Final answer:
Earth's oxygen-rich atmosphere was mainly created by biological activity, with cyanobacteria's photosynthesis playing a pivotal role around 2.4 billion years ago. This led to the formation of the ozone layer, enabling life to thrive on land.
Step-by-step explanation:
The Earth's atmosphere became oxygen-rich primarily due to biological activity, especially through the photosynthesis of cyanobacteria, or blue-green algae. This occurred approximately 2.4 billion years ago, a period known as the Great Oxygenation Event.
Before this time, Earth's atmosphere was anoxic and only supported anaerobic life forms. The oxygen produced also led to the formation of the ozone layer, which consists of a triple molecule of oxygen (O3), providing essential UV protection and enabling life to flourish on land.
While volcanic outgassing does release gases like carbon dioxide (CO2), water vapor (H2O), and sulfur dioxide (SO2), which are important for the atmosphere's formation and development, it was not the primary source of free oxygen.
Instead, the increase in plant life and their evolutionary sophistication over time played a critical role in oxygen accumulation. The geological activity also contributed by causing erosion that buried plant carbon before it could recombine with oxygen to form CO2.