Final answer:
Partial melting of subduction plate begins at a certain depth in the Earth's mantle, where the release of volatiles, such as water, leads to the formation of magma. This process can result in volcanic activity.
Step-by-step explanation:
The partial melting of subduction plate begins at a certain depth in the Earth's mantle. As the subducting plate is forced down into regions of high pressure and temperature, it eventually reaches a depth of several hundred kilometers below the surface where partial melting occurs. This melting is caused by the release of volatiles, such as water, from hydrated minerals in the subducting plate.
One of the primary factors influencing the depth at which partial melting begins is the presence of water. Water lowers the melting temperature of the mantle region directly above the subducting plate, leading to the formation of magma. This magma can then rise to the surface and result in volcanic activity.
It's important to note that the exact depth at which partial melting begins can vary depending on various factors, including the composition and temperature of the subducting plate.