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, causing profound physical and/or chemical change, the protolith may be a sedimentary, an igneous, or even an existing type of metamorphic rock. Metamorphic rocks make up a part of the Earths crust. They are classified by texture and by chemical and mineral assemblage and they may be formed simply by being deep beneath the Earths surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure and they are formed when rock is heated up by the intrusion of hot molten rock called magma from the Earths interior. The study of rocks provides information about the temperatures and pressures that occur at great depths within the Earths crust. Some examples of rocks are gneiss, marble, schist.
Metamorphic minerals are those that only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, staurolite and some garnet. Other minerals, such as olivines, amphiboles, micas and quartz, may be found in metamorphic rocks and these minerals formed during the crystallization of igneous rocks. They are stable at temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are only within certain limits. The change in the size of the rock during the process of metamorphism is called recrystallization. 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. The layering within metamorphic rocks is called foliation, 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 and 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 metamorphic rock, originating from shale
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. Thus it is a type of nesosilicate or orthosilicate and it is a common mineral in the Earths subsurface but weathers quickly on the surface. The ratio of magnesium and iron varies between the two endmembers of the solid solution series and fayalite, compositions of olivine are commonly expressed as molar percentages of forsterite and fayalite. Forsterite has a high melting temperature at atmospheric pressure, almost 1,900 °C. The melting temperature varies smoothly between the two endmembers, as do other properties, olivine incorporates only minor amounts of elements other than oxygen, silicon and iron. Manganese and nickel commonly are the elements present in highest concentrations. Olivine gives its name to the group of minerals with a structure which includes tephroite and kirschsteinite. It has a structure similar to magnetite but uses one quadravalent. Olivine gemstones are called peridot and chrysolite, olivine is named for its typically olive-green color, though it may alter to a reddish color from the oxidation of iron.
Translucent olivine is sometimes used as a gemstone called peridot, some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad island in the Red Sea. Olivine occurs in mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica and that magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite can be left after extraction of magmas. Olivine and high pressure structural variants constitute over 50% of the Earths upper mantle, the metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content produces Mg-rich olivine, or forsterite. In contrast, Mg-rich olivine does not occur stably with silica minerals, Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km within Earth. Mg-rich olivine has discovered in meteorites, on the Moon and Mars, falling into infant stars.
Such meteorites include chondrites, collections of debris from the early Solar System, the spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets often have the signature of olivine
It is bordered by the Atlantic Ocean to the east, the states of Connecticut and Rhode Island to the south, New Hampshire and Vermont to the north, and New York to the west. The state is named for the Massachusett tribe, which inhabited the area. The capital of Massachusetts and the most populous city in New England is Boston, over 80% of Massachusetts population lives in the Greater Boston metropolitan area, a region influential upon American history and industry. Originally dependent on agriculture and trade, Massachusetts was transformed into a manufacturing center during the Industrial Revolution, during the 20th century, Massachusetts economy shifted from manufacturing to services. Modern Massachusetts is a leader in biotechnology, higher education, finance. Plymouth was the site of the first colony in New England, founded in 1620 by the Pilgrims, in 1692, the town of Salem and surrounding areas experienced one of Americas most infamous cases of mass hysteria, the Salem witch trials. In 1777, General Henry Knox founded the Springfield Armory, which during the Industrial Revolution catalyzed numerous important technological advances, in 1786, Shays Rebellion, a populist revolt led by disaffected American Revolutionary War veterans, influenced the United States Constitutional Convention.
In the 18th century, the Protestant First Great Awakening, which swept the Atlantic World, in the late 18th century, Boston became known as the Cradle of Liberty for the agitation there that led to the American Revolution. The entire Commonwealth of Massachusetts has played a commercial and cultural role in the history of the United States. Before the American Civil War, Massachusetts was a center for the abolitionist, temperance, in the late 19th century, the sports of basketball and volleyball were invented in the western Massachusetts cities of Springfield and Holyoke, respectively. Many prominent American political dynasties have hailed from the state, including the Adams, both Harvard University and the Massachusetts Institute of Technology, in Cambridge, have been ranked among the most highly regarded academic institutions in the world. Massachusetts public school students place among the top nations in the world in academic performance, the official name of the state is the Commonwealth of Massachusetts.
