Weathering is the breaking down of rocks and minerals as well as wood and artificial materials through contact with the Earth's atmosphere and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, snow, wind and gravity and being transported and deposited in other locations. Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water and pressure; the second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals known as biological weathering in the breakdown of rocks and minerals. While physical weathering is accentuated in cold or dry environments, chemical reactions are most intense where the climate is wet and hot.
However, both types of weathering occur together, each tends to accelerate the other. For example, physical abrasion decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions; the various agents act in concert to convert primary minerals to secondary minerals and release plant nutrient elements in soluble forms. The materials left over after the rock breaks down combined with organic material creates soil; the mineral content of the soil is determined by the parent material. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition. Physical weathering called mechanical weathering or disaggregation, is the class of processes that causes the disintegration of rocks without chemical change; the primary process in physical weathering is abrasion. However and physical weathering go hand in hand. Physical weathering can occur due to temperature, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.
Abrasion by water and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material. Plant roots pry them apart, resulting in some disintegration. However, such biotic influences are of little importance in producing parent material when compared to the drastic physical effects of water, ice and temperature change. Thermal stress weathering, sometimes called insolation weathering, results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals; as some minerals expand more than others, temperature changes set up differential stresses that cause the rock to crack apart. Because the outer surface of a rock is warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers.
This process may be accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments. Thermal stress weathering comprises thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night; the repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is called exfoliation. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized heat can expand a boulder; the thermal heat from wildfire can cause significant weathering of rocks and boulders, heat can expand a boulder and thermal shock can occur.
The differential expansion of a thermal gradient can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material. If nothing stops this crack from propagating through the material, it will result in the object's structure to fail. Frost weathering called ice wedging or cryofracturing, is the collective name for several processes where ice is present; these processes include frost frost-wedging and freeze -- thaw weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses nea
In geology and physical geography, a plateau called a high plain or a tableland, is an area of a highland consisting of flat terrain, raised above the surrounding area with one or more sides with steep slopes. Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, erosion by water and glaciers. Plateaus are classified according to their surrounding environment as intermontane, piedmont, or continental. Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, erosion by water and glaciers. Volcanic plateaus are produced by volcanic activity; the Columbia Plateau in the northwestern United States is an example. They may be formed by upwelling of volcanic extrusion of lava; the underlining mechanism in forming plateaus from upwelling starts when magma rises from the mantle, causing the ground to swell upward. In this way, flat areas of rock are uplifted to form a plateau. For plateaus formed by extrusion, the rock is built up from lava spreading outward from cracks and weak areas in the crust.
Plateaus can be formed by the erosional processes of glaciers on mountain ranges, leaving them sitting between the mountain ranges. Water can erode mountains and other landforms down into plateaus. Dissected plateaus are eroded plateaus cut by rivers and broken by deep narrow valleys. Computer modeling studies suggest that high plateaus may be a result from the feedback between tectonic deformation and dry climatic conditions created at the lee side of growing orogens. Plateaus are classified according to their surrounding environment. Intermontane plateaus are the highest in the world, bordered by mountains; the Tibetan Plateau is one such plateau. Lava or volcanic plateaus are the plateau; the magma that comes out through narrow cracks or fissures in the crust spread over large area and solidifies. These layers of lava sheets form volcanic plateaus; the Antrim plateau in Northern Ireland, The Deccan Plateau in India and the Columbia Plateau in the United States are examples of lava plateaus. Piedmont plateaus are bordered on one side by mountains and on the other by a sea.
The Piedmont Plateau of the Eastern United States between the Appalachian Mountains and the Atlantic Coastal Plain is an example. Continental plateaus are bordered on all sides by oceans, forming away from the mountains. An example of a continental plateau is the Antarctic Polar Plateau in East Antarctica; the largest and highest plateau in the world is the Tibetan Plateau, sometimes metaphorically described as the "Roof of the World", still being formed by the collisions of the Indo-Australian and Eurasian tectonic plates. The Tibetan plateau covers 2,500,000 km2, at about 5,000 m above sea level; the plateau is sufficiently high to reverse the Hadley cell convection cycles and to drive the monsoons of India towards the south. The second-highest plateau is the Deosai Plateau of the Deosai National Park at an average elevation of 4,114 m, it is located in northern Pakistan. Deosai means'the land of giants'; the park protects an area of 3,000 km2. It is known for its rich flora and fauna of the Karakoram-West Tibetan Plateau alpine steppe ecoregion.
In spring it is covered by a wide variety of butterflies. The highest point in Deosai is Deosai Lake, or Sheosar Lake from the Shina language meaning "Blind lake" near the Chilim Valley; the lake lies at an elevation of 4,142 m, one of the highest lakes in the world, is 2.3 km long, 1.8 km wide, 40 m deep on average. Some other major plateaus in Asia are: Najd in the Arabian Peninsula elevation 762 to 1,525 m, Armenian Highlands, Iranian plateau, Anatolian Plateau, Mongolian Plateau, the Deccan Plateau. Another large plateau is the icy Antarctic Plateau, sometimes referred to as the Polar Plateau, home to the geographic South Pole and the Amundsen-Scott South Pole Station, which covers most of East Antarctica where there are no known mountains but rather 3,000 m high of superficial ice and which spreads slowly toward the surrounding coastline through enormous glaciers; this polar ice cap is so massive that the echolocation sound measurements of ice thickness have shown that large parts of the Antarctic "dry land" surface have been pressed below sea level.
Thus, if that same ice cap were removed, the large areas of the frozen white continent would be flooded by the surrounding Antarctic Ocean or Southern Ocean. On the other hand, were the ice cap melts away too the surface of the land beneath it would rebound away through isostasy from the center of the Earth and that same land would rise above sea level. A large plateau in North America is the Colorado Plateau, which covers about 337,000 km2 in Colorado, Utah and New Mexico. In northern Arizona and southern Utah the Colorado Plateau is bisected by the Colorado River and the Grand Canyon. How this came to be is that over 10 million years ago, a river was there, though not on the same cours
A volcanic plug called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of pressure if rising volatile-charged magma is trapped beneath it, this can sometimes lead to an explosive eruption. Glacial erosion can lead to exposure of the plug on one side, while a long slope of material remains on the opposite side; such landforms are called tail. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive upstanding landform. An example of two volcanic plugs can be found at the Pitons, in Saint Lucia, as they rise abruptly out of the eastern Caribbean Sea. Near the village of Rhumsiki in the Far North Province of Cameroon, Kapsiki Peak is an example of a volcanic plug and is one of the most photographed parts of the Mandara Mountains. Spectacular volcanic plugs are present in the center of La Gomera island in the Canary Islands archipelago, within the Garajonay National Park.
Borgarvirki is a volcanic plug located in north Iceland. A volcanic plug is situated in the town of Motta Sant'Anastasia in Italy. Saint Michel d'Aiguilhe chapel, whose construction started near Le Puy-en-Velay in France; the volcanic plug rises about 85 metres above the surroundings. Another building on a volcanic plug is the 14th century Trosky Castle in the Czech Republic. Strombolicchio, the northernmost of the Aeolian Islands, Rockall, a small, remote islet in the North Atlantic Ocean, are volcanic plugs. In the United Kingdom, two examples of a building on a volcanic plug are the Castle Rock in Edinburgh and Deganwy Castle, Wales; the Law, Ailsa Craig, Bass Rock, North Berwick Law and Dumgoyne hill are other examples of volcanic plugs located in Scotland. There are over 30 volcanic plugs in Northern Ireland, including Slemish in Ballymena, Scawt Hill, Carrickarede and Slieve Gallion. There are several volcanic plugs in the United States, including Morro Rock in California and Shiprock in New Mexico.
