What is the only major natural pathway for deep, geological carbon stored in rocks to return to the atmosphere?

Answer

Volcanism

Tectonic plate movement subjects carbon-bearing rocks to high heat and pressure, causing them to melt and release $ ext{CO}_2$ back into the atmosphere during volcanic eruptions.

What is the only major natural pathway for deep, geological carbon stored in rocks to return to the atmosphere?
oceanatmospherecyclecarbondriver