Final answer:
An increase in confining pressure helps keep a rock solid and prevents it from melting by forcing the atoms in a lattice structure more tightly together. Temperature increases and increases in lattice vibrations typically lead to melting, while pressure increases prevent it.
Step-by-step explanation:
The act that keeps a rock solid instead of melting is an increase in confining pressure. This is because, at the base of the Earth's crust, a rock remains solid due to high pressures keeping it from melting. When the pressure is increased, the atoms in the rock are more tightly packed together in the lattice structure. This additional pressure can prevent the atoms from moving into a liquid state. Conversely, an increase in temperature and an increase in the vibration within a lattice generally contribute to the melting of the rock by providing the necessary energy to overcome the forces holding the atoms together.
For example, at point 'X' in a pressure and temperature graph of crustal rocks, a rock can be prevented from melting by high confining pressure. To melt this rock, the temperature would need to increase or the pressure would need to decrease, or a combination of both. Alternatively, the melting conditions of the rock can be changed by adding water, which lowers its melting temperature, allowing the rock to melt without changing temperature or pressure conditions.
In the context of metamorphic rocks, changes in temperature and pressure can result in the recrystallization of minerals within the rock without reaching the melting point, transforming the rock's form and creating a new metamorphic rock from the protolith.