Final answer:
Decreasing the air content of rock is not an important mechanism for magma formation in the Earth. The important mechanisms include increasing rock temperature, decreasing rock pressure, and increasing the water content of rock.
Step-by-step explanation:
The option that is not an important mechanism for magma formation within the Earth is '4. decreasing the air content of rock'. The three main ways that rocks melt to form magma are:
- Increase in rock temperature, which may occur in regions known as hot spots.
- Decrease in rock pressure, also known as decompression melting, which can happen at mid-ocean ridges or other rift zones.
- Increasing the water content of rock, which lowers the melting temperature of rocks, a process called flux melting.
These processes correspond to the arrows 'a', 'b', and 'd' respectively, as mentioned in the provided text. In contrast, the air content of rocks does not play a significant role in the melting process, making it an incorrect answer for mechanisms of magma formation.