Silicon dioxide known as silica, silicic acid or silicic acid anhydride is an oxide of silicon with the chemical formula SiO2, most found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, aerogels, it is used in structural materials, as components in the food and pharmaceutical industries. Inhaling finely divided crystalline silica is toxic and can lead to severe inflammation of the lung tissue, bronchitis, lung cancer, systemic autoimmune diseases, such as lupus and rheumatoid arthritis. Uptake of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal. In the majority of silicates, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom.
The most common example is seen in the quartz polymorphs. It is a 3 dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms. For example, in the unit cell of α-quartz, the central tetrahedron shares all four of its corner O atoms, the two face-centered tetrahedra share two of their corner O atoms, the four edge-centered tetrahedra share just one of their O atoms with other SiO4 tetrahedra; this leaves a net average of 12 out of 24 total vertices for that portion of the seven SiO4 tetrahedra that are considered to be a part of the unit cell for silica. SiO2 has a number of distinct crystalline forms in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon–oxygen bond lengths vary between the different crystal forms; the Si-O-Si angle varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite.
In α-quartz, the Si-O-Si angle is 144°. Fibrous silica has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra. Stishovite, the higher-pressure form, in contrast, has a rutile-like structure where silicon is 6-coordinate; the density of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm3. The difference in density can be ascribed to the increase in coordination as the six shortest Si-O bond lengths in stishovite are greater than the Si-O bond length in α-quartz; the change in the coordination increases the ionicity of the Si-O bond. More any deviations from these standard parameters constitute microstructural differences or variations, which represent an approach to an amorphous, vitreous, or glassy solid; the only stable form under normal conditions is alpha quartz, in which crystalline silicon dioxide is encountered. In nature, impurities in crystalline α-quartz can give rise to colors; the high-temperature minerals and tridymite, have both lower densities and indices of refraction than quartz.
Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high-temperature minerals. As is common with many substances, the higher the temperature, the farther apart the atoms are, due to the increased vibration energy; the transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can induce fracturing of ceramics or rocks passing through this temperature limit; the high-pressure minerals, seifertite and coesite, have higher densities and indices of refraction than quartz. This is due to the intense compression of the atoms occurring during their formation, resulting in more condensed structure. Faujasite silica is another form of crystalline silica, it is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and surface area.
Faujasite-silica has high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or crystallinity after boiling in concentrated hydrochloric acid. Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: negative temperature expansion, density maximum at temperatures ~5000 °C, a heat capacity minimum, its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C. Molecular SiO2 with a linear structure is produced when molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge. Dimeric silicon dioxide, 2 has been prepared by reacting O2 with matrix isolated dimeric silicon monoxide. In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94° and bond length of 164.6 pm and the terminal Si-O bond length is 150.2 pm. The Si-O bond length is 148.3 pm.
The bond energy is estimated at 621.7 kJ/mol. Silica with the chemical formula SiO2 is most found in nature as quartz, which comprises more than 10% by mass of the earth's crust. Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure form
Ilmenite known as manaccanite, is a titanium-iron oxide mineral with the idealized formula FeTiO3. It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important ore of titanium. Ilmenite is the main source of titanium dioxide, used in paints, printing inks, plastics, sunscreen and cosmetics. Ilmenite crystallizes in the trigonal system; the ilmenite crystal structure consists of an ordered derivative of the corundum structure. Containing high spin ferrous centers, ilmenite is paramagnetic. Ilmenite is recognized in altered igneous rocks by the presence of a white alteration product, the pseudo-mineral leucoxene. Ilmenites are rimmed with leucoxene, which allows ilmenite to be distinguished from magnetite and other iron-titanium oxides; 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 and a brown-pink tinge. Samples of ilmenite exhibit a weak response to a hand magnet.
In 1791 William Gregor discovered ilmenite, in a stream that runs through the valley just south of the village of Manaccan, identified for the first time Titanium as one of the constituents of ilmenite. Ilmenite most contains appreciable quantities of magnesium and manganese and the full chemical formula can be expressed as O3. Ilmenite forms a solid solution with geikielite and pyrophanite which are magnesian and manganiferous end-members of the solid solution series. Although there appears evidence of the complete range of mineral chemistries in the O3 system occurring on Earth, the vast bulk of ilmenites are restricted to close to the ideal FeTiO3 composition, with minor mole percentages of Mn and Mg. A key exception is in the ilmenites of kimberlites where the mineral contains major amounts of geikielite molecules, in some differentiated felsic rocks ilmenites may contain significant amounts of pyrophanite molecules. At higher temperatures it has been demonstrated there is a complete solid solution between ilmenite and hematite.
