Planet Earth/6g. Common Rock Identification

What are Rocks?
All rocks can be classified into one of the three major groups, igneous (for rocks that cooled from molten materials), metamorphic (for rocks that were subjected to intense heat and pressure, but did not melt), and sedimentary (for rocks that form from Earth’s surface processes, such as transported grains, organic material and evaporation). All rocks can be grouped within one of these three types. Rock names on the other hand, are classifications that organize the many different varieties of rocks found in each of these three major types. There are many different ways that geologists name rocks, but it comes down to two things; the mineralogy and texture of the rock. In other words, the percentage of each mineral present in the rock (the mineralogy), and the nature of the crystal or grain sizes and shapes, or texture of the rock. There are many ways in which geologists name rocks, and several competing ideas and active research. The basic rock names are listed below for each type of rock.

Igneous Rocks
All igneous rocks are divided into two major divisions based on the minerals they contain. Felsic igneous rocks are composed mostly of feldspars (plagioclase/orthoclase) and quartz, while mafic igneous rocks are composed of mostly pyroxene, amphibole and olivine. Some rocks, particularly enriched in olivine, are called ultramafic, and have very little silica (quartz). Felsic minerals tend to be white, pink and clear (transparent) in color, and hence these rocks will be a lighter overall color, while mafic minerals tend to be dark black, dark red, or dark green colors, and these mafic rocks will be a darker color, often black. All igneous rocks are formed by crystals that form as the molten material cools. If the rock cooled very slowly (deep in the Earth or plutonic), the rock is called intrusive, while if the rock cooled very quickly (such as from lava on the surface of the Earth), the rock is called extrusive. The length of time the molten material cooled results in different sizes of the crystals in the rock. Intrusive rocks have large crystals that formed slowly, while extrusive rocks will have very tiny crystals. The size of the crystals in igneous rocks is called texture. We can generally divide all igneous rocks based on these two characteristics, whether they are mostly composed of felsic minerals or mafic minerals, and the texture of the rock.

Granite (Felsic Intrusive Igneous Rock)
Granite is the name of igneous rocks that are both felsic and intrusive, which form from cooled molten magma deep inside the Earth. Because these rocks form from the slow cooling of molten material, crystals can be very large. Granite is almost exclusively composed of quartz, feldspars (plagioclase/orthoclase) and mica (muscovite/biotite). When crystals are exceptionally large, geologists called this a pegmatite, or pegmatitic granitic rock. If the individual crystals can be seen by the naked eye, then it is called phanerite, or phaneritic granitic rock. Most granite is phaneritic granitic rock, with visible speckles of individual crystals of felsic minerals. Granite is typically found in the core of mountains and exclusively in continental, silica-rich crust. Granite is a useful building material, although its hardness makes it more difficult to carve and sculpt compared to other softer types of rocks.

Diorite (Intermediate Intrusive Igneous Rock)
Diorite is rock that contains slightly more mafic minerals than felsic granite, with an increase in darker colored minerals include biotite, pyroxene and amphibole. These minerals give the rock a black and white speckled appearance, when compared to the light-colored granite. Sometimes geologists will refer to granodiorite, a rock that is between a diorite and granite in mineral composition.

Gabbro (Mafic Intrusive Igneous Rock)
Gabbro is a rock that contains mostly mafic minerals including pyroxene, amphibole, and olivine, with less quartz and feldspar. Gabbro is black in color, but as an intrusive plutonic rock contains large crystals of these minerals. Gabbro is a common rock found in subduction zones and deep in oceanic crust. In continents, gabbro is often found in ophiolites, which are regions of oceanic crust that has been accreted or emplaced into continental crust. If the rock is composed of more olivine, and exhibits a dark green color, with little silica (quartz) the rock is considered ultramafic. An ultramafic rock name used for olivine-rich rocks is peridotite. Peridotite is a common rock found in the deeper mantle of the Earth, and exhibits more olivine and less quartz than gabbro, although both are considered mafic igneous rocks. Many of the lunar rocks brought back to Earth through the Apollo missions are classified as gabbro, with high amounts of olivine and other mafic minerals.

