Metamorphic Rock Metamorphic Rocks Used with permission of Tammy Peyton, Rock Hounds web editor. These rocks were once igneous or sedimentary rocks. The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you examine metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.
Metamorphic rocks are VERY uncommon in Iowa.