Garnet schist from the Cretaceous of Alaska, USA. (~4.1 centimeters across at its widest)

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Schist is an intermediate- to high-grade, foliated metamorphic rock. It is highly variable in appearance, depending on the mineral content, which is a function of the precursor rock and specific temperature-pressure conditions. Garden-variety schists form by metamorphism of phyllites. Schists typically have medium- to large-sized crystals, unlike the microcrystalline nature of slate & phyllite.

Schist's foliated texture (= crystals aligned into bands or stripes or layers) is often only seen when specimens are viewed on edge.

Garnet schist has conspicuous, large or small garnet crystals. Garnets in schists are typically very deep red-colored. The surrounding matrix is often a grayish and mica-rich. The specimen seen here is from Garnet Ledge, Alaska, which has near-gem-quality almandine garnets.

Garnet Ledge is a famous garnet collecting locality. The site is just south of the Stikine River mouth, near the town of Wrangell, in southeastern Alaska. Many thousands of deep red, gemmy almandine garnets have been collected from Garnet Ledge for over a century. The land at Garnet Ledge was given to the Boy Scouts in the early 1960s, and ever since, only the children of Wrangell may collect garnets there for free.

Garnet Ledge Geology - Almandine garnets occur in grayish-colored schists containing a mixture of sillimanite, muscovite, biotite, staurolite, chlorite, plagioclase, graphite, and ilmenite. Before metamorphism, the precursor rocks were Late Jurassic-Early Cretaceous fine-grained siliciclastic sedimentary rocks. These were cooked by contact metamorphism during intrusion of a quartz diorite pluton of the Admiralty-Revillagigedo Plutonic Belt (Alaskan Coast Plutonic Complex) during the early Late Cretaceous (at 89-92 million years). The garnets appear to have formed over a one million year interval, from 89 to 90 million years ago. The Garnet Ledge locality is a small, 1.3 square kilometer fragment of roof pendant over the quartz diorite pluton. [Summarized from information in Stowell et al. (2001).]