Rhyolite (Felsic Extrusive Igneous Rock)
Rhyolite is the name of igneous rock that is composed of felsic minerals (quartz and feldspars (orthoclase/plagioclase)), but also contains tiny crystals (only seen through magnification). These tiny uniform crystals that form the matrix of the rock is called aphanitic texture. Rhyolite tends to be a white to pink color, with a similar uniform color and texture with the presence of these tiny aphanitic crystals that compose the entirety of the rock. Extrusive rocks form from rapid cooling, usually associated with abrupt volcanic eruptions. Rhyolite is a common rock found in volcanic regions emerging from silica-rich continental crust, like that found around Mount Saint Helens, and other subduction zones. Rhyolite and other extrusive rocks can also be described as porphyritic. A porphyritic rock is an igneous rock that underwent two stages of cooling, an abrupt cooling event that lead to tiny crystals (aphanitic texture), and a slower cooling event that lead to larger crystals (pegmatitic/phaneritic textures). A porphyritic igneous rock is any igneous rock that contains both tiny and large crystals. These larger crystals often freeze at lower temperatures, for example the mineral quartz, but can also be due to intrusive rocks that had cooled slowly, but suddenly were cooled more rapidly during a volcanic eruption, resulting in both tiny and large crystals (porphyritic texture).

Andesite (Intermediate Extrusive Igneous Rock)
Andesite is an extrusive igneous volcanic rock of intermediate composition (between felsic and mafic) like diorite, but with aphanitic to porphyritic texture. It tends to be greyish in color, with speckles of small crystals, stemming from rapid cooling of magma. Andesite tends to exhibit large crystals of plagioclase, amphibole, and pyroxene. These large crystals are known as phenocrysts, which are large crystals surrounded by a matrix of smaller crystals in porphyritic rock. Andesite is named for the Andes Mountains in South America, a prominent region of active subduction, but andesite is also common in many volcanically active regions of the Earth, especially on continents.

Basalt (Mafic Extrusive Igneous Rock)
Basalt is one of the most common types of igneous rocks on Earth, formed from the rapid cooling of mafic-rich magma and lava. Basalt is especially common in oceanic crust that forms the seafloor, and found in island volcanic systems, like Hawaii. Basalt is common in continental volcanic regions where deeper mafic minerals are brought up in massive lava flows that can cover large regions. Basalt is molten rock that has cooled very rapidly, resulting in tiny aphanitic crystals of amphibole, pyroxene, and olivine, but can contain smaller amounts of quartz, biotite and feldspars. Basalt is nearly always black, dark green, to dark reddish in color. Since basalt forms from cooling lava flows that contain many gases, basalt often exhibits vesicular texture. Vesicular texture is where bubbles of gas have left behind holes or pores within the rock. These holes are often uniform in their distribution, and are left behind when volcanic gas escapes from the rapidly cooling lava. Sometimes these can be filled in with phenocrysts of quartz. Scoria is a rock name used for reddish vesicular basalt that is popular in landscaping. Not all basalt is vesicular, as much of the basalt found on Earth is aphanitic in texture, with a dark black color. Obsidian is a silica-rich rock formed from extrusive, rapidly cooling molten rock, and is often found in association with basalt. Obsidian is a silica rich rock, forming a glass like texture due to an increase in molten quartz, which can pool and rapidly cool within lava and magma flows, between mafic-rich basalt.

Nearly all igneous rocks can be grouped into one of these six subdivisions; granite, diorite, gabbro, rhyolite, andesite and basalt. Although there is often debate among geologists where these strict divisions should be defined as, especially regarding the proportions of various minerals and texture. These definitions are useful for a quick identification of igneous rocks that you are likely to encounter.

