How are rocks formed when heat and pressure are applied?

The correct answer reflects the process of metamorphism, where existing rocks are subjected to significant heat and pressure, leading to their transformation into new rock types. This process can alter the minerals and structures of the original rock, resulting in the formation of metamorphic rocks.

For instance, sedimentary rocks can become schist or gneiss, while igneous rocks may transform into granite or marble under these conditions. This ability of rocks to change into different types as a result of heat and pressure is fundamental to understanding the rock cycle and the dynamic nature of Earth's geology.

Conversely, the other options do not accurately describe the effects of heat and pressure on rocks. While heat and pressure can change the crystal structure of minerals, they do not solely result in the formation of crystals nor merely cause the rocks to become liquids or lose mineral content. Instead, the transformation leads to the creation of entirely new rock types, showcasing the versatile nature of geological processes.