Geologists subdivide timescale by events that happened in geological history . These time intervals are unequal in length, geological timescale describes the timings and relationship of those events that changed Earth’s geology. This time scale is developed by carefully studying and analysing the rock layers ,evolution of species, rock dating, fossil samples and atmosphere.

Eons

Eons are the largest divisions in geological Time and span to millions of years . There are four eons catalogued till present day, these are — Hadean, Archean, Protozoic and Phenerozoic eon . The first three eons are collectively referred to as the Precambrian super eon.

Hadean Eon

The Hadean eon began when the earth formed about 4.6 billion years ago and ended 4 billion years ago . The planet’s condition was very dynamic , earth was still very hot and frequently collided with other planetary bodies.

Archean Eon

It began about 4 billion years ago till 2.5 billion years . The earth’s crust slowly started cooling and stabilising to form landmass .The earliest lifeform probably anaerobic microbes called stromatolites survived. example – Cyanobacteria, Proteo bacteria , microbial mats. Volcanic activity was very high and Archean rocks were often metamorphosised water segments like volcanic segments and mud rocks.

It is further divided into eras:

(mya- million years ago)

• The first Supercontinent Vaalbara appeared.
• Earth possessed a firm crust for the first time.
• Prokaryotic microbes called Archaea appeared.

• Earliest confirmed evidence of life.
• Stromatolites may have been created.
• Microbial mats work present.

• Tectonic plates subduction.
• Expansion of microbial life.
• Ocean temperatures around 55-85°C
• High levels of methane and carbon dioxide in the atmosphere.

• Major development in complex life and continued formation.
• Rise of oxygen in the atmosphere.

Proterozoic Eon

It spanned the interval from 2500-541 mya . For the first time oxygen appeared in the earth’s atmosphere . Mountain formation activity began because of the continuation of continental accretion .Glaciers began to form . Processes like subduction were active due to high tectonic plate movement. It is further divided into 3 eras-

• Longest era
• Continents stabilised
• One day duration was 20 hours.
• Enormous increase in oxygen.
• Anaerobic life form largely died because oxygen was highly toxic for them.
• First mass extinction happened.
• Eukaryotes emerged.
• Periods in which this era is divided- Siderian, Phyacean and Orosirian.

• Evolution of sexual reproduction.
• Development of multi cellular organisms.
• Break up of Columbia supercontinent.
• Periods – Ectasian and Stenian .

• Most glaciation.
• Ice sheets reached equator.
• Snowball Earth formed.
• First appearance of Trilobites ans Sponges.
• Periods- Tonian, Cryogenian and Ediacaran.

Phanerozoic Eon

It covers timeline from 541 million years ago to the present. At the start of this eon , complex plants and multi cellular organisms with hardshells appeared . There was a single landmass known as Pangaea. It is divided into 4 eras:

• Most rapid diversification of life called Cambrian explosion.
• Evolution of arthropods ,fish and amphibians.
• Life transition to the land.
• Forests of primitive plants appeared.
• Largest extinction event called Permian- Triassic extinction event happened.
• Appearance of invertebrate animals phyla.
• Reptiles prospered.
• Periods- Cambrian, Ordovian, Silurian, Devonian, Carboniferrous and Permian.

• Dominance of reptiles like dinosaurs.
• Abundance of Conifers and Ferns.
• Breakup of Pangaea.
• Dinosaurs appeared in mid Triassic period and remained till early Jurassic period (about 150-135 mya).
• Archaic birds appeared in Jurassic period.
• Flowering plants appeared in early Cretaceous period.
• Periods- Triassic, Jurassic and Cretaceous.

• Dominance of mammals , birds and flowering plants.
• Cool and dry climate.
• Complex life evolved.
• This error started with mass extinction event called Cretaceous spell Eugene extension event in which many species including non-even dinosaurs became extinct possibly due to a large asteroid impact
• The Quarternary period saw Mammoths and wooly animals. It was marked by ice ages due to the cooling trend of earth.
• Around 50 million years ago , Earth entered a cooling period mainly due to the collision of Indian tectonic plate with Eurasia. This caused the rise in the Himalayas.
• Periods- Paleogene, Neogene and Quaternary.

The  Earth went through many violent and beautiful events before Homosapians arrived . We must make sure to protect and cherish our dear planet .