The most common type of rock that makes up metamorphic rocks is shale, accounting for approximately 75% of all metamorphic rocks.

Here's why:

* Abundance: Shale is a sedimentary rock formed from the compaction of mud and clay. These materials are extremely abundant on Earth, making shale a common source for metamorphic rocks.

* Transformation: Shale readily transforms under pressure and heat into various metamorphic rocks like slate, schist, and gneiss.

While shale is the most abundant, other sedimentary rocks like sandstone, limestone, and even some igneous rocks can also transform into metamorphic rocks.