While this designation is part of the official name, it has no practical implications. Massachusetts has the position and powers within the United States as other states. Massachusetts was originally inhabited by tribes of the Algonquian language family such as the Wampanoag, Nipmuc, Pocomtuc and Massachusett. While cultivation of crops like squash and corn supplemented their diets, villages consisted of lodges called wigwams as well as longhouses, and tribes were led by male or female elders known as sachems. Between 1617 and 1619, smallpox killed approximately 90% of the Massachusetts Bay Native Americans, the first English settlers in Massachusetts, the Pilgrims, arrived via the Mayflower at Plymouth in 1620, and developed friendly relations with the native Wampanoag people. This was the second successful permanent English colony in the part of North America that became the United States, the event known as the First Thanksgiving was celebrated by the Pilgrims after their first harvest in the New World which lasted for three days
Ultramafic are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earths mantle is composed of ultramafic rocks, ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks. Intrusive ultramafic rocks are found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent the chemistry of the magma from which they crystallized, the ultramafic intrusives include the dunites and pyroxenites. Other rare varieties include troctolite which has a percentage of calcic plagioclase. Gabbro and norite often occur in the portions of the layered ultramafic sequences. Hornblendite and, rarely phlogopite, are found, subvolcanic ultramafic rocks and dykes persist longer, but are rare. Many of the lavas being produced on Io may be ultramafic, mercury appears to have ultramafic volcanic rock.
Examples include komatiite and picritic basalt, komatiites can be host to ore deposits of nickel. Ultrapotassic, ultramafic rocks such as lamprophyre and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved, most of these rocks occur as dikes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lampproite occurrences occur as volcanic and subvolcanic diatremes and maars, vents of Proterozoic lamproite, and Cenozoic lamproite are known, as are vents of Devonian lamprophyre. Kimberlite pipes in Canada and South Africa have incompletely preserved tephra and these are generally diatreme events and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process to typical ultramafic rocks, metamorphism of ultramafic rocks in the presence of water and/or carbon dioxide results in two main classes of metamorphic ultramafic rock, talc carbonate and serpentinite.
When such metamorphic fluids have less than 10% molar proportion of CO2, reactions favor serpentinisation, the majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. Ultramafic magmas in the Phanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic, many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones. Serpentine soil is a rich, calcium and phosphorus poor soil that develops on the regolith derived from ultramafic rocks. Ultramafic rocks contain elevated amounts of chromium and nickel which may be toxic to plants, as a result, a distinctive type of vegetation develops on these soils
Quartz is the second most abundant mineral in Earths continental crust, behind feldspar. There are many different varieties of quartz, several of which are semi-precious gemstones, since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia. The word quartz is derived from the German word Quarz and its Middle High German ancestor twarc, the Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning icy cold, because some philosophers apparently believed the mineral to be a form of supercooled ice. Today, the rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end, well-formed crystals typically form in a bed that has unconstrained growth into a void, usually the crystals are attached at the other end to a matrix and only one termination pyramid is present.
However, doubly terminated crystals do occur where they develop freely without attachment, a quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward. α-quartz crystallizes in the crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the system, space group P6222 and P6422. These space groups are truly chiral, both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks. The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is an identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, common colored varieties include citrine, rose quartz, smoky quartz, milky quartz, and others.
The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties, the cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, heliotrope, amethyst is a form of quartz that ranges from a bright to dark or dull purple color. The worlds largest deposits of amethysts can be found in Brazil, Uruguay, France, sometimes amethyst and citrine are found growing in the same crystal. It is referred to as ametrine, an amethyst is formed when there is iron in the area where it was formed
Titanite, or sphene, is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present, commonly present are rare earth metals including cerium and yttrium, calcium may be partly replaced by thorium. Sphene was the most commonly used name until the IMA decision, the name sphene continues to be publishable in peer-reviewed scientific literature, e. g. a paper by Hayden et al. was published in early 2008 in the journal Contributions to Mineralogy and Petrology. Sphene persists as the name for titanite gemstones. Titanite, which is named for its content, occurs as translucent to transparent, reddish brown, yellow, green. These crystals are typically sphenoid in habit and are often twinned, possessing a subadamantine tending to slightly resinous luster, titanite has a hardness of 5.5 and a weak cleavage. Its specific gravity varies between 3.52 and 3.54, transparent specimens are noted for their strong trichroism, the three colours presented being dependent on body colour.
