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
Fire-cracked rock Fire-cracked rock In archaeology, fire-cracked rock (FCR) or fire-affected rock (FAR), is rock of any type that has been altered and split as the result of deliberate heating. It is a feature of many archaeological sites, throughout the United States. In many cases, fire-cracked rock results when stones were used to line hearths and earth ovens or were heated to provide a longer-lasting heat-source (similar to a modern hot water bottle). In other cases, fire-cracked rock results from stone being used to heat or boil water; the stones were heated and dropped directly into water held in containers made of