Sedimentary Rocks
All sedimentary rocks form from surface processes on Earth, including 1) cemented grains/sediment transported by wind and water, 2) burial of organic matter produced by living organisms, and 3) evaporation and recrystallization of minerals from a solution. All these processes occur only on the surface of the Earth, and through a process of lithification these materials will turn to stone.

The lithification process of turning buried material (such as sediment) into stone is called diagenesis. Diagenesis is the process that describes physical and chemical changes that sediments undergo due to increasing temperature, pressure, and groundwater flow as they get buried deeply into Earth’s subsurface. Diagenesis is also responsible for the petrification of fossilized bones of extinct animals such as dinosaurs. Sedimentary rocks are the only type of rock that contains fossilized remains of ancient animals and plants, but also the only type of rock that contains hydrocarbon fuels, such as petroleum, natural gas, and coal.

Sedimentary geologists divide most sedimentary rocks into two major groupings, clastic and carbonate sedimentary rocks. Clastic sedimentary rocks are those that are formed from transported grains or clasts of sediment resulting from the erosion of Earth materials by wind and water. Carbonate sedimentary rocks are those that are formed by organic carbonate minerals in the oceans, lakes, and ponds.

Carbonate sedimentary rocks
Calcium carbonate (CaCO3) easily precipitates from ocean and lake water, but is mostly utilized by aquatic lifeforms to forms shells and protective coatings. These sediments of organic calcium carbonate accumulate after burial on the ocean or lake floor, eventually turning into a rock called limestone. Limestone is a general term used to described rock composed of mostly calcium carbonate from organic matter that has been buried in ancient oceans and lakes. Limestone can be further subdivided by two classification schemes, the Folk classification and the Dunham classification, which classify limestone based on their texture. All limestone is composed of calcite or aragonite, the two naturally occurring minerals of calcium carbonate (CaCO3), however with diagenesis, the calcium (Ca) can be replaced with magnesium (Mg) forming dolomite, a harder mineral than either calcite and aragonite. Most aragonite undergoes a process of diagenesis in the subsurface to change into the more stable and compact mineral of calcite, so most naturally occurring limestone is composed of mostly calcite. Limestone easily dissolves when exposed to neutral or slightly acidic rain and ground water, producing karstification which is a terrane formed from this dissolution of the surrounding rock, including sinkholes, caverns and caves. Limestones form important aquafers for the passage of groundwater in Earth’s subsurface, as underground water will often dissolve out these layers of rock. They can also serve as important reservoirs for petroleum underground.

In Utah, along the Wasatch Mountain Range east of Salt Lake City, and within the Cache Valley in northern Utah, limestones are the predominate rock found in the region. These limestones, which today form the high mountains were deposited in a tropical shallow ocean that once covered western Utah 500 to 450 million years ago. The stacked layers of limestone were later trusted upward, forming mountains when the motion of the North American continent shifted westward, and begin to expand.

Clastic Sedimentary Rocks
Clastic sedimentary rocks are rocks composed of transported grains (called clasts), which are lithified together with a mineral glue (called cement). Grains can be either transported by wind (eolian sedimentary deposition), or by water (marine, fluvial, and lacustrine sedimentary deposition). Many rocks that you pick up from the surface of the Earth have been formed by these surface processes of erosion and re-deposition. This is due to Earth’s dynamic rock cycle, drive by water cycle and atmospheric winds. Most other planets in the solar system have rare if any sedimentary rocks on their surfaces. Sedimentary rocks are named based on their mineralogy and texture. Texture is the size and distribution of the individual grains that are glued together to form the sedimentary rock. These grains can be any size, from gigantic boulders to clay size particles, and sedimentary rock names are based on these sizes of the particles that form the rock. These grains can be well sorted (all of similar size) or poorly sorted (of various sizes), they can also be well rounded or angular in shape. Often sedimentary geologists will modify the name of the sedimentary rock based on the mineralogy of the individual grains that compose the rock. Studying sedimentary rocks can tell you how far the individual grains were transported before they were buried. Quartz is highly stable on the surface of the Earth, and will end up being transported the furthest. If a sedimentary rock contains only quartz grains, then that observation indicates that the rock is made of sediment that had travel far before burial. While if a sedimentary rock contains feldspars or lithic fragments of other rocks and minerals, then that observation indicates that the rock is made of sediment that traveled only a short distance before burial. Size and shape also are indictive of the total distance the sediment was transported before burial. Well-sorted and well-rounded sediments are found in sediment transported great distances, while poorly sorted and angular grains are found in sediment transported only a short distance, or by glaciers. You may observe that pebbles found in rivers, or beach sand will be rounded by the action of flowing water and crashing waves, while rocks that are found in alpine landslides will be angular and jumbled. Clastic sedimentary rocks are fascinating because they tell you how sediment was transported across the Earth’s surface throughout its long history, as mountains eroded and basins filled.

