__NOTOC__ An isograd is a concept used in the study of

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

s. The

metamorphic grade Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

of such a rock is a rough measure of the degree of

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

it has undergone, as characterised by the presence of certain

index mineral An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith (parent rock), chemical reactions between mine ...

s. An isograd is a theoretical

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is t ...

composed of points where the metamorphic grade is the same. It thus separates

metamorphic zone In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and press ...

s whose rocks contain different index minerals.Marchak (2009), p183 On

geologic map A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...

s focusing on metamorphic terranes (or landscapes underlain by metamorphic rocks), the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd lines. The

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

isograd, for example, would mark the first occurrence of garnet in the rocks. The

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

s present in a metamorphic rock are important because laboratory experiments at high pressures and temperatures have provided a lot of information on the pressure and temperature conditions under which certain metamorphic minerals form. For example, with increasing temperature and pressure the first minerals to form from a

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

are

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

s, particularly

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

and

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

. With increasing temperature and pressure

appears, and then

kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally ...

(at relatively high pressure) or

sillimanite Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut. Occurrence S ...

(at relatively high temperature). The metamorphic zone with chlorite can be referred to as the ''chlorite zone'', the zone with garnet as ''garnet zone'', and so forth. To communicate this easily, the dominant metamorphic minerals in

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

s are usually included in the name, as, for example, garnet schist, or garnet-staurolite schist, and so forth.

Notes

References

*, 2003, ''Igneous and Metamorphic Petrology'', Blackwell Publishing (2nd ed.), *Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. *, 1990, ''Principles of Igneous and Metamorphic Petrology'', Prentice Hall, {{petrology-stub Petrology

See also

Notes

References