A stratovolcano known as a composite volcano, is a conical volcano built up by many layers of hardened lava, tephra and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas; the lava flowing from stratovolcanoes cools and hardens before spreading far, due to high viscosity. The magma forming this lava is felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows have travelled as far as 15 km. Stratovolcanoes are sometimes called "composite volcanoes" because of their composite stratified structure built up from sequential outpourings of erupted materials, they are in contrast to the less common shield volcanoes. Two famous examples of stratovolcanoes are Krakatoa, known for its catastrophic eruption in 1883 and Vesuvius, whose eruption in AD79 caused destruction of Pompeii and Herculaneum in 79 AD.
Both eruptions claimed thousands of lives. In modern times, Mount St. Helens and Mount Pinatubo have erupted catastrophically, with lesser losses of lives; the possible existence of stratovolcanoes on other terrestrial bodies of the Solar System has not been conclusively demonstrated. The one feasible exception are the existence of some isolated massifs on Mars, for example the Zephyria Tholus. Stratovolcanoes are common at subduction zones, forming chains and clusters along plate tectonic boundaries where oceanic crust is drawn under continental crust or another oceanic plate; the magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths; the water freed from the rock lowers the melting point of the overlying mantle rock, which undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, pools temporarily at the base of the lithosphere.
The magma rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition. When the magma nears the top surface, it pools in a magma chamber within the crust below the stratovolcano. There, the low pressure allows water and other volatiles dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO2. Once a critical volume of magma and gas accumulates, the plug of the volcanic vent is broken, leading to a sudden explosive eruption. In recorded history, explosive eruptions at subduction zone volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, such as Mount St. Helens, Mount Etna and Mount Pinatubo erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases; as a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma. Following the breaching of the vent and the opening of the crater, the magma degasses explosively.
The magma and gases blast out with full force. Since 1600 CE, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and lahars, deadly hazards that accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash, fragmented lava and superheated gases that can travel at speeds in excess of 160 km/h. Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean. In March to April 1982, three explosive eruptions of El Chichón in the State of Chiapas in southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people. Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed an ash cloud 40 km into the air and produced huge pyroclastic surges and lahar floods that devastated a large area around the volcano.
Pinatubo, located in Central Luzon just 90 km west-northwest from Manila, had been dormant for 6 centuries before the 1991 eruption, which ranks as one of the largest eruptions in the 20th century. In 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km/h. Unzen is one of more than 75 active volcanoes in Japan; the eruption of Mount Vesuvius in 79 smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of pyroclastic surges and lava flows. Although death toll is estimated between 13,000 and 26,000 remains, the exact number still remains unknown. Vesuvius is recognized as one of the most dangerous volcanoes, due to its
Trans-Mexican Volcanic Belt
The Trans-Mexican Volcanic Belt known as the Transvolcanic Belt and locally as the Sierra Nevada, is a volcanic belt that covers central-southern Mexico. Several of its highest peaks have snow all year long, during clear weather, they are visible to a large percentage of those who live on the many high plateaus from which these volcanoes rise; the Trans-Mexican Volcanic Belt spans across Central-Southern Mexico from the Pacific Ocean to the Gulf of Mexico between 18°30'N and 21°30'N, resting on the southern edge of the North American Plate. This 1000 kilometer long, 90–230 km broad structure is an east-west, continental volcanic arc. Over several million years, the subduction of the Rivera and Cocos plates beneath the North American Plate along the northern end of the Middle America Trench formed the Trans-Mexican Volcanic Belt; the Trans-Mexican Volcanic Belt is a unique volcanic belt. In addition to the physiographic complexities, igneous compositions vary—dominant subduction related products contrast with intraplate geo-chemical signatures.