There is a miscibility gap at lower temperatures, resulting in a coexistence of these 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 homogeneously 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 indicative of ilmenite in the unaltered rock. Ilmenite is a common accessory mineral found in igneous rocks, it is found in large concentrations in layered intrusions where it forms as part of a cumulate layer within the silicate stratigraphy of the intrusion. Ilmenite occurs within the pyroxenitic 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 intergrown magnetite and ilmenite, formed by the oxidation of ulvospinel. Ilmenite occurs as discrete grains with some hematite in solid solution, complete solid solution exists between the two minerals at temperatures above about 950 °C. Titanium was identified for the first time by William Gregor in 1791 in ilmenite from the Manaccan valley in Cornwall, southwest England. Ilmenite is named after the locality of its discovery near Miass, Russia. Most ilmenite is mined for titanium dioxide production. In 2011, about 47% of the titanium dioxide produced worldwide were based on this material. Ilmenite and/or titanium dioxide are used in the production of Titanium metal. Titanium dioxide is most used as a white pigment and the major consuming industries for TiO2 pigments are paints and surface coatings and paper and paperboard. Per capita consumption of TiO2 in China is about 1.1 kilograms per year, compared with 2.7 kilograms for Western Europe and the United States.
Ilmenite can be converted into pigment grade titanium dioxide via either the sulfate process or the chloride process. Ilmenite can be improved and purified to Rutile using the Becher process. Ilmenite ores can be converted to liquid iron and a titanium rich slag using a smelting process. Ilmenite ore is used as a flux by steelmakers to line blast furnace hearth refractory. Ilmenite sand is used as a sandblasting agent in the cleaning of diecasting dies. Ilmenite can be used to produce ferrotitanium via an aluminothermic reduction. Australia was the world's largest ilmenite ore producer in 2011, with about 1.3 million tonnes of production, followed by South Africa, Mozambique, China, Ukraine, Norway and United States. Although most ilmenite is recovered from heavy mineral sands ore deposits, ilmenite can be recovered from layered intrusive sources or "hard rock" titanium ore sources; the top four ilmenite and rutile feedstock producers in 2010 were Rio Tinto Group, Iluka Resources and Kenmare Resources, which collectively accounted for more than 60% of world's supplies.
The world's two largest open cast ilmenite mines are: The Tellnes mine located in Sokndal and run by Titania AS with 0.55 Mtpa capacity and
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is applied to what are metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks. Volcanic rocks are among the most common rock types on Earth's surface in the oceans. On land, they are common at plate boundaries and in flood basalt provinces, it has been estimated. Lava Tephra Volcanic bomb Lapilli Volcanic ash Volcanic rocks are fine-grained or aphanitic to glass in texture, they contain clasts of other rocks and phenocrysts.
Phenocrysts are crystals that are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a fine grained matrix. Volcanic rocks have a vesicular texture caused by voids left by volatiles trapped in the molten lava. Pumice is a vesicular rock produced in explosive volcanic eruptions. Most modern petrologists classify igneous rocks, including volcanic rocks, by their chemistry when dealing with their origin; the fact that different mineralogies and textures may be developed from the same initial magmas has led petrologists to rely on chemistry to look at a volcanic rock's origin. The chemistry of volcanic rocks is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most volcanic rocks tends to increase the silica content by crystal fractionation; the initial composition of most volcanic rocks is basaltic, albeit small differences in initial compositions may result in multiple differentiation series.
The most common of these series are tholeiitic, calc-alkaline, alkaline. Most volcanic rocks share a number of common minerals. Differentiation of volcanic rocks tends to increase the silica content by fractional crystallization. Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as phyllo and tectosilicates including the feldspars, quartz polymorphs and muscovite. While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as olivine and the pyroxenes. Bowen's reaction series predicts the order of formation of the most common minerals in volcanic rocks. A magma may pick up crystals that crystallized from another magma. Diamonds found in kimberlites are well-known xenocrysts. Volcanic rocks are named according to both texture. Basalt is a common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro.
