Question

the replacing of methane gas with oxygen gas into the atmosphere for millions of years is known as blank

Answer 1

The process you are referring to is known as "methanogenesis" or "methane oxidation." In natural environments, methane can be oxidized by microorganisms in a process called anaerobic oxidation of methane (AOM), where it is converted into carbon dioxide and other compounds in the presence of sulfate, nitrate, or other oxidants. Over geological timescales, methane that escapes into the atmosphere can also be oxidized by atmospheric oxygen into carbon dioxide and water through reactions catalyzed by sunlight and certain atmospheric conditions. Thus, the replacement of methane with oxygen in the atmosphere can also be broadly related to the processes of methane cycling and oxidation. If you are looking for a specific term or concept, please clarify.