The many intriguing aspects of the belt has spurred several hypotheses based on a typical subduction scenario. These features are related to the reactivation of early fault systems during the Trans-Mexican Volcanic belt's evolution; the main brittle fault system's geometry and age define a complex array of what could be multiple factors affecting the deformation of the belt. It exhibits many volcanic features, not limited to large stratovolcanoes, including monogenetic volcano cones, shield volcanoes, lava dome complexes, major calderas. Prior to the formation of the Trans-Mexican Volcanic Belt, Robby F an older, but related volcanic belt, the Sierra Madre Occidental occupied the area. Resuming in the Eocene, post-Laramide deformation, subduction related volcanism formed the Sierra Madre Occidental silic volcanic arc at a paleo-subduction zone off the coast of Baja California, before the peninsula rifted away. From the ocene to the Middle Miocene, counterclockwise rotation of the volcanic arc transitioned the once active Sierra Madre Occidental to a now active Trans-Mexican Volcanic Belt.
By the Middle Miocene, the transition from the silicic to more mafic compositions was complete, can be considered the beginning of the Trans-Mexican Volcanic Belt. Due to the orthogonal orientation of the Trans-Mexican Volcanic Belt in relation to the trend of Mexican tectonic provinces, its Pre-Cretaceous basement is heterogeneous; the Trans-Mexican Volcanic Belt east of 101°W rests upon Precambrian terranes, assembled into the Oaxaquia microcontinent and on the Paleozoic Mixteco terrane. West of 101°W, the Trans-Mexican Volcanic Belt resides on top of the Guerro composite terrane - a make up of Jurassic to Cretaceous marine marginal arcs, which are built on Triassic - Early Jurassic siliclastic turbidites. Assemblage of these basement rocks results with a thickness of 50–55 km east of 101°W and 35–40 km west of 101°W; the subducting plates originated from the breakup of the Farallon Plate at 23 Ma, which created two plates at equatorial latitudes, the Cocos Plate and southern Nazca Plate.
The Rivera Plate was the last fragment detached from the Cocos Plate, becoming a microplate at around 10 Ma. This small plate is bounded by the Rivera fracture zone, the East Pacific Rise, the Tamayo fracture zone, the Middle American Trench; the larger Cocos Plate is bordered by the North American Plate and the Caribbean Plate to the northeast, the Pacific Plate to the west, to the south by the Nazca Plate. The Cocos and Rivera are young oceanic plates that are subducting along the Middle American Trench at different convergence rates. Found subduction related rocks such as calc-alkaline rocks volumetrically occupy a majority of the Trans-Mexican Volcanic Belt but smaller volumes of intraplate-like lavas, potassium rich rocks, adakites are associated with the area. Middle Miocene adakitic rocks are found furthest from the trench and along the volcanic front of the central Trans-Mexican Volcanic Belt during the Pliocene-Quaternary, it has been suggested that slab melting contributed to the adakitic imprint on the Trans-Mexican Volcanic Belt, prompted by the prolonged flat subduction of the Cocos Plate.
1) From the early to mid Miocene ~20 to 8 Ma, the initial Trans-Mexican Volcanic Belt volcanic arc consisted of intermediate effusive volcanism, producing andesitic and dacitic polygenetic volcanoes extending from western Michoacan to the Palma Sola area. The plate boundary geometry and sub-horizontal subducting slab's thermal structure are the controlling factors for initial arc volcanism. Magmatism migrated away from the trench, moving northeast towards the Gulf of Mexico—giving the arc its characteristic E-W orientation, the inland push of the arc showed progressively drier melting, slab melting began to occur—suggesting flattening of the subducted slab; the oldest rocks of this age may be exposed in Central Mexico. 2) A Late Miocene ~11 Ma eastward traveling pulse of mafic volcanism swept across the whole of Central Mexico, north of the formed arc, ending ~ 3 Ma. The onset of the mafic lavas indicates lateral propagation of slab tear, prompted by the end of subduction beneath Baja California, allowing the influx of
Archaeology, or archeology, is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, biofacts or ecofacts and cultural landscapes. Archaeology can be considered a branch of the humanities. In North America archaeology is a sub-field of anthropology, while in Europe it is viewed as either a discipline in its own right or a sub-field of other disciplines. Archaeologists study human prehistory and history, from the development of the first stone tools at Lomekwi in East Africa 3.3 million years ago up until recent decades. Archaeology is distinct from palaeontology, the study of fossil remains, it is important for learning about prehistoric societies, for whom there may be no written records to study. Prehistory includes over 99% of the human past, from the Paleolithic until the advent of literacy in societies across the world. Archaeology has various goals, which range from understanding culture history to reconstructing past lifeways to documenting and explaining changes in human societies through time.
