Regional (pressure-dominated) metamorphism occurs over large areas during periods of mountain building. Regional metamorphism causes a layered or platy structure in rocks, called foliation. Foliated rocks are the result of intense pressure (and sometimes, to a lesser degree, are also the result of heat). Different grades of metamorphism are demonstrated when a shale is subjected to increasingly greater pressure and heat - first it becomes slate, then phyllite, then schist, and finally, gneiss.
Contact (heat-dominated) metamorphism occurs when molten
rock (magma) comes into contact with other surrounding rocks (called the
rock). The heat causes a localized "baked zone" of metamorphosed rock.
Metamorphic rocks produced by contact metamorphism are not foliated,
as the major factor here involves heat rather than pressure.
|Summary Chart of Common Metamorphic Rocks|
|Original Rocks||Metamorphic Equivalent||Foliated?||Metamorphism|
|sandstone||quartzite||no||regional & contact|
|shale||slate >> phyllite >> schist >> gneiss||yes||regional|
|bituminous coal||anthracite coal||no||regional|
Some characteristics of metamorphic rocks to look for:
1. They are invariably bright and lustrous.
2. Foliated metamorphic rocks appear layered.
3. Coarse-grained metamorphic rocks may show crystals that grew larger during metamorphism.