Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray depending on their mineralogy; the word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar; the term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic.
A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids; the extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive and tough; these properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source; the rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations. Sills parallel beds and foliations in the surrounding country rock.
They can be emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations. Sills can be confused with solidified lava flows. Intruded sills will show partial incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed. Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will show evidence of vesicles where gases escaped into the atmosphere; because sills form at shallow depths below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock do not. Certain layered intrusions are a variety of sill that contain important ore deposits. Precambrian examples include the Bushveld and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, the Stillwater igneous complex of the United States.
Phanerozoic examples are smaller and include the Rùm peridotite complex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusions contain concentrations of gold, platinum and other rare elements. Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by short dike-like segments; such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin. The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data; such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive. Aquatic sill Batholith Dike Laccolith Sheet intrusion Sill swarm Stock
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
Sydney is the state capital of New South Wales and the most populous city in Australia and Oceania. Located on Australia's east coast, the metropolis surrounds Port Jackson and extends about 70 km on its periphery towards the Blue Mountains to the west, Hawkesbury to the north, the Royal National Park to the south and Macarthur to the south-west. Sydney is made up of 40 local government areas and 15 contiguous regions. Residents of the city are known as "Sydneysiders"; as of June 2017, Sydney's estimated metropolitan population was 5,230,330 and is home to 65% of the state's population. Indigenous Australians have inhabited the Sydney area for at least 30,000 years, thousands of engravings remain throughout the region, making it one of the richest in Australia in terms of Aboriginal archaeological sites. During his first Pacific voyage in 1770, Lieutenant James Cook and his crew became the first Europeans to chart the eastern coast of Australia, making landfall at Botany Bay and inspiring British interest in the area.
In 1788, the First Fleet of convicts, led by Arthur Phillip, founded Sydney as a British penal colony, the first European settlement in Australia. Phillip named the city Sydney in recognition of 1st Viscount Sydney. Penal transportation to New South Wales ended soon after Sydney was incorporated as a city in 1842. A gold rush occurred in the colony in 1851, over the next century, Sydney transformed from a colonial outpost into a major global cultural and economic centre. After World War II, it experienced mass migration and became one of the most multicultural cities in the world. At the time of the 2011 census, more than 250 different languages were spoken in Sydney. In the 2016 Census, about 35.8% of residents spoke a language other than English at home. Furthermore, 45.4% of the population reported having been born overseas, making Sydney the 3rd largest foreign born population of any city in the world after London and New York City, respectively. Despite being one of the most expensive cities in the world, the 2018 Mercer Quality of Living Survey ranks Sydney tenth in the world in terms of quality of living, making it one of the most livable cities.
It is classified as an Alpha+ World City by Globalization and World Cities Research Network, indicating its influence in the region and throughout the world. Ranked eleventh in the world for economic opportunity, Sydney has an advanced market economy with strengths in finance and tourism. There is a significant concentration of foreign banks and multinational corporations in Sydney and the city is promoted as Australia's financial capital and one of Asia Pacific's leading financial hubs. Established in 1850, the University of Sydney is Australia's first university and is regarded as one of the world's leading universities. Sydney is home to the oldest library in Australia, State Library of New South Wales, opened in 1826. Sydney has hosted major international sporting events such as the 2000 Summer Olympics; the city is among the top fifteen most-visited cities in the world, with millions of tourists coming each year to see the city's landmarks. Boasting over 1,000,000 ha of nature reserves and parks, its notable natural features include Sydney Harbour, the Royal National Park, Royal Botanic Garden and Hyde Park, the oldest parkland in the country.
Built attractions such as the Sydney Harbour Bridge and the World Heritage-listed Sydney Opera House are well known to international visitors. The main passenger airport serving the metropolitan area is Kingsford-Smith Airport, one of the world's oldest continually operating airports. Established in 1906, Central station, the largest and busiest railway station in the state, is the main hub of the city's rail network; the first people to inhabit the area now known as Sydney were indigenous Australians having migrated from northern Australia and before that from southeast Asia. Radiocarbon dating suggests human activity first started to occur in the Sydney area from around 30,735 years ago. However, numerous Aboriginal stone tools were found in Western Sydney's gravel sediments that were dated from 45,000 to 50,000 years BP, which would indicate that there was human settlement in Sydney earlier than thought; the first meeting between the native people and the British occurred on 29 April 1770 when Lieutenant James Cook landed at Botany Bay on the Kurnell Peninsula and encountered the Gweagal clan.
