Chapter 6 – The Origin and Early Evolution of the Earth (1) PDF

# The Origin and Early Evolution of the Earth ## Major Concepts * The earth formed from spinning matter in a solar cloud or nebula about 5 billion years ago. * The planet picked up further matter from the solar gas cloud, and by 4.6 billion years ago gravitational attraction had condensed a small...

# The Origin and Early Evolution of the Earth ## Major Concepts * The earth formed from spinning matter in a solar cloud or nebula about 5 billion years ago. * The planet picked up further matter from the solar gas cloud, and by 4.6 billion years ago gravitational attraction had condensed a small protoplanet. * The earth was heated by gravitational condensation by the rapid decay of abundant radioactive elements, and by numerous asteroid impacts. * The dense iron and nickel derived from the original solar nebula settled to the core of the planet, and the silicate-rich portion remained as the mantle. * By 3.8 billion years ago the earth was no longer constantly bombarded by meteorites, and its surface was cool enough for bodies of liquid water and primitive sedimentary rocks to form. * Chemical differentiation of materials in the upper mantle produced the first stable oceanic and continental crust, however, the early protocontinents were constantly being metamorphosed and remelted. * Lighter gases, such as hydrogen and helium, escaped from the atmosphere because the earth’s gravity was too weak to retain them. * The rest of the atmospheric gases came from the mantle by volcanic eruptions or by sunlight breakdown of other gases. * Free oxygen did not exist in the early atmosphere but was added later by the photosynthetic activities of plants and bacteria. ## Distribution of Elements * Approximately one-quarter of the 100-odd known elements are common - only about 95 percent of the universe is composed of only two: hydrogen and helium. * Heavier elements tend to be progressively less abundant. * Most elements evolved successively from hydrogen through a series of complex thermonuclear reactions in stars. * Only large celestial bodies develop sufficient internal gravitational pressure and temperature to trigger reactions and form stars, which is why the planets are just planets. * Practically all the elements making up the earth originated in an exploding star before the solar system formed. * Only daughter elements from radioactive decay of initially incorporated unstable isotopes have evolved within the earth. * Fifteen to twenty elements can be called major constituents of the earth: iron, oxygen, magnesium, silicon, sulfur, nickel, calcium, aluminum, cobalt, sodium, manganese, potassium, titanium, phosphorus, and chromium. * Although hydrogen and helium make up most of the universe, they are almost totally missing from the earth because these elements escaped to space. * Much of the earth’s original hydrogen supply was trapped by reacting with oxygen to make water H₂O. * Water is the most unique chemical characteristic of earth, which justifies the nickname “blue planet.” ## Probable Origin of the Earth ### Hot Origins * Many theories have been proposed for the origin of the earth. * It was once thought that the planets formed from hot material torn from the sun by the gravitational pull of a passing comet, however, this theory was abandoned. * Subsequent theories called for condensation of planets from a hot gaseous cloud or solar nebula surrounding the sun, however, some problems remain. ### A Cold Beginning * The planetesimal hypothesis was proposed early in the 20th century. * It states that another star passed near our sun and its gravitational pull extracted solar gaseous materials which then condensed to form small solid bodies known as *planetesimals*. * These planetesimals aggregated to form planets. * The *solar nebula* hypothesis, which considered both the differences and similarities among the planets, was proposed in the mid-twentieth century. * It states that the planets formed about 5 to 6 billion years ago from a disc-shaped interstellar cloud of gases and dust that was spinning, which resulted in turbulence in the solar nebula that induced a temperature gradient. ## Recent Modifications * It has been argued that the planetesimals and protoplanets accreted with initially different compositions. * Meteorites are thought to provide clues to the composition of planetesimals. * Condensation temperatures vary, with calcium, aluminum, and titanium condensing first, then iron, nickel, cobalt, magnesium, and silicon. * Water would condense last, below 100°C. * The relative iron content of the planets seems to decrease with distance from the sun, suggesting that there was a temperature gradient in the solar nebula. * Some geochemists believe that the earth is too rich in Fe and Ni to be explained by a single-stage accretion and differentiation. * They propose a *two-stage accretion* for the earth. * The first 80% of the earth accreted and the metallic core differentiated, forming a mantle rich in lighter elements but still containing considerable iron. * The outer 20% of the earth then accreted to form the mantle. ## Planetology of the Inner Solar System * Space exploration has provided much information about the solar system. * The moon is almost as large as Mercury. * The moon is differentiated into three zones: crust, mantle, and possibly a