Clastic sedimentary rocks are typically glued together by two types of cement minerals, calcite and quartz. Calcite is a softer mineral than quartz, and will result in a weaker over all rock that more easily erodes. A few other minerals can be found in between grains, acting as a glue or cement, including many of the iron oxide minerals (such as hematite), sulfides like pyrite, as well as other minerals that may form in hydrothermal deposits through the passage of extremely hot groundwaters and dissolved minerals. Often these accessory minerals will form what are called concretions, where a strong cement mineral holds together a strangely shaped group of grains, producing unusual weathering patterns and unique colors. Concretions are often mistake for fossil eggs or fossil bones, because of their highly variable shapes and colors. This is caused by the unique way in which the individual grains of sediment are glued together by different minerals in the subsurface, and becomes evident as the rock weathers on the surface.

Claystone
Claystone is simply rock formed from clay size grains or clasts. Clay sized grains are about a thousand times smaller than typical sand grains and smaller than 3.9 μm micrometers. Clay size grains are so small that when you bite the rock with your teeth, you will not feel any grit between your teeth. This has led many geologists to bite pieces of rock they suspect is claystone, to see if they can feel any grit between their teeth (to the amusement of non-geologists). Claystone will have a smooth texture, often chalky and soft as it is frequently composed of clay minerals, that form from the weathering of silicate minerals.

Shale
Shale is sedimentary rocks deposited in low energy systems at the bottom of oceans and lakes, and forming flat tabular layers. It is most often composed of mud to silt sized clasts, which are stacked during deposition into finely laminated layers. Shale is a dominate rock found in ancient oceans and lakes, and often rich in hydrocarbon molecules, giving the rock a black color. These hydrocarbon molecules can serve as a source material for hydrocarbon fuels, such as natural gas and petroleum in the subsurface. Highly enriched hydrocarbon shales are frequently called oil shale, due to the high hydrocarbon content. Shale is often formed from marine deposits that are deeper than the carbonate composition depth, the photic zone or in colder waters, where carbonate sedimentation is more limited.

Mudstone
Mudstone is simply rock formed from mud size grains or clasts. Mud size grains are between 3.9 μm to about 62.5 μm micrometers, and somewhat more gritty texture than clay. Mud is found in soils, flood plains, river banks, and even in small ponds and wetlands. This eroded mud material when buried forms mudstone. Mudstone is often blocky or massive, when compared to shale, which is platy or thinly layered. Mudstone is the dominate rock that forms from sediments in a flood plain near meandering rivers, as well as within soils.

Siltstone
Siltstone is simply rock formed from silt sized grains or clasts. Silt sized grains are about 50 to 63 μm micrometers, which cannot be easily seen by the naked eye, but coarser than mud with more grit and can be felt as a rough texture. It is often found in a slightly higher energy system than mudstone, but in similar depositional environments.

