<P> In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occurs in the bedrock . These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure . </P> <P> The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic terrane . The two parameters together determine the metamorphic grade . The difference in grade between two locations is called the metamorphic gradient . Planes that connect points with the same metamorphic grade are called isograds . The secant lines of isograds with the surface form lines on a geologic map . </P> <P> Changes in mineralogical composition in a terrane reflect differences in metamorphic grade of the rocks . Minerals that are characteristic for a certain metamorphic grade are called index minerals . The first or last appearance of an index mineral (the place where a metamorphic reaction is observed) forms an easily recognizable isograd . A metamorphic zone is the region between two such easily recognizable isograds . Often they are named for the most characteristic index mineral of the zone . </P> <P> Whether a certain index mineral occurs is also dependent on the composition of the rock itself . Many index minerals have complicated chemical compositions . If not all necessary elements are abundant, the mineral will not grow . When mapping the metamorphic grade of a terrane, a geologist has to take the lithology of the rock in account . Lithologies are mainly dependent on the protolith, the original rock before metamorphism . The main lithologies are ultramafic, mafic, felsic (or quartzo - feldspatic), pelitic and calcareous . In all of these (and other) lithologies, different combinations of minerals occur at a certain grade . The metamorphic zones in these lithologies can also be different . </P>

Where do most metamorphic changes to rock occur