Final answer:
As magma rises inside a volcano, the magmatic pressure decreases because there is less overlying rock weight exerting pressure on it, allowing gas molecules within the magma to expand.
Step-by-step explanation:
The question asks what happens as magma rises inside a volcano. When magma rises, there is a change in the conditions it experiences, specifically in terms of pressure and gas content. The correct answer is C. The magmatic pressure decreases as the magma ascends towards the Earth's surface because there is less overlying rock weight exerting pressure on the magma.
Physically, this means the gas molecules within the magma that were under high pressure at depth can now expand as they move upwards, further decreasing the pressure on the magma. This expansion mirrors the behavior of gases in other contexts where they expand when pressure decreases. With respect to volcanoes, this reduction in pressure can also contribute to the melting of rock if the conditions are right, as melting can occur with a combination of increased temperature and decreased pressure (arrow "b" and "c") or through the addition of water which lowers the rock's melting point (arrow "d").