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Facts About Metamorphic Rocks For Kids


Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means “change in form”. The original rock is subjected to heat and pressure, (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change.

  • They may be formed simply by being deep beneath the Earth’s surface, subjected to high temperatures and the great pressure of the rock layers above it.
  • Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.
  • Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism.
  • For instance, the small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble, or in metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite, also known as metaquartzite, in which the often larger quartz crystals are interlocked.
  • The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning “leaves”), and it occurs when a rock is being shortened along one axis during recrystallization.
  • This causes the platy or elongated crystals of minerals, such as mica and chlorite, to become rotated such that their long axes are perpendicular to the orientation of shortening.
  • For example, slate is a foliated metamorphic rock, originating from shale.
  • Slate is an example of a very fine-grained, foliated metamorphic rock, while phyllite is medium, schist coarse, and gneiss very coarse-grained.
  • Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the processes of metamorphism.
  • It is due to the introduction of chemicals from other surrounding rocks.
  • Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock).
  • Many altered rocks of this type were formerly called hornstones, and the term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.
  • A shale may become a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component.
  • A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar.
  • Recrystallization of the rock will destroy the textures and fossils present in sedimentary rocks.
  • In gneisses, minerals also tend to be segregated into bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral.
  • In gneisses these alternating folia are sometimes thicker and less regular than in schists, but most importantly less micaceous; they may be lenticular, dying out rapidly.