Final answer:
Carbon trapped in the lithosphere is mostly released into the atmosphere through volcanic eruptions, which are a major natural mechanism for this process.
Step-by-step explanation:
Carbon trapped in the lithosphere can be released directly into the atmosphere through a few different processes. One of the primary natural mechanisms is volcanic eruptions, which release carbon dioxide and other gases that are stored in the Earth's mantle. Other processes, such as the burning of fossil fuels, also contribute to the release of carbon, but this is a result of human activities rather than a direct geological process like volcanic activity.