Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Human prehistory is the period between the use of the first stone tools c. 3.3 million years ago by hominins and the invention of writing systems. The earliest writing systems appeared c. 5,300 years ago, but it took thousands of years for writing to be adopted, it was not used in some human cultures until the 19th century or until the present. The end of prehistory therefore came at different dates in different places, the term is less used in discussing societies where prehistory ended recently. Sumer in Mesopotamia, the Indus valley civilization, ancient Egypt were the first civilizations to develop their own scripts and to keep historical records. Neighboring civilizations were the first to follow. Most other civilizations reached the end of prehistory during the Iron Age; the three-age system of division of prehistory into the Stone Age, followed by the Bronze Age and Iron Age, remains in use for much of Eurasia and North Africa, but is not used in those parts of the world where the working of hard metals arrived abruptly with contact with Eurasian cultures, such as the Americas, Oceania and much of Sub-Saharan Africa.
These areas with some exceptions in Pre-Columbian civilizations in the Americas, did not develop complex writing systems before the arrival of Eurasians, their prehistory reaches into recent periods. The period when a culture is written about by others, but has not developed its own writing is known as the protohistory of the culture. By definition, there are no written records from human prehistory, so dating of prehistoric materials is crucial. Clear techniques for dating were not well-developed until the 19th century; this article is concerned with human prehistory, the time since behaviorally and anatomically modern humans first appeared until the beginning of recorded history. Earlier periods are called "prehistoric". Beginning The term "prehistory" can refer to the vast span of time since the beginning of the Universe or the Earth, but more it refers to the period since life appeared on Earth, or more to the time since human-like beings appeared. End The date marking the end of prehistory is defined as the advent of the contemporary written historical record.
The date varies from region to region depending on the date when relevant records become a useful academic resource. For example, in Egypt it is accepted that prehistory ended around 3200 BCE, whereas in New Guinea the end of the prehistoric era is set much more at around 1900 common era. In Europe the well-documented classical cultures of Ancient Greece and Ancient Rome had neighbouring cultures, including the Celts and to a lesser extent the Etruscans, with little or no writing, historians must decide how much weight to give to the highly prejudiced accounts of these "prehistoric" cultures in Greek and Roman literature. Time periods In dividing up human prehistory in Eurasia, historians use the three-age system, whereas scholars of pre-human time periods use the well-defined geologic record and its internationally defined stratum base within the geologic time scale; the three-age system is the periodization of human prehistory into three consecutive time periods, named for their respective predominant tool-making technologies: Stone Age Bronze Age Iron Age The notion of "prehistory" began to surface during the Enlightenment in the work of antiquarians who used the word'primitive' to describe societies that existed before written records.
The first use of the word prehistory in English, occurred in the Foreign Quarterly Review in 1836. The use of the geologic time scale for pre-human time periods, of the three-age system for human prehistory, is a system that emerged during the late nineteenth century in the work of British and Scandinavian archeologists and anthropologists; the main source for prehistory is archaeology, but some scholars are beginning to make more use of evidence from the natural and social sciences. This view has been articulated by advocates of deep history; the primary researchers into human prehistory are archaeologists and physical anthropologists who use excavation and geographic surveys, other scientific analysis to reveal and interpret the nature and behavior of pre-literate and non-literate peoples. Human population geneticists and historical linguists are providing valuable insight for these questions. Cultural anthropologists help provide context for societal interactions, by which objects of human origin pass among people, allowing an analysis of any article that arises in a human prehistoric context.
Therefore, data about prehistory is provided by a wide variety of natural and social sciences, such as paleontology, archaeology, geology, comparative linguistics, molecular genetics and many others. Human prehistory differs from history not only in terms of its chronology but in the way it deals with the activities of archaeological cultures rather than named nations or individuals. Restricted to material processes and artifacts rather than written records, prehistory is anonymous; because of this, reference terms that prehistorians use, such as Neanderthal or Iron Age are modern labels with definitions sometimes subject to debate. The concept of a "Stone Age" is found useful in the archaeology of most of the world, though in the archaeology of the Americas it is called by different names and begins with a Lithic sta
Prehistoric technology is technology that predates recorded history. History is the study of the past using written records. Anything prior to the first written accounts of history is prehistoric, including earlier technologies. About 2.5 million years before writing was developed, technology began with the earliest hominids who used stone tools, which they may have used to start fires and bury their dead. There are several factors that made the evolution of prehistoric technology necessary. One of the key factors is behavioral modernity of the developed brain of Homo sapiens capable of abstract reasoning, language and problem solving; the advent of agriculture resulted in lifestyle changes from nomadic lifestyles to ones lived in homes, with domesticated animals, land farmed using more varied and sophisticated tools. Art, architecture and religion evolved over the course of the prehistoric periods; the Stone Age is a broad prehistoric period during which stone was used in the manufacture of implements with a sharp edge, a point, or a percussion surface.