Sandstone
Sandstone is simply rock formed from sand sized grains or clasts, and one of the most common types of clastic sedimentary rocks. Sandstone comes many different colors, but is composed of grains between 63 μm micrometers to 2 millimeters. Sandstone can be subdivided from very fine grained to coarse grained. Most sand size grains can be seen with the naked eye, giving the rock a speckled appearance. One of the challenges of identifying sandstone is that it can look like igneous rhyolite and andesite, which are composed of crystals that have grown together, whereas in sandstone the individual grains have been glued or cemented together. Sandstone forms from the burial and diagenesis of sand, which can be deposited in beach, nearshore coastal, river, lake, and eolian (desert sand dunes) environments. The study of the individual grains or clasts within sandstone can reveal key information about how the sand was transported across the Earth’s surface. Sedimentary structures, like cross beds and trace fossils are also important for the reconstruction of ancient environments. Sandstone is also an important reservoir rock for oil and water, because it tends to have more space between individual grains than in finer grained mudstone and claystone allowing the rock to behave like a sponge, with accommodation space for oil deposits, as well as an important aquifer for ground water. Most sandstone is composed of a predominance quartz sand grains, which are resistive to weathering on Earth’s surface, but other clasts of fragments of other rocks and minerals can be present as well. Sandstones are typically cemented or glued together with calcite and/or silica (quartz) minerals which grow between individual grains, binding the sand together into rock.

Conglomerate
A conglomerate is simply a rock composed of grains or clasts that are greater than about 2 millimeters in width, larger than sand grains and about the size of pebbles, but can include grains and clasts upward to the size of large boulders. Conglomerates form from deposits of river pebbles, landslides and high energy fluvial environments proximal to mountains or topographically steep environments. Conglomerates can be formed from well-rounded pebbles or clasts, but also include highly angular clasts and grains like that found in tillites, which are lithified till from sediments transported by glaciers. Conglomerates are very useful for reconstructing the Earth’s past topography, as these deposits are often adjacent to mountainous terrain which may have eroded away over long spans of time. Overall grain size will decrease in size the farther clasts are transported by water and wind, so large grains sizes tend to be found in closer proximity to their original source. Very angular, jagged or pointy large sized grains are given a unique rock name, they are called breccia.

Other types of sedimentary rocks
Beside the two major groups of carbonate and clastic sedimentary rocks are a few types of rocks that are a result of organic material and evaporation. These rocks include coal, which is composed of carbonized buried plant material, mostly ancient vegetation that has been buried and compacted, with an abundance of hydrocarbon molecules that can be combusted as a fuel source. There are different grades of coal, lignite coal is low grade brownish coal that contains the least amount of hydrocarbons, subbituminous coal is mid-grade coal with slightly more hydrocarbons, bituminous coal is higher grade coal that is shiny black and smooth and frequently mined for combustion. The highest grade of coal is anthracite coal, which is sometimes classified as a metamorphic rock, since it has been subjected to more heat and pressure. It tends to be much harder than bituminous coal. Anthracite is more common in the eastern United States, while in Utah most of the mined coal is the softer bituminous grade coal from Cretaceous rock layers. Evaporite is the name used to describe rocks formed from evaporation of ocean or lake water, leaving behind evaporate minerals like gypsum and halite. These types of rocks are common in ancient lake and ocean basins where large regions were subjected to evaporation and the deposition and burial of these minerals. One unique aspect of evaporite is that it often flows into dome like structures in the subsurface which are called a salt diapir. The upwelling of evaporite layers in the Paradox Basin of southeastern Utah gives the region around Moab some of its unique geological features, including the famous Upheaval Dome in Canyonlands National Park.

All sedimentary rocks are deposited in layers, which is referred to as bedding. These layers are stacked on top of each other over the course of millions of years of deposition. One important characteristic of sedimentary rocks is their regional layer cake appearance, which records a lengthy history of Earth.

