The geologic processes that created the North Cascades changed the original rock into metamorphic rock.

There are several types of metamorphism, the process by which rocks change into new rocks. The two most general types are contact metamorphism and regional metamorphism.

CONTACT METAMORPHISM occurs when a rock is exposed to hot magma inside the Earth. The intense heat of the magma alters the rock, often causing its minerals to recrystallize. Thus, the new rock has new or larger mineral crystals than the older rock. Sometimes, the hot magma will even introduce new minerals and modify the entire chemical composition of the original rock. The area of rock affected by contact metamorphism is appropriately known as the baked zone.

Compression from tectonic forces causes the parent rock to foliate.

REGIONAL METAMORPHISM, on the other hand, occurs during the formation of mountain ranges. As tectonic plates collide and converge, intense pressure deforms and alters sedimentary and igneous rocks already buried in the Earth. The mountain ranges of east Greenland are an example of where this has taken place. Often, folds or curves in the rocks indicate the direction of the intense pressure.

Whether metamorphic rocks are formed by contact with hot magma or by pressure from colliding plates in the Earth, the result is that mineral crystals in the original rock are rearranged. The new rock is classified into two basic types: foliated and non-foliated.

Gneiss, a regional metamorphic rock, has a banded, or foliated, appearance.

FOLIATED metamorphic rocks are those in which the minerals have been flattened and pushed down into parallel layers. The bands in foliated metamorphic rock look like pages in a book. Slate, one of the most common foliated metamorphic rocks, splits easily into thin slabs. Whereas slate is smooth and fine-textured, schist is a medium- to coarse-textured foliated rock. Schist is made mostly of mica minerals; it, too, splits readily into slabs.

Cupid and Psyche, by Antonio Canova, was carved from marble, a metamorphic rock.

A third example of a foliated metamorphic rock is gneiss (pronounced "nice"), a coarse-grained rock with a rough feel. It is characterized by bands of light and dark minerals. Although gneiss often looks much like schist, it does not split as easily into thin slabs. Of the three foliated rocks discussed here, gneiss results from the greatest and most intense metamorphism.

NON-FOLIATED metamorphic rocks do not display layers. Rather, they are massive structures with no obvious banding. A good example of non-foliated rock is quartzite, the smooth-textured, metamorphosed form of the mineral quartz. A coarse-textured non-foliated rock is marble. Anthracite, or hard coal, is a non-foliated rock that forms when intense pressure drives gases out of soft coal, causing it to harden.

Note that a parent rock may metamorphose into several different metamorphic rocks. For example, shale (a sedimentary rock) can be transformed into slate, schist, or gneiss, depending on the degree of metamorphism. The greater the pressure or heat, the more likely the shale will change into gneiss.

The rock cycle does not end with a metamorphic rock. Processes of change are always at work, and a metamorphic rock can be broken or eroded into sediment, or melted and reformed as an igneous rock. The environmental forces that drive the rock cycle are responsible for the ever-changing face of Earth's landscape.

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