Owing to the effect of iron, sphene exhibits no fluorescence under ultraviolet light. Some titanite has been found to be metamict, in consequence of damage due to radioactive decomposition of the often significant thorium content. When viewed in section with a petrographic microscope, pleochroic halos can be observed in minerals surrounding a titanite crystal. Titanite occurs as an accessory mineral in intermediate and felsic igneous rocks. It occurs in rocks such as gneiss and schists. Titanite is a source of titanium dioxide, TiO2, used in pigments, as a gemstone, titanite is usually some shade of chartreuse, but can be brown or black. Hue depends on Fe content, with low Fe content causing green and yellow colours and it is prized for its exceptional dispersive power which exceeds that of diamond. Jewelry use of titanite is limited, both because the stone is uncommon in gem quality and is relatively soft, titanite can be used as a U-Pb geochronometer, specifically in metamorphic terranes
Lava is the molten rock expelled by a volcano during an eruption. The resulting rock after solidification and cooling is called lava. The molten rock is formed in the interior of planets, including Earth. The source of the heat melts the rock within the earth is geothermal energy. When first erupted from a vent, lava is a liquid usually at temperatures from 700 to 1,200 °C. A lava flow is an outpouring of lava, which is created during a non-explosive effusive eruption. When it has stopped moving, lava solidifies to form igneous rock, the term lava flow is commonly shortened to lava. Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic, explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows. The word lava comes from Italian, and is derived from the Latin word labes which means a fall or slide. The first use in connection with extruded magma was apparently in an account written by Francesco Serao on the eruption of Vesuvius between May 14 and June 4,1737.
Serao described a flow of lava as an analogy to the flow of water. The composition of almost all lava of the Earths crust is dominated by silicate minerals, mostly feldspars, pyroxenes, micas, igneous rocks, which form lava flows when erupted, can be classified into three chemical types, felsic and mafic. These classes are primarily chemical, the chemistry of lava tends to correlate with the temperature, its viscosity. Felsic or silicic lavas such as rhyolite and dacite typically form lava spines, most silicic lava flows are extremely viscous, and typically fragment as they extrude, producing blocky autobreccias. Felsic magmas can erupt at temperatures as low as 650 to 750 °C, unusually hot rhyolite lavas, may flow for distances of many tens of kilometres, such as in the Snake River Plain of the northwestern United States. Intermediate or andesitic lavas are lower in aluminium and silica, and usually somewhat richer in magnesium, intermediate lavas form andesite domes and block lavas, and may occur on steep composite volcanoes, such as in the Andes.
Poorer in aluminium and silica than felsic lavas, and commonly hotter, greater temperatures tend to destroy polymerized bonds within the magma, promoting more fluid behaviour and a greater tendency to form phenocrysts. Higher iron and magnesium tends to manifest as a darker groundmass, mafic or basaltic lavas are typified by their high ferromagnesian content, and generally erupt at temperatures in excess of 950 °C
Aragonite is a carbonate mineral, one of the two most common, naturally occurring, crystal forms of calcium carbonate, CaCO3. It is formed by biological and physical processes, including precipitation from marine, aragonites crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal. Repeated twinning results in pseudo-hexagonal forms, Aragonite may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri from their association with the ores at the Carinthian iron mines. The type location for aragonite is Molina de Aragón,25 km from Aragon for which it was named in 1797, an aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the US, aragonite in the form of stalactites and cave flowers is known from Carlsbad Caverns, massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas. Aragonite is the high pressure polymorph of calcium carbonate, as such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones.
Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm-, because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the shell is aragonite, in others. The nacreous layer of the fossil shells of some extinct ammonites forms an iridescent material called ammolite. Aragonite forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, Aragonite is not uncommon in serpentinites where high Mg in pore solutions apparently inhibits calcite growth and promotes aragonite precipitation. Aragonite is metastable at the low pressures near the Earths surface and is commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown and it can be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above 60 °C or in water-ethanol mixtures at ambient temperatures.
Aragonite is thermodynamically unstable at temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years. The mineral vaterite, known as μ-CaCO3, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earths surface, in aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the necessary for much sea life and keeps the pH of the water close to its natural level. Aragonite has been tested for the removal of pollutants like zinc, cobalt. Aragonite sea Ikaite, CaCO3·6H2O Monohydrocalcite, CaCO3·H2O Nacre, otherwise known as Mother-of-Pearl Oolitic aragonite sand The Ochtinska aragonite cave Kosovo Caves Aragonite Formations
Magnetite is a mineral and one of the main iron ores. With the chemical formula Fe3O4, it is one of the oxides of iron, Magnetite is ferrimagnetic, it is attracted to a magnet and can be magnetized to become a permanent magnet itself. It is the most magnetic of all the minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, today it is mined as iron ore. Small grains of magnetite occur in almost all igneous and metamorphic rocks, Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and leaves a black streak. The chemical IUPAC name is iron oxide and the chemical name is ferrous-ferric oxide. In addition to rocks, magnetite occurs in sedimentary rocks, including banded iron formations and in lake. Magnetite nanoparticles are thought to form in soils, where they probably oxidize rapidly to maghemite, Magnetite has an inverse spinel crystal structure. As a member of the group, it can form solid solutions with similarly structured minerals, including ulvospinel.
Titanomagnetite, known as titaniferous magnetite, is a solution between magnetite and ulvospinel that crystallizes in many mafic igneous rocks. Titanomagnetite may undergo oxyexsolution during cooling, resulting in ingrowths of magnetite and ilmenite, Magnetite has been important in understanding the conditions under which rocks form. Magnetite reacts with oxygen to produce hematite, and the pair forms a buffer that can control oxygen fugacity. Commonly, igneous rocks contain solid solutions of both titanomagnetite and hemoilmenite or titanohematite, Magnetite is produced from peridotites and dunites by serpentinization. Lodestones were used as a form of magnetic compass. At low temperatures, magnetite undergoes a crystal structure phase transition from a structure to a cubic structure known as the Verwey transition. The Verwey transition occurs around 121 K and is dependent on size, domain state. An isotropic point occurs near the Verwey transition around 130 K, the Curie temperature of magnetite is 858 K.
Magnetite is sometimes found in large quantities in beach sand. Such black sands are found in places, such as Lung Kwu Tan of Hong Kong, California of the United States
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have different mineralogical compositions, it is a classification based on the textural appearance of the rock. Mylonites are ductilely deformed rocks formed by the accumulation of large shear strain, in fault zones. Mechanical abrasion of grains by milling does not occur, although this was thought to be the process that formed mylonites. There are many different mechanisms that accommodate crystal-plastic deformation, in crustal rocks the most important processes are dislocation creep and diffusion creep. Dislocation generation acts to increase the energy of crystals. This process tends to organize dislocations into subgrain boundaries and this process, sometimes referred to as subgrain rotation recrystallization, acts to reduce the mean grain size. Volume and grain-boundary diffusion, the mechanisms in diffusion creep, become important at high temperatures.
Mylonites generally develop in ductile shear zones where high rates of strain are focused and they are the deep crustal counterparts to cataclastic brittle faults that create fault breccias. Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding, ultramylonites usually have undergone extreme grainsize reduction. In structural geology, ultramylonite is a kind of defined by modal percentage of matrix grains more than 90%. Ultramylonite is often hard, cherty to flinty in appearance and sometimes resemble pseudotachylite, in reverse, ultramylonite-like rocks are sometimes deformed pseudotachylyte. Mesomylonites have undergone an appreciable amount of reduction, and are defined by their modal percentage of matrix grains being between 50 and 90%. Protomylonites are mylonites which have experienced limited grainsize reduction, and are defined by their percentage of matrix grains being less than 50%. Because mylonitisation is incomplete in these rocks, relict grains and textures are apparent and they typically have a well-developed secondary shear fabric.
