Competitive Exams: Metamorphic Rocks

Metamorphic rocks are those formed by changes in pre-existing rocks under the influence of high temperature, pressure, and chemically active solutions. The changes can be chemical (compositional) and physical (textural) in character.

Features of Metamorphic

  1. The change is due to change in texture and mineral composition of the pre-existing rocks.

  2. After metamorphism, some rocks become more harder than its original structure: Marble is harder than limestone, quartzite from sandstone, and diamond from carbon.

  3. They do not have fossils

  4. The coarse-grained metamorphic rocks are imperfectly foliated, e. g, gneises from granites while fine-grained metamorphic rocks are perfectly foliated, for example schist from shales.

  5. It may split along the bedding planes, for example mica-schist.

  6. Some of them are impervious (marble and slate) and some of them are previous for example gneiss.

  7. Most of it comprises bands of granular quartz and felspar.

Rock Type Description

  1. Slate: Shale exposed to heat and pressure that splits into hard flat plates.

  2. Schist: Shale exposed to intense heat and pressure that shows the evidence of shearing

  3. Quartzite: Sandstone that is welded by a silica cement into a very hard rock of solid quartz.

  4. Marble: Limestone exposed to heat and pressure, resulting in larger more uniform crystals.

  5. Gnesis: Rock resulting from the exposure of clastic sedimentary or intrusive igneous rocks to heat and pressure.

Chemical Composition

Despite the wide variety of igneous and sedimentary rock types that can recrystallize into metamorphic rocks, most metamorphic rocks can be described with reference to only four chemical systems: Pelitic, calcareous, felsic, and mafic.

  1. Pelitic rocks are derived from mudstone (shale) protoliths and are rich in potassium (K), aluminum (Al), silicon (Si), iron (Fe), magnesium (Mg), and water (H20), with lesser amounts of manganese (Mn), titanium (Ti), calcium (Ca). and other constituents.

  2. Calcareous rocks are formed from a variety of chemical and detrital sediments such as limestone, dolostone etc. And are largely composed of calcium oxide (CaO), magnesium oxide (MgO), and carbon dioxide (C02), with varying amounts of aluminum, silicon, iron, and water.

  3. Felsic rocks can be produced by metamorphism of both igneous and sedimentary protoliths (e. g. granite and arkose, respectively) and are rich in silicon, sodium, potassium, calcium, aluminum, and lesser amounts of iron and magnesium.

  4. Mafic rocks derive from basalt protoliths and some volcanogenic sediments and contain an abundance of iron, magnesium, calcium, silicon, and aluminum.

Rock composition

Thermodynamics of metamorphic assemblages

  1. The number of mineral phases that can coexist stably in a metamorphic rock at a particular set of pressure-temperature conditions is given by the Gibbs phase rule.

  2. A typical pelitic rock made up of the six chemical components silica, aluminum oxide, ferrous oxide, magnesium oxide, potash, and water would contain no more than six minerals; the identity of those minerals would be controlled by the pressure and temperature at which recrystallization occurred.

  3. The process of chemical mixing is referred to as metasomatism.

Distribution metamorphic rocks

The central and often dominant feature of most continents is their vast Precambrian-shield area; examples include the Canadian Shield, Brazilian Shield, African Shield, and Australian Shield. They consist of vast areas of granitic or granodioritic gneisses. Inside them, between them, and overlapping onto them are belts of sedimentary rocks. These rocks are frequently metamorphosed in the greenschist, amphibolite, and granulite facies. The Caledonian orogeny (at the close of the Silurian Period) produced tectonic metamorphic events along the east coast of North America. Greenland, the British Isles, Fennoscandia, Central Asia, and Australia. The Hercynian, or Variscan, orogeny followed about 300 million years ago, affecting subparallel regions and the Urals and European Alps. The rock cycle: It reflects the basic relationships among igneous, metamorphic, and sedimentary rocks.

Erosion includes weathering (the physical and chemical breakdown of minerals) and transportation to a site of deposition. Diagenesis is the process of forming sedimentary rock by compaction and natural cementation of grains, or crystallization from water or solutions, or recrystallization. The conversion of sediment to rock is termed lithification.