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.

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.

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.

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.

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.

Words

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

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