This is referred to as determining the shear sense and it is common practice to assume that the deformation is plane strain simple shear deformation. This type of strain field assumes that deformation occurs in a zone where displacement is parallel to the shear zone boundary. Furthermore, during deformation the incremental strain axis maintains a 45 degree angle to the zone boundary
Ilmenite is the titanium-iron oxide mineral with the idealized formula FeTiO3. It is a weakly magnetic black or steel-gray solid, from the commercial perspective, ilmenite is the most important ore of titanium. Ilmenite is the source of titanium dioxide, which is used in paints, plastics, sunscreen, food. Ilmenite crystallizes in the trigonal system, containing high spin ferrous centers, ilmenite is paramagnetic. Ilmenite is commonly recognized in altered igneous rocks by the presence of an alteration product. Often ilmenites are rimmed with leucoxene, which allows ilmenite to be distinguished from magnetite, the example shown in the image at right is typical of leucoxene-rimmed ilmenite. In reflected light it may be distinguished from magnetite by more pronounced reflection pleochroism, samples of ilmenite exhibit a weak response to a hand magnet. Ilmenite most often contains appreciable quantities of magnesium and manganese and the chemical formula can be expressed as O3. Ilmenite forms a solution with geikielite and pyrophanite which are magnesian.
At higher temperatures it has been demonstrated there is a solid solution between ilmenite and hematite. There is a miscibility gap at lower temperatures, resulting in a coexistence of two minerals in rocks but no solid solution. This coexistence may result in exsolution lamellae in cooled ilmenites with more iron in the system than can be accommodated in the crystal lattice. Altered ilmenite forms the mineral leucoxene, an important source of titanium in heavy mineral sands ore deposits, leucoxene is a typical component of altered gabbro and diorite and is generally indicative of ilmenite in the unaltered rock. Ilmenite is an accessory mineral found in metamorphic and igneous rocks. It is found in large concentrations in layered intrusions where it forms as part of a layer within the silicate stratigraphy of the intrusion. Ilmenite generally occurs within the portion of such intrusions. Magnesian ilmenite is indicative of kimberlitic paragenesis and forms part of the MARID association of minerals assemblage of glimmerite xenoliths, manganiferous ilmenite is found in granitic rocks and in carbonatite intrusions where it may contain anomalous niobium.
Many mafic igneous rocks contain grains of magnetite and ilmenite
Basalt is a common extrusive igneous rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. Flood basalt describes the formation in a series of basalt flows. By definition, basalt is an igneous rock with generally 45-55% silica and less than 10% feldspathoid by volume. Basalt commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains, the average density is 3.0 gm/cm3. Basalt is defined by its content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic minerals into hematite, although usually characterized as dark, basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured and these phenocrysts usually are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt.
Basalt with a texture is called vesicular basalt, when the bulk of the rock is mostly solid. Gabbro is often marketed commercially as black granite and these ultramafic volcanic rocks, with silica contents below 45% are usually classified as komatiites. Agricola applied basalt to the black rock of the Schloßberg at Stolpen. Tholeiitic basalt is relatively rich in silica and poor in sodium, included in this category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau. Basalt rocks are in some cases classified after their content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka traps and it has greater than 17% alumina and is intermediate in composition between tholeiite and alkali basalt, the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase. Alkali basalt is relatively poor in silica and rich in sodium and it is silica-undersaturated and may contain feldspathoids, alkali feldspar and phlogopite.
Boninite is a form of basalt that is erupted generally in back-arc basins. Ocean island basalt Lunar basalt On Earth, most basalt magmas have formed by melting of the mantle. Basalt commonly erupts on Io, the third largest moon of Jupiter, and has formed on the Moon, Venus. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below, the mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene