78-680: Parkfield may refer to: Parkfield, California , an unincorporated community in California, United States Parkfield earthquake , various earthquakes that have occurred near Parkfield Parkfield Interventional EQ Fieldwork , land art near Parkfield Parkfield, Cornwall , a hamlet in Cornwall, United Kingdom Parkfield, a park and stadium serving Potters Bar Town F.C. in Potters Bar, Hertfordshire, United Kingdom Parkfield Colliery ,
156-624: A 6.0 Mw earthquake struck at 10:15 am Pacific Daylight Time . Parkfield is the most closely observed earthquake zone in the world. Scientists constantly measure the strain in rocks, heat flow, microseismicity, and geomagnetism around Parkfield. The observations of the San Andreas fault in Parkfield are hoped to help scientists better understand the physics of earthquakes and faulting; information gathered from Parkfield may still be used someday to issue predictions for major earthquakes along
234-439: A consequence, a powerful source generating plate motion is the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into
312-450: A few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that Earth would be much older, and that its core was still sufficiently hot to be liquid. By 1915, after having published a first article in 1912, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans . In that book (re-issued in four successive editions up to
390-636: A former coal mine in South Gloucestershire, United Kingdom Parkfield High School , a former secondary school in Wolverhampton, West Midlands, United Kingdom Parkfield School, a free school at Bournemouth Airport in Dorset, United Kingdom Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Parkfield . If an internal link led you here, you may wish to change
468-579: A layer of basalt (sial) underlies the continental rocks. However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by
546-400: A misnomer as there is no force "pushing" horizontally, indeed tensional features are dominant along ridges. It is more accurate to refer to this mechanism as "gravitational sliding", since the topography across the whole plate can vary considerably and spreading ridges are only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of
624-558: A secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of van Bemmelen . This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin. Alfred Wegener , being a meteorologist , had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift ; however, these forces were considered far too small to cause continental motion as
702-407: A solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Many distinguished scientists of the time, such as Harold Jeffreys and Charles Schuchert , were outspoken critics of continental drift. Despite much opposition, the view of continental drift gained support and a lively debate started between "drifters" or "mobilists" (proponents of
780-478: A static Earth without moving continents up until the major breakthroughs of the early sixties. Two- and three-dimensional imaging of Earth's interior ( seismic tomography ) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density
858-438: Is mantle convection from buoyancy forces. How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of mechanisms that are thought to exist related to the dynamics of the mantle that influence plate motion which are primary (through
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#1732765546487936-471: Is also due to the presence of serpentinite. No drilling has been done at Hollister to confirm this. Plate tectonics Plate tectonics (from Latin tectonicus , from Ancient Greek τεκτονικός ( tektonikós ) 'pertaining to building') is the scientific theory that Earth 's lithosphere comprises a number of large tectonic plates , which have been slowly moving since 3–4 billion years ago. The model builds on
1014-750: Is an unincorporated community in Monterey County , California . It is located on Little Cholame Creek 21 miles (34 km) east of Bradley , at an elevation of 1,529 feet (466 m). As of 2007, road signs announce the population as 18. Parkfield is located in the Temblor Range between the San Joaquin Valley and the Central Coast , at an elevation of 1,529 feet (466 m) above sea level. Mining and homesteading used to be prosperous activities in this community, but
1092-527: Is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers. Continental crust is formed through arc volcanism and accretion of terranes through plate tectonic processes. Oceanic crust is denser than continental crust because it has less silicon and more of the heavier elements than continental crust . As a result of this density difference, oceanic crust generally lies below sea level , while continental crust buoyantly projects above sea level. Average oceanic lithosphere
1170-461: Is called a plate boundary . Plate boundaries are where geological events occur, such as earthquakes and the creation of topographic features such as mountains , volcanoes , mid-ocean ridges , and oceanic trenches . The vast majority of the world's active volcanoes occur along plate boundaries, with the Pacific plate's Ring of Fire being the most active and widely known. Some volcanoes occur in
1248-533: Is called the geosynclinal theory . Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves —have similar shapes and seem to have once fitted together. Since that time many theories were proposed to explain this apparent complementarity, but
1326-440: Is hoped that SAFOD observations will provide insight into the source mechanisms of these small earthquakes, which can be scaled up in an effort to understand larger events. The drilling discovered a mineral called serpentinite at the site of origin of the 2004 earthquake. Serpentinite can transform into talc , a very soft mineral, thus facilitating easy slippage of plates. It has been hypothesized that aseismic creep at Hollister
1404-492: Is in motion, presents a problem. The same holds for the African, Eurasian , and Antarctic plates. Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them (see the paragraph on Mantle Mechanisms). This gravitational sliding represents
1482-408: Is invoked as the major driving force, through slab pull along subduction zones. Gravitational sliding away from a spreading ridge is one of the proposed driving forces, it proposes plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from
1560-415: Is still advocated to explain the break-up of supercontinents during specific geological epochs. It has followers amongst the scientists involved in the theory of Earth expansion . Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below Earth's crust, which then provide basal friction to the lithosphere. This theory, called "surge tectonics",
1638-488: Is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate. One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than other types of plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than
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#17327655464871716-407: Is typically 100 km (62 mi) thick. Its thickness is a function of its age. As time passes, it cools by conducting heat from below, and releasing it raditively into space. The adjacent mantle below is cooled by this process and added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it
1794-435: Is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas find their roots in the early 1930s in the works of Beloussov and van Bemmelen , which were initially opposed to plate tectonics and placed the mechanism in a fixed frame of vertical movements. Van Bemmelen later modified the concept in his "Undation Models" and used "Mantle Blisters" as
1872-567: The Appalachian Mountains of North America are very similar in structure and lithology . However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. As it
1950-470: The Pacific Plate (see plate tectonics ). There is a bridge across the creek with piers on either side that have shifted more than five feet relative to one another due to aseismic creep since the bridge was constructed in 1936. Google satellite images show that the bridge, which Simon Winchester described as "bent" and having cracked and patched pavement, has been replaced since Winchester's book
2028-671: The San Andreas Fault Observatory at Depth (SAFOD). The goal of SAFOD is to drill a hole nearly 2.5 miles (4 kilometers) into the Earth's crust, across the San Andreas Fault. The specific target of the probe was a patch of fault known to generate sequences of repeating microearthquakes of around magnitude 1.0. The drilling was completed in the midyear of 2005, and an array of sensors was installed to capture and record earthquakes that happen near this area. It
2106-422: The chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates , which ride on
2184-736: The fluid-like solid the asthenosphere . Plate motions range from 10 to 40 millimetres per year (0.4 to 1.6 in/year) at the Mid-Atlantic Ridge (about as fast as fingernails grow), to about 160 millimetres per year (6.3 in/year) for the Nazca plate (about as fast as hair grows). Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium ) and continental crust ( sial from silicon and aluminium ). The distinction between oceanic crust and continental crust
2262-473: The lithosphere and asthenosphere . The division is based on differences in mechanical properties and in the method for the transfer of heat . The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction , whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with
2340-546: The Earth's rotation and the Moon as main driving forces for the plates. The vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate. The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem
2418-549: The San Andreas fault and around the world. Since 1985, the United States Geological Survey has been working on a project known as "The Parkfield Experiment", a long-term research project on the San Andreas Fault. "The experiment's purpose is to better understand the physics of earthquakes — what actually happens on the fault and in the surrounding region before, during and after an earthquake." In 2004, work began just north of Parkfield on
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2496-541: The actual motions of the Pacific plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces. The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics . The development of
2574-478: The assumption of a solid Earth made these various proposals difficult to accept. The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of Earth . This had previously been estimated by its cooling rate under the assumption that Earth's surface radiated like a black body . Those calculations had implied that, even if it started at red heat , Earth would have dropped to its present temperature in
2652-399: The asthenosphere. This theory was launched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the 20th century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged
2730-476: The base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches. Recent models indicate that trench suction plays an important role as well. However, the fact that the North American plate is nowhere being subducted, although it
2808-495: The bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals , published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley. Simultaneous advances in early seismic imaging techniques in and around Wadati–Benioff zones along
2886-416: The concept of continental drift , an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s. The processes that result in plates and shape Earth's crust are called tectonics . Tectonic plates also occur in other planets and moons. Earth's lithosphere, the rigid outer shell of
2964-413: The concept was of continents plowing through oceanic crust. Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929. However, in the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s),
3042-415: The deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. For much of the first quarter of the 20th century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through
3120-534: The discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model. In 1973, George W. Moore of the USGS and R. C. Bostrom presented evidence for a general westward drift of Earth's lithosphere with respect to the mantle, based on the steepness of the subduction zones (shallow dipping towards the east, steeply dipping towards the west). They concluded that tidal forces (the tidal lag or "friction") caused by Earth's rotation and