small core. * Moon rocks are similar in composition to that of terrestrial rocks. * The lowland areas of the moon, called *maria*, have rocks most like terrestrial basalt, but richer in titanium, zirconium, yttrium, and chromium and relatively depleted in sodium, potassium, and rubidium. * Their densities are 3.1 to 3.5 g/cm³. * The lunar highlands, containing less-dense rocks rich in plagioclase feldspar, are depleted in sodium, potassium, silicon, rubidium, and other volatile constituents. * The highlands also have many more impact craters than do the lowlands. * Isotopic dating of lunar rocks suggests that extensive melting of the moon’s surface occurred 4.5 to 4.2 billion years ago. * Differentiation of the moon’s interior produced the basalts which were erupted between about 3.8 and 3.2 billion years ago. * The lowlands of the terrestrial planets other than the earth are also thought to contain basalt. * Highlands are consistently more cratered. * There is evidence that the intense bombardment by asteroid-size bodies triggered the large-scale melting that resulted in basaltic eruptions. * Differentiation and cratering were universal during the first 1 to 2 billion years of the solar system’s history. * The record has been obscured by erosion here on the earth. * The other terrestrial planets show very different styles of structural activity than Earth, and they are depleted in volatile constituents. * Neither Mercury nor the moon has any atmosphere at all, and Mars has only a very tenuous one of carbon dioxide with minor amounts of nitrogen, oxygen, and water vapor. * Venus has an atmosphere ninety times denser than Earth’s and with a very different composition (97 percent CO2). * Earth is unique in having abundant H₂O, most of which is present in the liquid state most of the time. * Mars is the only other planet with any significant water, and most of that is now locked up in polar ice caps or permafrost. * Earth’s size was such that its gravity could retain enough hydrogen to combine with oxygen to form water. * The distance from the sun was just right for most of that water to remain liquid. * Venus apparently lost all but a trace of its hydrogen, while Mars is too small to hold much volatile hydrogen and oxygen and farther from the sun, so the water that is present remains mostly frozen. * Early Earth was also large enough to generate sufficient internal heat to cause not only differentiation but also profound structural deformation, which persists even today. ## Leftovers - Asteroids, Meteorites, and Comets * As protoplanets grew, they cleaned up their respective neighborhoods by accreting or ejecting smaller bodies. * Accretion occurred rapidly during the first million years. * We still have meteorites, asteroids, and comets. * Asteroids are millions of small objects circling the sun between the orbits of Mars and Jupiter, ranging from about 1 to 950 kilometers in diameter. * Meteorites are miniature asteroids that have fallen to earth. * They are composed either of metallic iron-nickel (25 percent) or stony (75 percent) composition. * Their combined overall composition approaches that of the total earth. * Comets are low-density objects consisting mainly of ice crystals and some stony particles. * They are often described as "dirty snowballs." * Most comets seem to originate in the outermost solar system, where ice and dust are thought to orbit in the Oort Cloud. ## Nature of the Earth’s Interior ### Early Speculations * The discovery of the magnetic field in 1600 led to the suggestion that a gigantic, spherical, iron lodestone was the source of the earth’s magnetic field. * Other internal compositions, like those of hydrogen and silicon, were also proposed. * Theories included all-liquid, all-solid, partly solid-partly liquid, and even gaseous. * Twentieth-century geophysical studies provided more accurate information about the earth’s interior. ### Analogies from Meteorites * The earth’s overall density of 5.5 g/cm³ requires that the interior must contain denser material than that of the crust, whose average density is only 2.8 g/cm³. * It was suggested that the interior may be composed of materials like those that make up meteorites. * This is supported by the fact that radiogenic 143Nd is produced in the earth at the same rate as in stony meteorites. * The fact that meteorites have five times as much iron but only three-fourths as much oxygen and silica as crustal rocks is explained by the postulated early gravitational differentiation of protoplanets, which presumably concentrated heavier elements in interiors and lighter ones in crusts and atmospheres. ### Seismological Evidence of Internal Structure * Indirect geophysical evidence of internal zonation was discovered from deeper mines and wells. * Seismologists have recognized three zones in the earth’s interior: the crust, the mantle, and the core. ## Chemical Composition of the Deep Interior * The interior consists primarily of common elements (Table 6.1) arranged in mineral phases compatible with seismic evidence, temperatures, and pressures. * The observation of heat flow from the interior indicates that radioactive isotopes are not concentrated below the crust. * Analogies have been drawn from the densities and seismic properties of known surface-rock materials to help speculate about conditions at depth. * Bulk composition of crust beneath