Cooling History or Geological time scale of earth is divided and subdivided in different means like, Eons, Eras, Periods, Epochs, Ages, etc. Eons are being
divided into Eras, Eras are being subdivided into Periods, Periods into Epochs and so on. The complete geologic time scale can be divided into sequence of
four Eons, namely Hadean, Archean, Proterozoic and Phanerozoic. The first three Eons can be collectively known as the Precambrian Supereon. The Phanerozoic
Eon is divided into three Eras namely, Paleozoic, Mesozoic and Cenozoic Eras.
The Precambrian supereon duration accounts for 85 to 89% of earth's geological timescale dating back from 5 billion years to 600 million years. During this
length of time, the Earth's solid crust was broken and Tectonic plates were formed. No fossils are available of this time scale and no idea of flora and
fauna. Around 700 million years back life began on earth when Precambrian supereon was about to complete. Life began in oceans and the first life that was
formed, was unicellular organism. First rain occurred around 800 million years back and Atmosphere was formed at the same time.
The Phanerozoic Eon is being divided into three Eras namely, Paleozoic, Mesozoic and Cenozoic Eras.
- Paleozoic Era dates from 600 million years back to 225 million years back comprising around 375 million years. It is divided into 6 Periods namely,
Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian Periods.
- In Cambrian Period, first sedimentary rocks were formed and Metamorphic rocks also began to form. Fossils are available of this Period which
are the oldest fossils that are found on the earth. In Ordovician period, oceans got expanded and it was the period of volcanism. Devonian is
dubbed as the "Age of Fish".
- Carboniferous Period is known for Carbonization and formation of Coal seams. Continents were united up to this period and united continents were called
Pangaea. Formation of coal fields took place in Gondwanaland, a part of Pangaea. That is why these coal fields are known as Gondwana coal fields. In
Permian Period, reptiles started spreading.
- Mesozoic Era dates from 225 million years back to 65 million years back which is equivalent to 160 million years. It is divided into 3 Periods namely,
Triassic, Jurassic and Cretaceous Periods. It is also called the "Age of Dinosaurs".
- The first dinosaurs appeared in the Triassic Period. By the time of Jurassic Period, they were at peak. During this Period, different types of
dinosaurs were present including flying dinosaurs. During Cretaceous Period, dinosaurs got ended.
- The main reasons for their extinction are sudden climate changes, cosmic rays, overpopulation, poaching of their eggs by mammals, etc.
- Cenozoic Era dates from 65 million years back to till date. It is the "Age of Mammals". It is divided into 3 Periods namely, Paleogene, Neogene and
- Paleogene Period is further divided into 3 Epochs namely, Paleocene, Eocene and Oligocene. During Paleocene, large fruit bearing trees appeared.
During Eocene, Carnivorous mammals appeared and the first one to appear is a primitive dog. During Oligocene, Himalayan elevation took
- Neogene Period is further divided into 2 Epochs namely, Miocene and Pliocene. During Miocene, Apes appeared and got diversified. During Pliocene,
- Quaternary Period -
- Quaternary Period comprises of 2 Epochs namely, Pleistocene ( 3 million years to 10000 years back ) and Holocene ( 10000 years to till now ).
- Pleistocene is also called "Ice Age" as large ice sheets covered the earth surface. Homo sapiens got evolved during this period.
- Holocene Epoch -
- During Holocene, earth's surface temperature started rising. This is a warm period. Continents got sinked and large drainage system ( river systems )
started forming. Glaciers started melting as a result of which there was rise in sea levels and numerous lakes were formed, e.g Great lakes of US.
- Biodiversity has expanded because of warm climate. Human civilization got initiated.
- The Pleistocene ice sheets are found now in Canada, Antarctica, Greenland and Scandinavian countries. Some geographers believe that Pleistocene is still
not ended and Holocene is an intermission only. From 1400 – 1700 AD, there happened sudden cooling of the earth known as "Little Ice Age" and expansion
of ice shields took place.