Metamorphic Rocks
Metamorphic rocks are the product of intense heat and pressure, which makes them more difficult to name and classify, as these conditions can produce a wide range of unique mineralogy. A skilled geologist can use metamorphic rock mineralogy to not only discern the original rock, but also the amount of heat and pressure the rock was subjected to during burial to produce the observed minerals. Metamorphic rocks are ranked based on the amount of heat and pressure they were subjected to in the ground, from low-grade metamorphic rocks, which exhibit some recrystallization of their grains due to heat and pressure, up to high-grade metamorphic rocks in which minerals may have partially melted, with major recrystallization of the mineralogy of the rock. One of the most important features that identifies almost all metamorphic rocks is foliation. Foliation is a wavy pattern of differing mineral crystals resulting from differential pressure. Foliation is unique in being caused by the crushing force of the intense pressure placed on the rock as it was buried deep in the Earth. These wavy lines can be seen in most metamorphic rocks, and differ from bedding that exhibit straight horizontal lines often observed in sedimentary rocks, like sandstone. Metamorphic rocks tend to sparkle more than sedimentary rocks and igneous rocks, as the grains or crystals are partially melted under heat or recrystallized under pressure, this is true of most high-grade metamorphic rocks, which often exhibit beautiful crystals. Most metamorphic rocks are found in the cores of mountains, or in regional metamorphic belts caused by volcanic or tectonic activity, such as subduction zones, or hot spots. Metamorphic rocks are also some of the oldest rocks found on Earth. Metamorphic rocks have not completely melted into magma or lava as part of the rock cycle, and so they are exceptionally old and common within continental cratons. Cratons are the ancient core or heart of continental crust that initially formed the of today continents, and were never subjected to subduction during plate tectonic motion. These regions are rich in metamorphic rock which are very ancient with rocks that are upwards to 4 billion years old.

Metamorphic rocks are named based on their original source rock, and ranked from low-grade to high-grade. A mudstone or shale will metamorphize into slate, phyllite, schist, then finally gneiss, while limestone will metamorphize into marble. Sandstone will metamorphize into quartzite. Igneous rocks will also metamorphize with burial and pressure, with basalt changing into blueschist or greenschist, peridotite changing into serpentinite, and amphibole rich mafic rocks turning into amphibolite, and pyroxene rich igneous rocks into hornfels. Granite and other felsic rich igneous rocks will metamorphize into gneiss. Often rock names will include accessory minerals in their descriptions, for example a schist might contain garnet crystals, and the rock would be called a garnet schist.

Slate
Historically slate was used as a chalk-board writing surface and still frequently used for roofing material. It is a hard, black metamorphic rock derived from shale which has been subjected to low-grade metamorphism in the subsurface of the Earth. The rock may exhibit fine-grained foliation or be fairly homogeneous, but is harder than shale and breaks in thin sheets.

Phyllite
Phyllite is a slightly higher-grade metamorphic rock in which the minerals chlorite, biotite and muscovite start to crystallize from clay minerals. These new minerals give the rock a silver sparkle, as if covered in glitter. The rock tends to exhibit more foliation than slate, with wavy bands of parallel crystal arrangements of mica. The rock can also contain accessory minerals of garnet and staurolite at higher grade of metamorphism.

Schist
Schist is a medium-grade metamorphic rock which exhibits wavy sheet-like crystals, including crystals of muscovite and biotite, but also chlorite, talc, hornblende, graphite and quartz. Schist forms at higher temperatures with larger crystals than phyllite, but is also is silver in color and with bright sparkly crystals. Quartz becomes more liquid at these higher temperatures and can flow into foliation bands, as light colored or translucent wavy lines. Garnet and staurolite are common in schist. Garnet will form these red to black crystals, which resemble lumps like that found in chocolate chips in a cookie. Schist is often named based on accessory minerals when they occur, such as garnet schist, staurolite schist, or tourmaline schist. Schist is the common rock that is exposed in Central Park in New York City, but can be found in many continental crustal rocks in areas with a history of region metamorphism.