He noted in his journal that they were somewhat hostile towards the foreign visitors. Cook was not commissioned to start a settlement, he spent a short time collecting food and conducting scientific observations before continuing further north along the east coast of Australia and claiming the new land he had discovered for Britain. Prior to the arrival of the British there were 4,000 to 8,000 native people in Sydney from as many as 29 different clans; the earliest British settlers called the natives Eora people. "Eora" is the term the indigenous population used to explain their origins upon first contact with the British. Its literal meaning is "from this place". Sydney Cove from Port Jackson to Petersham was inhabited by the Cadigal clan; the principal language groups were Darug and Dharawal. The earliest Europeans to visit the area noted that the indigenous people were conducting activities such as camping and fishing, using trees for bark and food, collecting shells, cooking fish. Britain—before that, England—and Ireland had for a long time been sending their convicts across the Atlantic to the American colonies.
That trade was ended with the Declaration of Independence by the United States in 1776. Britain decided in 1786 to found a new penal outpost in the territory discovered by Cook some 16 years ear
Wyoming is a state in the mountain region of the western United States. The state is the 10th largest by area, the least populous, the second most sparsely populated state in the country. Wyoming is bordered on the north by Montana, on the east by South Dakota and Nebraska, on the south by Colorado, on the southwest by Utah, on the west by Idaho and Montana; the state population was estimated at 577,737 in 2018, less than 31 of the most populous U. S. cities including Denver in neighboring Colorado. Cheyenne is the state capital and the most populous city, with an estimated population of 63,624 in 2017; the western two-thirds of the state is covered by the mountain ranges and rangelands of the Rocky Mountains, while the eastern third of the state is high elevation prairie called the High Plains. Half of the land in Wyoming is owned by the U. S. government, leading Wyoming to rank sixth by area and fifth by proportion of a state's land owned by the federal government. Federal lands include two national parks—Grand Teton and Yellowstone—two national recreation areas, two national monuments, several national forests, historic sites, fish hatcheries, wildlife refuges.
Original inhabitants of the region include the Crow, Arapaho and Shoshone. Southwestern Wyoming was in the Spanish Empire and Mexican territory until it was ceded to the United States in 1848 at the end of the Mexican–American War; the region acquired the name Wyoming when a bill was introduced to the U. S. Congress in 1865 to provide a "temporary government for the territory of Wyoming"; the name was used earlier for the Wyoming Valley in Pennsylvania, is derived from the Munsee word xwé:wamənk, meaning "at the big river flat". The main drivers of Wyoming's economy are mineral extraction—mostly coal, natural gas, trona—and tourism. Agricultural commodities include livestock, sugar beets and wool; the climate is semi-arid and continental and windier than the rest of the U. S. with greater temperature extremes. Wyoming has been a politically conservative state since the 1950s, with the Republican Party candidate winning every presidential election except 1964. Wyoming's climate is semi-arid and continental, is drier and windier in comparison to most of the United States with greater temperature extremes.
Much of this is due to the topography of the state. Summers in Wyoming are warm with July high temperatures averaging between 85 and 95 °F in most of the state. With increasing elevation, this average drops with locations above 9,000 feet averaging around 70 °F. Summer nights throughout the state are characterized by a rapid cooldown with the hottest locations averaging in the 50–60 °F range at night. In most of the state, most of the precipitation tends to fall in early summer. Winters are cold, but are variable with periods of sometimes extreme cold interspersed between mild periods, with Chinook winds providing unusually warm temperatures in some locations. Wyoming is a dry state with much of the land receiving less than 10 inches of rainfall per year. Precipitation depends on elevation with lower areas in the Big Horn Basin averaging 5–8 inches; the lower areas in the North and on the eastern plains average around 10–12 inches, making the climate there semi-arid. Some mountain areas do receive a good amount of precipitation, 20 inches or more, much of it as snow, sometimes 200 inches or more annually.
The state's highest recorded temperature is 114 °F at Basin on July 12, 1900 and the lowest recorded temperature is −66 °F at Riverside on February 9, 1933. The number of thunderstorm days vary across the state with the southeastern plains of the state having the most days of thunderstorm activity. Thunderstorm activity in the state is highest during early summer; the southeastern corner of the state is the most vulnerable part of the state to tornado activity. Moving away from that point and westwards, the incidence of tornadoes drops with the west part of the state showing little vulnerability. Tornadoes, where they occur, tend to be small and brief, unlike some of those that occur farther east; as specified in the designating legislation for the Territory of Wyoming, Wyoming's borders are lines of latitude 41°N and 45°N, longitude 104°3'W and 111°3'W, making the shape of the state a latitude-longitude quadrangle. Wyoming is one of only three states to have borders along only straight latitudinal and longitudinal lines, rather than being defined by natural landmarks.
