Answer:
Heat and pressure
Explanation:
There are mainly three types of rocks- igneous, sedimentary, metamorphic. These rocks continuously change their form by the rock cycle due to heat and pressure. The rock cycle indicates a rock can't stay in its previous form and changes with time. The newly formed rock is the metamorphic rock. The metamorphic rock forms by application of high heat and pressure. These two energy is the catalytic agent to change the volume and composition of the rock.
It happens when the tectonic plates displace, and the upper plates go down, the lower plates come up. Due to this displacement, the underlying rocks come under high pressure. As a result of their shape and composition changes. Sometimes, this crushing of earth causes rising of molten lava to the crust. This lava comes in contact with the crustal rock changes its forms. The newly metamorphic rocks are a combination of new and old rock. They have recrystallized and reorganized.
e.g. Granite changes to gneiss and limestone form marble under high pressure and heat.