Devils Tower in Wyoming and Little Devils Postpile in Yosemite National Park, are believed to be volcanic plugs by many geologists. In Canada, the Northern Cordilleran Volcanic Province gives rise to several confirmed and suspected plugs. Chief among these is Castle Rock, located in British Columbia, which last erupted during the Pleistocene; the southern coast of Saint Lucia is dominated by a UNESCO World Heritage site. The twin peaks, Gros Piton and Petit Piton, steeply rise more than 770 metres above the Caribbean. There are several volcanic plugs in the North Island of New Zealand, including: the Pinnacles in the Coromandel Peninsula Bream Head in Northland Paritutu and the adjacent Sugar Loaf Islands in Taranaki St. Paul's Rock at Whangaroa Harbour Piha's Lion Rock, which hosted a fortified Maori pa. In New Zealand's South Island, Ōnawe Peninsula on Banks Peninsula is a prominent volcanic plug, erosion of Saddle Hill near Dunedin has revealed a plug. Dunedin's Mount Cargill displays two plugs: its main summit and the subsidiary summit of Buttar's Peak.
In Australia, the Nut and Table Cape in Tasmania are further examples along with Mount Warning in New South Wales
A monastery is a building or complex of buildings comprising the domestic quarters and workplaces of monastics, monks or nuns, whether living in communities or alone. A monastery includes a place reserved for prayer which may be a chapel, church, or temple, may serve as an oratory. Monasteries vary in size, comprising a small dwelling accommodating only a hermit, or in the case of communities anything from a single building housing only one senior and two or three junior monks or nuns, to vast complexes and estates housing tens or hundreds. A monastery complex comprises a number of buildings which include a church, cloister, library and infirmary. Depending on the location, the monastic order and the occupation of its inhabitants, the complex may include a wide range of buildings that facilitate self-sufficiency and service to the community; these may include a hospice, a school, a range of agricultural and manufacturing buildings such as a barn, a forge, or a brewery. In English usage, the term monastery is used to denote the buildings of a community of monks.
In modern usage, convent tends to be applied only to institutions of female monastics communities of teaching or nursing religious sisters. A convent denoted a house of friars, now more called a friary. Various religions may apply these terms in more specific ways; the word monastery comes from the Greek word μοναστήριον, neut. of μοναστήριος – monasterios from μονάζειν – monazein "to live alone" from the root μόνος – monos "alone". The earliest extant use of the term monastērion is by the 1st century AD Jewish philosopher Philo in On The Contemplative Life, ch. III. In England the word monastery was applied to the habitation of a bishop and the cathedral clergy who lived apart from the lay community. Most cathedrals were not monasteries, were served by canons secular, which were communal but not monastic. However, some were run by monasteries orders, such as York Minster. Westminster Abbey was for a short time a cathedral, was a Benedictine monastery until the Reformation, its Chapter preserves elements of the Benedictine tradition.
See the entry cathedral. They are to be distinguished from collegiate churches, such as St George's Chapel, Windsor. In most of this article, the term monastery is used generically to refer to any of a number of types of religious community. In the Roman Catholic religion and to some extent in certain branches of Buddhism, there is a somewhat more specific definition of the term and many related terms. Buddhist monasteries are called vihara. Viharas may be occupied by men or women, in keeping with common English usage, a vihara populated by females may be called a nunnery or a convent. However, vihara can refer to a temple. In Tibetan Buddhism, monasteries are called gompa. In Thailand and Cambodia, a monastery is called a wat. In Burma, a monastery is called a kyaung. A Christian monastery may be a priory, or conceivably a hermitage, it may be a community of men or of women. A charterhouse is any monastery belonging to the Carthusian order. In Eastern Christianity, a small monastic community can be called a skete, a large or important monastery can be given the dignity of a lavra.
The great communal life of a Christian monastery is called cenobitic, as opposed to the anchoretic life of an anchorite and the eremitic life of a hermit. There has been under the Osmanli occupation of Greece and Cyprus, an "idiorrhythmic" lifestyle where monks come together but being able to own things individually and not being obliged to work for the common good. In Hinduism monasteries are called matha, koil, or most an ashram. Jains use the Buddhist term vihara. In most religions the life inside monasteries is governed by community rules that stipulate the gender of the inhabitants and require them to remain celibate and own little or no personal property; the degree to which life inside a particular monastery is separate from the surrounding populace can vary widely. Others focus on interacting with the local communities to provide services, such as teaching, medical care, or evangelism; some monastic communities are only occupied seasonally, depending both on the traditions involved and the local weather, people may be part of a monastic community for periods ranging from a few days at a time to an entire lifetime.
