METAMORPHIC ROCKS
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
country
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 |
limestone | marble | no | contact |
clay-rich rock | hornfels | no | contact |
granite | gneiss | yes | regional |
basalt | schist | 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.