Geography for Competitive Exams

ORIGIN OF THE EARTH

Earth formed from a small part of the sun, it originated as a hot gaseous mass, which on cooling, turned first into a liquid and then into solid mass.

Gaseous Hypothesis Theory – Kant

Nebural Hypothesis Theory – Laplace

Tidal Hypothesis Theory – Jeans and Jeffreys

•      The denser matter forms the central part of the earth and the surface rocks are made up of lighter materials.


•      The rocks forming the surface layer of the earth which are richer in lighter minerals are called SIAL (Silicon and Aluminium).


•      The denser rocks forming the inner layers of the earth are called SIMA (Silicon and Magnesium) and NIFE (Nickel and Iron).


•      The solid crust of the earth came into existence about 4.6 billion years ago and is considered the beginning of the history of the earth.


•      Most popular method of estimating the age of the rocks forming the crust is carbon dating.

The History of Earth is divided into 3 Eras,

1. Palaeozoic Era


2. Mesozoic Era


3. Cenozoic Era

The period before the Palaeozoic Era is called PRE-CAMBRIAN TIME.

Various Eras and Periods

1)      Pre-Cambrian Time

2)     Palaeozoic Era

a)      Cambrian Period – 4.5 billion years ago – Bacteria appeared

b)     Ordovician Period – 570 million years ago

c)      Silurian Period – 500 million years ago

d)     Devonian Period – 435 million years ago

e)      Carboniferous Period –

i)        Lower Carboniferous Period – 410 million years ago – Fish appeared

ii)      Upper Carboniferous Period – 360 million years ago – Frogs appeared

f)       Permian Period – 290 million years ago – Seed Plants appeared

3)      Mesozoic Era

a)      Triassic Period – 240 million years ago – Dinosaurs appeared

b)     Jurassic Period – 205 million years ago – Birds appeared

c)      Cretaceous Period – 138 million years ago – Flowering plants appeared

4)     Cenozoic Era

a)      Tertiary Period

i)        Palaeocene Epoch – 63 million years ago – Small Mammals appeared

ii)      Eocene Epoch – 55 million years ago – Grasses appeared

iii)    Oligocene Epoch – 38 million years ago – Primitive Apes appeared

iv)    Miocene Epoch – 24 million years ago – Apes

v)      Pliocene Epoch – 5 million years ago – Human like creatures appeared

b)     Quaternary Period

i)       Pleistocene Epoch – 2million years ago – Human beings developed

ii)    Holocene Epoch – began 10,000 years ago and is still continuing.