Final answer:
Carbon dioxide (CO₂) is the gas that primarily escapes from magma as it rises and the external pressure decreases.
Step-by-step explanation:
The gas that primarily escapes as magma rises toward the surface, causing pressure drops, is carbon dioxide (CO₂). As the magma ascends, the pressure exerted on it decreases, allowing gases dissolved in the magma to exsolve, or come out of solution.
This decrease in pressure can cause the release of CO₂ and other volcanic gases like water vapor (H₂O) and sulfur dioxide (SO₂) into the atmosphere. Historically, volcanic activity is acknowledged as a significant source for Earth's early atmosphere.