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How did volcanic activity contribute gases to Earth’s early atmosphere?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Earth’s early atmosphere was significantly shaped by the process of outgassing, which occurred as the planet differentiated into its core, mantle, and crust. During this period, volatile materials that had been trapped within the solid particles forming the planet were released through volcanic activity and related processes.1

The composition of these early gases was influenced by the oxidation state of the materials they contacted. If the early crust contained metallic iron, the outgassed products would have included molecular hydrogen, water vapour, carbon monoxide, and hydrogen sulfide. As the planet evolved and metallic iron became less prevalent, these gases shifted toward more oxidized forms, primarily water vapour, carbon dioxide, and molecular nitrogen.2

This volcanic outgassing was essential for establishing the early atmosphere, providing the water vapour that eventually condensed to form oceans and the carbon dioxide that contributed to an enhanced greenhouse effect, which helped keep the planet warm despite a weaker early Sun. Unlike modern volcanic emissions, which contain no oxygen, these early releases were entirely devoid of molecular oxygen.23