The mountain-building processes of the Earth’s crust involve tremendous pressures and high temperatures. These extreme conditions alter igneous or sedimentary rocks, transforming them into metamorphic rocks.
In many cases, the mineral components of the parent rock are changed into different mineral varieties. In some cases, the original minerals may recrystallize.
Extreme heat and pressure transform shale into slate or schist, sandstone into quartzite, and limestone into marble. Gneiss forms when an intrusive magma cools next to igneous or sedimentary rocks.