Prior to 2001, no complete Phanerozoic-like ophiol-ite sequences had been recognized from Archean rock sequences around the world, leading some workers to the conclusion that no Archean ophiolites or oceanic crustal fragments are preserved. These ideas were challenged by the discovery of a complete 2.5 billion-year-old ophiolite sequence in the North China craton. This remarkable rock sequence includes chert and pillow lava, a sheeted dike complex, gabbro and layered gabbro, cumulate ultramafic rocks, and a suite of strongly deformed mantle harzburgite tec-tonites, all complexly deformed in a series of fault blocks. The mantle rocks include a distinctive type of intrusion with metallic chrome nodules called a podiform chromite deposit, known to form only in oceanic crust.
Well-preserved black smoker chimney structures in metallic sulfide deposits have also been discovered in some sections of the Dongwanzi ophiolite belt, and these ancient seafloor hydrothermal vents are among the oldest known. Deep-sea hydrothermal vents host the most primitive thermo-phyllic, chemosynthetic, sulfate-reducing organisms known, believed to be the closest relatives of the oldest life on Earth, with similar vents having possibly provided nutrients and protected environments for the first organisms. These vents are associated with some unusual microscale textures that may be remnants of early life-forms, most likely bacteria. These ancient fossils provide tantalizing suggestions that early life may have developed and remained sheltered in deep-sea hydrothermal vents until surface conditions became favorable for organisms to inhabit the land.
Archean oceanic crust was possibly thicker than Proterozoic and Phanerozoic counterparts, resulting in accretion predominantly of the upper basaltic section of oceanic crust. The crustal thickness of Archean oceanic crust may in fact have resembled modern oceanic plateaus. If this were the case, complete Phanerozoic-like ophiolite sequences would have been very unlikely to be accreted or obduced during Archean orogenies. In contrast, only the upper, pillow lava-dominated sections would likely be accreted. Remarkably, Archean greenstone belts contain an abundance of tectonic slivers of pillow lavas, gabbros, and associated deep-water sedimentary rocks. The observation that Archean greenstone belts have such an abundance of accreted ophiolitic fragments compared to Phanerozoic orogens suggests that thick, relatively buoyant, young Archean oceanic lithosphere may have had a rheological structure favoring delamination of the uppermost parts during subduction and collisional events.
See also African geology; Arabian geology; Asian geology; convergent plate margin processes; divergent plate margin processes; orogeny.
Anonymous. "Ophiolites." Geotimes 17 (1972): 24-25. Dewey, John F., and John M. Bird. "Origin and Emplacement of the Ophiolite Suite: Appalachian Ophiolites in Newfoundland, in Plate Tectonics." Journal of Geophysical Research 76 (1971): 3,179-3,206. Kusky, T. M., ed. Precambrian Ophiolites and Related Rocks. Developments in Precambrian Geology Vol. 13. Amsterdam: Elsevier Publishers, 2003. Kusky, Timothy M., Jianghai Li, and Robert T. Tucker. "The Archean Dongwanzi Ophiolite Complex, North
China Craton: 2.505 Billion Year Old Oceanic Crust and Mantle." Science 292 (2001): 1,142-1,145. Moores, Eldridge M. "Origin and Emplacement of Ophiol-
ites." Review Geophysics 20 (1982): 735-750. U.S. Geological Survey. "This Dynamic Earth: The Story of Plate Tectonics." Available online. URL: http://pubs. usgs.gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.
Ordovician The Ordovician is the second period of the Paleozoic Era and refers to the corresponding rock series, falling between the Cambrian and the Silurian. Commonly referred to as the age of marine invertebrates, the base of the Ordovician is defined on the Geological Society of America time scale (1999) as 490 million years ago, and the top or end of the Ordovician is defined at 444 million years ago. Charles Lapworth named the period, in 1879, after the Ordovices, a Celtic tribe that inhabited the Arenig-Bala area of northern Wales, where rocks of this series are well exposed.
By the Early Ordovician, North America had broken away from the supercontinent of Gondwana that amalgamated during the latest Precambrian and early Cambrian Period. It was surrounded by shallow water passive margins, and being at equa torial latitudes, these shallow seas were well suited for the proliferation of marine life-forms. The Iape-tus Ocean separated what is now the east coast of North America from the African and South American segments of the remaining parts of Gond-wana. By the Middle Ordovician, convergent tectonics brought an island arc system to the North American margin, initiating the Taconic orogeny as an arc/continent collision. This was followed by a sideways sweep of parts of Gondwana past the North American margin, leaving fragments of Gondwana attached to the modified eastern margin of North America.
During much of the Ordovician, carbonate sediments produced by intense organic productivity covered shallow epeiric seas in the tropical regions, including most of North America. This dramatic increase in carbonate sedimentation reflects a combination of tectonic activities that brought many low-lying continental fragments into the Tropics, high-sea level stands related to the breakup of Gond-wana, and a sudden increase in the number of different organisms that started to use calcium carbonate to build their skeleton or shell structures.
