Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, evidence of magmatism has been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, once in Hawaii.
Most magmatic liquids are rich in silica. Silicate melts are composed of silicon, aluminium, magnesium, calcium and potassium; the physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition. Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are more polymerized, with more linkage of silica tetrahedra, so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous. Speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to less explosive eruptions. Characteristics of several different magma types are as follows: Ultramafic SiO2 < 45% Fe–Mg > 8% up to 32%MgO Temperature: up to 1500°C Viscosity: Very Low Eruptive behavior: gentle or explosive Distribution: divergent plate boundaries, hot spots, convergent plate boundaries.
At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock; the geothermal gradient averages about 25 °C/km with a wide range from a low of 5–10 °C/km within oceanic trenches and subduction zones to 30–80 °C/km under mid-ocean ridges and volcanic arc environments. It is very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure; the composition of a rock may be considered to include volatile phases such as water and carbon dioxide. The presence of volatile phases in a rock under pressure can stabilize a melt fraction; the presence of 0.8% water may reduce the temperature of melting by as much as 100 °C. Conversely, the loss of water and volatiles from a magma may cause it to freeze or solidify.
A major portion of all magma is silica, a compound of silicon and oxygen. Magma contains gases, which expand as the magma rises. Magma, high in silica resists flowing, so expanding gases are trapped in it. Pressure builds up until the gases blast out in a dangerous explosion. Magma, poor in silica flows so gas bubbles move up through it and escape gently. Melting of solid rocks to form magma is controlled by three physical parameters: temperature and composition; the most common mechanisms of magma generation in the mantle are decompression melting and lowering of the solidus. Mechanisms are discussed further in the entry for igneous rock; when rocks melt, they do so and because most rocks are made of several minerals, which all have different melting points. As a rock melts, for example, its volume changes; when enough rock is melted, the small globules of melt soften the rock. Under pressure within the earth, as little as a fraction of a percent of partial melting may be sufficient to cause melt to be squeezed from its source.
Melts can stay in place long enough to melt to 20% or 35%, but rocks are melted in excess of 50%, because the melted rock mass becomes a crystal-and-melt mush tha
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make concrete include both sand and gravel. Aggregate composites tend to be much easier to fabricate, much more predictable in their finished properties, than fiber composites. Fiber orientation and continuity can have an overwhelming effect, but can be difficult to control and assess. Fabrication aside, aggregate materials themselves tend to be less expensive. Not all composite materials include aggregate. Aggregate particles tend to have about the same dimensions in every direction, so that aggregate composites do not display the level of synergy that fiber composites do. A strong aggregate held together by a weak matrix will be weak in tension, whereas fibers can be less sensitive to matrix properties if they are properly oriented and run the entire length of the part.
Most composites are filled with particles whose aspect ratio lies somewhere between oriented filaments and spherical aggregates. A good compromise is chopped fiber, where the performance of filament or cloth is traded off in favor of more aggregate-like processing techniques. Ellipsoid and plate-shaped aggregates are used. In most cases, the ideal finished. A given application's most desirable quality is most prominent in the aggregate itself; the matrix is chosen to serve this role, but its abilities should not be abused. Experiments and mathematical models show that more of a given volume can be filled with hard spheres if it is first filled with large spheres the spaces between are filled with smaller spheres, the new interstices filled with still smaller spheres as many times as possible. For this reason, control of particle size distribution can be quite important in the choice of aggregate; the upper limit to particle size depends on the amount of flow required before the composite sets, whereas the lower limit is due to the thickness of matrix material at which its properties change.
Particle size distribution is the subject of much study in the fields of ceramics and powder metallurgy. Some exceptions to this rule include: Toughness is a compromise between the requirements of strength and plasticity. In many cases, the aggregate will have one of these properties, will benefit if the matrix can add what it lacks; the most accessible examples of this are composites with an organic matrix and ceramic aggregate, such as asphalt concrete and filled plastic, although most metal matrix composites benefit from this effect. In this case, the correct balance of hard and soft components is necessary or the material will become either too weak or too brittle. Many materials properties change radically at small length scales. In the case where this change is desirable, a certain range of aggregate size is necessary to ensure good performance; this sets a lower limit to the amount of matrix material used. Unless some practical method is implemented to orient the particles in micro- or nano-composites, their small size and high strength relative to the particle-matrix bond allows any macroscopic object made from them to be treated as an aggregate composite in many respects.