Gneiss
Gneiss is a high-grade metamorphic rock that closely resembles granite. This is due to the intense heat and pressure that results in the partial melting of many of the minerals in the rock, including quartz, feldspars, and muscovite. Gneiss can also form from the metamorphism of granite, which results in foliation, or bands of minerals due to differential stress and pressure subjected to the rock. Gneiss can be identified by the wavy bands of crystalline layers of minerals, often with bands enriched in quartz, while other bands are enriched in other minerals like felspar and muscovite and biotite. Gneiss can also exhibit many large pegmatitic crystals of a wide variety of minerals. Gneiss will lack the uniform texture found in granite, and feature wavy layers of differing mineralogical compositions.

Marble
Marble forms from metamorphism of limestone, as the minerals of calcite and dolomite recrystallize into a denser rock than sedimentary limestone. Because of this increase in density, marble retains its softness, but becomes more uniform in its texture, resulting in a rock that can be easily sculpted and carved. Marble is ideal for statutes and building materials, and has been used since antiquity for construction projects, from ancient Egypt to Greece. Much of the marble used in the United States is mined from Marble, Colorado, which is a rich source of a cool gray-blue color marble (called Yule Marble), that exhibits some foliation and was used in the construction of governmental buildings and monuments in the Washington District of Columbia. Italian marble comes from Tuscany (called Carrara Marble), while Greek marble comes from Attica around Mount Pentelicus, northwest of Athens (called Pentelicus Marble), both of which tend to be a warmer yellow color of marble.

Quartzite
Quartzite is the metamorphic rock of quartz rich sandstone, which results in the individual quartz crystals to fuse together removing any porosity and permeability between the individual grains, resulting in a framework of highly recrystallized quartz. A result of quartz being highly stable on Earth’s surface, quartzite is extremely resistant to any weathering. Often quartzite will form steep jagged outcrops and be commonly found as river pebbles and cobbles which are found in high energy rivers. Since quartzite contains very little other minerals, and quartz exhibits a hardness of 7 on Mohs scale, it frequently remains a common rock found on the surface of Earth due to this hardness, especially in mountainous regions. Quartzite can sometimes be enriched in gold and other rare precious metals, when the metamorphism was accompanied by hydrothermal activity, bringing dissolved metal cations through the rock with extremely hot ground waters resulting in small veins of gold hosted within quartzite rock.

Metamorphic Rocks from Mafic Igneous Material
Just as sedimentary rocks are subjected to metamorphism with intense heat and pressure, so too can igneous rocks also be subject to changes in the subsurface, leading to a diverse group of metamorphic rocks. Collectively these metamorphic rocks stem from original material of basalt, gabbro, and the ultramafic rock of peridotite. These rocks exhibit minerals that are mafic, and hence this group of metamorphic rocks formed from these types of will tend to be green to black in color, and contain many of the same minerals. Collectively geologists might refer to these mafic metamorphic rocks as greenstone, or greenstone belts. These regions form from mafic rich igneous rocks being subjected to heat and pressure, and often are thought of as rocks formed when the separation of felsic rich continental and mafic rich oceanic crust was still undivided early in Earth’s history. These rocks are given a host of different names depending on the amount of pressure and temperature they were subjected to as well as their mineralogical make up, including Greenschist, Blueschist, Amphibolite, Hornfels, Eclogite, Granulite, and Zeolite. Serpentinite is a metamorphic rock that originates from ultramafic rock with a dominate mineralogy of olivine. Greenstone belts are important precious metal ores in Australia, Canada and Africa, they also reveal a history of the early formation of continental crust.

Rock names describe a rocks mineralogy, texture and the processes that formed the rock with natural processes that work in Earth’s interior. Naming rocks in geology requires the ability to identify the common minerals present in the rock, as well as a description of the process of formation of the rock. This can be challenging when you first begin to pick up rocks and try to discern how it may have formed inside Earth. With experience, and being observant, every rock can be named and classified.