The life within the walls of a monastery may be supported in several ways: by manufacturing and selling goods agricultural products, by donations or alms, by rental or investment incomes, by funds from other organizations within the religion, which in the past formed the traditional support of monasteries. There has been a long tradition of Christian monasteries providing hospitable and hospital services. Monasteries have been associated with the provision of education and the encouragement of scholarship and research, which has led to the establishment of schools and colleges and the association with universities. Christian monastic life has adapted to modern society by offering computer services, accounting services and management as well as modern hospital and educational administration. Buddhist monasteries, known as vihāra i
Diatoms are a major group of algae microalgae, found in the oceans and soils of the world. Living diatoms number in the trillions: they generate about 20 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live, contribute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half mile deep on the ocean floor, the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by east-to-west transatlantic winds from the bed of a dried up lake once covering much of the African Sahara. Diatoms are unicellular: they occur either as solitary cells or in colonies, which can take the shape of ribbons, zigzags, or stars. Individual cells range in size from 2 to 200 micrometers. In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles every 24 hours by asexual multiple fission. Diatoms have two distinct shapes: a few are radially symmetric, while most are broadly bilaterally symmetric.
A unique feature of diatom anatomy is that they are surrounded by a cell wall made of silica, called a frustule. These frustules have structural coloration due to their photonic nanostructure, prompting them to be described as "jewels of the sea" and "living opals". Movement in diatoms occurs passively as a result of both water currents and wind-induced water turbulence. Similar to plants, diatoms convert light energy to chemical energy by photosynthesis, although this shared autotrophy evolved independently in both lineages. Unusually for autotrophic organisms, diatoms possess a urea cycle, a feature that they share with animals, although this cycle is used to different metabolic ends in diatoms; the study of diatoms is a branch of phycology. Diatoms are classified as eukaryotes, organisms with a membrane-bound cell nucleus, that separates them from the prokaryotes archaea and bacteria. Diatoms are a type of plankton called phytoplankton, the most common of the plankton types. Diatoms grow attached to benthic substrates, floating debris, on macrophytes.
They comprise an integral component of the periphyton community. Another classification divides plankton into eight types based on size: in this scheme, diatoms are classed as microalgae. Several systems for classifying the individual diatom species exist. Fossil evidence suggests that diatoms originated during or before the early Jurassic period, about 150 to 200 million years ago. Diatoms are used to monitor past and present environmental conditions, are used in studies of water quality. Diatomaceous earth is a collection of diatom shells found in the earth's crust, they are soft, silica-containing sedimentary rocks which are crumbled into a fine powder and have a particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, as a dynamite stabilizer. Diatoms are 2 to 200 micrometers in length, their yellowish-brown chloroplasts, the site of photosynthesis, are typical of heterokonts, having four membranes and containing pigments such as the carotenoid fucoxanthin.
Individuals lack flagella, but they are present in male gametes of the centric diatoms and have the usual heterokont structure, except they lack the hairs characteristic in other groups. Diatoms are referred as "jewels of the sea" or "living opals" due to their photonic crystal properties; the biological function of this structural coloration is not clear, but it is speculated that it may be related to communication, thermal exchange and/or UV protection. Diatoms build intricate hard but porous cell walls called frustules composed of silica; this siliceous wall can be patterned with a variety of pores, minute spines, marginal ridges and elevations. The cell itself consists of two halves, each containing an flat plate, or valve and marginal connecting, or girdle band. One half, the hypotheca, is smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is circular, some cells may be triangular, square, or elliptical, their distinguishing feature is a hard mineral frustule composed of opal.
Most diatoms are nonmotile, as their dense cell walls cause them to sink. Planktonic forms in open water rely on turbulent mixing of the upper layers of the oceanic waters by the wind to keep them suspended in sunlit surface waters; the only mechanism for regulating buoyancy is an ionic pump. Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures. Diatoms are photosynthetic. Diatom cells are contained within a unique silica cell wall known as a frustule made up of two valves called thecae, that overlap one another; the biogenic silica composing the cell wall is synthesised intracellularly by the polymerisation of silicic acid monomer
Neanderthals are an extinct species or subspecies of archaic humans in the genus Homo, who lived within Eurasia from circa 400,000 until 40,000 years ago. The earliest fossils of Neanderthals in Europe are dated between 450,000 and 430,000 years ago, thereafter Neanderthals expanded into Southwest and Central Asia, they are known from numerous fossils, as well as stone tool assemblages. All assemblages younger than 160,000 years are of the so-called Mousterian techno-complex, characterised by tools made out of stone flakes; the type specimen is Neanderthal 1, found in Neander Valley in the German Rhineland, in 1856. Compared to modern humans, Neanderthals were stockier, with bigger bodies. In conformance with Bergmann's rule, as well as Allen's rule, this was was an adaptation to preserve heat in cold climates. Male and female Neanderthals had cranial capacities averaging 1,600 cm3 and 1,300 cm3 within the range of the values for anatomically modern humans. Average males stood around females 152 to 156 cm tall.
There has been growing evidence for admixture between Neanderthals and anatomically modern humans, reflected in the genomes of all modern non-African populations but not in the genomes of most sub-Saharan Africans. This suggests that interbreeding between Neanderthals and anatomically modern humans took place after the recent "out of Africa" migration, around 70,000 years ago. Recent admixture analyses have added to the complexity, finding that Eastern Neanderthals derived up to 2% of their ancestry from anatomically modern humans who left Africa some 100,000 years ago. Neanderthals are named after one of the first sites where their fossils were discovered in the mid-19th century in the Neander Valley, just east of Düsseldorf, at the time in the Rhine Province of the Kingdom of Prussia; the valley itself was named for Joachim Neander, Neander being the graecicized form of the surname Neumann. The German spelling of Thal "Valley" was current in the 19th century. Neanderthal 1 was known as the "Neanderthal cranium" or "Neanderthal skull" in anthropological literature, the individual reconstructed on the basis of the skull was called "the Neanderthal man".
The binomial name Homo neanderthalensis—extending the name "Neanderthal man" from the individual type specimen to the entire group—was first proposed by the Anglo-Irish geologist William King in a paper read to the British Association in 1863, although in the following year he stated that the specimen was not human and rejected the name. King's name had priority over the proposal put forward in 1866 by Homo stupidus. Popular English usage of "Neanderthal" as shorthand for "Neanderthal man", as in "the Neanderthals" or "a Neanderthal", emerged in the popular literature of the 1920s. Since the historical spelling -th- in German represents the phoneme /t/ or /tʰ/, not the fricative /θ/, standard British pronunciation of "Neanderthal" is with /t/; because of the usual sound represented by digraph ⟨th⟩ in English, "Neanderthal" is pronounced with the voiceless fricative /θ/, at least in "layman's American English". The spelling Neandertal is seen in English in scientific publications. Since "Neanderthal", or "Neandertal", is a common name, there is no authoritative prescription on its spelling, unlike the spelling of the binominal name H. neanderthalensis, predicated by King 1864.
The common name in German is always invariably Neandertaler, not Neandertal, but the spelling of the name of the Neander Valley itself has been affected by the species name, the names of the Neanderthal Museum and of Neanderthal station persisting with pre-1900 orthography. Since the discovery of the Neanderthal fossils, expert opinion has been divided as to whether Neanderthals should be considered a separate species or a subspecies relative to modern humans. Pääbo described such "taxonomic wars" as unresolveable in principle, "since there is no definition of species describing the case." The question depends on the definition of Homo sapiens as a chronospecies, in flux throughout the 20th century. Authorities preferring classification of Neanderthals as subspecies have introduced the subspecies name Homo sapiens sapiens for the anatomically modern Cro-Magnon population which lived in Europe at the same time as Neanderthals, while authorities preferring classification as separate species use Homo sapiens as equivalent to "anatomically modern humans".