Marine life included diverse forms of articulate brachiopods, communities of echinoderms such as the crinoids or sea lilies, and reef-building stro-
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Ordovician paleogeography showing the distribution of continents approximately 500 million years ago
Two Ordovician trilobite fossils from the Wolchow River in Russia. Left is Cerauinella ingrica (12.5 cm long), right is Pseudobasilica lawrowi (François Gohier/Photo Researchers, Inc.)
matoporoids, rugose and tabulate corals. Trilobites roamed the shallow seafloors, and many forms emerged. The ordovician saw rapid diversification and wide distribution of several planktonic and pelagic faunas, especially the graptolites and con-odonts, which form useful index fossils for this period. Nautiloids floated across the oceans and some attained remarkably large sizes, reaching up to more than 10 feet (several meters) across. Fish fossils are not common from ordovician deposits, but some primitive armored types may have been present. The end of the ordovician is marked by a marine extinction event, apparently caused by rapid cooling of the shallow seas, perhaps related to continental glaciation induced by tectonic plate movements. The end-ordovician extinction is one of the greatest of all Phanerozoic time. About half of all species of brachiopods and bryozoans died off, and more than 100 other families of marine organisms disappeared forever.
The cause of the mass extinction at the end of the ordovician appears to have been largely tectonic. The major landmass of Gondwana had been resting in equatorial regions for much of the Middle ordovician, but migrated toward the south Pole at the end of the ordovician. This caused global cooling and glaciation, lowering sea levels from the high stand they had been resting at for most of the Cambrian and ordovician. The combination of cold climates with lower sea levels, leading to a loss of shallow shelf environments for habitation, probably was enough to cause the mass extinction at the end of the ordovician.
See also North American geology; Paleozoic.
origin and evolution of the Earth and solar system FURTHER READING
Condie, Kent C., and Robert E. sloan. Origin and Evolution of Earth: Principles of Historical Geology. upper saddle River, N.J.: Prentice Hall, 1997. Geological society of America. Geologic Time scale. Available online, uRL: http://www.geosociety.org/science/ timescale/timescl.htm. Accessed January 25, 2009.
origin and evolution of the Earth and solar system understanding the origin of the Earth, planets, sun, and other bodies in the solar system is a fundamental yet complex problem that has intrigued scientists and philosophers for centuries. Most of the records from the earliest history of the Earth have been lost to tectonic reworking and erosion, so most information about the formation of the Earth and solar system comes from the study of meteorites, the Earth's moon, and observations of the other planets and interstellar gas clouds. In addition, isotope geochemistry can be used to understand some of the conditions on the early Earth.
The solar system displays many general trends with increasing distance from the sun, and systematic changes like these imply that the sun did not gravitationally capture planets, but rather the sun and planets formed from a single event that occurred about 4.6 billion years ago. The nebular theory for the origin of the solar system suggests that a large spinning cloud of dust and gas formed and began to collapse under its own gravitational attraction. As it collapsed, it began to spin faster to conserve angular momentum (much like ice skaters spin faster when they pull their arms in to their chests) and eventually formed a disk. Collisions between particles in the disk formed protoplanets and a protosun, which then had larger gravitational fields than surrounding particles and began to sweep up and accrete loose particles.
The condensation theory states that particles of interstellar dust (many of which formed in older supernovas) act as condensation nucleii that grow through accretion of other particles to form small planetesimals that then have a greater gravitational field that attracts and accretes other planetesimals and dust. some collisions cause accretion, other collisions are hard and cause fragmentation and breaking up of the colliding bodies. The Jovian planets became so large that their gravitational fields were able to attract and accrete even free hydrogen and helium in the solar nebula.
This condensation theory explains the main differences between the planets due to distance from the sun, since the temperature of the solar nebula would have decreased away from the center where the sun formed. The temperature determines which materials
origin and evolution of the Earth and solar system condense out of the nebula, so the composition of the planets was determined by the temperature at their position of formation in the nebula. The inner terrestrial planets are made of rocky and metallic material because high temperatures near the center of the nebula allowed only the rocky and metallic material to condense from the nebula. Farther out, water and ammonia ices also condensed out of the nebula, because temperatures were cooler at greater distances from the early sun.
Early in the evolution of the solar system, the sun was in a T-Tauri stage and possessed a strong solar wind that blew away most gases from the solar nebula, including the early atmospheres of the inner planets. Gravitational dynamics moved many of the early planetesimals into orbits in the oort Cloud, where most comets and many meteorites are found. some of these bodies have eccentric orbits that occasionally bring them into the inner solar system, and collisions with comets and smaller molecules likely brought the present atmospheres and oceans to Earth and the other terrestrial planets. Thus air and water, some of the basic building blocks of life, were added to the planet after it formed, being thrown in from deep space of the oort Cloud.
The Hadean is the term used for the first of the four major eons of geological time: the Hadean, Archean, Proterozoic, and Phanerozoic. some time classification schemes use an alternative division of early time, in which the Hadean is considered the earliest part of the Archean. As the earliest phase of Earth's evolution, ranging from accretion until approximately the age of first rocks [4.55 to 4.0 Ga (Ga = giga annee, or 109 years)], the hadean is the most poorly understood interval of geologic time. only a few mineral grains and rocks have been recognized from this Eon, so most of what is thought to be known about the hadean is based on indirect geochemical evidence, meteorites, and models.
