Our Earth is about 4.5 billion years old. It is roughly represented by the column of sedimentary rocks now present on the earth. In this record the time elasped during the formation of unconformities is missing. The unconformities are however, important because they subdivide the geological time into smaller units. On this basis a standard geological Time scale has been prepared which is used universally for the correlation of rock formations. Geological time scale is divided into Eon, Era, Period and Epoch.
Image taken from http://www.stratigraphy.org/index.php/ics-chart-timescale
Geological Timescale Free Download ( Special Thanks to International Commission on Stratigraphy)
There are 2 Eons namely
1. Hadean (Age – 4.6 billion years)
- Indirect photosynthesis evidence of precambrian life.
- Oldest known mineral. i.e ZIRCON
- Crust was thin in the 1st stage of Earth’s history.
- Composition of atmosphere was- vapour, methane, Co2, Nitrogen, Hydrogen, Inert gases and acidic fumes.
- Hydrogen and Helium was later dissipated into universe.
- Free Oxygen was 1st formed from the disintegration of H2o and Co2 under the influence of Sun rays in the upper layer of the earth’s atmosphere.
- Proportion of Oxygen in the atmosphere increased to its present leavel.
- Formation of hydrosphere and atmosphere lead to intensive erosion and deposition of sedimentary rocks intercalated with volcanic rocks.
2. Archean (Age – 400 my to 2500 my)
- Stabilization of most modern cratons.
- First stromatolite (probably colonial Cyanobacteria).
- Oldest Microfossil was of this era.
- First known Oxygen producing bacteria.
- Hydrosphere contained dissolved gaseous product of volcanism such as HCL , HF, Basic Acid, H2S, Co2, CH4, Highly acidic water with silica in dissolved state.
- Extensive volcanism and unusual composition of atmosphere are reflected in the chemical composition of rocks making them distinct from younger rock formations.
3. Proterozoic (Age 2500 my to 541 my)
- It begins with tectonic zonation of earth surface into sedimentary basins of platform and geosynclinal types. Such zonation was accomplished at somewhat differing time aver different continents.
- Composition of Hydrosphere and atmosphere under wet very little change during the transition period of Archean and Proterozoic eras.
- Carbon Dioxide content in hydrosphere declined.
- Volcanic products such as Sulphur and H2S of hydrosphere were converted into sulphates in the presence of Oxygen which insure deposited in the sedimentary succession of the time.
- Deposition of carbonate rocks( limestone and Dolomite) 1st rock of evaporite facies was phosphorite deposited in regressing marine basin.
- Early forms were not having hard parts so could not preserved as fossils.
- Imprints of Algal life preserved in the form of Stromatolites commonly observed in proterozoic succession.
Precambrian is divided into 3 Era
Paleozoic ( Age 541 my to 262 my)
Paleozoic is divided into 6 Periods
1. Cambrian ( Age 541 my to 485 my)
- Major diversification of life in cambrian explosion.
- Numerous fossils, most modern phyla apppeared.
- 1st Chordates appeared.
- Trilobites, Sponges, Inarticulate sponges brachiopods and various other animals appeared.
- Gondwana Emerged.
- Climate was mild.
2.Ordovician ( Age 485 my to 443 my)
- Invertebrates diversified into many new types.
- Early Corals, Bivalves, Nautiloids and many types of Echinoids were common.
- Conodonts (early planktonic vertebrate) appeared.
- First green plant and fungi on land appeared.
- Ice age at the end of the period
3.Silurian ( Age 443 my to 419 my)
- First vesicular plant appeared.
- First Inwed Fishes appeared as well as many armoured jawless fishes populated the seas.
- Brachiopods and Crinoids were abundant.
- Trilobites and Mollusks diversified.
4.Devonian ( Age 419 my to 358 my)
- 1st Club mosses and ferns appeared, as do the first seed bearing plant.
- Trilobites declined while jawless fishes and Sharks ruled the sea.
- 1st Amphibian still aquatic.
- Mountains of North America and Appalachian mountains of North America were seen.
5.Carboniferous ( Age 358 my to 298 my)
- First reptiles and Coal Forests.
- Highest ever atmospheric Oxygen levels.
- First land Vertebrate.
- Rhizodonts were dominate big freshwater predators.
- In Ocean early sharks are common and quite diverse.3.
6.Permian ( Age 298 my to 262 my)
- Landmasses units into supercontinent Pangea, creating the Appalachians.
- End of Permo Carboniferous Glaciation.
- Reptiles become plentiful while amphibians remain common.
- In the mid permian, coal age flora are replaced b cone bearing gymnosperms and by 1st true mosses.
- Marine life flourished in warm shallow reefs.
- Triassic extinction event occurred 257 mya: 95% of life on earth become extinct.
MESOZOIC ( Age 262 my to 66 my)
Mesozoic is divided into 3 Periods
1.Triassic ( Age 262 my to 201 my)
- Archosaurs dominated on land as Dinosaurs, and in air as Pterosaurs.
- 1st mammal and Crocodile appeared.
- Modern Corals and fish appeared.
- Andean orogeny in South America.
2.Jurassic ( Age 201 my to 145 my)
- Gymnosperms, Conifers and ferns were common.
- Many types of Dinosaurs evolved.
- Mammals were common but small.
- 1st Bird and lizard appeared.
- Sea Urchins were common.
- Carbon Dioxide levels in Atmosphere were 4-5 times mo re than the present day.
- Breakup of Pangea into Gondwana and Laurasia.
3.Cretaceous ( Age 145 my to 66 my)
- Flowering plant along with new type of insects evolved.
- Ammonoidea, Belemnites, Bivalves, Echinoids were common.
- New type of Dinosaur appeared on land and modern snake in sea,
- Atmospheric Co2 close to present day levels.
CENOZOIC ( Age 66 my to Recent)
Cenozoic is divided into 3 Periods
1.Paleogene ( Age 66 my to 23 my)
- Climate was warm.
- Rapid evolution and diversification of fauna.
- Appearance of several ” Modern” mammal families.
- Primitive Whales appeared and diversified.
- First Grass.
- Modern plant appeared.
- Mammals diversified into numbers of primitive lineages following the extinction of dinosaurs.
2.Neogene ( Age 23 my to 2.5 my)
- Widespread forests slowly draw in massive amounts of Co2, lowering the level of atmospheric Co2.
- First APE appeared.
- Horses and Mastodons diversified.
- Modern mammals and bird families become recognizable.
- Moderate climate, punctuated by Iceage.
- Orogeny in Northern Hemisphere.
3.Quaternary ( Age 2.5 my to Recent)
- Human Stone age culture with increasing technical relative to previous Ice age cukture in Pleistocene.
- Quaternary age continues with glaciation and fluctuation from 100 to 300 ppmv in atmospheric Co2 levels, further intensification of Icehouse earth conditions, roughly 1.6My.
- Last Glaciation period (18000-15000 yrs ago).
- In Holocene, Quaternary Ice Age reduces and the current interglacial bigens, rise of human civilization.
- Agriculture begins in Holocene.
- Flourishing and then extinction of many large mammals.