Due to surveying inaccuracies during the 19th century, Wyoming's legal border deviates from the true latitude and longitude lines by up to half of a mile in some spots in the mountainous region along the 45th parallel. Wyoming is bordered on the north by Montana, on the east by South Dakota and Nebraska, on the south by Colorado, on the southwest by Utah, on the west by Idaho, it is the tenth largest state in the United States in total area, containing 97,814 square miles and is made up of 23 counties. From the north border to the south border it is 276 miles; the Great Plains meet the Rocky Mountains in Wyoming. The state is a great plateau broken by many mountain ranges. Surface elevations range from the summit of Gannett Peak in the Wind River Mountain Range, at 13,804 feet, to the Belle Fourche River val
Diorite is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. Diorite is grey to dark-grey in colour, but it can be black or bluish-grey, has a greenish cast, it is distinguished from gabbro on the basis of the composition of the plagioclase species. Diorite may contain small amounts of quartz and olivine. Zircon, titanite, magnetite and sulfides occur as accessory minerals. Minor amounts of muscovite may be present. Varieties deficient in hornblende and other dark minerals are called leucodiorite; when olivine and more iron-rich augite are present, the rock grades into ferrodiorite, transitional to gabbro. The presence of significant quartz makes the rock type quartz-diorite or tonalite, if orthoclase is present at greater than 10 percent, the rock type grades into monzodiorite or granodiorite. A dioritic rock containing feldspathoid mineral/s and no quartz is termed foid-bearing diorite or foid diorite according to content.
Diorite has a phaneritic speckled, texture of coarse grain size and is porphyritic. Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus, within a diorite porphyry matrix. Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from the partial melting of a mafic rock above a subduction zone, it is produced in volcanic arcs, in cordilleran mountain building, such as in the Andes Mountains, as large batholiths. The extrusive volcanic equivalent rock type is andesite. Diorite, although not rare, underlies comparatively small areas. An orbicular variety found in Corsica is called corsite. Diorite is an hard rock, making it difficult to carve and work with, its hardness, however allows it to be worked finely and take a high polish, to provide a durable finished work. One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary.
The most famous diorite work extant is the Code of Hammurabi, inscribed upon a 2.23 m pillar of black diorite. The original can be seen today in Paris' Musée du Louvre; the use of diorite in art was most important among early Middle Eastern civilizations such as Ancient Egypt, Babylonia and Sumer. It was so valued in early times that the first great Mesopotamian empire—the Empire of Sargon of Akkad—listed the taking of diorite as a purpose of military expeditions. Pallavas mamallapuram is one of the great example for diorite relief sculptures. Although one can find diorite art from periods, it became more popular as a structural stone and was used as pavement due to its durability. Diorite was used by both the Inca and Mayan civilizations, but for fortress walls, etc, it was popular with medieval Islamic builders. In times, diorite was used as cobblestone. Although diorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of St. Paul's Cathedral, where centuries of foot traffic have polished the steps to a sheen.
List of rock types
An impact crater is an circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, bowl-shaped depressions to large, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth. Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Callisto and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics over time. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more used.
In early literature, before the significance of impact cratering was recognised, the terms cryptoexplosion or cryptovolcanic structure were used to describe what are now recognised as impact-related features on Earth. The cratering records of old surfaces, such as Mercury, the Moon, the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been lower, but it is appreciable nonetheless; this indicates that there should be far more young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are sent cascading into the inner solar system. Formed in a collision 160 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate causing the Chicxulub impact that may have triggered the extinction of the non-avian dinosaurs 66 million years ago.
Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System. Although Earth's active surface processes destroy the impact record, about 190 terrestrial impact craters have been identified; these range in diameter from a few tens of meters up to about 300 km, they range in age from recent times to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are selectively found in the stable interior regions of continents. Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, the subduction of the ocean floor into Earth's interior by processes of plate tectonics. Impact craters are not to be confused with landforms that may appear similar, including calderas, glacial cirques, ring dikes, salt domes, others. Daniel M. Barringer, a mining engineer, was convinced that the crater he owned, Meteor Crater, was of cosmic origin.
Yet, most geologists at the time assumed. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States, he concluded they had been created by some great explosive event, but believed that this force was volcanic in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were formed by impacts. Grove Karl Gilbert suggested in 1893. Ralph Baldwin in 1949 wrote that the Moon's craters were of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy, Gene "saw the craters on the Moon as logical impact sites that were formed not in eons, but explosively, in seconds." For his Ph. D. degree at Princeton, under the guidance of Harry Hammond Hess, Shoemaker studied the impact dynamics of Barringer Meteor Crater. Shoemaker noted Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at the Nevada Test Site, notably Jangle U in 1951 and Teapot Ess in 1955.
In 1960, Edward C. T. Chao and Shoemaker identified at Meteor Crater, proving the crater was formed from an impact generating high temperatures and pressures, they followed this discovery with the identification of coesite within suevite at Nördlinger Ries, proving its impact origin. Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Victoria, British Columbia and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon; because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.
Impact cratering invo