The geological time scale is divided into Eras which has the longest span which is further divided into periods and the periods are divided into Epochs. Understanding the Pre-Cambrian era and subsequent geological periods is crucial for comprehending Earth's evolutionary history, the formation of landforms, and the development of life on our planet.
PRE-CAMBRIAN ERA
Archeozoic Era
The Archeozoic Era started 3.8K-600 million years ago.
Earth was very hot & had violent volcanic Eruptions.
Proterozoic Era
Started 2K million Years ago (MYA).
It is also known as pre-cambrian era.
The world's oldest fold mountain Aravalli belongs to the pre-cambrian era.
PALEOZOIC ERA
Overview
Started around 600 MYA.
It is divided into 6 periods.
Life started originating in the pre-cambrian era.
Cambrian Period
This was the earliest period in which majority of the oceans were flooded and certain mountain building activities took place.
Ordovician Period
First evidences of vertebrates.
No life on the land (life was in water).
Silurian Period
During this period Caledonian Mountains were formed.
The mountain formation took place in the central part of Asia, North-America (Appalachian mountain), Mountains of Scandinavia.
Devonian Period
Also known as age of fishes.
Carboniferous Period
Carboniferous period corresponded to the period which began around 320 MYA.
This period lasted for 65 MYA.
Carboniferous period further divided into:
- Upper carboniferous period
- Lower carboniferous period
Two major events occur in the carboniferous period:
- The Carboniferous Period saw the formation of Pangaea, and this idea was later explained by Wegener’s Continental Drift Theory (1912)
- During this period Pangea broke into two parts i.e., Laurasia & Gondwana land, resulting into the formation of Tethys sea
This period is also known for very dense luxurious tropical vegetation due to dominance of hot and humid climate.
Reptiles also emerged during this period.
Due to the movement of the landmass there was repeated several emergence and submergence of forested areas. So the submergence of this forest resulted into the formation of coal, and hence the name carboniferous.
According to Wegener the equator during this period was passing through South-America (Brazil, North sea), Central Eurasia and then again to Antarctica which explains huge coal deposits in this region.
Permian Period
Cockroach originated in the Permian Period.
MESOZOIC ERA
Three Periods
It is divided into three periods:
- Triassic
- Jurassic
- Cretaceous
Triassic Period
The Gondwana landmass started further breaking.
Jurassic Period
This period was around 180 MYA.
This was the period of Dinosaurs.
Cretaceous Period
This period is approx of 114 MYA.
During this period the temp. increased & therefore there was rise in the sea level. Hence many parts of land went below the sea.
There was large scale volcanic eruptions due to cooling and degassing of the earth as well as large scale meteor impact. Hence,
(1) This was the period of mass extinction which resulted into the extinction of dinosaurs.
(2) Large scale fissure eruption gave rise to vast volcanic plateaus such as Deccan lava Plateau, Siberian Plateau & Kimberley Plateau (South-Africa).
During this time the Indian plate was in the southern hemisphere passing over Reunion Islands which during that time was an example of Hotspot and the western part of Indian Plate came over the Reunion Hotspot.
This resulted in large scale volcanic eruption covering an area of 10 Lakh sq km. This now has been eroded and reduced to half of its original size and is the region of black cotton soil.
QUATERNARY PERIOD
This is the most recent & the shortest period of the earth's geological history with a duration of only 1 million Years.
It is divided into 3 Epochs:
- Pleistocene
- Holocene
- Anthropocene
Pleistocene Epoch
Started around 1 MYA and ended 10K years ago.
This was the period of great ice age which affected physical features as well as biological life on the earth.
EXTENT OF PLEISTOCENE ICE AGE
The Pleistocene ice age covered major part of North-America, Canada, Central Asia and extended right up to northern part of India in the KAREWA region of Kashmir.
This Pleistocene ice age which is also known as quaternary ice age was formed due to different factors such as variation in the solar radiation, volcanic dust, Hypothesis, carbon dioxide decrease etc.
During this period 1/5 of the earth's surface was covered by ice and sea level was 300 feet lower than today.
This ice age was having 4 phases of advancement of the glacier and 4 phases of glacier retreat.
Penck has identified 4 glacial periods in Europe and they are:
- Gunz
- Mindel
- Riss
- Wurm
separated by interglacial periods.
EFFECT ON TOPOGRAPHY
Glaciers by the process of scratching the earth surface cause immense changes on the topography. During the pleistocene period glaciers modified huge areas of North-America and North-Europe and Siberia by scratching the Earth's surface causing the formation of large number of lakes.
Great lakes of North-America
Similar lakes are formed even in Europe.
As the glaciers advanced huge mass of ice caused subsidence producing the features such as Baltic sea (Europe).
Due to the advancement of the glaciers there was large scale erosion of the Canadian shield, Siberian Plateau and after melting the glacier deposited huge amount of glacier till and moraine.
It also forms Drumlins & Eskers in the major regions of Europe.
Isostatic adjustments were caused due to the weight of ice as well as after the melting of the ice causing huge/large earthquakes, which were not associated with plate tectonics.
CHANGE IN THE SEA LEVEL
Pleistocene ice age caused fluctuation in the sea level. Eustatic changes were caused due to waxing and waning of ice sheets.
This changed the course of major rivers, changed their erosional & depositional capacities giving rise to polycyclic relief due to river rejuvenation or river becoming old.
Evidences of Pleistocene glaciation are seen even in India such as Kargil Basin & Bangar & Khadar region in north.
IMPACT ON THE CLIMATE
Large scale glaciation decreased the Albedo of Earth and hence cooled the climate even beyond the areas of glaciation.
Due to glaciation the pattern of global wind was modified. Cold air originating from the glacier moved with great velocity and deposited glacial sediments outside the region of glaciation giving rise to huge Loess deposits (मुल्तानी मिट्टी).
Yellow River region of China.
Due to changes in the temperature, hydrological cycle and the rainfall patterns of the world were affected.
The pattern of ocean currents changed because global heat balance was changed.
CONCLUSION
Thus the Pleistocene ice age not only resulted into extinction of the animals, migration of the humans but also large scale topography and climatic changes increasing the complexity of the earth.
Key Points to Remember
- Archeozoic Era: 3.8K-600 million years ago
- Proterozoic Era: 2K million years ago
- Paleozoic Era: 600 MYA, divided into 6 periods
- Carboniferous Period: 320 MYA, lasted 65 MYA
- Jurassic Period: 180 MYA, period of Dinosaurs
- Cretaceous Period: 114 MYA, mass extinction event
- Pleistocene: 1 MYA - 10K years ago
- Ice covered 1/5 of Earth's surface
- Sea level was 300 feet lower than today
- Four glacial periods: Gunz, Mindel, Riss, Wurm
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