Geology in a Minute - What are Metamorphic Rocks?

Rocks that change shape and texture when they are squeezed or heated are called metamorphic rocks.

Imagine you have a soft clay cupcake. If you press it between two heavy books for a while, it might flatten out and get squishy, but still keep its identity as a cupcake. That's kind of what happens to metamorphic rocks. They start life as other kinds of rocks, maybe sedimentary or igneous, but then they are squeezed by the Earth’s movements, or heated up deep inside the planet.

When something is squeezed really hard, like being pushed between giant hands, it can get flattened and shiny. That's what happens to shale, which turns into schist when it gets squeezed. Or think of chocolate chips in cookie dough, if you press them flat, the chips might stretch out and become smooth.

If they are heated up instead, like being baked in a very hot oven for a long time, rocks can get twisted or banded, like when cake layers mix together. That’s how marble is made from limestone, just like chocolate chip cookies can turn into fudge if you bake them longer.

Metamorphic rocks are like the rock version of a transformed cupcake, still familiar, but now with new texture and shape!