Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemically active fluids. This process is called metamorphism.

Here's a breakdown of the process:

1. Existing Rocks: The starting material for metamorphic rocks can be any type of rock:

* Igneous rocks (formed from cooled magma or lava)

* Sedimentary rocks (formed from compacted and cemented sediments)

* Other metamorphic rocks (that have already undergone metamorphism)

2. The Metamorphic Agents: These agents cause the changes in the existing rocks:

* Heat: Heat can come from nearby magma intrusions, deep burial within the Earth, or tectonic plate movement.

* Pressure: Pressure arises from the weight of overlying rocks or from tectonic forces.

* Chemically Active Fluids: Hot water, often rich in dissolved minerals, can circulate through rocks and alter their composition.

3. Transformation: The heat, pressure, and fluids cause the original minerals in the rock to recrystallize, change their structure, and sometimes form new minerals. This process occurs in the solid state, meaning the rock doesn't melt completely.

4. Types of Metamorphism: There are different types of metamorphism based on the intensity of the agents and the environment:

* Contact metamorphism: Occurs when rocks are heated by contact with magma. This typically creates smaller areas of metamorphic rocks.

* Regional metamorphism: Occurs over large areas due to heat and pressure from tectonic plate collisions. This can create large mountain ranges.

* Hydrothermal metamorphism: Occurs when hot water reacts with existing rocks, often altering their chemical composition.

5. Classification of Metamorphic Rocks: Metamorphic rocks are classified based on their mineral composition, texture, and the degree of metamorphism:

* Foliated rocks: Have a layered or banded appearance due to the alignment of minerals under pressure. Examples include slate, schist, and gneiss.

* Non-foliated rocks: Do not have a layered appearance. Examples include marble, quartzite, and anthracite coal.

In essence, metamorphic rocks are the "transformed" versions of existing rocks, born from intense heat, pressure, and chemical activity within the Earth.