Everything about Metamorphic Rock totally explained
Metamorphic rock is the result of the transformation of a pre-existing
rock type, the
protolith, in a process called
metamorphism, which means "change in form ".The protolith 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. The protolith may be
sedimentary rock,
igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the
Earth's
crust and are classified by texture and by
chemical and
mineral assemblage (
metamorphic facies). 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. They can be formed by
tectonic processes such as continental collisions which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the
intrusion of hot molten rock called
magma from the Earth's interior.
The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides us with very valuable information about the temperatures and pressures that occur at great depths within the Earth's crust.
Some examples of metamorphic rocks are
gneiss,
slate,
marble and
schist.
Metamorphic minerals
Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as
index minerals, include
sillimanite,
kyanite,
staurolite,
andalusite, and some
garnet.
Other minerals, such as
olivines,
pyroxenes,
amphiboles,
micas,
feldspars, and
quartz, may 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 were 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 change into larger crystals in the metamorphic rock
marble, or in metamorphosed sandstone, recrystallisation of the original quartz sand grains results in very compact quartzite, 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
The layering within metamorphic rocks is called
foliation (derived from the
Latin word
folia, meaning "leaves"), and it occurs when a the rock is being compressed from one direction to a recrystallizing rock. This causes the platy or elongated crystals of minerals, such as
mica and
chlorite, to grow with their long axes perpendicular to the direction of the force. 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 doesn't have planar patterns of stress.
Rocks that were subjected to uniform pressure from all sides, or those which lack minerals with distinctive growth habits, won't be foliated. Slate is an example of a very fine-grained, foliated metamorphic rock, while
phyllite is coarse,
schist coarser, and
gneiss very coarse-grained. Marble is generally not foliated, which allows its use as a material for sculpture and architecture.
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 that were absent from the original rock, and lack some which were originally present. Still, the introduction of new chemicals isn't necessary for recrystallization to occur.
Types of metamorphism
Contact metamorphism
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-honrfels 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.
Chert or
flint may become a finely crystalline quartz rock;
sandstones lose their
clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called
quartzite.
If the rock was originally banded or
foliated (as, for example, a laminated sandstone or a foliated calc-
schist) this character may not be obliterated, and a banded hornfels is the product;
fossils even may have their shapes preserved, though entirely recrystallized, and in many contact-altered
lavas the
vesicles are still visible, though their contents have usually entered into new combinations to form minerals which were not originally present. The minute structures, however, disappear, often completely, if the thermal alteration is very profound; thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed.
By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thus shales may pass into
cordierite rocks, or may show large crystals of
andalusite (and
chiastolite),
staurolite,
garnet,
kyanite and
sillimanite, all derived from the aluminous content of the original shale. A considerable amount of
mica (both muscovite and biotite) is often simultaneously formed, and the resulting product has a close resemblance to many kinds of schist. Limestones, if pure, are often turned into coarsely crystalline marbles; but if there was an admixture
of clay or sand in the original rock such minerals as garnet,
epidote,
idocrase, wollastonite, will be present. Sandstones when greatly heated may change into coarse quartzites composed of large clear grains of quartz. These more intense stages of alteration are not
so commonly seen in igneous rocks, because their minerals, being formed at high temperatures, are not so easily transformed or recrystallized.
In a few cases rocks are fused and in the dark glassy product minute crystals of
spinel, sillimanite and
cordierite may separate out. Shales are occasionally thus altered by basalt
dikes, and feldspathic sandstones may be completely vitrified. Similar changes may be induced in shales by the burning of
coal seams or even by an ordinary furnace.
There is also a tendency for
metasomatism between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other. Granites may absorb fragments of shale or pieces of basalt. In that case hybrid rocks called
skarn arise which have not the characters of normal igneous or sedimentary rocks. Sometimes an invading granite magma permeates the rocks around, filling their joints and planes of bedding, etc., with threads of quartz and feldspar. This is very exceptional but instances of it are known and it may take place on a large scale.
Regional metamorphism
Regional metamorphism is the name given to changes in great masses of rock over a wide area. Rocks can be metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Much of the lower continental crust is metamorphic, except for recent igneous intrusions. Horizontal tectonic movements such as the collision of continents create
orogenic belts, and cause high temperatures, pressures and deformation in the rocks along these belts. If the metamorphosed rocks are later uplifted and exposed by
erosion, they may occur in long belts or other large areas at the surface. The process of metamorphism may have destroyed the original features that could have revealed the rock's previous history.
Recrystallization of the rock will destroy the textures and
fossils present in sedimentary rocks. Metasomatism will change the original composition.
Regional metamorphism tends to make the rock more indurated and at the same time to give it a foliated, shistose or gneissic texture, consisting of a planar arrangement of the minerals, so that platy or prismatic minerals like mica and hornblende have their longest axes arranged parallel to one another. For that reason many of these rocks split readily in one direction along mica-bearing zones (
schists). 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. Along the mineral layers composed of soft or fissile minerals the rocks will split most readily, and the freshly split specimens will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at facewise might be supposed to consist entirely of shining scales of mica. On the
edge of the specimens, however, the white folia of granular quartz will be visible. 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. Gneisses also, as a rule, contain more feldspar than schists do, and they're tougher and less fissile. Contortion or crumbling of the foliation is by no means uncommon, and then the splitting faces are undulose or puckered. Schistosity and gneissic banding (the two main types of foliation) are formed by directed pressure at elevated temperature, and to interstitial movement, or internal flow arranging the mineral particles while they're crystallizing in that directed pressure field.
Rocks which were originally sedimentary and rocks which were undoubtedly igneous are converted into schists and gneisses, and if originally of similar composition they may be very difficult to distinguish from one another if the metamorphism has been great. A
quartz-porphyry, for example, and a fine feldspathic sandstone, may both the converted into a grey or pink mica-schist.
Metamorphic rock textures
The five basic metamorphic textures with typical rock types are:
Further Information
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