These minerals, known as index mineralsinclude sillimanitekyanitestauroliteandalusiteand some garnet.
These minerals, known as index mineralsinclude sillimanitekyanitestauroliteandalusiteand some garnet. Other minerals, such as olivinespyroxenesamphibolesmicasfeldsparsand quartzmay be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism.
These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process.
However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed.
The change in the particle size of the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble ; 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.
Both high temperatures and pressures contribute to recrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact.
Foliation geology 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 chloriteto become rotated such that their long axes are perpendicular to the orientation of shortening.
This results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them. Textures are separated into foliated and non-foliated categories. Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage.
For example, slate is a foliated metamorphic rock, originating from shale. Non-foliated rock does not have planar patterns of strain.
Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated.
Where a rock has been subject to differential stress, the type of foliation that develops depends on the metamorphic grade. For instance, starting with a mudstonethe following sequence develops with increasing temperature: Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting.
In the process atoms are exchanged between the minerals, and thus new minerals are formed. Many complex high-temperature reactions may take place, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism. 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. Water may transport these chemicals rapidly over great distances. Because of the role played by water, metamorphic rocks generally contain many elements absent from the original rock, and lack some that originally were present.
Still, the introduction of new chemicals is not necessary for recrystallization to occur.
Types of metamorphism Contact metamorphism A contact metamorphic rock made of interlayered calcite and serpentine from the Precambrian of Canada. Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock country rock.
The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it.
Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole.
Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed unchanged country rock some distance away.
The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone. When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline.
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 marbletough and splintery, with abundant augitegarnetwollastonite 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.Formation of Metamorphic Rocks Even though least common of the three types, these rocks are a bit unique as they form from an existing sedimentary rock, igneous rock or an old metamorphic rock.
In lithology, i.e. the branch of geology which studies various attributes of rocks, the parent rock of any metamorphic rock is referred to as the 'protolith'. FORMATION OF METAMORPHIC ROCKS. Metamorphic rocks record how temperature and pressure affected an area when it was forming.
The rocks provide clues to their transformation into a metamorphic rocks. Metamorphic rocks are best identified when looking at the rock as you see them in nature.
Metamorphic rocks formed from direct magma heating and intrusions are termed as thermal or contact metamorphic rocks. Those formed as a result of widely distributed pressure and temperature changes induced by tectonic movements are known as regional metamorphic rocks.
Metamorphic rocks are largely grouped into foliated and non-foliated rocks. Types of Metamorphic Rocks. There are two main types . Slate is a metamorphic rock that is formed from the sedimentary rock mudstone. Granulite is a metamorphic rock that is formed from the igneous rock basalt.
Learn about sedimentary rocks, igneous rocks, other rocks and minerals or fossils. Feb 23, · Overview of metamorphic rocks and their formation. Including explanation of contact and regional metamorphism and the conditions in which they commonly form.
For example, slate is a foliated metamorphic rock, Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away.
The formation of important ore minerals may occur by the process of .