During the early 20th century, a prevailing view of Neanderthals as "simian", influenced by Arthur Keith and Marcellin Boule, tended to exaggerate the anatomical differences between Neanderthals and Cro Magnon. Beginning in the 1930s, revised reconstructions of Neanderthals emphasized the similarity rather than differences from modern humans. From the 1940s throughout the 1970s, it was common to use the subspecies classification of Homo sapiens neanderthalensis vs. Homo sapiens sapiens; the hypothesis of "multiregional origin" of modern man was formulated in the 1980s on such grounds, arguing for the presence of an unbroken succession of fossil sites in both Europe and Asia. Hybridization between Neanderthals and Cro Magnon had been suggested on skeletal and craniological grounds since the early 20th century, found increasing support in the 20th century, until Neanderthal admixture was found to be present in modern populations genet
A meteoroid is a small rocky or metallic body in outer space. Meteoroids are smaller than asteroids, range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars; when a meteoroid, comet, or asteroid enters Earth's atmosphere at a speed in excess of 20 km/s, aerodynamic heating of that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. This phenomenon is called a meteor or "shooting star". A series of many meteors appearing seconds or minutes apart and appearing to originate from the same fixed point in the sky is called a meteor shower. If that object withstands ablation from its passage through the atmosphere as a meteor and impacts with the ground, it is called a meteorite. An estimated 25 million meteoroids and other space debris enter Earth's atmosphere each day, which results in an estimated 15,000 tonnes of that material entering the atmosphere each year.
In 1961, the International Astronomical Union defined a meteoroid as "a solid object moving in interplanetary space, of a size smaller than an asteroid and larger than an atom". In 1995, Beech and Steel, writing in the Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid would be between 100 µm and 10 m across. In 2010, following the discovery of asteroids below 10 m in size and Grossman proposed a revision of the previous definition of meteoroid to objects between 10 µm and one meter in diameter in order to maintain the distinction. According to Rubin and Grossman, the minimum size of an asteroid is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy; some of the smallest asteroids discovered are 2008 TS26 with H = 33.2 and 2011 CQ1 with H = 32.1 both with an estimated size of one m. In April 2017, the IAU adopted an official revision of its definition, limiting size to between 30 µm and one meter in diameter, but allowing for a deviation for any object causing a meteor.
Objects smaller than meteoroids are classified as micrometeoroids and interplanetary dust. The Minor Planet Center does not use the term "meteoroid". All meteoroids contain extraterrestrial nickel and iron, they have three main classifications: iron and stony-iron. Some stone meteoroids are called chondrites. Stony meteoroids without these features are called "achondrites", which are formed from extraterrestrial igneous activity; the composition of meteoroids can be inferred as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals give information useful for daytime meteors, which are otherwise difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams associated with a parent comet, others sporadic. Debris from meteoroid streams may be scattered into other orbits; the light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.
The study of meteorites gives insights into the composition of non-ephemeral meteoroids. Most meteoroids come from the asteroid belt, having been perturbed by the gravitational influences of planets, but others are particles from comets, giving rise to meteor showers; some meteoroids are fragments from bodies such as Mars or our moon, that have been thrown into space by an impact. Meteoroids travel around the Sun at various velocities; the fastest move at about 42 km/s through space in the vicinity of Earth's orbit. This is escape velocity from the Sun, equal to the square root of two times Earth's speed, is the upper speed limit of objects in the vicinity of Earth, unless they come from interstellar space. Earth travels at about 29.6 km/s, so when meteoroids meet the atmosphere head-on the combined speed may reach about 71 km/s. Meteoroids moving through Earth's orbital space average about 20 km/s. On January 17, 2013 at 05:21 PST, a one meter-sized comet from the Oort cloud entered Earth atmosphere over California and Nevada.
The object had a retrograde orbit with perihelion at 0.98 ± 0.03 AU. It approached from the direction of the constellation Virgo, collided head-on with Earth's atmosphere at 72 ± 6 km/s vapourising more than 100 km above ground over a period of several seconds; when meteoroids intersect with Earth's atmosphere at night, they are to become visible as meteors. If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are called meteorites. Meteorites are transformed in chemistry by the heat of entry and force of impact. A noted 4-metre asteroid, 2008 TC3, was observed in space on a collision course with Earth on 6 October 2008 and entered Earth's atmosphere the next day, striking a remote area of nor