The period lasted 2.5 million years, from the time of early hominids to Homo sapiens in the Pleistocene era, ended between 6000 and 2000 BCE with the advent of metalworking. The Stone Age lifestyle was that of hunter-gatherers who traveled to hunt game and gather wild plants, with minimal changes in technology; as the last glacial period of the current ice age neared its end, large animals like the mammoth and bison antiquus became extinct and the climate changed. Humans adapted by maximizing the resources in local environments and eating a wider range of wild plants and hunting or catching smaller game. Domestication of plants and animals with early stages in the Old World Mesolithic and New World Archaic periods led to significant changes and reliance on agriculture in the Old World Neolithic and New World Formative stage; the agricultural life led to significant technological advancements. Although Paleolithic cultures left no written records, the shift from nomadic life to settlement and agriculture can be inferred from a range of archaeological evidence.
Such evidence includes ancient tools, cave paintings, other prehistoric art, such as the Venus of Willendorf. Human remains provide direct evidence, both through the examination of bones, the study of mummies. Though concrete evidence is limited and historians have been able to form significant inferences about the lifestyle and culture of various prehistoric peoples, the role technology played in their lives; the Lower Paleolithic period was the earliest subdivision of the Old Stone Age. It spans the time from around 2.5 million years ago when the first evidence of craft and use of stone tools by hominids appears in the current archaeological record, until around 300,000 years ago, spanning the Oldowan and Acheulean lithic technology. Early human used stone tool technology, such as a hand axe, similar to that used by primates, which are found to have intelligence levels of modern children aged 3 to 5 years. Intelligence and use of technology did not change much for millions of years; the first "Homo" species began with Homo habilis about 2.4 to 1.5 million years ago.
Homo habilis created. Homo ergaster lived in eastern and southern Africa about 2.5 to 1.7 million years ago and used more diverse and sophisticated stone tools than its predecessor, Homo habilis, including having refined the inherited Oldowan tools and developed the first Acheulean bifacial axes. Homo erectus lived about 1.8 to 1.3 million years ago in West Asia and Africa and is thought to be the first hominid to hunt in coordinated groups, use complex tools, care for infirm or weaker companions. Homo antecessor the earliest hominid in Northern Europe lived from 1.2 million to 800,000 years ago and used stone tools. Homo heidelbergensis lived between 600,000 and 400,000 years ago and used stone tool technology similar the Acheulean tools used by Homo erectus. European and Asian sites dating back 1.5 million years ago seem to indicate controlled use of fire by Homo erectus. A northern Israel site from about 690,000 to 790,000 years ago suggests. Homo heidelbergensis may have been the first species to bury their dead about 500,000 years ago.
The Middle Paleolithic period occurred in Europe and the Near East, during which the Neanderthals lived. The earliest evidence of settlement in Australia dates to around 40,000 years ago when modern humans crossed from Asia by island-hopping; the Bhimbetka rock shelters exhibit the earliest traces of human life in India, some of which are 30,000 years old. Homo neanderthalensis used Mousterian Stone tools that date back to around 300,000 years ago and include smaller, knife-like and scraper tools, they buried their dead in shallow graves along with stone tools and animal bones, although the reasons and significance of the burials are disputed. Homo sapiens, the only living species in the genus Homo, originated in Africa about 200,000 years ago; as compared to their predecessors, Homo sapiens had greater mental capability and ability to walk erect, which provided freed hands for manipulating objects and far greater use of tools. There was art created during this period. Intentional burial with grave goods, may be one of the earliest detectable forms of religious practice since it may signify a "concern for the dead that transcends daily life."
The earliest undisputed human burial so far dates back 130,000 years. Human skeletal remains stained with red ochre w
In Old World archaeology, Mesolithic is the period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is used synonymously for outside northern Europe, for the corresponding period in the Levant and Caucasus; the Mesolithic has different time spans in different parts of Eurasia. It refers to the final period of hunter-gatherer cultures in Europe and Western Asia, between the end of the Last Glacial Maximum and the Neolithic Revolution. In Europe it spans 15,000 to 5,000 BP; the term is less used of areas further east, not at all beyond Eurasia and North Africa. The type of culture associated with the Mesolithic varies between areas, but it is associated with a decline in the group hunting of large animals in favour of a broader hunter-gatherer way of life, the development of more sophisticated and smaller lithic tools and weapons than the heavy chipped equivalents typical of the Paleolithic. Depending on the region, some use of pottery and textiles may be found in sites allocated to the Mesolithic, but indications of agriculture are taken as marking transition into the Neolithic.
The more permanent settlements tend to be close to the sea or inland waters offering a good supply of food. Mesolithic societies are not seen as complex, burials are simple; the terms "Paleolithic" and "Neolithic" were introduced by John Lubbock in his work Pre-historic Times in 1865. The additional "Mesolithic" category was added as an intermediate category by Hodder Westropp in 1866. Westropp's suggestion was controversial. A British school led by John Evans denied any need for an intermediate: the ages blended together like the colors of a rainbow, he said. A European school led by Louis Laurent Gabriel de Mortillet asserted that there was a gap between the earlier and later. Edouard Piette claimed to have filled the gap with his naming of the Azilian Culture. Knut Stjerna offered an alternative in the "Epipaleolithic", suggesting a final phase of the Paleolithic rather than an intermediate age in its own right inserted between the Paleolithic and Neolithic. By the time of Vere Gordon Childe's work, The Dawn of Europe, which affirms the Mesolithic, sufficient data had been collected to determine that a transitional period between the Paleolithic and the Neolithic was indeed a useful concept.
However, the terms "Mesolithic" and "Epipalaeolitic" remain in competition, with varying conventions of usage. In the archaeology of Northern Europe, for example for archaeological sites in Great Britain, Scandinavia and Russia, the term "Mesolithic" is always used. In the archaeology of other areas, the term "Epipaleolithic" may be preferred by most authors, or there may be divergences between authors over which term to use or what meaning to assign to each. In the New World, neither term is used. "Epipaleolithic" is sometimes used alongside "Mesolithic" for the final end of the Upper Paleolithic followed by the Mesolithic. As "Mesolithic" suggests an intermediate period, followed by the Neolithic, some authors prefer the term "Epipaleolithic" for hunter-gatherer cultures who are not succeeded by agricultural traditions, reserving "Mesolithic" for cultures who are succeeded by the Neolithic Revolution, such as the Natufian culture. Other authors use "Mesolithic" as a generic term for post-LGM hunter-gatherer cultures, whether they are transitional towards agriculture or not.
In addition, terminology appears to differ between archaeological sub-disciplines, with "Mesolithic" being used in European archaeology, while "Epipalaeolithic" is more common in Near Eastern archaeology. The Balkan Mesolithic begins around 15,000 years ago. In Western Europe, the Early Mesolithic, or Azilian, begins about 14,000 years ago, in the Franco-Cantabrian region of northern Spain and southern France. In other parts of Europe, the Mesolithic begins by 11,500 years ago, it ends with the introduction of farming, depending on the region between c. 8,500 and 5,500 years ago. Regions that experienced greater environmental effects as the last glacial period ended have a much more apparent Mesolithic era, lasting millennia. In northern Europe, for example, societies were able to live well on rich food supplies from the marshlands created by the warmer climate; such conditions produced distinctive human behaviors that are preserved in the material record, such as the Maglemosian and Azilian cultures.
Such conditions delayed the coming of the Neolithic until some 5,500 BP in northern Europe. The type of stone toolkit remains one of the most diagnostic features: the Mesolithic used a microlithic technology – composite devices manufactured with Mode V chipped stone tools, while the Paleolithic had utilized Modes I–IV. In some areas, such as Ireland, parts of Portugal, the Isle of Man and the Tyrrhenian Islands, a macrolithic technology was used in the Mesolithic. In the Neolithic, the microlithic technology was replaced by a macrolithic technology, with an increased use of polished stone tools such as stone axes. There is some evidence for the beginning of construction at sites with a ritual or astronomical significance, including Stonehenge, with a short row of large post holes aligned east-west, a possible "lunar calendar" at Warren Field in Scotland, with pits of post holes of varying sizes, thought to reflect the lunar phases. Both are dated to before c. 9,000 BP. As the "Neolithic package" (including farming, polished stone axes, timber longhouses and pot
Mortar and pestle
Mortar and pestle are implements used since ancient times to prepare ingredients or substances by crushing and grinding them into a fine paste or powder in the kitchen and pharmacy. The mortar is a bowl made of hard wood, ceramic, or hard stone, such as granite; the pestle is a blunt club-shaped object. The substance to be ground, which may be wet or dry, is placed in the mortar, where the pestle is pressed and rotated onto it until the desired texture is achieved. Scientists have found ancient mortars and pestles that date back to 35000 BC; the English word mortar derives from classical Latin mortarium, among several other usages, "receptacle for pounding" and "product of grinding or pounding". The classical Latin pistillum, meaning "pounder", led to English pestle; the Roman poet Juvenal applied both mortarium and pistillum to articles used in the preparation of drugs, reflecting the early use of the mortar and pestle as a symbol of a pharmacist or apothecary. The antiquity of these tools is well documented in early writing, such as the Egyptian Ebers Papyrus of ~1550 BC and the Old Testament.
Mortars and pestles were traditionally used in pharmacies to crush various ingredients prior to preparing an extemporaneous prescription. The mortar and pestle, with the Rod of Asclepius, the Green Cross, others, is one of the most pervasive symbols of pharmacology, along with the show globe. For pharmaceutical use, the mortar and the head of the pestle are made of porcelain, while the handle of the pestle is made of wood; this is known as a Wedgwood mortar and pestle and originated in 1759. Today the act of reducing the particle size is known as trituration. Mortars and pestles are used as drug paraphernalia to grind up pills to speed up absorption when they are ingested, or in preparation for insufflation. To finely ground drugs, not available in liquid dosage form is used if patients need artificial nutrition such as parenteral nutrition or by nasogastric tube. Mortars are used in cooking to prepare wet or oily ingredients such as guacamole and pesto, as well as grinding spices into powder.
The molcajete, a version used by pre-Hispanic Mesoamerican cultures including the Aztec and Maya, stretching back several thousand years, is made of basalt and is used in Mexican cooking. Other Native American nations use mortars carved into the bedrock to other nuts. Many such depressions can be found in their territories. In Japan large mortars are used with wooden mallets to prepare mochi. A regular sized Japanese mortar and pestle are called surikogi, respectively. Granite mortars and pestles are used in Southeast Asia, as well as India. In India, it is used extensively to make spice mixtures for various delicacies as well as day to day dishes. With the advent of motorized grinders, use of the mortar and pestle has decreased, it is traditional in various Hindu ceremonies to crush turmeric in these mortars. In Malay, it is known as batu lesung. Large stone mortars, with long wood pestles were used in West Asia to grind meat for a type of meatloaf, or kibbeh, as well as the hummus variety known as masabcha.
In Indonesia and the Netherlands mortar is known as Cobek or Tjobek and pestle is known as Ulekan or Oelekan. It is used to make fresh sambal, a spicy chili condiment, hence the sambal ulek/oelek denote its process using pestle, it is used to grind peanut and other ingredients to make peanut sauce for gado-gado. Large mortars and pestles are used in developing countries to husk and dehull grain; these are made of wood, operated by one or more persons. Good mortar and pestle-making materials must be hard enough to crush the substance rather than be worn away by it, they can not be too brittle either. The material should be cohesive, so that small bits of the mortar or pestle do not mix in with the ingredients. Smooth and non-porous materials are trap the substances being ground. In food preparation, a rough or absorbent material may cause the strong flavour of a past ingredient to be tasted in food prepared later; the food particles left in the mortar and on the pestle may support the growth of microorganisms.