Intermediate volcanic rocks include andesite, dacite and latite. Pyroclastic rocks are the product of explosive volcanism, they are felsic. Pyroclastic rocks are the result of volcanic debris, such as ash and tephra, other volcanic ejecta. Examples of pyroclastic rocks are ignimbrite. Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks, are considered to be volcanic, shading into subvolcanic; the terms lava stone and lava rock are more used by marketers than geologists, who would say "volcanic rock". "Lava stone" may describe anything from a friable silicic pumice to solid mafic flow basalt, is sometimes used to describe rocks that were never lava, but look as if they were. To convey anything about the physical or chemical properties of the rock, a more specific term should be used; the sub-family of rocks that form from volcanic lava are called igneous volcanic rocks. The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition.
If a rhyolite lava-stream cools it can freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy appearing pumice. Allowed to cool it forms a light-colored, uniformly solid rock called rhyolite; the lavas, having cooled in contact with the air or water, are finely crystalline or have at least fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow, still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, the steam and other gases, which they contained in great quantity were free to escape; as crystal
A phenocryst is an early forming large and conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, the adjective porphyritic is used to describe them. Phenocrysts have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization; the term phenocryst is not used unless the crystals are directly observable, sometimes stated as greater than.5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Large phenocrysts are termed megaphenocrysts; some rocks contain both megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts. Phenocrysts are more found in the lighter igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics; the largest crystals found in some pegmatites are phenocrysts being larger than the other minerals.
Rocks can be classified according to the nature and abundance of phenocrysts, the presence or absence of phenocrysts is noted when a rock name is determined. Aphyric is a term used to describe rocks that have no phenocrysts, or more where the rock consists of less than 1% phenocrysts. Porphyritic rocks are named using mineral name modifiers in decreasing order of abundance, thus when olivine forms the primary phenocrysts in a basalt, the name may be refined from basalt to porphyritic olivine basalt or olivine phyric basalt. A basalt with olivine as the dominate phenocrysts, but with lesser amounts of plagioclase phenocrysts, might be termed a olivine-plagioclase phyric basalt. In more complex nomenclature, a basalt with 1% plagioclase phenocrysts, but 4% olivine microphenocrysts, might be termed an aphyric to sparsely plagioclase-olivine phyric basalt, where plagioclase is listed before the olivine, because of its larger crystals. Categorizing a rock as aphyric or as sparsely phyric is a question of whether a significant number of crystals exceed the minimum size.
Geologists use phenocrysts to help determine rock origins and transformations, as when and whether crystals form depends on pressure and on temperature. Fumiko Shido first applied this technique to oceanic basalts, further development came from Tsugio Shibata, from W. B. Bryan. Plagioclase phenocrysts exhibit zoning with a more calcic core surrounded by progressively more sodic rinds; this zoning reflects the change in magma composition. In rapakivi granites, phenocrysts of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase. In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix; these volcanic phenocrysts show flow banding, a parallel arrangement of lath-shaped crystals. These characteristics provide clues to the rocks' origins. Intragranular microfractures and any intergrowth among crystals provide additional clues. Best, Myron. Igneous and Metamorphic Petrology. Oxford, England: Blackwell Publishing.
ISBN 978-1-4051-0588-0. Williams, Howel. Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. ISBN 978-0-7167-0206-1; the Integrated Ocean Drilling Program. Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports
Porphyritic is an adjective used in geology for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals larger than another group. Porphyritic rocks may be aphanites or extrusive rock, with large crystals or phenocrysts floating in a fine-grained groundmass of non-visible crystals, as in a porphyritic basalt, or phanerites or intrusive rock, with individual crystals of the groundmass distinguished with the eye, but one group of crystals much bigger than the rest, as in a porphyritic granite. Most types of igneous rocks may display some degree of porphyritic texture. One main type of rock that has a porphyritic texture are porphyry, though not all porphyritic rocks are porphyries. Porphyritic rocks are formed. In the first stage, the magma is cooled deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled at shallow depth or as it erupts from a volcano, creating small grains that are invisible to the unaided eye referred to as the ground mass
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate, its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y. Zircon forms in silicate melts with large proportions of high field strength incompatible elements. For example, hafnium is always present in quantities ranging from 1 to 4%; the crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, brown and green. Colorless specimens that show gem quality are a popular substitute for diamond and are known as "Matura diamond"; the name derives from the Persian zargun, meaning "gold-hued". This word is corrupted into "jargoon", a term applied to light-colored zircons; the English word "zircon" is derived from Zirkon, the German adaptation of this word. Yellow and red zircon is known as "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.