While bulk synthesis of such nanoparticles as carbon nanotubes is too expensive for widespread use, some less extreme nanostructured materials can be synthesized by traditional methods, including electrospinning and spray pyrolysis. One important aggregate made by spray pyrolysis is glass microspheres. Called microballoons, they consist of a hollow shell several tens of nanometers thick and one micrometer in diameter. Casting them in a polymer matrix yields syntactic foam, with high compressive strength for its low density. Many traditional nanocomposites escape the problem of aggregate synthesis in one of two ways: Natural aggregates: By far the most used aggregates for nano-composites are occurring; these are ceramic materials whose crystalline structure is directional, allowing it to be separated into flakes or fibers. The nanotechnology touted by General Motors for automotive use is in the former category: a fine-grained clay with a laminar structure suspended in a thermoplastic olefin; the latter category includes fibrous asbestos composites with matrix materials such as linoleum and Portland cement.
In-situ aggregate formation: Many micro-composites form their aggregate particles by a process of self-assembly. For example, in high impact polystyrene, two immiscible phases of polymer are mixed together. Special molecules include separate portions which
Pumice, called pumicite in its powdered or dust form, is a volcanic rock that consists of vesicular rough textured volcanic glass, which may or may not contain crystals. It is light colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles, thicker vesicle walls and being dark colored and denser. Pumice is created when super-heated pressurized rock is violently ejected from a volcano; the unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases that are dissolved in the lava, causing the gases to exsolve; the simultaneous cooling and depressurization freezes the bubbles in a matrix. Eruptions under water are cooled and the large volume of pumice created can be a shipping hazard for cargo ships. Pumice is composed of microvesicular glass pyroclastic with thin, translucent bubble walls of extrusive igneous rock, it is but not of silicic or felsic to intermediate in composition, but basaltic and other compositions are known.
Pumice is pale in color, ranging from white, blue or grey, to green-brown or black. It forms when volcanic gases exsolving from viscous magma form bubbles that remain within the viscous magma as it cools to glass. Pumice is a common product of explosive eruptions and forms zones in upper parts of silicic lavas. Pumice has a porosity of 64–85% by volume and it floats on water for years, until it is waterlogged and sinks. Scoria differs from pumice in being denser. With larger vesicles and thicker vesicle walls, scoria sinks rapidly; the difference is the result of the lower viscosity of the magma. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a volcanic glass. Pumice varies in density according to the thickness of the solid material between the bubbles. After the explosion of Krakatoa, rafts of pumice drifted through the Indian Ocean for up to 20 years, with tree trunks floating among them. In fact, pumice rafts support several marine species.
In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 kilometers that floated hundreds of kilometres to Fiji. There are two main forms of vesicles. Most pumice contains tubular microvesicles; the elongation of the microvesicles occurs due to ductile elongation in the volcanic conduit or, in the case of pumiceous lavas, during flow. The other form of vesicles are subspherical to spherical and result from high vapor pressure during eruption. Pumice is igneous rock with a foamy appearance; the name is derived from the Latin word "pumex" which means "foam" and through history has been given many names because its formation was unclear. The old English term was “Spuma Maris”, meaning froth of the sea, because it was a frothy material thought to be hardened sea foam, it was known as “écume de mer” in French and “Meerschaum” in German for the same reason. Around 80 B. C. in Greek civilization it was called “lapis spongiae” for its vesicular properties.
Many Greek scholars decided there were different sources of pumice, one of, in the sea coral category. Pumice can be found all around the globe deriving from continental volcanic occurrence and submarine volcanic occurrence. Floating stones can be distributed by ocean currents; as described earlier pumice is produced by the eruption of explosive volcanoes under certain conditions, natural sources occur in volcanically active regions. Pumice is transported from these regions. In 2011, Italy and Turkey led pumice mining production at 3 million tonnes respectively. Total world pumice production in 2011 was estimated at 17 million tonnes. There are large reserves of pumice in Asian countries including Afghanistan, Japan, Syria and eastern Russia. Considerable amounts of pumice can be found at the Kamchatka Peninsula on the eastern flank of Russia; this area contains 19 active volcanoes and it lies in close proximity with the Pacific volcanic belt. Asia is the site of the second-most dangerous volcanic eruption in the 20th century, Mount Pinatubo, which erupted on June 12, 1991 in the Philippines.
Ash and pumice lapilli were distributed over a mile around the volcano. These ejections filled trenches. So much magma was displaced from the vent than the volcano became a depression on the surface of the Earth. Another well-known volcano that produces pumice is Krakatoa. An eruption in 1883 ejected so much pumice that kilometers of sea were covered in floating pumice and in some areas rose 1.5 meters above sea level. Europe is the largest producer of pumice with deposits in Italy, Greece and Iceland. Italy is the largest producer of pumice because of its numerous eruptive volcanoes. On the Aeolian Islands of Italy, the island of Lipari is made up of volcanic rock, including pumice. Large amounts of igneous rock on Lipari are due to the numerous extended periods of volcanic activity from the Upper Pleistocene/Tyrrhenian to the Post-Pleistocene periods. Pumice can be found all across North America including on the Caribbean Islands. In the United States, pumice is mined in Nevada, Idaho, California, New
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is 277 miles long, up to 18 miles wide and attains a depth of over a mile; the canyon and adjacent rim are contained within Grand Canyon National Park, the Kaibab National Forest, Grand Canyon-Parashant National Monument, the Hualapai Indian Reservation, the Havasupai Indian Reservation and the Navajo Nation. President Theodore Roosevelt was a major proponent of preservation of the Grand Canyon area, visited it on numerous occasions to hunt and enjoy the scenery. Nearly two billion years of Earth's geological history have been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted. While some aspects about the history of incision of the canyon are debated by geologists, several recent studies support the hypothesis that the Colorado River established its course through the area about 5 to 6 million years ago.
Since that time, the Colorado River has driven the down-cutting of the tributaries and retreat of the cliffs deepening and widening the canyon. For thousands of years, the area has been continuously inhabited by Native Americans, who built settlements within the canyon and its many caves; the Pueblo people considered the Grand Canyon a holy site, made pilgrimages to it. The first European known to have viewed the Grand Canyon was García López de Cárdenas from Spain, who arrived in 1540; the Grand Canyon is a river valley in the Colorado Plateau that exposes uplifted Proterozoic and Paleozoic strata, is one of the six distinct physiographic sections of the Colorado Plateau province. It is not the deepest canyon in the world. However, the Grand Canyon is known for its visually overwhelming size and its intricate and colorful landscape. Geologically, it is significant because of the thick sequence of ancient rocks that are well preserved and exposed in the walls of the canyon; these rock layers record much of the early geologic history of the North American continent.
Uplift associated with mountain formation moved these sediments thousands of feet upward and created the Colorado Plateau. The higher elevation has resulted in greater precipitation in the Colorado River drainage area, but not enough to change the Grand Canyon area from being semi-arid; the uplift of the Colorado Plateau is uneven, the Kaibab Plateau that Grand Canyon bisects is over one thousand feet higher at the North Rim than at the South Rim. All runoff from the North Rim flows toward the Grand Canyon, while much of the runoff on the plateau behind the South Rim flows away from the canyon; the result is deeper and longer tributary washes and canyons on the north side and shorter and steeper side canyons on the south side. Temperatures on the North Rim are lower than those on the South Rim because of the greater elevation. Heavy rains are common on both rims during the summer months. Access to the North Rim via the primary route leading to the canyon is limited during the winter season due to road closures.
The Grand Canyon is part of the Colorado River basin which has developed over the past 70 million years, in part based on apatite /He thermochronometry showing that Grand Canyon reached a depth near to the modern depth by 20 Ma. A recent study examining caves near Grand Canyon places their origins beginning about 17 million years ago. Previous estimates had placed the age of the canyon at 5–6 million years; the study, published in the journal Science in 2008, used uranium-lead dating to analyze calcite deposits found on the walls of nine caves throughout the canyon. There is a substantial amount of controversy because this research suggests such a substantial departure from prior supported scientific consensus. In December 2012, a study published in the journal Science claimed new tests had suggested the Grand Canyon could be as old as 70 million years. However, this study has been criticized by those who support the "young canyon" age of around six million years as " attempt to push the interpretation of their new data to their limits without consideration of the whole range of other geologic data sets."The canyon is the result of erosion which exposes one of the most complete geologic columns on the planet.
The major geologic exposures in the Grand Canyon range in age from the 2-billion-year-old Vishnu Schist at the bottom of the Inner Gorge to the 230-million-year-old Kaibab Limestone on the Rim. There is a gap of about a billion years between the 500-million-year-old stratum and the level below it, which dates to about 1.5 billion years ago. This large unconformity indicates a long period. Many of the formations were deposited in warm shallow seas, near-shore environments, swamps as the seashore advanced and retreated over the edge of a proto-North America. Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group were deposited in non–marine environments; the great depth of the Grand Canyon and the height of its strata can be attributed to 5–10 thousand feet of uplift of the Colorado Plateau, starting about 65 million years ago. This uplift has steepened the stream gradient of the Colorado River