Metamorphic rocks form when existing rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. This causes profound physical and/or chemical changes to the original rocks. The process of metamorphism does not melt the rocks completely, but instead transforms them into new types of rocks with distinct characteristics.
There are several key characteristics that distinguish metamorphic rocks from other rock types:
1. Foliated Texture
One of the most distinctive features of metamorphic rocks is a banded or foliated texture. This is caused by the realignment of mineral grains under heat and pressure. It results in the metamorphic rocks splitting or fracturing along mineral grain boundaries. The foliated texture allows metamorphic rocks to be easily split into thin slabs or sheets. Some examples of foliated metamorphic rocks include slate, schist and gneiss. The extent of foliation depends on the types of minerals present and the intensity of the metamorphic process. Higher grades of metamorphism generally result in more pronounced foliation.
2. Recrystallization
Metamorphic rocks exhibit recrystallization, which is the formation of new minerals from existing ones. Recrystallization occurs because the minerals in the original rocks become unstable when subjected to high temperatures and pressures. New minerals grow in the places where the old minerals react. These new minerals have structures that are stable under the new temperature-pressure conditions. Common new minerals formed during metamorphism include micas, garnet, staurolite and kyanite. The type of new minerals that form depends on the composition of the original rocks and the specific metamorphic conditions.
3. No Primary Structures
In contrast to sedimentary rocks, metamorphic rocks do not retain any primary structures from the original rocks. Features like bedding planes, ripple marks and mud cracks are destroyed by metamorphism. Any sedimentary structures present in the unmetamorphosed precursor rocks are erased by the growth of new minerals during recrystallization.
4. Rare Fossils
Fossils and organic matter are rarely found in metamorphic rocks. The high temperatures and pressures of the metamorphic environment cause any fossils present in the precursor rocks to become distorted, destroyed or smeared out. However, fossils can very occasionally be found preserved in some metamorphic rocks if the organism was rapidly buried and subjected to minimal metamorphic effects. This fossil preservation indicates that little recrystallization occurred in that specific area of rock.
5. Mineral Changes
There are several characteristic mineral changes that take place during the metamorphism of existing rock types. For example, the mineral hematite, which is common in sedimentary iron-rich rocks, gets replaced by magnetite in metamorphic conditions. Calcium-rich plagioclase feldspar gets converted to higher-grade anorthite or microcline. Pyroxenes like augite get replaced by minerals like hornblende. Clay minerals like kaolinite are transformed into more complex minerals like biotite, chlorite or garnet that are stable in high-temperature environments.
6. Formation of New minerals
In addition to mineral changes, wholly new minerals can also grow in metamorphic rocks that were not originally present in the precursor rocks. As mentioned earlier, common new minerals that appear include micas (biotite and muscovite), garnet, staurolite, kyanite, sillimanite, hornblende and more. Index or facies minerals like these indicate the precise metamorphic grade and formation conditions.
7. Development of Schistosity
In schists, micas and other platy or elongated minerals become aligned perpendicular to the direction of pressure. This realignment of mineral grains produces a type of cleavage called schistosity. Schistosity allows schists to be split smoothly parallel to the aligned micas. The extent of schistosity present is reflective of the intensity of metamorphism. Higher grades of metamorphism lead to stronger schistosity.
8. Banded Appearance
Alternating light and dark bands are a very characteristic feature of gneisses and metamorphic marble. This banded appearance arises because different types of minerals have separated into distinct bands. In gneisses, light-colored bands contain quartz and feldspars while dark bands contain micas and amphiboles. In metamorphic marble, impurities are segregated into thin layers which alternate with pure calcite or dolomite bands. The specific banding pattern depends on the chemical composition of the original rocks.
9. Layered Texture
Metamorphic rocks like gneisses display a layered texture representing the foliation planes that formed parallel to pressure during metamorphism. The light and dark bands produce a gneissic layering that is inclined or folded based on the tectonic forces present. Small-scale compositional layering can result within the bands themselves. Very high temperatures during regional metamorphism can sometimes destroy the gneissic layering and produce a massive-looking metamorphic rock instead.
10. Aligned Crystals
In many metamorphic rocks, platy or elongated minerals like micas become aligned in a consistent orientation. This is especially common in schists where mica grains align perpendicular to pressure sources. The aligned crystals result in properties like schistosity and gneissic banding. This crystal alignment is evidence of solid-state deformation that occurred as the rocks underwent compression and shearing during metamorphism.
So, key characteristics that can be used to identify metamorphic rocks include foliation, recrystallization, lack of fossils/sedimentary structures, mineral changes, formation of new minerals, schistosity, banding, layering, and aligned crystals. The exact features observed depend on the starting rocks and the specific pressures, temperatures, fluids and stresses present during metamorphism. Together, these characteristic textures and mineralogies provide evidence for the dynamic metamorphic processes that reshape existing rocks into new types.