oceans is close to that of basalt, continental crust is close to that of granitic rocks, and the upper mantle is closest to that of ultramafic rocks. * Ultramafic rocks consist almost exclusively of dark minerals, such as pyroxene and olivine, which make up peridotite. * Ultramafic rocks lack feldspar and quartz. * They are similar in composition to the stony meteorites. * The core must be much denser because it makes up only 16 percent of the earth’s volume, and iron and nickel are abundant enough in nature to form the major part of the core. * Lighter elements such as sulfur and silica are thought to be present in some iron-nickel alloy. * Large masses of peridotite are seen in many mountain belts. * Oceanic basalts also contain chunks of peridotite carried to the surface intact. * This provides direct evidence of the makeup of the upper mantle. * Unusual small bodies of peridotite, called kimberlite, contain the form of pure carbon called diamond. * Minute inclusions of other minerals found within host diamond crystals have yielded surprisingly old 147Sm-143Nd dates suggesting that some diamonds may date from the early formation of the mantle. * Eruptions of magma derived from the crust should give direct samples of mantle material, but these lavas yield basalt, which is more siliceous than peridotite. * The volcanic equivalent of peridotite is olivine-rich komatiite. * Komatiite was produced early, but basalt was produced later by partial melting. * The origin of continental crust is less well understood. * Continental crust appears to have been formed by partial melting after cooling had progressed for a couple of billion years. * If the entire earth contained as much radioactive materials as do the continents, the globe would be entirely melted. * Continental crust might have been formed after 4.3 to 4.4 billion years ago. * Isotopic dating suggests that continental crust was not stable until then. * The oldest crustal rocks on earth are gneisses from southern Greenland which are dated at 3.8 billion years, and detrital zircons from Australia dated at 4.3 to 4.4 billion years. * Detrital zircons from the Acasta Gneiss in northwestern Canada dated at 4.03 billion years. * The oldest known crustal fragments on the earth’s surface at Hudson Bay are dated at 4.28 billion years old. * Some sort of granitic or dioritic protocrust was being weathered and eroded from the earth’s surface around 4.3 to 4.4 billion years ago. ## Dawn of Earth History ### Chemical and Thermal Evolution * The early earth must have undergone a great deal of heating. * Initial heating, caused by gravitational contraction of the protoplanet, would have raised the temperature at the center to about 1,000°C. * Radioactive decay would have added another 2,000°C to the temperature. * Chondritic meteorites have yielded information about the rare isotope 26Mg, which decays from 26A1 with a half-life of only 700,000 years. * It is believed that the rapid and intense decay of 26A1 to 26Mg in the first million years of the condensation of the earth was enough to melt the planet. * The rapid decay of 238U, 235U, and 40K, and large meteorite impacts would have been additional heat sources. * A distinct core and mantle existed by at least 4.5 billion years ago. * Persistence of a liquid outer core beneath a solid mantle may seem puzzling, but magnesium and iron-rich silicate minerals typical of the mantle have higher melting points than iron at any pressure and temperature combination. * The outer core temperature is estimated to be near 2,500°C, which is above the melting point of core material but below that of mantle silicate minerals. * The earth is still flowing from the interior, and although much of the original heat-producing radioactive decay has ceased, insulation by the crust and mantle impedes heat dissipation. * The interior is still evolving, and consequences of this chemical and thermal evolution are important. * Mountain building is the most conspicuous of structural disturbances. * Structural turmoil must eventually decline as the thermal energy reservoir is exhausted. ### Origin of the Crust * The crust must have formed by chemical differentiation of light elements from the mantle. * Bowen suggested in 1928 that the crust of continents, whose average properties approach those of granite, originated by chemical fractionation from an ultramafic-rich mantle. * Lines of evidence suggest that the crust derived from the denser mantle, but the mechanisms are more complex. * Lighter elements such as silicon, oxygen, aluminum, potassium, sodium, calcium, carbon, nitrogen, hydrogen, and helium, and lesser amounts of other elements rose to the surface to form the crust, seawater, and atmosphere. * The most familiar rock-forming minerals are produced in the proportions shown in Table 6.4, but atomic mass is not the only factor to determine their ultimate residence, however. * Atomic size and electrical charge are more important than atomic mass for heavy elements, such as uranium, thorium, and the rare earth elements. * Such elements are too large to fit into the closely packed crystal structures of dense silicate minerals found in ultramafic rocks. * They are more similar in size to potassium and calcium, and therefore tend to find them preferentially concentrated in the more open structures of the minerals found in crustal rocks. * Oceanic crust