Geologic History of the Earth


Geologists are scientists who study the structure of rocks and the history of the earth. By looking at and examining layers of rocks and the fossils they contain they are able to tell us what the earth looked like at a certain time in history and what kind of plants and animals lived at that time.

Scientists think that the earth probably was formed at the same time as the rest of our solar system, about 4.6 billion years ago. The solar system may have begun as a cloud of dust, from which the sun and the planets evolved. Small particles crashed into each other to create bigger objects, which then turned into smaller or larger planets. Our earth is made up of three basic layers. The centre has a core made of iron and nickel. Around it is a thick layer of rock called the mantle and around that is a thin layer of rock called the crust.


Layers of the earth


When the earth formed over 4 billion years ago it was totally different from the planet we live on today. There were no plants or animals, only rock, desert, water and ice. The atmosphere probably consisted of carbon dioxide and steam with almost no oxygen to breathe.


The Precambrian Time

The oldest period of the earth’s history lasted from the beginnings four and a half billion years ago to about 600 million years ago. At first simple forms of one-celled life developed in the oceans. Later on bacteria and algae evolved. Towards the middle of the Precambrian, about 2 billion years ago, more complex organisms, sponge-like creatures and soft-bodied animals lived in the seas. During this time there was no life on land because there was not enough oxygen to breathe.

As the Precambrian came to an end the oceans were full of life. Plants started absorbing the carbon dioxide from the atmosphere and turned it into oxygen. Early continents formed, but they looked quite different than they do today.


The Paleozoic Era

The Paleozoic Era lasted from about 600 million to about 240 million years ago. Geologists divide this era into six periods. From the earliest to the latest these are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and the Permian.

Although most animals and plants still lived in the oceans, life started to develop on land and by the end of this era there was life in both the sea and on land. The earliest living things on land were simple plants and mosses, the first creatures to appear on land were animals that looked like spiders, scorpions and insects.

The middle of the era was dominated by all sorts of fish and invertebrates. Early amphibians, animals that could live on land and in the water, appeared. During the Carboniferous period the first reptiles evolved and insects grew to an enormous size.

The end of the era was the time of big forests and swamps. The earth got hot and wet. Plants and big trees died and were buried in sediments.

Over millions of years they turned into gigantic coal deposits which we find in the eastern United States, Europe, Russia and China.

During the Paleozoic era the land masses were constantly moving and by the end of the era they joined together to become a single continent called Pangaea. As these land masses collided several mountain chains, like the Appalachian and Ural Mountains emerged.


Pangaea-continents start drifting apart


The Mesozoic Era


The Mesozoic era lasted from about 240 million to about 65 million years ago. At the end of the Paleozoic about 90 % of all living creatures on earth died out. We don’t really know what caused this to happen but many scientists think that our climate started to change dramatically. Maybe a big volcanic eruption thrust gas into the earth’s atmosphere or maybe a large asteroid hit the earth and dust blocked out sunlight for many years.

The Mesozoic era is often called the age of dinosaurs because they dominated the earth’s landmasses. Reptiles were the most powerful and fearsome creatures of that time. The archaeopteryx was a flying reptile, probably the first bird on earth. Some dinosaurs like the Tyrannosaurus Rex were meat eating predators; others only ate plants and leaves. The 30 meter long brontosaurus was the largest land animal that ever lived. During the Mesozoic era the first mammals also appeared on earth but they were very small and could not match the size and greatness of dinosaurs.


Stegosaurus - Nobu Tamura (


In this era Pangaea started breaking up and land masses formed the continents we know today. They started moving in all directions. By the end of the Mesozoic era South America had separated from Africa; Australia and Antarctica was one continent and North America had started to move away from Eurasia. Just like the Paleozoic era before it, the Mesozoic also ended abruptly. About 65 million years ago 75 per cent of all animals on earth, including the dinosaurs died out. Geologists are pretty sure that a large asteroid hit Mexico and sent dust into the atmosphere that blocked out sunlight for years. It killed off many plants and animals could not survive without food.


The Cenozoic era

The Cenozoic era started about 65 million years ago and continues on into the present. It is divided into the Tertiary period which ended about 1.8 million years ago and Quaternary period.

After the death of the dinosaurs and other reptiles mammals started to dominate life on earth. In the early Cenozoic era horses, rhinoceroses, pigs, camels, deer and cattle started to evolve. As time went on mammals got bigger and bigger. Elephants and mammoths roamed the plains and forests.

wooly mammoth

The wooly mammoth
Mauricio Antón -


About 2 to 4 million years ago apelike creatures lived in Africa. Apes that looked like humans appeared 2 million years ago, but the first real humans came to earth much later, maybe even less than 200 000 years ago.

