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Changes to the Earth and Atmosphere (Higher)
 
The Atmosphere | The Early Atmosphere | Photosynthesis | Rocks and Fossil Fuels
 
The Atmosphere

The Earth is surrounded by a thick layer of gases called the atmosphere.

It keeps the temperature on Earth fairly constant, and contains gases which allow organisms to live.

The proportions of the different gases in the atmosphere are:
  • About four-fifths (80%) nitrogen
  • About one-fifth (20%) oxygen
  • Small proportions of various other gases, including carbon dioxide, water vapour and noble gases
These proportions have been the same for about 200 million years due to the activities of living organisms.

The Early Atmosphere

During the first billion years of the Earth's existence there was a lot of volcanic activity which released gases that formed the early atmosphere.

The main gas would probably have been carbon dioxide, with water vapour and small amounts of ammonia and methane.

There would have been little or no oxygen.

The atmosphere on Mars and Venus today is mainly carbon dioxide with little or no oxygen.

When the Earth cooled, the water vapour condensed to form the oceans.

Early forms of life developed in the water.

These included algae (simple plants) and bacteria.

Photosynthesis

About 400 million years ago land plants appeared.

These plants eventually colonised most of the Earth's surface.                                      

They absorbed some of the carbon dioxide and produced oxygen in the process of photosynthesis.

 

 

 

light

 

 

 

carbon dioxide

+

water

   —›

glucose

+

oxygen

 

 

 

chlorophyll

      

This reaction is vital for the existence of animals.

As the levels of oxygen increased there were fewer habitats available for microorganisms, which could not tolerate oxygen.

Some of the oxygen was converted into ozone gas that formed a layer around the Earth and filtered out much of the harmful ultra-violet radiation from the Sun. This allowed new organisms to evolve.

Methane and ammonia decreased because they reacted with the oxygen. The reaction of ammonia with oxygen released some nitrogen.

ammonia

+

oxygen

—›

nitrogen

+

water

Most of the nitrogen was produced by living organisms, including denitrifying bacteria, which convert nitrates into nitrogen gas.


Rocks and Fossil Fuels

The level of carbon dioxide was also reduced by chemical reactions which converted it to carbonates in sedimentary rocks.

These rocks are sometimes moved deep into the Earth by geological activity.

The carbonates decompose due to the high temperature releasing carbon dioxide into the atmosphere again via volcanoes.

Fossil fuels were formed over millions of years.

The process of fossilisation trapped large amounts of carbon.

When fossil fuels are burnt, carbon is converted into carbon dioxide which enters the atmosphere.

Some of this additional carbon dioxide reacts with sea water producing insoluble (mainly calcium) carbonates, which are deposited as sediment.

Soluble hydrogencarbonates (mainly calcium and magnesium) are also formed.  However, burning fuels is slowly causing an increase in the level of carbon dioxide in the atmosphere.