Question

volcanoes can erupt when mantle rock begins to melt. this can occur by an increase in temperature or a decrease in pressure, but in oceanic crust, the melting temperature is also lowered if is introduced into the magma.

Explanation:

In oceanic crust settings, the addition of water (or volatiles like water) to mantle rock lowers its melting temperature, triggering melting that can lead to volcanic eruptions. This is a key process in subduction zone volcanism, where water is released from the subducting oceanic plate into the overlying mantle.

Answer:

water (or water/volatile compounds)