The discipline involves surveying and analysis of data collected to learn more about the past. In broad scope, archaeology relies on cross-disciplinary research, it draws upon anthropology, art history, ethnology, geology, literary history, semiology, textual criticism, information sciences, statistics, paleography, paleontology and paleobotany. Archaeology developed out of antiquarianism in Europe during the 19th century, has since become a discipline practiced across the world. Archaeology has been used by nation-states to create particular visions of the past. Since its early development, various specific sub-disciplines of archaeology have developed, including maritime archaeology, feminist archaeology and archaeoastronomy, numerous different scientific techniques have been developed to aid archaeological investigation. Nonetheless, archaeologists face many problems, such as dealing with pseudoarchaeology, the looting of artifacts, a lack of public interest, opposition to the excavation of human remains.
The science of archaeology grew out of the older multi-disciplinary study known as antiquarianism. Antiquarians studied history with particular attention to ancient artifacts and manuscripts, as well as historical sites. Antiquarianism focused on the empirical evidence that existed for the understanding of the past, encapsulated in the motto of the 18th-century antiquary, Sir Richard Colt Hoare, "We speak from facts not theory". Tentative steps towards the systematization of archaeology as a science took place during the Enlightenment era in Europe in the 17th and 18th centuries. In Europe, philosophical interest in the remains of Greco-Roman civilization and the rediscovery of classical culture began in the late Middle Age. Flavio Biondo, an Italian Renaissance humanist historian, created a systematic guide to the ruins and topography of ancient Rome in the early 15th century, for which he has been called an early founder of archaeology. Antiquarians of the 16th century, including John Leland and William Camden, conducted surveys of the English countryside, drawing and interpreting the monuments that they encountered.
One of the first sites to undergo archaeological excavation was Stonehenge and other megalithic monuments in England. John Aubrey was a pioneer archaeologist who recorded numerous megalithic and other field monuments in southern England, he was ahead of his time in the analysis of his findings. He attempted to chart the chronological stylistic evolution of handwriting, medieval architecture and shield-shapes. Excavations were carried out by the Spanish military engineer Roque Joaquín de Alcubierre in the ancient towns of Pompeii and Herculaneum, both of, covered by ash during the Eruption of Mount Vesuvius in AD 79; these excavations began in 1748 in Pompeii, while in Herculaneum they began in 1738. The discovery of entire towns, complete with utensils and human shapes, as well the unearthing of frescos, had a big impact throughout Europe. However, prior to the development of modern techniques, excavations tended to be haphazard; the father of archaeological excavation was William Cunnington. He undertook excavations in Wiltshire from around 1798.
Cunnington made meticulous recordings of Neolithic and Bronze Age barrows, the terms he used to categorize and describe them are still used by archaeologists today. One of the major achievements of 19th-century archaeology was the development of stratigraphy; the idea of overlapping strata tracing back to successive periods was borrowed from the new geological and paleontological work of scholars like William Smith, James Hutton and Charles Lyell. The application of stratigraphy to archaeology first took place with the excavations of prehistorical and Bronze Age sites. In the third and fourth decades of the 19th-century, archaeologists like Jacques Boucher de Perthes and Christian Jürgensen Thomsen began to put the artifacts they had found in chronological order. A major figure in the development of archaeology into a rigorous science was the army officer and ethnologist, Augustus Pitt Rivers, who began excavations on his land in England in the 1880s, his approach was methodical by the standards of the time, he is regarded as the first scientific archaeologist.
