Development Of The Earth’s Atmosphere (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Evolution of the atmosphere

• Theories on the development of Earth’s atmosphere have altered and developed over time as instrumental analysis has improved

  • It is difficult to gather evidence about the early atmosphere because it happened 4.6 billion years ago

• The surface of the early Earth was molten for millions of years with no atmosphere 

• As cooling slowly occurred, the molten surface began to slowly solidify into land masses

• Volcanoes formed on the land masses

• One theory of how the early atmosphere formed suggests that the volcanoes released gases from the Earth’s interior through violent eruptions

• These eruptions released large amounts of carbon dioxide and water vapour, as well as nitrogen, hydrogen, and other gases which may have included small proportions of ammonia and methane

• At this stage, the Earth's atmosphere may have been similar to the atmospheres of Mars and Venus today, consisting mainly of carbon dioxide, with little or no oxygen

How volcanoes affected the atmosphere

 Volcanoes spewed out water, carbon dioxide and other gases from the Earth's interior

• While the surface of the Earth was still very hot the large quantities of water vapour remained in the gaseous state

• When conditions cooled sufficiently, the water vapour later condensed and fell to the surface of the Earth, forming the oceans

• Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and dissolves readily

• When the water vapour condensed large amounts of CO₂ dissolved in the oceans

• Carbonate substances were precipitated during this process which later formed sediments

• These processes reduced the amount of carbon dioxide in the atmosphere

Why did oxygen levels increase?

• Primitive plants and algae began photosynthesising which used up carbon dioxide from the atmosphere and released oxygen

• Algae first evolved around 2.7 billion years ago and during the next billion years or so small green plants began to appear

• As more and more plants began to appear the levels of oxygen began to increase which allowed for more complex life forms to evolve

• The balanced symbol equation for photosynthesis is:

• Over billions of years, photosynthesis caused the amount of oxygen in the atmosphere to increase and the amount of carbon dioxide to decrease

• This trend continued until around 200 million years ago when the composition of the atmosphere reached similar characteristics as today

Removal of carbon dioxide

• Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and dissolves readily

• There were high concentrations of carbon dioxide in the early atmosphere 

• ,The levels of carbon dioxide have decreased because: 

  • When the water vapour in Earth’s early atmosphere condensed, large amounts of CO₂ dissolved in the oceans

  • Carbonates were precipitated during this process which later formed sediments on the seabed

  • Green plants and algae began to evolve and absorbed considerable amounts of carbon dioxide during photosynthesis

  • Animals fed on the plants which transferred carbon to their tissues including bones and shells and when these organisms died, their remains formed sedimentary rocks

  • Dead organisms turn into fossil fuels, such as crude oil, natural gas and coal which 'locked up' the carbon