was produced by simple, complete melting when the mantle was very hot, but basalt was produced later by partial melting after cooling had progressed for a couple of billion years. * The origin of the continental crust is less well understood and apparently has been more complex. * If the entire earth contained as much radioactive material as the continents do, the globe would be entirely melted. * Uranium is more abundant in granite than in basalt, which is more abundant than in ultramafic rocks. * Continental crust was not stable until about 4.3 to 4.4 billion years ago. * The oldest crustal rocks known on earth are gneisses from southern Greenland, where dates of about 3,8 billion years were from gnisses that had once been sediments. * Scientists found detrital zircons in Australia that yielded dates of 4.3 to 4.4 billion years. * The oldest dated crustal minerals are not found in the oldest rocks. * In 1999, dates of 4.03 billion years were obtained from detrital zircons in the Acasta Gneiss in northwestern Canada. * Rocks on the eastern shore of Hudson Bay are dated at 4.28 billion years old. * These dates indicate that some sort of granitic or dioritic protocrust was being weathered and eroded from the earth’s surface around 4.3 to 4.4 billion years ago. ## Origin and Evolution of the Atmosphere and Seawater ### The Problem * The atmosphere and oceans must have originated by chemical differentiation. * The origin of life was also linked with differentiation and required certain unique chemical characteristics. * Once formed, life influenced the further development of both the atmosphere and seawater. ### Outgassing Hypothesis * Most atmospheric gases reached the atmosphere from the interior by a process called outgassing. * It was suggested that the trace amounts of helium found in the atmosphere originated from radioactive decay of uranium in the crust, and that atmospheric argon was derived from the decay of potassium 40 in the earth. * Volcanoes and hot springs are known to expel steam, carbon dioxide, nitrogen, and carbon monoxide. * It is believed that the atmospheric nitrogen, helium, argon, and water vapor were released by outgassing. * The preponderance of steam in the expelled gases provides a ready source of seawater. * Oxygen had a separate origin from photosynthesis. ### Photochemical Dissociation Hypothesis * An early atmosphere similar to that of Jupiter, with methane, ammonia, and some water vapor, would be devoid of any molecular oxygen and therefore of ozone (O3). * Ozone filters out most of the lethal, short-wavelength ultraviolet radiation from the sun, making the earth habitable. * Ozone is manufactured in the atmosphere by high-energy ultraviolet radiation from the sun. Oxygen molecules are broken, and the free oxygen atoms then combine with other molecules to form ozone. * Ozone is unstable, and the third atom soon splits off to combine with another free O to form O2. * The process is a steady-state one. * High-energy ultraviolet radiation can trigger photochemical reactions. * Primeval water vapor dissociated into hydrogen and oxygen, with most hydrogen escaping to space. * Newly formed molecular oxygen reacts with methane to form carbon dioxide and more water vapor. * Oxygen reacts with ammonia to form nitrogen and water. * After all CH4 and NH3 were converted to CO2 and N2, O2 could accumulate as more water vapor dissociated. ## Oxygen from Photosynthesis * The early earth probably had a great deal of CO2 in its atmosphere. * Photosynthetic organisms require CO2 and release O2. * A close balance between the abundance of carbon in sedimentary rocks and molecular oxygen in the atmosphere seems to confirm that most O2 has been released through photosynthesis. * Oxidation of stored carbon, by burning or respiration, returns CO2 to the atmosphere, from where it is recycled by photosynthesis. * Today photosynthesis is universally regarded as the major source of atmospheric oxygen. * Primitive photosynthetic cyanobacteria, which could produce O2, appeared no later than 3.5 billion years ago. * Red-colored strata containing strongly oxidized iron appeared by at least 2.5 to 2.8 billion years ago. * Once molecular oxygen was present, ozone could form and provide an ultraviolet shield for further life development. ## Origin of Seawater * Seawater is not difficult to explain because any hypothesis for the atmosphere also provides abundant water. * The origin of the oceans becomes largely a question of when seawater began to accumulate, and the rate of accumulation of water and dissolved salts. ## Ocean–Atmosphere Regulatory Systems * The global chemostat. Seawater undergoes chemical exchanges with the atmosphere, the solid earth, and life, which result in a complex chemical regulatory system. * Global exchanges with the atmosphere, the solid earth, and life result in a continuous chemical flux, maintaining a near steady-state, or global chemostat system. * The proportions of salts in seawater remain nearly constant through precipitation of any overly concentrated salts as sediments. * If seawater becomes depleted in something, that element will be redissolved from sediments. * Important exchanges also occur between life and the atmosphere. * Bacteria are important in overall cycling of nitrogen, some release it, whereas others fix it in nitrogenous compounds. * Photosynthesis extracts carbon