During the Cenozoic era continents continued to move and crash into each other. Layers of rock folded and moved upward. During this era the biggest mountains of the world, the Alps, Himalayas, Rocky Mountains and Andes have taken shape. In the last 2 million years large parts of the earth have been covered by huge ice sheets. In four Ice Ages, which were separated by warmer periods, glaciers moved across the northern hemisphere. The surface of the seas sank by about 100 meters and turned many shallow parts of the oceans, like the North Sea, into land. Great Britain, for example, was a part of the European mainland and became an island when the ice melted about 20,000 years ago. The glaciers built up huge deposits of rock and reshaped mountains and valleys into today’s form. On the southern continents it rained a lot and turned these areas, like the Sahara desert, into green forests and grasslands.


Related Topics



  • abruptly = very quickly
  • absorb = to take something in
  • algae = very simple plants that grow in or near water
  • although =while
  • amphibian = animal that lives on land and in the sea
  • apelike = like an ape or monkey
  • appear = to be seen for the first time
  • asteroid = a small planet like object that moves around the sun
  • at first = in the beginning
  • basic = main, important
  • beginning = start
  • billion = one thousand million
  • block out = keep out, not let in
  • breathe = to take air into your lungs and send it out again
  • bury = if something is hidden under the ground
  • carbon dioxide = gas that is produced when you breathe out
  • Carboniferous = later part of the Paleozoic era
  • cattle = cows
  • cause = lead to
  • certain = special
  • collide = to crash against
  • complex = something that has many different parts
  • consist of =to be made up of
  • constantly = always
  • contain = have
  • continue = to go on
  • core = the hard centre of an object
  • cover =if something is over something else
  • create = make
  • creature = a living thing
  • crust = the hard outer layer of the earth
  • deer = a large wild animal that can run very fast, has thin legs , eats grass and has horns
  • deposit =layer of rock that has metal or minerals in it
  • desert = a large area of hot, dry land
  • develop = grow
  • direction =route, way
  • divide =break up, separate
  • dominate = control, to be the most important
  • drift = to move slowly away from each other
  • dust = dry powder made up of dirt
  • emerge = to come into being
  • enormous = very very big
  • era = time in history
  • eruption = if a volcano explodes and sends out smoke, fire and rock into the sky
  • Eurasia = the land mass of Europe and Asia
  • evolve = grow, develop
  • examine = to look at something very carefully because you want to find out more about it
  • fearsome = very frightening
  • fold = to bend something
  • fossil = an animal or plant that lived many thousands of years ago and that has been preserved in rock
  • gigantic = very big
  • glacier = a large mass of ice which moves slowly down a mountain valley
  • grassland = a large area of grass covered with wild grass
  • greatness = here: power
  • hemisphere = half of the earth north or south of the equator
  • huge = very big
  • human = a person
  • ice sheet = layer of ice
  • including =together with
  • invertebrate = a living thing that does not have a backbone
  • join = to become one
  • layer =material that lies between two other substances
  • location = place
  • mainland = the area of land that forms a country , not the islands
  • mammal = an animal that drinks milk from its mother’s body when it is young
  • mammoth = an animal like a large hairy elephant that lived on earth thousands of years ago
  • match = to be the same as
  • melt = to become water
  • moss = a very small green plant that that grows on the ground or wet rocks
  • mountain chain = group of mountains , usually in a line
  • nickel = a hard silver-white metal
  • oxygen = a gas that is in the air and which we need to breathe
  • particle = a very small piece of something
  • period = a part of an era
  • plains = a large area of flat land
  • predator = an animal that kills and eats other animals
  • present = today
  • probably =likely
  • quite = very
  • real =true, original
  • reptile = animal whose body temperature always changes. It usually lays eggs to have babies
  • reshape = to get a new form
  • rhinoceros = a large heavy African or Asian animal with thick skin and one or two horns on its nose
  • roam = to walk or travel for a long time without having a place to go
  • scientist =a person who is trained in science
  • sediment = material that falls down to the bottom of the sea
  • separate = divide into parts
  • separate = to move away from
  • shallow = not deep
  • size =how big something is
  • solar system = the sun and the planets that go around it
  • spider = a small creature that has eight legs and catches insects
  • sponge =a soft material with small holes in it that can absorb water or other liquids
  • steam = gas that comes up from hot water
  • sure =certain, there is no doubt
  • surface = the top layer of something
  • survive = to go on living
  • swamp = land that is always wet or covered with water
  • take shape = form
  • thrust = to send out quickly and with a lot of power
  • towards =just before
  • upward = to the top
  • valley = an area of land between mountains, usually with a river that flows through it