Wednesday, 31 August 2017

Wednesday, 7 March 2018

The Geological Time Scale (background of geography)

The vast expanse of geological time has been separated into eras, periods, and epochs. The Geological time scale is an essential tool for understanding the history of Earth and the evolution of life. The geology or deep time of Earth’s past has been organized into various units according to events which took place in each period.

The Geological Time Scale


1. Precambrian era -This is the oldest era of geological history. The duration of this era is 4600 million years. The rocks are igneous and metamorphic in nature, which are complex. The climatic conditions is hot and broken down by series of ice ages. Sea weeds are the form of vegetation. Life originated in the warm seas and not on land.

2. Paleozoic era - The duration of this era is 370 million years i.e, it ranged from 600 to 230 million years. The major orogeniesare the Calcdonian in the lower Paleozoic and Variscan in the upper Paleozoic. The lower Paleozoic era is comprised of the Cambrian, Ordovician and Silurian systems, and the upper Paleozoic era comprise of Davonian, Carboniferous and Permian systems.

(a) Cambrian period - The duration of this period is 70 million years i.e, it started 570 million years ago. The word Cambrian came from Cambria which is the Latin name of Wales. This period was characterized by shallow seas which covered much of the earth. Cambrian rocks are found in Wales, north-west Scotland and western England. There was volcanic activity in Europe but no evidence of mountain building. The climatic conditions were moderately warm and equable, plant life was confined to sea, major invertebrates evolved and there was no life on land.

The-Geological-Time-Scale


(b) Ordovician period - The duration of this period is 65 million years i.e, it ranged from 500 to 435 million years. The word Ordovician had been named after the Ordovices which is an ancient Celtic tribe of central Wales. There was widespread vulcanicity and the onset of Caledonian orogeny. The rocks have been found not only in Wales but also in parts of North America and north-west Europe. The climatic conditions were warm, plant life was confined to sea, vertebrates appeared and there was no life on land.

(c) Silurian period - The duration of this period is 40 million years i.e, it ranged from 435 to 395 million years. The word Silurian had been named after Silures, which is an ancient Celtic tribe of the Wales. The level of seas tended to rise and fall, causing regular changes in the land areas. Silurian rocks are found in Shropshire, Baltic region and Niagara Falls. There is less volcanic activity in this period. The climatic conditions were warm and equable. Plants first appeared on land but were leafless. New species of vertebrates were developed in the sea.

(d) Devonian period - The duration of this period is 50 million years i.e, it ranged from 395 to 345 million years. The word Devonian had been named after the country of Devon in south- west England. The land area increased at the expense of sea. Mountain building and volcanic activity was extensive. Old red sandstone of Europe developed during this period. The climatic conditions was warm and semi-arid. Plants with roots, stems and leaves evolved. There was rapid evolution of vertebrate animals and sharks. This period was known as the 'Age of Fish.'

(e) Carboniferous period - The duration of this period is 65 million years i.e, it ranged from 345 to 280 million years. The word Carboniferous was named from the widespread occurrence of carbon. This period was known as the 'Coal Age.' Carboniferous rocks are divided into Pennsylvanian and Mississippian. Clear, shallow seas were widespread and parts of Europe and Russia were under water. Sea beds began to rise. The climatic conditions were dry but in some areas it was moist. Evergreen trees flourished that measured up to 33 meters. Both plants and animals developed in varieties. The reptiles were first to be developed on land.

(f) Permian Period - The duration of this period is 55 million years i.e, it ranged from 280 to 225 million years. This period was named after the province of Perm in Russia. Lofty mountains formed in Europe, Asia and eastern USA. The climatic conditions was arid with warm zones in Northern hemisphere and ice ages in Southern hemisphere. Evergreen plants reduced due to climatic variations. Marine creatures ended as animal and plant life on land increased. This period ended the Paleozoic era.

3. Mesozoic era - The duration of this era ranged from 250 to 65 million years. Giant reptiles and mammals appeared for the first time. This era can be classified into Triassic, Jurassic and Cretaceous.

(a) Triassic period - The duration of this period is 32 million years i.e, it ranged from 225 to 195 million years. This period has been named after the three fold mountain in Germany. There was formation of marl and sandstone deposits in the warm seas. The climatic conditions were hot, dry wet in some areas. The first carnivores, fish shaped reptiles and flying fish evolved.

(b) Jurassic period - The duration of this period is 60 million years i.e, it ranged from 195 to 135 million years. This period was named after the Jura mountains of France by Humboldt in 1795. The seas advanced and most land areas consisted of forests or swampy plains with lakes and meandering rivers. The climatic conditions was mild and also sufficient rainfall to support vegetation. Flowers evolved for the first time. Reptiles got increased in size and birds evolved feathers.

(c) Cretaceous period - The duration of this period is 72 million years i.e, it ranged from 136 to 64 million years. This period was named after the Greek word Creta which means chalk. The rivers started flowing slowly and formed deltas. The climatic conditions was mild due to which vegetation grew abundantly as Greenland. Australia was covered with glaciers. Deciduous trees and turtles developed. Dinosaurs came to an extinct.

4. Cainozoic era - The duration of this era is 63 million years i.e it ranged from 65 to 2 million years. It is divided into Palaeocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene epochs.
(a) Paleocene epoch - It started 60 million years ago, which lasted for 27 million years. Mountain ranges in Cretaceous period continued to grow. Volcanic activity led to the formation of the Atlantic and Indian oceans. Deciduous trees were dominant. The climatic conditions was warm. Modern mammals like ancestors of elephants, rhinoceros, pig, cattle etc evolved.

(b) Oligocene epoch - It started 38 million years ago, which lasted for 12 million years. The Alps began to form. Grass eating animals increased due to the spread of grasslands. The ancestors of cats, dogs and also ancestors of man evolved.

(c) Miocene epoch - It started 26 million years ago, which lasted for 19 million years. The Mediterranean sea became landlocked ocean. Increased rainfall led to erosion in some parts of European and Asian landmasses that were joined together. Powerful earth movements led to the complete formation of Alps and Himalayas. Sharks and elephants grew in size.

(d) Pliocene epoch - It started 7 million years ago, which lasted for 5 million years. Continents and oceans took their present form. The North sea, Black sea, Caspian sea and the sea of Aral was formed. Giant sharks became extinct.

(e) Pleistocene epoch - It started 2 million years ago. Ice sheets and glaciers covered most of Europe, America, Antarctica and the Himalayas. There was abnormal climatic changes. Ape like creatures developed enough intelligence to make stone implements. Primitive man spread to Asia and Europe. True elephants, horses and oxen appeared for the first time.

(f) Holocene epoch - It started 10 thousands years ago. The ice continued to spread, causing the sea level to rise further. The climatic conditions became more equable. Deserts in North Africa and Middle East came into existence due to dryness. Man learned to domesticate animals and cultivate plants.

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