Amphibolite facies - medium grade regional metamorphism. Characterized by amphibolite rocks if a mafic parent, schists and some gneisses if a complex siliciclastic sedimentary parent, plus other metamorphic rocks from other parents.

Amphibolite (rock) - a medium grade rock rich in amphibole derived from the metamorphism of a mafic igneous parent rock.

Barrovian (Regional) Metamorphism - large scale, regional metamorphism characterized by belts of progressively increasing metamorphic grade. Increasing grade may be measured either by index minerals or facies.

Banding (mineral) - in high grade metamorphism the separation of minerals into separate bands. Typically the micaceous minerals like biotite separate from bands of quartz and feldspar.

Blueschist facies - lower temperature, high pressure regional metamorphism.

Cleavage (slaty, rock) - a tendency of a rock to split along secondary, aligned fractures or other closely spaced planar structures or textures produced by deformation, or metamorphism. In low grade metamorphic rocks the cleavage follows the fine grained foliation. Metamorphism under hydrostatic pressure typically results in foliation and cleavage parallel to bedding. With directed pressure the sedimentary layers typically become folded, but the foliation and cleavage continues to form at right angles to the stress and cuts across bedding.

Contact Facies Metamorphism - metamorphism produced in a restricted zone next to an igneous intrusion. May involve heat or hydrothermal fluids, but not directed pressures.

Country rock - the rock intruded by and surrounding an igneous intrusion.

Diagenesis - the processes of compaction, cementation, and crystal growth which convert a sediment into a sedimentary rock, short of metamorphic processes.

Directed stress - stress which comes from a preferred direction. Typically occurs in areas of compressive mountain building.

Eclogite Facies Metamorphism - high temperature, very high pressure metamorphism. Typically occurs to mafic and ultramafic rocks at the base of subduction zones, producing a rock rich in pink garnets and pale green pyroxenes.

Facies (metamorphic) - a group of rocks metamorphosed under similar temperature and pressure conditions resulting in a characteristic metamorphic mineral assemblage. Any particular facies (e.g. greenschist, or blueschist, or eclogite, etc.) represents the same temperature and pressure conditions regardless of the original parent rock or the specific metamorphic rock each has transformed into.

Foliation - any planar set of minerals, or banding of mineral concentrations, especially the planar structure that results from flattening of the mineral grains, like micas.

Grade - a measure of the intensity of metamorphism, typically low grade (greenschist facies), medium grade (amphibolite facies), and high grade (granulite facies).

Granulite Facies - high grade regional metamorphism. Characterized by pyroxene rich rocks if a mafic parent, quartz and feldspar rich gneisses if a complex siliciclastic sedimentary parent, plus other rocks from other parents.

Granulite (rock) - a high grade rock rich in pyroxene derived from the metamorphism of a mafic igneous parent rock.

Geothermal Gradient - the rate of increase of temperature in the earth with depth. Varies from place to place, but average rate approximates 25^o C/km of depth.

Greenschist Facies - low grade regional metamorphism. Characterized by greenstone (greenschist) rocks if a mafic parent, slates and phyllites if a complex siliciclastic sedimentary parent, plus other metamorphic rocks from other parents.

Greenschist (rock) - a low grade rock rich in chlorite derived from the metamorphism of a mafic igneous parent rock.

Hydrothermal Metamorphism - a local type of metamorphism caused by the percolation of hot solutions or gases through fractures, causing mineralogic changes in the neighboring rock (see metasomatism). Many varieties, conditions of formation, and resulting rocks and deposits.

Hydrostatic Stress - stress which is uniform in all directions resulting from burial or submergence in a liquid.

Index Mineral - a mineral developed under a particular set of temperature and pressure conditions, thus characterizing a particular degree (grade) of metamorphism. In Barrovian metamorphism, the sequence is (low grade to high grade) Chlorite Biotite Garnet Staurolite Kyanite Sillimanite.

Isograd - a line on a map connecting points of first occurrence of a given (index) mineral in a metamorphic rock.

Metamorphism - the mineralogical and textural changes which occur in a rock in response to changes in temperature and pressure. Includes only those changes occurring after diagenesis and before melting.

Metasomatism - the presence of interstitial, chemically active pore liquids or gasses introduced into a rock body from external sources, such as those expelled from a cooling magma.

Metasomatic Halo (Aureole) - the zone of metasomatism accompanying contact metamorphism which alters a parent country rock.

Metamorphic Grade - the intensity of metamorphism measured by the amount or degree of change from the parent rock. From low grade to high grade = Greenschist Amphibolite Granulite.

Metamorphic Zone - a measure of metamorphic intensity based on the appearance of specific index minerals, in order Chlorite Biotite Garnet Staurolite Kyanite Sillimanite.

Migmatite - a rock composed of an unmelted metamorphic portion and a melted igneous portion.

Non-foliated - said of a metamorphic rock in which all the grains are granular and equi-dimensional. Typical of rocks composed of a simple minerals such as quartz (sandstone) or calcite (limestone).

Parent Rock - an original rock from which a metamorphic rock is derived. May be an igneous, sedimentary, or other metamorphic rock of any composition.

Porphoroblast - a large crystal surrounded by smaller crystals, having the appearance of a phenocryst in an igneous rock, but produced by metamorphism. Garnets are typical.

Prograde - the metamorphic changes that occur when a rock heats up.

Regional Metamorphism - see Barrovian.

Retrograde - the metamorphic changes that occur as the temperature and pressure of metamorphism decline. Typically, high grade textures are retained, but high grade minerals are replaced by lower grade ones. A typical example are large garnet porphoroblasts where the dodecahedral crystal form is retained but the garnet has retrograded to chlorite.

Schist - this term has several distinct and overlapping meanings, making it the most confusing and complicated metamorphic term. Frequently the term is combined with another giving the conjoined terms a distinct meaning and interpretation

Schist - a type of layering or foliation typical of metamorphic rocks where minerals large enough to be seen by eye show definite orientations. Specific examples include Greenschist (chlorite dominated), Blueschist (glaucophane dominated, plus many other minerals), and just plain Schist (biotite or muscovite, quartz, and feldspar dominated.) To add confusion some rocks with a schist-like texture do not include the term in the name, like amphibolite.

Blueschist (facies) - lower temperature, high pressure regional metamorphism.

Greenschist (facies) - low grade regional metamorphism. Characterized by greenstone (greenschist) rocks if a mafic parent, slates and phyllites if a complex siliciclastic sedimentary parent, plus other metamorphic rocks from other parents.

Greenschist (rock) - a low grade rock rich in chlorite derived from the metamorphism of a mafic igneous parent rock.

Schistosity - the foliation in schist or other coarse-grained crystalline rocks due to the parallel, planar arrangement of platy or prismatic mineral grains, usually micas and amphiboles. The parallel arrangement of sheety or prismatic minerals like micas and amphiboles resulting from non-hydrostatic stress in metamorphism.

Skarn - carbonate bearing silicates derived from nearly pure limestones and dolomites altered by the introduction of large amounts of hydrothermal Si, Al, Fe, and Mg.

Zeolite Facies Metamorphism - low temperature, low pressure metamorphism. Sometimes considered lowest level of regional metamorphism, coming right after diagenesis.

Zone - in regional (Barrovian) metamorphism the belts between isograds.