dioxide and releases oxygen, whereas animal respiration consumes oxygen and releases CO2. * O2 is used in large quantities for the oxidation of minerals at the crust surface, a weathering process of major importance. * Carbon is temporarily stored in coal and carbonate rocks. * Earth would have a CO2-dominated atmosphere like that of Venus if it were not for the abundance of carbonate rocks. * The rate of accumulation of seawater is tied directly to the atmospheric production of water vapor and therefore to chemical fractionation of the solid earth. * The rate of release of helium and argon 40 to the atmosphere suggests that seawater accumulated over a long period of time. * The rate of accumulation was probably fast early in history but then slowed down. * There is isotopic evidence that the earth had an ocean as early as 4.4 billion years ago, suggesting that most of the outgassing of the atmosphere and seawater was completed by then. ## Global Thermostat and Climate * The global thermostat. Average global temperature depends upon many factors, some of which are terrestrial and some extraterrestrial in origin. * The most important of these factors are solar output, orbital variations, reflectivity of the earth, location of the poles and oceans, and the transparency of the atmosphere. * Global temperature is affected by the ratio between incoming and outgoing radiation. * The magnitude of radiation reflected from different surfaces, expressed as a percentage of the incoming radiation, is termed albedo. * Land areas have far less heat retention capacity than water. * Surface seawater absorbs solar radiation to form the photic zone, and oceanic circulation distributes that heat widely over the earth. * The larger the ratio of ocean to land surface, the warmer will be the overall global temperature. * If the earth’s poles are located in open oceanic areas, they will be warmer than if they are landlocked. * The configuration of continents as it affects atmospheric and oceanic circulation is another important factor. * Transparency of the atmosphere is affected mainly by cloud cover and dust, as well as certain chemical compounds. * Fine dust and smoke particles are effective inhibitors of radiation. * The 1982 eruption of El Chichon in Mexico produced one of the largest ever observed discharges of sulfur dioxide. * The trapping of long-wavelength infrared radiation in the atmosphere causes the atmospheric greenhouse effect. * If all long-wave radiation from the earth’s surface could escape to space, the earth would be cold and uninhabitable. * Escape is inhibited by molecules of CH4, CO2, water vapor, and ozone. * Variations of atmospheric CO2 content might explain major climatic changes. ## The Moho Discontinuity * The Moho discontinuity defines the base of the earth’s crust. * Continental crust has resulted from more complex, poorly understood processes of differentiation from the upper mantle. * The processes were driven by thermal energy resulting in large part from radioactive decay. * The earth has experienced the distinctive form of crustal turmoil and differentiation that resulted in plate tectonics. * Such activity must cease as thermal reservoirs become depleted. * Lighter elements such as K, Na, Ca, Al, Si, and O, have been concentrated together with traces of the heavier elements U and Th. * The atmosphere and seawater also must have formed by global differentiation. * The atmosphere and seawater also must have formed by global differentiation, but their present compositions have resulted from several processes. * Outgassing of the lightest elements from the interior could have given rise to N2, CO2, H2, H₂O and He. * Photochemical dissociation of early compounds, such as CH4, NH3, and water vapor, also could have given rise to N2, CO2, H₂O, and some O2. * Although small amounts of oxygen apparently were present in the early atmosphere, most of it has been generated by photosynthesis. * The global chemostat, thermostat, and ozone shield. The ocean–atmosphere system provides important regulatory functions. * Feedback interactions among the atmosphere, seawater, crust, and life have maintained a near steady-state chemical and thermal equilibrium through most of geologic time. * If heat or a particular chemical element increases in one realm, the others take it up to maintain overall balance. * The ozone layer in the lower atmosphere provides an important shield from ultraviolet radiation. * The global heat budget depends upon the ratio of incoming to outgoing radiation. * The most important factors regulating climate are the reflectivity of the earth, or albedo, the location of poles, and the transparency of the atmosphere. ## Earth’s Core * A dense core rich in iron and nickel is inferred from earth’s bulk density. * Earth’s magnetic field is best explained by interaction between a spinning solid mantle and a liquid outer core. * The earth’s mantle is composed chiefly of the ultramafic peridotite, which is rich in magnesium and iron. * The bulk composition of the mantle approximates that of stony meteorites. * Komatiite, a volcanic rock formed by complete melting of peridotite, was the dominant composition of oceanic crust in early Precambrian time. * A low seismic velocity zone of low rigidity defines the base of the lithosphere. ## Readings * Ahrens, L. 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