the plate tectonic activity was more vigorous. Nevertheless, the formation of continental plates had begun in this period.

Formation of rocks and continental plates

4. Magnetic field:

About 3.5 billion years ago during the Archean Eon, a significant development took place – the Earth’s magnetic field was formed. It was important because the magnetic field prevented the Earth’s atmosphere from being stripped away by the solar wind flux. The field then formed was much weaker than it is today but it saved the planet from what likely happened to Mars.

Magnetic field

3. Formation of Oxygen:

In the Archean Eon again, another significant event in Earth formation took place which led to an oxygen rich atmosphere that is so precious to every living being on the planet. Photosynthesizing cyanobacteria evolved to produce oxygen by using water as a reducing agent. These cyanobacteria were capable of photosynthesis which resulted in the conversion of carbon dioxide into organic compounds. Oxygen was a byproduct and thus began its accumulation in the atmosphere.

Formation of Oxygen

2. Life:

Life on Earth began with the common ancestor which first appeared more than 3.5 billion years ago. The first living organism was formed from simple molecules present in the Earth’s vast oceans between 3.8 and 3.5 billion years ago. It was a primitive life form; the starting point of life. That common ancestor is from which life on Earth evolved, including you!


1. Appearance of Genus Homo:

What would the Earth be if it weren’t for Homo sapiens? Yes, the genus homo is an integral part of human evolution on Earth. Hence, the appearance of this genus is certainly an important event in Earth’s chronological calendar. It was around 2.5 million years ago that the first member of the genus Homo, Homo habilis, evolved from whom the Homo sapiens came into being around 500,000 years ago.

Appearance of Genus Homo

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