Metamorphic rock the foliation is called "gneissosity"), granoblastic (includes granulite, some marbles and quartzite), and hornfelsic (includes hornfels and skarn). Metamorphic rock Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure ( or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by
Slate types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. These were traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage. Before the mid-19th century, the terms "slate", "shale" and "schist" were not sharply distinguished. In the context of underground coal mining in the United States, the term slate was commonly used to refer to shale well into