Zircon is ubiquitous in the crust of Earth. It occurs as a common accessory mineral in igneous rocks, in metamorphic rocks and as detrital grains in sedimentary rocks. Large zircon crystals are rare, their average size in granite rocks is about 0.1–0.3 mm, but they can grow to sizes of several centimeters in mafic pegmatites and carbonatites. Zircon is very resistant to heat and corrosion; because of their uranium and thorium content, some zircons undergo metamictization. Connected to internal radiation damage, these processes disrupt the crystal structure and explain the variable properties of zircon; as zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, the color changes. Zircon occurs in many colors, including reddish brown, green, blue and colorless; the color of zircons can sometimes be changed by heat treatment. Common brown zircons can be transformed into colorless and blue zircons by heating to 800 to 1000 °C. In geological settings, the development of pink and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers.
Color in this red or pink series is annealed in geological conditions above temperatures of around 400 °C. Zircon is consumed as an opacifier, has been known to be used in the decorative ceramics industry, it is the principal precursor not only to metallic zirconium, although this application is small, but to all compounds of zirconium including zirconium dioxide, one of the most refractory materials known. Other applications include use in refractories and foundry casting and a growing array of specialty applications as zirconia and zirconium chemicals, including in nuclear fuel rods, catalytic fuel converters and in water and air purification systems. Zircon is one of the key minerals used by geologists for geochronology. Zircon is a part of the ZTR index to classify highly-weathered sediments. Zircon is a common accessory to trace mineral constituent of most felsic igneous rocks. Due to its hardness and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands.
Zircon is rare within mafic rocks and rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites and lamprophyre, where zircon can be found as a trace mineral owing to the unusual magma genesis of these rocks. Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, within some rare alkaline volcanic rocks, for example the Toongi Trachyte, New South Wales Australia in association with the zirconium-hafnium minerals eudialyte and armstrongite. Australia leads the world in zircon mining, producing 37% of the world total and accounting for 40% of world EDR for the mineral. South Africa is Africa’s main producer, with 30% of world production, second after Australia. Zircon has played an important role during the evolution of radiometric dating. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques; because zircons can survive geologic processes like erosion, transport high-grade metamorphism, they contain a rich and varied record of geological processes.
Zircons are dated by uranium-lead, fission-track, cathodoluminescence, U+Th/He techniques. For instance, imaging the cathodoluminescence emission from fast electrons can be used as a prescreening tool for high-resolution secondary-ion-mass spectrometry to image the zonation pattern and identify regions of interest for isotope analysis; this is done scanning electron microscope. Zircons in sedimentary rock can identify the sediment source. Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth. In addition, the oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was water on the surface of the Earth. This interpretation is supported by additional trace element data, but is the subject of debate. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in the Jack Hills of Western Australia.
According to one of the researchers, "If life arose quickly on Earth... it could be common in the universe
Continental crust is the layer of igneous and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima, richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, more mafic in character; the continental crust consists of various layers, with a bulk composition, intermediate. The average density of continental crust is about 2.83 g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3. Continental crust is less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is thicker than oceanic crust, which has an average thickness of around 7–10 km.
About 40% of Earth's surface is occupied by continental crust. It makes up about 70% of the volume of Earth's crust; because the surface of continental crust lies above sea level, its existence allowed land life to evolve from marine life. Its existence provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion. All continental crust is derived from mantle-derived melts through fractional differentiation of basaltic melt and the assimilation of pre-existing continental crust; the relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur at magmatic arcs associated with subduction. There is little evidence of continental crust prior to 3.5 Ga. About 20% of the continental crust's current volume was formed by 3.0 Ga. There was rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga.
During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. In contrast to the persistence of continental crust, the size and number of continents are changing through geologic time. Different tracts rift apart and recoalesce as part of a grand supercontinent cycle. There are about 7 billion cubic kilometers of continental crust, but this quantity varies because of the nature of the forces involved; the relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is subducted back into the mantle. Continental crust is subducted. For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in recycled oceanic crust. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history.
The height of mountain ranges is related to the thickness of crust. This results from the isostasy associated with orogeny; the crust is thickened by the compressive forces related to continental collision. The buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion; this forms a keel or mountain root beneath the mountain range, where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and severed, replaced by oceanic crust; the edges of continental fragments formed. The high temperatures and pressures at depth combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic - the main exception to this being recent igneous intrusions. Igneous rock may be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer beneath it. Continental crust is produced and destroyed by plate tectonic processes at convergent plate boundaries.
Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Material can be accreted horizontally when volcanic island arcs, seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is lost through erosion and sediment subduction, tectonic erosion of forearcs and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction, it is a mat