Earth and atmosphere, Year 11: how the atmosphere formed

volcanic

HintThe gases came from inside the Earth, through openings in its crust.

The whyThe same eruptions are thought to have released water vapour, and nitrogen, which slowly built up. There may also have been small amounts of methane and ammonia. This is the only theory that GCSE requires.

Carbon dioxide

HintIt is the same gas that dominates the air on Mars and Venus now.

The whyThe early atmosphere is thought to have been mainly carbon dioxide, with little or no oxygen. Today carbon dioxide is a very small part of the air (a little over 0.04%, or 420 parts per million, in 2022, NASA), so nearly all of it has been removed.

Water vapour released by volcanoes condensed

HintThink of what happens to steam when its surroundings cool.

The whyVolcanoes gave out water vapour along with the other gases. As the Earth cooled, the vapour condensed to liquid and collected as the oceans. Carbon dioxide then began to dissolve in them.

It existed billions of years ago — the Earth is about 4.6 billion years old

HintConsider how far into the past scientists are trying to look.

The whyNobody was there to measure it and very little survives from so long ago, so scientists rely on indirect clues such as ancient rocks and the atmospheres of other planets. That is why theories about the early atmosphere have changed and developed over time.

Mars; Venus

HintThey are our two nearest planetary neighbours, one on each side of us.

The whyNeither planet has algae or plants, so nothing there has removed the carbon dioxide or added oxygen. They show what an atmosphere can be like when life has not changed it.

Algae

HintSimple water-living organisms that photosynthesise, older than land plants.

The whyAlgae were the first organisms to produce oxygen by photosynthesis, and soon afterwards oxygen began to appear in the atmosphere. Plants evolved later and added more. (Scientists often call these first oxygen-producers cyanobacteria; GCSE calls them algae.)

2.7

HintThe Earth is about 4.6 billion years old; this happened well after it formed, between two and three billion years before now.

The whyThe Earth formed about 4.6 billion years ago, so for nearly its first two billion years there was little or no oxygen in the air. Soon after algae began to photosynthesise, oxygen started to appear in the atmosphere.

It removed carbon dioxide and added oxygen

HintOne gas is on the left of the arrow and one is on the right.

The whyCarbon dioxide is a reactant, so it is used up; oxygen is a product, so it is released. Six molecules of oxygen are released for every six molecules of carbon dioxide taken in. The reaction needs light; Biology teaches it in full.

Volcanoes released little or no oxygen; it was produced by photosynthesis in algae and plants

HintAsk which living things make this gas today.

The whyThe gases from volcanoes gave an atmosphere of mainly carbon dioxide. Oxygen appeared only after living things began to photosynthesise, and its percentage then rose gradually.

Algae evolve, then oxygen builds up, then animals evolve

HintDecide which event each of the others depended on.

The whyAlgae made the first oxygen, plants later added more, and over about a billion years the percentage rose. Only when there was enough oxygen could animals evolve.

Carbon dioxide dissolved in the oceans and formed carbonates, which settled as sediment and became rock

HintStart with the gas meeting sea water, and finish on the sea bed.

The whyLimestone is mostly calcium carbonate. Some carbonate was precipitated directly, and sea creatures also used it to build shells and skeletons. These collected on the sea bed and were compressed into sedimentary rock, locking the carbon away.

Coal

HintIt is a black solid, dug from mines.

The whyThe plants, mostly trees and ferns, had taken in carbon dioxide by photosynthesis. Buried in swamps without oxygen they could not rot, and heat and pressure slowly turned them into a sedimentary rock that is mostly carbon.

plankton

HintThe word comes from the Greek for 'drifting'.

The whyLayers of mud and sediment covered the dead organisms and kept oxygen out. Over millions of years, heat and pressure turned the remains into crude oil and natural gas, which became trapped in the rocks.

They were buried before they could decay, so their carbon stayed locked underground

HintNormally rotting returns it to the air; here something prevented that.

The whyPhotosynthesis moved carbon from the air into living things. Usually decay returns it, but burial without oxygen stopped that, so the carbon stayed stored for millions of years — until the fuels are burned.

Carbon dioxide has fallen to a very small proportion; oxygen has risen to about one-fifth

HintOne gas went from being the main ingredient to a trace; the other went the opposite way.

The whyEarly on the air was mainly carbon dioxide with little or no oxygen. Now it is about four-fifths nitrogen and about one-fifth oxygen, with far less than 1% carbon dioxide. Photosynthesis, and the locking of carbon into sedimentary rocks and fossil fuels, caused the change.