____ formed on the land masses and released gases from the Earth’s interior through violent ____ Earth’s gravity prevented the gases from escaping into outer space and they formed the ____ Analysis of the minerals in the earth’s crust enables scientists to deduce the gases present millions of years ago that helped form those minerals It is thought that the atmosphere at that moment in Earth’s history was similar to that of ____ or Mars today which consist mainly of ____ During a period of intense ____ activity, large amounts of carbon dioxide and water ____ were released, as well as nitrogen, hydrogen, ____, methane and other gases. The early atmosphere therefore contained mainly CO₂ and ____ vapour There was little or no ____ present While the surface of the Earth was still very hot the large quantities of water vapour remained in the ____ stateWhen conditions cooled sufficiently, the water vapour later ____ and fell to the surface of the Earth, forming the ____ Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and ____ readily When the water vapour condensed large amounts of ____ dissolved in the oceans ____ substances were ____ during this process which later formed sediments As ____ life began to evolve sea creatures began to appear which used up the carbonates to form ____ and skeletons ____ plants and ____ began to evolve and absorbed considerable amounts of carbon dioxide during photosynthesis When these organisms died, their remains locked the atmospheric carbon into the ____ (also as ____ fuels). This is how the large amounts of carbon dioxide in the early atmosphere were ____ Primitive plants and algae began ____ which used up carbon dioxide from the atmosphere and released ____:Carbon dioxide and water --> ____ and oxygen Through these processes over a long period of time, the amount of ____ in the atmosphere increased and the amount of CO₂ decreased ____ 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 ____ which allowed for more ____ life forms to evolve This trend continued until around 200 million years ago the composition of the atmosphere reached similar characteristics as today: around ____ oxygen, ____ nitrogen and tiny amounts of other gases (mainly ____) Marine algae are thought to be responsible for about 90% of all the atmospheric oxygen produced

Atmosphere evolution and modern composition

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