When dealing with medications, the prepared drugs may interact or mix, contaminating the used ingredients. Rough ceramic mortar and pestle sets can be used to reduce substances to fine powders, but stain and are brittle. Porcelain mortars are sometimes conditioned for use by grinding some sand to give them a rougher surface which helps to reduce the particle size. Glass mortars and pestles are fragile, but suitable for use with liquids. However, they do not grind as finely as the ceramic type. Other materials used include stone marble or agate, bamboo, steel and basalt. Mortar and pestle sets made from the wood of old grape vines have proved reliable for grinding salt and pepper at the dinner table. Uncooked rice is sometimes ground in mortars to clean them; this process must be repeated until the rice comes out white. Some stones, such as molcajete, need to be seasoned first before use. Metal mortars are kept oiled. Since the results obtained with hand grinding are neither reproducible nor reliable, most laboratories work with automatic mortar grinders.
Grinding time and pressure of the mortar can be adjusted and fixed, saving time and labor. The first automatic Mortar Grinder was invented by F. Kurt
Sandstone is a clastic sedimentary rock composed of sand-sized mineral particles or rock fragments. Most sandstone is composed of quartz or feldspar because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in Bowen's reaction series. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, yellow, grey, pink and black. Since sandstone beds form visible cliffs and other topographic features, certain colors of sandstone have been identified with certain regions. Rock formations that are composed of sandstone allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity. Quartz-bearing sandstone can be changed into quartzite through metamorphism related to tectonic compression within orogenic belts.
Sandstones are clastic in origin. They are formed from cemented grains that may either be fragments of a pre-existing rock or be mono-minerallic crystals; the cements binding these grains together are calcite and silica. Grain sizes in sands are defined within the range of 0.0625 mm to 2 mm. Clays and sediments with smaller grain sizes not visible with the naked eye, including siltstones and shales, are called argillaceous sediments; the formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation, either from water or from air. Sedimentation occurs by the sand settling out from suspension. Once it has accumulated, the sand becomes sandstone when it is compacted by the pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains; the most common cementing materials are silica and calcium carbonate, which are derived either from dissolution or from alteration of the sand after it was buried.
Colors will be tan or yellow. A predominant additional colourant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red, with additional manganese imparting a purplish hue. Red sandstones are seen in the Southwest and West of Britain, as well as central Europe and Mongolia; the regularity of the latter favours use as a source for masonry, either as a primary building material or as a facing stone, over other forms of construction. The environment where it is deposited is crucial in determining the characteristics of the resulting sandstone, which, in finer detail, include its grain size and composition and, in more general detail, include the rock geometry and sedimentary structures. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings: Terrestrial environmentsRivers Alluvial fans Glacial outwash Lakes Deserts Marine environmentsDeltas Beach and shoreface sands Tidal flats Offshore bars and sand waves Storm deposits Turbidites Framework grains are sand-sized detrital fragments that make up the bulk of a sandstone.
These grains can be classified into several different categories based on their mineral composition: Quartz framework grains are the dominant minerals in most clastic sedimentary rocks. These physical properties allow the quartz grains to survive multiple recycling events, while allowing the grains to display some degree of rounding. Quartz grains evolve from plutonic rock, which are felsic in origin and from older sandstones that have been recycled. Feldspathic framework grains are the second most abundant mineral in sandstones. Feldspar can be divided into two smaller subdivisions: plagioclase feldspars; the different types of feldspar can be distinguished under a petrographic microscope. Below is a description of the different types of feldspar. Alkali feldspar is a group of minerals in which the chemical composition of the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a complete solid solution. Plagioclase feldspar is a complex group of solid solution minerals that range in composition from NaAlSi3O8 to CaAl2Si2O8.
Lithic framework grains are pieces of ancient source rock that have yet to weather away to individual mineral grains, called lithic fragments or clasts. Lithic fragments can be any fine-grained or coarse-grained igneous, metamorphic, or sedimentary rock, although the most common lithic fragments found in sedimentary rocks are clasts of volcanic rocks. Accessory minerals are all other mineral grains in a sandstone. Common accessory minerals include micas, olivine and corundum. Many of these accessory grains are more dense than the silicates that
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca