When rocks are heated up or put under a lot of pressure, they can change drastically. This is because the minerals that make up the rocks form only at certain temperatures and pressures. At high temperatures or pressures, the elements will rearrange their crystal structure to form a different mineral.
Graphite and diamond are two minerals that are both made entirely out of carbon. If we put graphite under a huge amount of pressure, the carbon atoms will be squeezed together and will rearrange themselves into the more compact crystal structure of diamonds.
This change happens without any melting of the rock. Everything remains solid while the metamorphism occurs.
So why are these rocks interesting?
We know from experiments that certain minerals form only at very specific temperatures and pressures. If we find a rock that has minerals (such as a diamond) that only form at high pressures, we know that it must have formed deep in the Earth. Something must have happened to bring the rock up to the surface where we could find it.
If we find a rock that formed at high temperatures (such as marble), we know that the rock must have been heated up. This often occurs deep in the Earth or near magma underground.
We often find metamorphic rocks in mountain ranges where high pressures squeezed the rocks together and they piled up to form ranges such as the Himalayas, Alps, and the Rocky Mountains. Metamorphic rocks are forming deep in the core of these mountain ranges. As the mountain ranges are eroded away over millions of years, the metamorphic rocks are eventually uncovered. A good example of where this has happened is the Appalachian Mountains in the Eastern United States. The Appalachian Mountains used to be a very large mountain range. Over time the mountains have eroded away and we now can see many different types of metamorphic rocks in the region.