History of the earth and geological time scale

July 7, 2018 Sushil Humagain Geology and Astronomy 0

  • Earth is a member of the solar system and it revolves round the sun.
  • It is very difficult to give the exact date of the origin of the earth but it is probably at least 4.5 billion years old.
  • Although no universal theory of the evolution or origin of earth and the solar system has as yet been propounded, few opinions and hypotheses are put forward by different people.
  1. George Wofan’s hypothesis (old planetisimal hypothesis):
  • It was proposed by a French scientist George Wofan in 1749 AD.
  • According to this hypothesis, the earth, along with other planets and satellites was formed when a comet moving around the universe stroke the sun millions of years ago.
  • A large amount of matter was thrown out of the sun during the collision, which further cooled to form earth and other planets.
  1. Kant and Laplace’s Nebular hypothesis:
  • A German philosopher, Immanuel Kant in 1755 AD put forward this hypothesis which was later improved by a French astronomer Pierre Simon Laplace in 1796 AD.
  • According to this hypothesis, the solar system developed from a huge revolving nebula (cloud of gas and interstellar dust).
  • When the cloud cooled and contracted, it condensed to form rings of matter with the central huge mass.
  • The central mass became the sun and the outer collection of gas and dust ultimately gave rise to planets and the satellites revolving round the sun.
  1. Jean’s and Jeffrey’s hypothesis (Tidal hypothesis):
  • It was put forward by James Jean and Harold Jeffrey in 1917 AD.
  • According to this hypothesis, a bigger star came closer to the sun causing a tide because of its gravitational force.
  • The molten mass of sun elongated in the direction of the star as it moved far away from the sun separating some of the materials (in pieces) from the sun.
  • These pieces started revolving round the sun and became planets, satellites and other members of the solar system.

Some important data related to earth

Estimated age 4.5 billion years
Mass 6×1021 metric tons
Total surface area 509,700,000 square km
Land area (lithosphere) 148,400,000 square km
Water area (hydrosphere) 361,300,000 square km
Highest land Mount Everest (8848m above sea level in Nepal)
Lowest land Dead sea (about 399 below the sea level)
Diameter 12,756.3 km
Important gases Nitrogen, Oxygen, Carbon dioxide
Average density 5.7
Number of satellite 1 (Moon)

Source: The world book of encyclopedia

Geological time scale:

  • The history of the earth is recorded in the rocks of the earth’s crust.
  • Geologists have been able to estimate the age of sedimentary rocks and fossils in them with the use of sophisticated technology like carbon dating.
  • A geological time table of the earth has been prepared and this time scale which covers the earth’s history from its origin to the present day is called geological time scale.
  • The geological time scale is divided into four different lengths of time according to the origin of the living organisms; eon, era, period and epoch.

Eon:

  • It is the longest duration of time with billions of years in the geological history which is divided into eras.
  • The entire geological history is divided into four eons namely;
    • Hadean eon (4.5 to 4 billion years ago)
    • Archean eon (4 to 2.5 billion years ago)
    • Proterozoic eon (2500 to 541 million years ago)
    • Phanerozoic eon (541 years ago to present)

Era:

  • It is a division of an eon in which important changes took place.
  • The entire geological history is divided into four geological eras;
    • Pre-cambrian era
    • Palaezoic era
    • Mesozoic era
    • Cenozoic era

Pre-cambrian era:

  • Duration: 4.5 billion years to 570 million years ago
  • The origin of earth took place in which the crust repeatedly melted and hardened.
  • No existence of living organisms before 3.8 billion years ago.
  • Rocks like gneiss and granite were formed in this era.
  • Bacteria (3.8 billion years ago) and algae (3.2 billion years ago) came into existence.

Palaezoic era:

  • Duration: 570 to 250 million years ago
  • This era is divided into six periods; Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
  • Sedimentary rocks were formed in this era.
  • Change in atmosphere and weather took place.
  • Biological characteristics in this era are as follows:

 

S.N. Periods Time (milli0n years ago) Biological characteristics
1. Cambrian 540 to 490 Development of some marine organisms
2. Ordovician 490 to 440 Evolution of marine invertebrates
3. Silurian 440 to 420 Evolution of terrestrial  animals, fish and green plants
4. Devonian 420 to 360 Evolution of winged insects, plants and fish
5. Carboniferous 360 to 300 Evolution of reptiles and amphibian

Development of fern plants (pteridophytes)
6. Permian 300 to 250 Extinction of many vertebrates and invertebrates.

 

Mesozoic era:

  • Duration: 250 to 65 million years ago
  • A large number of reptiles evolved on the land and sea and is therefore Mesozoic era is also called the golden era of the reptiles.
  • This era is divided into three periods; Triassic period, Jurassic period and Cretaceous period
  • Different types of hills and mountains were formed in this era.
  • Climatic condition was suitable to sustain aquatic, terrestrial and aerial animals like dinosaurs, tortoises and toothed birds etc.
  • Biological characteristics in this era are as follows:
S.N. Period Time (million years ago) Biological characteristics
1. Triassic 250 to 200 First dinosaurs,  marine reptiles (turtles),  mammal like reptiles and dominant conifers
2. Jurassic 200 to 150 Giant dinosaurs, reptiles dominant, first mammals, first toothed birds
3. Cretaceous 150 to 65.5 Extinction of dinosaurs and giant land and marine reptiles, different insects developed, angiosperms developed, gymnosperms declined

 

Cenozoic era:

  • Duration: 65.5 million years ago to the present
  • It is also known as the recent era or era of the human beings.
  • This era has been divided into two periods (tertiary and quaternary).
  • Epoch is the shortest duration in the geological  time scale and this era is divided into seven epochs; Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene.
  • Evolution of human beings took place in this era.
  • Breaking and expansion of rocks, volcanic eruptions took place.
  • The temperature was decreasing and mountain peaks were covered with ice.
  • Neanderthal man, toothed animals, gymnosperms and angiosperms evolved.
  • Biological characteristics in this era are as follows:
S.N. Period Epoch Time Biological characteristics
1.  

 

 

 

Quaternary

 Holocene 12 thousand years ago to present day Human beings hunted and tamed animals, age of humans
2. Pleistocene 1.8 million years to 12 thousand years ago Primitive human, first human social life.

Extinction of great mammals such as sabre toothed tiger and the giant mammals
3.  

 

 

 

 

Tertiary

 Pliocene 5.33 to 1.8 million years ago Man evolved from man like apes
4. Miocene 23 to 5.33 million years ago Apes appeared and grasses developed
5. Oligocene 33.9 to 23 million years ago Rise of monocotyledons and flowering plants.

Sabre toothed cats and monkeys appeared
6. Eocene 55.8 to 33.9 million years ago Whales, bats, rhinoceros, camels etc. appeared
7. Paleocene 65.5 to 55.8 million years ago Flowering plants became plentiful.

First primates and horses

History of the earth and geological time scale