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Metamorphic rocks are rocks much changed by various combinations of extreme heat, high pressure, and/or hot mineral-rich fluids. The changes take place deep within the Earth, or where tectonic plates collide. The original rock can be sedimentary, igneous or previously metamorphosed rock. The map shows where metamorphic rocks outcrop in the US.

A METAMORPHIC ROCK OUTCROP EXAMPLE

Metamorphic rock outcrops in Elk Knob State Park, in the Appalachian Mountains of western North Carolina, USA. That outcrope, known as the "Ashe Metamorphic Suite," has been well studied, and provides insights into what can be known about metamorphic outcrops in general.

For example, a rock from there, shown at the left, has been identified as amphibolite. Amphibolite is described as a grouping of rocks composed mainly of amphibole and plagioclase, with little or no quartz. Typically it's dark-colored and dense, weakly foliated or flaky. Small flakes of black and white minerals in the rock often give it a salt-and-pepper appearance.

In the above description, the word grouping is worth considering. Instead of providing a chemical formula as one might with a mineral, it's said that amphibolite is a grouping of rocks composed mainly of amphibole and plagioclas. Moreover, if you look up the names amphibole and plagioclase, you'll find that they themselves are described as groupings of various kinds. Plagioclase, for example, is "a series of tectosilicate minerals within the feldspar group." The term "tectosilicate" refers to a certain arrangement of the silicon atom with its four oxygen atoms.

So, this reminds us that geological metamorphism starts with rock that already probably consists of a complex of ingredients, which then is much changed with varying degrees of high temperature, pressure and chemical action, which normally leads to many forms of in-between states you just can't describe in concise, easily definable terms. Descriptions can be made, as was presented above, but using such terms as tectosilicate, and maybe having groupings within groupings.

However, getting your head around such concepts gives whole new insights into how things in Nature work, and that feels good. It especially feels good if you keep in mind that what you're dealing with is the comfortable-looking outcrop seen at the right.

Finally, if you're interested enough in knowing how the Ashe Metamorphic Suite was formed, and you're willing to deal with such concepts as the Iapetus Sea at the beginning of the Cambrian, the Cranberry Gneiss, and with subduction-related mélanges, check out Wikipedia's Formation & Origin page for the Ashe Metamorphic Suite.

BEST KNOWN METAMORPHIC ROCKS

Marble may be the best known metamorphic rock. A cut slab of marble is shown at the left, nicely showing marble's typical banding -- its "marbling." Marble forms when sedimentary limestone and dolomite are metamorphosed.

Quartzite, shown at the right, is an usually hard rock mostly composed of metamorphosed sandstone or siltstone. Its grainy, sandpaper-like surface can be glassy in appearance, and of almost any color.

Slate is metamorphosed shale. Like shale, a commonly occurring sedimentary rock, it's often layered, but unlike shale, slate is much harder and breaks with sharp edges. When slate is metamorphosed much more intensely than has been the case here, the resulting rock type, characterized by larger mica crystals, can be called phyllite.

Schist, shown at the right, results from the metamorphosis of many kinds of rocks and is composed of mineral grains easily seen with a hand lens. The grains are oriented in such a way that hunks of shist can be split into fairly thin flakes or plates. This feature causes dangerous instability when tunneling, making roadcuts through it, or building dams in schist country. In the pictures, schist looks similar to quartzite; But remember that schist breaks easily into flakes or plates, while quartzite is very hard.