He arranged his artifacts by type or "typologically, within types by date or "chronologically"
Nahuatl, known as Aztec, is a language or group of languages of the Uto-Aztecan language family. Varieties of Nahuatl are spoken by about 1.7 million Nahua peoples, most of whom live in central Mexico. Nahuatl has been spoken in central Mexico since at least the seventh century CE, it was the language of the Aztecs, who dominated what is now central Mexico during the Late Postclassic period of Mesoamerican history. During the centuries preceding the Spanish conquest of the Aztec Empire, the Aztecs had expanded to incorporate a large part of central Mexico, their influence caused the variety of Nahuatl spoken by the residents of Tenochtitlan to become a prestige language in Mesoamerica. At the conquest, with the introduction of the Latin alphabet, Nahuatl became a literary language, many chronicles, works of poetry, administrative documents and codices were written in it during the 16th and 17th centuries; this early literary language based on the Tenochtitlan variety has been labeled Classical Nahuatl, is among the most studied and best-documented languages of the Americas.
Today, Nahuan languages are spoken in scattered communities in rural areas throughout central Mexico and along the coastline. There are considerable differences among varieties, some are not mutually intelligible. Huasteca Nahuatl, with over one million speakers, is the most-spoken variety. All varieties have been subject to varying degrees of influence from Spanish. No modern Nahuan languages are identical to Classical Nahuatl, but those spoken in and around the Valley of Mexico are more related to it than those on the periphery. Under Mexico's General Law of Linguistic Rights of the Indigenous Peoples promulgated in 2003, Nahuatl and the other 63 indigenous languages of Mexico are recognized as lenguas nacionales in the regions where they are spoken, enjoying the same status as Spanish within their regions. Nahuan languages exhibit a complex morphology characterized by polysynthesis and agglutination. Through a long period of coexistence with the other indigenous Mesoamerican languages, they have absorbed many influences, coming to form part of the Mesoamerican language area.
Many words from Nahuatl have been borrowed into Spanish and, from there, were diffused into hundreds of other languages. Most of these loanwords denote things indigenous to central Mexico which the Spanish heard mentioned for the first time by their Nahuatl names. English words of Nahuatl origin include "avocado", "chayote", "chili", "chocolate", "atlatl", "coyote", "peyote", "axolotl" and "tomato"; as a language label, the term "Nahuatl" encompasses a group of related languages or divergent dialects within the Nahuan branch of the Uto-Aztecan language family. The Mexican Instituto Nacional de Lenguas Indígenas recognizes 30 individual varieties within the "language group" labeled Nahuatl; the Ethnologue recognizes 28 varieties with separate ISO codes. Sometimes the label is used to include the Pipil language of El Salvador. Regardless of whether "Nahuatl" is considered to label a dialect continuum or a group of separate languages, the varieties form a single branch within the Uto-Aztecan family, descended from a single Proto-Nahuan language.
Within Mexico, the question of whether to consider individual varieties to be languages or dialects of a single language is political. This article focuses on describing the general history of the group and on giving an overview of the diversity it encompasses. For details on individual varieties or subgroups, see the individual articles. In the past, the branch of Uto-Aztecan to which Nahuatl belongs has been called "Aztecan". From the 1990s onward, the alternative designation "Nahuan" has been used as a replacement in Spanish-language publications; the Nahuan branch of Uto-Aztecan is accepted as having two divisions: "General Aztec" and Pochutec. General Aztec encompasses the Pipil languages. Pochutec is a scantily attested language, which became extinct in the 20th century, which Campbell and Langacker classify as being outside of general Aztec. Other researchers have argued that Pochutec should be considered a divergent variant of the western periphery."Nahuatl" denotes at least Classical Nahuatl together with related modern languages spoken in Mexico.