3198-466: The driving force for horizontal movements, invoking gravitational forces away from the regional crustal doming. The theories find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators). The mechanism
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3276-473: The final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this ( Abraham Ortelius , Antonio Snider-Pellegrini , Eduard Suess , Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and
3354-695: The forces acting upon it by the Moon are a driving force for plate tectonics. As Earth spins eastward beneath the Moon, the Moon's gravity ever so slightly pulls Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). Since 1990 this theory is mainly advocated by Doglioni and co-workers ( Doglioni 1990 ), such as in a more recent 2006 study, where scientists reviewed and advocated these ideas. It has been suggested in Lovett (2006) that this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on
3432-588: The geographical latitudinal and longitudinal grid of Earth itself. These systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to Earth's equator and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page), therefore, invoked many of
3510-718: The interiors of plates, and these have been variously attributed to internal plate deformation and to mantle plumes. Tectonic plates may include continental crust or oceanic crust, or both. For example, the African plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. Some pieces of oceanic crust, known as ophiolites , failed to be subducted under continental crust at destructive plate boundaries; instead these oceanic crustal fragments were pushed upward and were preserved within continental crust. Three types of plate boundaries exist, characterized by
3588-412: The large scale convection cells) or secondary. The secondary mechanisms view plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. This is due to the inflow of mantle material related to the downward pull on plates in subduction zones at ocean trenches. Slab pull may occur in a geodynamic setting where basal tractions continue to act on
3666-410: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Parkfield&oldid=1230486927 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Parkfield, California Parkfield (formerly Russelsville )
3744-421: The lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges. Mantle plumes and hot spots are also postulated to impinge on the underside of tectonic plates. Slab pull : Scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along
3822-403: The lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open, and a recent paper by Hofmeister et al. (2022) revived the idea advocating again the interaction between
3900-405: The many geographical, geological, and biological continuities between continents. In 1912, the meteorologist Alfred Wegener described what he called continental drift, an idea that culminated fifty years later in the modern theory of plate tectonics. Wegener expanded his theory in his 1915 book The Origin of Continents and Oceans . Starting from the idea (also expressed by his forerunners) that
3978-429: The matching of the rock formations along these edges. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris , and the therapsid or mammal-like reptile Lystrosaurus , all widely distributed over South America, Africa, Antarctica, India, and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in
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#17327655464874056-584: The mines were exhausted below economic recovery levels, and the industry moved elsewhere. Today, Parkfield is a small town of about 18 people, most of whom are ranchers and farmers. There is a small tourism industry in the town based on equine-related events, hunting, a bluegrass music festival, and Parkfield's unique earthquake history (see the Geology section below). The Parkfield motto is, "Be here when it happens." A post office operated at Parkfield from 1884 to 1954. The town's original name of Russelsville
4134-568: The motion picture of the Atlantic region", processes that anticipated seafloor spreading and subduction . One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism . This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies,
4212-438: The motion. At a subduction zone the relatively cold, dense oceanic crust sinks down into the mantle, forming the downward convecting limb of a mantle cell , which is the strongest driver of plate motion. The relative importance and interaction of other proposed factors such as active convection, upwelling inside the mantle, and tidal drag of the Moon is still the subject of debate. The outer layers of Earth are divided into
4290-470: The north pole, and each continent, in fact, shows its own "polar wander path". During the late 1950s, it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956, and by Warren Carey in a symposium held in March 1956. The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on
4368-407: The oceanic crust is suggested to be in motion with the continents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are: Forces that are small and generally negligible are: For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and
4446-437: The oceanic lithosphere and the thicker continental lithosphere, each topped by its own kind of crust. Along convergent plate boundaries , the process of subduction carries the edge of one plate down under the other plate and into the mantle . This process reduces the total surface area (crust) of the Earth. The lost surface is balanced by the formation of new oceanic crust along divergent margins by seafloor spreading, keeping
4524-589: The planet including the crust and upper mantle , is fractured into seven or eight major plates (depending on how they are defined) and many minor plates or "platelets". Where the plates meet, their relative motion determines the type of plate boundary (or fault ): convergent , divergent , or transform . The relative movement of the plates typically ranges from zero to 10 cm annually. Faults tend to be geologically active, experiencing earthquakes , volcanic activity , mountain-building , and oceanic trench formation. Tectonic plates are composed of
