Final answer:
Volcanic activity returns carbon to the atmosphere as carbon dioxide, through the eruption of volcanoes, which releases carbon stored from the Earth's mantle.
Other geological processes include rock weathering and limestone heating during cement production. While photosynthesis removes CO2 from the atmosphere, volcanic eruptions, together with human activities, contribute to the natural and altered carbon cycle.
Step-by-step explanation:
The geological process that returns carbon to the atmosphere in the form of carbon dioxide is volcanic activity. When volcanoes erupt, they release carbon dioxide that has been stored in the Earth's mantle.
The carbon dioxide is released into the atmosphere, contributing to the natural carbon cycle. Other geological processes that release carbon dioxide include the weathering of rocks and the heating of limestone during the production of cement.
Additionally, human activities like burning fossil fuels also add significant amounts of carbon dioxide to the atmosphere, altering the natural carbon cycle.
This is a part of the larger carbon cycle, which includes biological processes such as cellular respiration and decomposition that also release carbon dioxide.
It is important to recognize that photosynthesis plays a vital role in removing carbon dioxide from the atmosphere, although it is not a process that directly returns carbon to the atmosphere.