The inclusion of Pipil into the group is debated. Lyle Campbell classified Pipil as separate from the Nahuatl branch within general Aztecan, whereas dialectologists like Una Canger, Karen Dakin, Yolanda Lastra and Terrence Kaufman have preferred to include Pipil within the General Aztecan branch, citing close historical ties with the eastern peripheral dialects of General Aztec. Current subclassification of Nahuatl rests on research by Canger and Lastra de Suárez. Canger introduced the scheme of a Central grouping and two Peripheral groups, Lastra confirmed this notion, differing in some details. Canger & Dakin demonstrated a basic split between Eastern and Western branches of Nahuan, considered to reflect the oldest division of the proto-Nahuan speech community. Canger considered the central dialect area to be an innovative subarea within the Western branch, but in 2011, she suggested that it arose as an urban koiné language with features from both Western and Eastern dialect areas. Canger tentatively included dialects of La Huasteca in the Central group, while Lastra de Suárez places them in the Eastern Periphery, followed by Kaufman.
The terminology used to describe varieties of spoken Nahuatl is inconsistently applied. Many terms are used with multiple denotations, or a single dialect grou
Nevado de Toluca National Park
The Nevado de Toluca National Park is located southwest of the city of Toluca, Mexico State. It was decreed a park in 1936 to protect the Nevado de Toluca volcano, which forms nearly the park's entire surface and is the fourth highest peak in Mexico, it is 135 from Mexico City. The park was established with the aim of conservation, but the park is under increasing pressure from the growth of the Toluca metropolitan area as well as from illegal logging done by local communities who need the income; the volcano has a large crater in which are two shallow lakes. The crater and lakes are popular with visitors from Mexico State and Mexico City when there is snow. There are a number of archeological sites in the park, including the lakes themselves, which contain numerous offerings of copal and other items that were deposited during the pre-Hispanic period; the park offers activities such as hiking, mountain biking and horseback riding as well as limited skiing facilities. Due to its altitude, the summit of the Nevado is colder than the surrounding area.
The Nevado de Toluca National Park was established in 1936, with the aim of preserving a 51,000-ha area surrounding the Xinantécatl, or Nevado de Toluca volcano. This park is one of the most important protected areas in Mexico; when the park was created, the lands were not expropriated by the federal government and they continued to be used as ejido and private lands with restrictions. The decree contained provisions to encourage those with lands here to plant with trees. In the 1930s, the park covered an area of 54,000 hectares, but now it covers only 17,000. Much of the lost land was forested which has diminished the area's ability to replenish local aquifers. In the remaining park lands, more than half of the original tree biomass has been lost; the rural population which lives in and around the park has been involved with agriculture, illegal logging and soil extraction here. From 1972 to 2000, the forest cover of the park was analyzed. Although the total forest area has remained more or less the same, the density of the forest has changed significantly.
Density in fir and broadleaved forests have remained stable but large areas of the forest dominated by pine trees have lost density so that most are now rated as being semi-dense or fragmented forests. The reason for this is. Many rural communities can no longer make a living with agriculture due to declining soil quality. Most of the damage to the park's lands has come from illegal logging, illegal mining and damage done by visitor's automobiles in the crater of the volcano. Illegal logging has had the most detrimental effect; the park overall is subject to an annual deforestation rate of 0.5%, but over 13,000 of the parks 17,000 hectares show some degree of disturbance in its ecosystem, such as new meadows being created where forests have been cleared. Agriculture has diminished in the park trout farms have appeared. In areas where there is still forest, most of this forest is more sparse; until 2000, about 40% of dense pine forests were lost to logging. Another problem that the trees being cut are the healthiest, leaving the weakest and sick behind, making the remaining forests susceptible to pests and diseases.