4602-470: The planet. In a paper by it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to
4680-399: The plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab). Furthermore, slabs that are broken off and sink into the mantle can cause viscous mantle forces driving plates through slab suction. In the theory of plume tectonics followed by numerous researchers during the 1990s, a modified concept of mantle convection currents
4758-426: The plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted. This view however has been contradicted by a recent study which found that
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#17327655464874836-408: The present continents once formed a single land mass (later called Pangaea ), Wegener suggested that these separated and drifted apart, likening them to "icebergs" of low density sial floating on a sea of denser sima . Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast Antonio Snider-Pellegrini had drawn on his maps, and from
4914-459: The relationships recognized during this pre-plate tectonics period to support their theories (see reviews of these various mechanisms related to Earth rotation the work of van Dijk and collaborators). Of the many forces discussed above, tidal force is still highly debated and defended as a possible principal driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond
4992-428: The relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called "polar wander" (see apparent polar wander ) (i.e., it was assumed that the north pole location had been shifting through time). An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to
5070-399: The ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis. This force is regarded as a secondary force and is often referred to as " ridge push ". This is
5148-491: The southern hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents , and went further than Wegener in recognising the strong links between the Gondwana fragments. Wegener's work was initially not widely accepted, in part due to a lack of detailed evidence but mostly because of the lack of a reasonable physically supported mechanism. Earth might have
5226-481: The theory of plate tectonics was the scientific and cultural change which occurred during a period of 50 years of scientific debate. The event of the acceptance itself was a paradigm shift and can therefore be classified as a scientific revolution, now described as the Plate Tectonics Revolution . Around the start of the twentieth century, various theorists unsuccessfully attempted to explain
5304-502: The theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements of plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove. In 1941, Otto Ampferer described, in his publication "Thoughts on
5382-476: The total surface area constant in a tectonic "conveyor belt". Tectonic plates are relatively rigid and float across the ductile asthenosphere beneath. Lateral density variations in the mantle result in convection currents, the slow creeping motion of Earth's solid mantle. At a seafloor spreading ridge , plates move away from the ridge, which is a topographic high, and the newly formed crust cools as it moves away, increasing its density and contributing to
5460-429: The trenches bounding many continental margins, together with many other geophysical (e.g., gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins. All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift
5538-467: The way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere . Dissipation of heat from the mantle is the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. As
5616-531: Was feasible. The theory of plate tectonics was defined in a series of papers between 1965 and 1967. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology . In the late 19th and early 20th centuries, geologists assumed that Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what
5694-599: Was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones. For shorter or longer distances, the subduction zone, and therefore also the mean, thickness becomes smaller or larger, respectively. Continental lithosphere is typically about 200 km (120 mi) thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The location where two plates meet
5772-424: Was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt , the prevailing concept during the first half of the twentieth century was that there were two types of crust, named "sial" (continental type crust) and "sima" (oceanic type crust). Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that
5850-443: Was popularized during the 1980s and 1990s. Recent research, based on three-dimensional computer modelling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere. Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity
5928-563: Was published, and a sliding joint installed; however, the west piers of the bridge remain displaced to the north. The bridge sports signage announcing to drivers that they are crossing the plate boundary. Since at least 1857, Parkfield has experienced a magnitude 6 or greater earthquake about every 22 years. In 1985, the US Geological Survey predicted that there would be a comparably-sized earthquake in this community by 1993, but no such event came until September 28, 2004, when
6006-619: Was rejected by the post office, and Parkfield was chosen from the town's park-like setting among oak trees. The ZIP Code is 93451, and the community is inside area code 805 . Parkfield lies along the San Andreas Fault , one of the longest and most active faults in the United States, which appears in the town as a seasonally dry creek bed. The fault marks the divide between the North American Plate and
6084-575: Was supported in this by researchers such as Alex du Toit ). Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick . Furthermore, the Caledonian Mountains of Europe and parts of
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