Final answer:
The transfer of heat from a hot magma to surrounding wall rock will cause solid rock to melt and form magma, especially when this process increases the temperature or decreases the pressure of the rock or adds water to lower the melting temperature.
Step-by-step explanation:
The phenomenon that will cause melting of solid rock to produce magma is the transfer of heat from a hot magma to surrounding wall rock. When the temperature of rock increases while the pressure remains constant, melting occurs. Conversely, if the pressure decreases while the temperature remains stable, the rock may also melt. Additionally, the presence of water can lower the melting temperature of rock, facilitating the formation of magma without a significant change in temperature or pressure.
To understand this concept, imagine a pressure and temperature graph of crustal rocks with a line called the solidus that separates regions of solid and liquid rock. Melting can occur by crossing this line either by increasing temperature, reducing pressure, a combination of both, or by changing the melting conditions, such as adding water. Tectonic activities, like mantle plumes causing hot spots or diverging tectonic plates reducing pressure, often result in these conditions which lead to magma generation.