The deforestation of the park follows the general trend of this populated area, were 100 hectares of wild lands are lost due to development. The metropolitan area of Toluca has over 1,350,000 people, four times the population it had in 1930 and continues to have population growth rates above average for Mexico. Industrialization has been intensive here since the 1940s, which has led to ecological consequences in the Toluca Valley. In recent decades 10,000 hectares of forests have disappeared as well as bodies of water such as the seasonal lakes of the Lerma River and numerous fresh water springs due to over pumping of groundwater. Soil acidity and erosion are serious problems. Aside from logging, the park faces other problems due to the large population. Mining in Mexico State of sand and gravel, is concentrated in the area around the volcano and it is known that these materials have been mined illegally in the park itself. There are a number of roads in the park with are exclusively used by trucks which take out illegally mined sand and gravel with at least eight illegal open pit mines.
While the area is only supposed to be forest, mining sand and gravel is easy as it is near the surface. Some of these mines are on ejido land. Local government officials claim that they have been closing mines but environmental groups state that the practiced is tolerated by officials. Other problems are due to the many visitors the park receives, which can be as high as 10,000 per day; this includes the illegal dumping of trash and damage to the crater area by automobiles, allowed to drive onto the soft sandy soil here. In 2008, part of the park was closed to allow for the recuperation of the forests there. More than 20% of the park is considered to be damaged in the higher elevations due to automobiles and grazing by livestock. Since July 2008, motorcycles and ATVs have been forbidden to enter the crater area due to the damage they cause here. Visitors to the crater must now walk two to six km from the parking area outside the crater. There are two routes from the parking area to the crater, a longer, flatter one and the Paso del Quetzal, steeper.
Cattle are forbidden to enter the crater area. The damage to the crater area has been deemed “critical” by a number of experts, it is unknown. In 2010, it was suggested that it would be necessary to d
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, sleet, snow and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates", thus and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."Moisture, lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is active when freezing rain occurs. A stationary front is present near the area of freezing rain and serves as the foci for forcing and rising air.
Provided necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely stratus and cumulonimbus. The cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. Thundersnow is possible within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation is caused by convection; the movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes.
Precipitation is a major component of the water cycle, is responsible for depositing the fresh water on the planet. 505,000 cubic kilometres of water falls as precipitation each year. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres, but over land it is only 715 millimetres. Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Precipitation may occur on other celestial bodies, e.g. when it gets cold, Mars has precipitation which most takes the form of frost, rather than rain or snow. Precipitation is a major component of the water cycle, is responsible for depositing most of the fresh water on the planet. 505,000 km3 of water falls as precipitation each year, 398,000 km3 of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres. Mechanisms of producing precipitation include convective and orographic rainfall.
Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation, while stratiform processes involve weaker upward motions and less intense precipitation. Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Mixtures of different types of precipitation, including types in different categories, can fall simultaneously. Liquid forms of precipitation include drizzle. Rain or drizzle that freezes on contact within a subfreezing air mass is called "freezing rain" or "freezing drizzle". Frozen forms of precipitation include snow, ice needles, ice pellets and graupel; the dew point is the temperature to which a parcel must be cooled in order to become saturated, condenses to water. Water vapor begins to condense on condensation nuclei such as dust and salt in order to form clouds. An elevated portion of a frontal zone forces broad areas of lift, which form clouds decks such as altostratus or cirrostratus.
Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can form due to the lifting of advection fog during breezy conditions. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, evaporative cooling. Adiabatic cooling occurs when air expands; the air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain. Conductive cooling occurs when the air comes into contact with a colder surface by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation; the main ways water vapor is added to the air are: wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet lan
A mammoth is any species of the extinct genus Mammuthus, one of the many genera that make up the order of trunked mammals called proboscideans. The various species of mammoth were equipped with long, curved tusks and, in northern species, a covering of long hair, they lived from the Pliocene epoch into the Holocene at about 4,000 years ago, various species existed in Africa, Europe and North America. They were members of the family Elephantidae, which contains the two genera of modern elephants and their ancestors; the oldest representative of Mammuthus, the South African mammoth, appeared around 5 million years ago during the early Pliocene in what is now southern and eastern Africa. Descendant species of these mammoths moved north and continued to propagate into numerous subsequent species covering most of Eurasia before extending into the Americas at least 600,000 years ago; the last species to emerge, the woolly mammoth, developed about 400,000 years ago in East Asia, with some surviving on Russia's Wrangel Island in the Arctic Ocean until as as 3,700 to 4,000 years ago, still extant during the construction of the Great Pyramid of ancient Egypt.
