* Many Starting Points: Nonfoliated metamorphic rocks can form from a variety of parent rocks (also called protoliths). These include:
* Igneous rocks: Granite, basalt, gabbro, etc.
* Sedimentary rocks: Sandstone, limestone, shale, etc.
* Even other metamorphic rocks: If they're subjected to further metamorphism.
* The Importance of Metamorphism: What truly matters is the type of metamorphism that occurred. Nonfoliated metamorphic rocks are formed under conditions that don't lead to the alignment of minerals into layers. This can happen due to:
* Contact metamorphism: Heat from an intrusion changes the surrounding rocks.
* Regional metamorphism: Pressure is applied evenly over a large area, but without significant shear stress.
Examples:
* Marble: Formed from limestone (sedimentary) through contact metamorphism.
* Quartzite: Formed from sandstone (sedimentary) through regional metamorphism.
* Hornfels: Formed from a variety of parent rocks through contact metamorphism.
In short: It's less about the specific parent rock and more about the metamorphic process that determines whether a rock is foliated or nonfoliated.