Between 4.55 and 3.8 Ga, meteorites bombarded the Earth; some were large enough to severely disrupt the surface, vaporize the atmosphere and ocean, and even melt parts of the mantle. By about 4.5 Ga, it appears as if a giant impactor, about the size of Mars, hit the protoearth. This impact ejected a huge amount of material into orbit around the protoearth, and some undoubtedly escaped. The impact probably also formed a new magma ocean, vaporized the early atmosphere and ocean (if present), and changed the angular momentum of the Earth as it spins and orbits
Q Solar nebula forms from a supernova
Q Solar nebula begins to rotate
Q Solar nebula forms from a supernova
Q Solar nebula begins to rotate
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Formation of the solar system from condensation and collapse of a solar nebula
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Formation of the solar system from condensation and collapse of a solar nebula origin and evolution of the Earth and solar system
Sketch of the late great impact hypothesis for the origin of the Moon and melting of parts of the outer shell of the Earth early in its evolution
the sun. The material in orbit coalesced to form the Moon, and the Earth-Moon system was born. Although not certain, this impact model for the origin of the Moon is the most widely accepted hypothesis, and it explains many divergent observations.
• The Moo^s orbit is inclined by 5.1° from the ecliptic plane, whereas the Earth's orbit is inclined 23.4° from the ecliptic, suggesting that some force, such as a collision, disrupted the angular momentum and rotational parameters of the Earth-moon system.
• The moon is retreating from the Earth, resulting in a lengthening of the day by 15 seconds per year, but the moon has not been closer to the Earth than 149,129 miles (240,000 km).
• The moon is significantly less dense than the Earth and other terrestrial planets, being depleted in iron and enriched in aluminum, titanium, and other related elements.
• The oxygen isotopes of igneous rocks from the moon are the same as from the Earth's mantle, suggesting a common origin.
These relationships suggest that the moon did not form by accretion from the solar nebula at its present location in the solar system. The age of the moon rocks shows that it formed at 4.5 Ga, with some magmatism continuing until 3.1 Ga, consistent with the impactor hypothesis.
The atmosphere and oceans of the Earth probably formed from early degassing of the interior by volca-nism within the first 50 million years of Earth history. It is likely that the present atmosphere is secondary, in that the first or primary atmosphere would have been vaporized by the late great impact that formed the moon, if it survived being blown away by an intense solar wind when the sun was in a T-Tauri stage of evolution. The primary atmosphere would have been composed of gases left over from accretion, including primarily hydrogen, helium, methane, and ammonia, along with nitrogen, argon, and neon. The fact that the atmosphere has much less than the expected amount of these elements and is quite depleted in these volatile elements relative to the sun suggests that the primary atmosphere has been lost to space.
Gases are presently escaping from the Earth during volcanic eruptions and also being released by weathering of surface rocks. Degassing of the mantle by volcanic eruptions, and perhaps also cometary impact, produced the secondary atmosphere. Gases released from volcanic eruptions include nitrogen (N), sulfur (S), carbon dioxide (CO2), and water vapor (H2O), closely matching the suite of volatiles that compose the present atmosphere and oceans. Most models show that little or no free oxygen was present in the early atmosphere, as oxygen was not produced until later, by photosynthetic life.
The early atmosphere was dense, with water vapor (H20), carbon dioxide (C02), sulfur (S), nitrogen (N), and hydrochloric acid (HCl). The mixture of gases in the early atmosphere would have caused greenhouse conditions similar to those presently existing on Venus. Since the early Sun during the Hadean era was approximately 25 percent less luminous than today, the atmospheric greenhouse served to keep temperatures close to their present range, where water is stable, and life can form and exist. As the Earth cooled, water vapor condensed to make rain that chemically weathered igneous crust, making sediments. Gases dissolved in the rain produced acids, including carbonic acid (H2CO3), nitric acid (HNO3), sulfuric acid (H2SO4), and hydrochloric acid (HCl). These acids were neutralized by minerals (which are bases) that became sediments, and chemical cycling began. These waters plus dissolved components became the early hydrosphere, and chemical reactions gradually began changing the composition of the atmosphere, getting close to the dawn of life.
Speculation about the origin of life on Earth is of great intellectual interest. In the context of the Hadean, when life most likely arose, scientists are forced to consider different options for the initial trigger of life. Life could have come to Earth on late accreting planetesimals (comets) as complex organic compounds, or perhaps it came from interplanetary dust. If true, this would show how life got to Earth, but not how, when, where, or why it originated. Biological evidence supports the origination of life on Earth, in the deep sea near a hydrothermal vent or in shallow pools with the right chemical mixture. To start, life probably needed an energy source, such as lightning, or perhaps submarine hydrothermal vents, to convert simple organic compounds into building blocks of life—ribonucleic acids (RNA) and amino acids.
See also Archean; Earth; life's origins and early evolution; meteor, meteorite; plate tectonics; solar system.
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