The earliest known proboscideans, the clade that contains the elephants, existed about 55 million years ago around the Tethys Sea area. The closest relatives of the Proboscidea are the hyraxes; the family Elephantidae is known to have existed six million years ago in Africa, includes the living elephants and the mammoths. Among many now extinct clades, the mastodon is only a distant relative of the mammoths, part of the separate Mammutidae family, which diverged 25 million years before the mammoths evolved; the following cladogram shows the placement of the genus Mammuthus among other proboscideans, based on hyoid characteristics: Since many remains of each species of mammoth are known from several localities, it is possible to reconstruct the evolutionary history of the genus through morphological studies. Mammoth species can be identified from the number of enamel ridges on their molars. At the same time, the crowns of the teeth became longer, the skulls become higher from top to bottom and shorter from the back to the front over time to accommodate this.
The first known members of the genus Mammuthus are the African species Mammuthus subplanifrons from the Pliocene and Mammuthus africanavus from the Pleistocene. The former is thought to be the ancestor of forms. Mammoths entered Europe around 3 million years ago. Only its molars are known -- 10 enamel ridges. A population evolved 12–14 ridges and split off from and replaced the earlier type, becoming M. meridionalis. In turn, this species was replaced by the steppe mammoth, M. trogontherii, with 18–20 ridges, which evolved in East Asia ca. 1 million years ago. Mammoths derived from M. trogontherii evolved molars with 26 ridges 200,000 years ago in Siberia, became the woolly mammoth, M. primigenius. The Columbian mammoth, M. columbi, evolved from a population of M. trogontherii that had entered North America. A 2011 genetic study showed that two examined specimens of the Columbian mammoth were grouped within a subclade of woolly mammoths; this suggests that the two populations produced fertile offspring.
It suggested that a North American form known as "M. jeffersonii" may be a hybrid between the two species. By the late Pleistocene, mammoths in continental Eurasia had undergone a major transformation, including a shortening and heightening of the cranium and mandible, increase in molar hypsodonty index, increase in plate number, thinning of dental enamel. Due to this change in physical appearance, it became customary to group European mammoths separately into distinguishable clusters: Early Pleistocene – Mammuthus meridionalis Middle Pleistocene – Mammuthus trogontherii Late Pleistocene – Mammuthus primigeniusThere is speculation as to what caused this variation within the three chronospecies. Variations in environment, climate change, migration played roles in the evolutionary process of the mammoths. Take M. primigenius for example: Woolly mammoths lived in opened grassland biomes. The cool steppe-tundra of the Northern Hemisphere was the ideal place for mammoths to thrive because of the resources it supplied.
With occasional warmings during the ice age, climate would change the landscape, resources available to the mammoths altered accordingly. The word mammoth was first used in Europe during the early 17th century, when referring to maimanto tusks discovered in Siberia. John Bell, on the Ob River in 1722, said that mammoth tusks were well known in the area, they were called "mammon's horn" and were found in washed-out river banks. Some local people claimed to have seen a living mammoth, but they only came out at night and always disappeared under water when detected, he presented it to Hans Sloan who pronounced it an elephant's tooth. The folklore of some native peoples of Siberia, who would find mammoth bones, sometimes frozen mammoth bodies, in eroding river banks, had various interesting explanations for these finds. Among the Khanty people of the Irtysh River basin, a belief existed that the mammoth was some kind of a water spirit. According to other Khanty, the mammoth was a creature that lived underground, burrowing its tunnels as it went, would die if it accidentally came to the surface.
The concept of the mammoth as an underground creature was known to the Chinese, who received some mammoth ivory from the