Theories on how Earth formed

Plate Tectonics Concentrations of earthquakes outline several large segments of the lithosphere called plates.  The lithospheric plates "float" on the asthenosphere and move about the Earth's surface. Some plates carry whole continents with them. The theory that describes these plates and their movement is called plate tectonics. Moving Plates At the mid-ocean ridges, new rock is produced by volcanism and the plates move away from each other. Where two plates approach each other, one is thrust downward into the mantle where it is heated and melted. 114k JPEG from Earth's Dynamic System.  Image courtesy Center for Earth and Planetary Studies, National Air and Space Museum. On this map of the Earth, each red triangle represents the location of an active volcano.  Volcanoes are concentrated along plate boundaries.  Oceanic ridges are found where plates spread apart (diverge).  Most surface volcanoes are located near converging plate boundaries (subduction zones), where two plates collide and one plate is driven beneath the other.  Exceptions include volcanic Islands in the mid-Pacific Ocean, which are formed as the plate moves over hotspots in the Earth's mantle.   Drifting Continents Two hundred million years ago all the Earth's continents formed a single land mass called Pangea. The continents began to drift apart about 150 million years ago. Today, the drifting continues. For example, every year North America moves 2-3 centimeters (about 1 inch) farther from Europe. Plate Tectonics The popular theory of drifting continents and oceans is called "plate tectonics."1 (Tectonics is the field of geology which studies the processes which deform the earth’s crust.) The general tenets of the popular theory may be stated as follows. The outer lithospheric shell of the earth consists of a mosaic of rigid plates, each in motion relative to adjacent plates. Deformation occurs at the margins of plates by three basic types of motion: horizontal extension, horizontal slipping, and horizontal compression. Sea-floor spreading occurs where two plates are diverging horizontally (e.g., the Mid-Atlantic Ridge and East Pacific Rise) with new material from the earth's mantle being added between them to form a new oceanic crust. Transform faulting occurs where one plate is slipping horizontally past another (e.g., the San Andreas fault of California and the Anatolian fault of northern Turkey). Subduction occurs where two plates are converging with one plate underthrusting the other producing what is supposed to be compressional deformation (e.g., the Peru-Chile Trench and associated Andes Mountains of South America). In conformity with evolutionary-uniformitarian assumption, popular plate tectonic theory supposes that plates move very slowly — about 2 to 18 centimeters per year. At this rate it would take 100 million years to form an ocean basin or mountain range. Fitting of Continents The idea that the continents can be fitted together like a jigsaw puzzle to form a single super continent is an old one. Especially interesting is how the eastern "bulge" of South America can fit into the southwestern "concavity" of Africa. Recent investigators have used computers to fit the continents. The "Bullard fit"2 gives one of the best reconstructions of how Africa, South America, Europe, and North America may have once touched. There are, however, areas of overlap of continents and one large area which must be omitted from consideration (Central America). There are a number of ways to fit Africa, India, Australia, and Antarctica (only one can be correct!). Reconstructions have been shown to be geometrically feasible which are preposterous to continental drift (e.g., rotation of eastern Australia fits nicely into eastern North America).3 Those who appreciate the overall fit of continents call the evidence "compelling," while others who note gaps, overlaps, or emissions remain skeptical. It is difficult to place probability on the accuracy of reconstructions and one's final judgment is largely subjective. Sea-Floor Spreading Evidence suggesting sea-floor spreading is claimed by many geologists to be the most compelling argument for plate tectonics. In the ocean basins along mid-ocean ridges or rises (and in some shallow seas) plates are thought to be diverging slowly and continuously at a rate of several centimeters yearly. Molten material from the earth's mantle is injected continuously between the plates and cools to form new crust. The youngest crust is claimed to be at the crest of the ocean rise or ridge with older crust farther from the crest. At the time of cooling, the rock acquires magnetism from the earth's magnetic field. Since the magnetic field of earth is supposed by many geologists to have reversed numerous times, during some epochs cooling oceanic crust should be reversely magnetized. If sea-floor spreading is continuous, the ocean floor should possess a magnetic "tape recording" of reversals. A "zebra stripe" pattern of linear magnetic anomalies parallel to the ocean ridge crest has been noted in some areas and potassium-argon dating has been alleged to show older rocks farther from the ridge crest. There are some major problems with this classic and "most persuasive" evidence of sea-floor spreading. First the magnetic bands may not form by reversals of the earth's magnetic field. Asymmetry of magnetic stripes, not symmetry, is the normal occurrence.4 It has been argued that the linear patterns can be caused by several complex interacting factors (differences in magnetic susceptibility, magnetic reversals, oriented tectonic stresses).5 Second, it is doubtful that the magnetic anomalies have been successfully dated. Wesson6 says that potassium-argon dating when correctly interpreted shows no evidence of increasing age with distance from the ridge system. The greater argon content (giving older apparent age) of ocean basalt on the flanks of the ocean ridges can be explained easily by the greater depth and pressure at the time of solidification incorporating original magmatic argon.7 Subduction Corollary to the idea of plate accretion by sea-floor spreading is the notion of plate destruction by subduction. (If sea-floor spreading occurs without plate destruction, the quantity of crust will increase and the volume of the earth must increase!). Subduction theory supposes that converging plates are destroyed below ocean trenches. The island arc or coastal mountain range associated with ocean trench subduction zones is claimed to form by compression as one plate is underthrusting another. The plate that is "subducted" below the trench is thought to be remelted at a depth of up to 700 kilometers. Gravity data indicate low density material of crustal character on the landward side below trenches. (Also, deep and high intensity earthquakes (i.e., earthquakes in Alaska, Peru, Nicaragua, etc.) are assumed to indicate break-up of the underthrust plate. Two major difficulties are encountered by models supposing subduction to explain the modern tectonic phenomena in ocean trenches. First, if subduction theory is correct, there should be compressed, deformed, and thrust faulted sediment on the floors of trenches. Studies of the Peru-Chile Trench and the eastern Aleutian Trench,8 however, show soft flat lying sediment without compression structures. Second, seismic first-motion data indicate that modern earthquakes occurring approximately under trenches and island arcs are often tensional, but only rarely compressional.9 dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. The conductive fluid in the geodynamo is liquid iron in the outer core, and in the solar dynamo is ionized gas at the tachocline. Dynamo theory of astrophysical bodies uses magnetohydrodynamic equations to investigate how the fluid can continuously regenerate the magnetic field. geosyncline Linear trough of subsidence of the Earth's crust, in which vast amounts of sediment accumulate. The filling of a geosyncline with thousands or tens of thousands of feet of sediment is accompanied by folding, crumpling, and faulting of the deposits. Intrusion of crystalline igneous rock and regional uplift complete the transformation into a belt of folded mountains. The concept was introduced by James Hall in 1859 and is basic to the theory of mountain building.