Misplaced Pages

#476523

29-537: The Smartville Block , also called the Smartville Ophiolite , Smartville Complex , or Smartville Intrusive Complex , is a geologic terrane formed in the ocean from a volcanic island arc that was accreted onto the North American Plate during the late Jurassic (~160–150 million years ago). The collision created sufficient crustal heating to drive mineral-laden water up through numerous fissures along

58-500: A bulk composition that is intermediate (SiO 2 wt% = 60.6). The average density of the continental crust is about, 2.83 g/cm (0.102 lb/cu in), less dense than the ultramafic material that makes up the mantle , which has a density of around 3.3 g/cm (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm (0.10 lb/cu in). At 25 to 70 km (16 to 43 mi) in thickness, continental crust

87-631: A certain depth (the Conrad discontinuity ), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character. Most continental crust is dry land above sea level. However, 94% of the Zealandia continental crust region is submerged beneath the Pacific Ocean , with New Zealand constituting 93% of the above-water portion. The continental crust consists of various layers, with

116-561: A layer immediately beneath it. Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries . Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Also, material can be accreted horizontally when volcanic island arcs , seamounts or similar structures collide with

145-439: A steady-state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapid planetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation of cratons (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics). However, this is not generally accepted. In contrast to

174-575: Is considerably thicker than oceanic crust, which has an average thickness of around 7 to 10 km (4.3 to 6.2 mi). Approximately 41% of Earth's surface area and about 70% of the volume of Earth's crust are continental crust. Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what

203-541: Is heated geothermally and injected towards the surface through fractures in the rock created from faulting. Gold and other minerals are precipitated from this fluid and have accumulated in the Sierra Nevadan metamorphic belt region in features called veins . The Smartville Block's eastern margin is the Rescue Lineament-Bear Mountains fault zone , and generally defines the range of gold-bearing veins of

232-409: Is little evidence of continental crust prior to 3.5 Ga . About 20% of the continental crust's current volume was formed by 3.0 Ga. There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga. During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. Proponents of

261-554: Is now called the Cambrian explosion . All continental crust is ultimately derived from mantle-derived melts (mainly basalt ) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur primarily at magmatic arcs associated with subduction . There

290-455: Is that the present spatial relations are incompatible with the inferred geologic histories. Where terranes that lie next to each other possess strata of the same age, they are considered separate terranes only if it can be demonstrated that the geologic evolutions are different and incompatible. There must be an absence of intermediate lithofacies that could link the strata. The concept of tectonostratigraphic terrane developed from studies in

319-494: Is the layer of igneous , metamorphic , and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves . This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust , called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at

SECTION 10

#1732856022477

348-598: The Appalachian belt of North America.... Support for the new hypothesis came not only from structural and lithological studies, but also from studies of faunal biodiversity and palaeomagnetism . When terranes are composed of repeated accretionary events, and hence are composed of subunits with distinct history and structure, they may be called superterranes . Africa Asia Taiwan Tibet Australasia Europe Fennoscandia North America South America Continental crust Continental crust

377-456: The Atlantic Ocean , for example) are termed passive margins . The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic – the main exception to this being recent igneous intrusions . Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming

406-484: The Mediterranean Sea at about 340 Ma. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history. The height of mountain ranges is usually related to the thickness of crust. This results from the isostasy associated with orogeny (mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision. The buoyancy of

435-636: The Mother Lode region, important in the history of the California Gold Rush . The Smartville Ophiolite is divided into sections based on lithology that describe the origin of the rocks it is made up of. In order of increasing depth: the top most unit is called the upper volcanic unit, then the lower volcanic unit, followed by the dike complex, and the plutonic suite at the bottom. The upper volcanic unit consists mainly of pillow lava and massive lavas with turbidites also being common. The turbidites of

464-449: The 1970s of the complicated Pacific Cordilleran orogenic margin of North America , a complex and diverse geological potpourri that was difficult to explain until the new science of plate tectonics illuminated the ability of crustal fragments to "drift" thousands of miles from their origin and attach themselves, crumpled, to an exotic shore. Such terranes were dubbed " accreted terranes " by geologists . Geologist J. N. Carney writes: It

493-709: The contact zone. When these cooled, among the precipitating minerals was gold . Associated with the Western Metamorphic Belt of the Sierra Nevada foothills it extends from the central Sierra Nevada mountain range, due west, under a section of the Central Valley and California Coast Ranges , in northern California. The ophiolitic sequence found in this terrane is one of several major ophiolites found in California. Ophiolites are crustal and upper-mantle rocks from

522-432: The crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of

551-419: The dominant mode of continental crust formation and destruction. It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount. By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it

580-429: The full thickness of the lithosphere . It is a piece of crust that has been transported laterally, usually as part of a larger plate, and is relatively buoyant due to thickness or low density. When the plate of which it was a part subducted under another plate, the terrane failed to subduct, detached from its transporting plate, and accreted onto the overriding plate. Therefore, the terrane transferred from one plate to

609-588: The ocean floor that have been moved on land. Ophiolites have been studied extensively regarding the movement of crustal rocks by plate tectonics . Gold was discovered in California in the mid 1800s. The Smartville Block is named for the small Gold Rush town of Smartville in Yuba County , California. Gold found in the Smartville Block and within the metamorphic belts of the Sierra Nevada foothills were created through hydrothermal processes. Mineral rich groundwater

SECTION 20

#1732856022477

638-615: The oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust ; the oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga , whereas the oldest large-scale oceanic crust (located on the Pacific plate offshore of the Kamchatka Peninsula ) is from the Jurassic (≈180 Ma ), although there might be small older remnants in

667-474: The other. Typically, accreting terranes are portions of continental crust which have rifted off another continental mass and been transported surrounded by oceanic crust, or they are old island arcs formed at some distant subduction zones. A tectonostratigraphic terrane is a fault-bounded package of rocks of at least regional extent characterized by a geologic history that differs from that of neighboring terranes. The essential characteristic of these terranes

696-466: The persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle . There are currently about 7 billion cubic kilometres (1.7 billion cubic miles) of continental crust, but this quantity varies because of the nature of the forces involved. The relative permanence of continental crust contrasts with

725-589: The rocks of the lower volcanic unit. At the base is the plutonic suite that are made up of gabbro , diorite , and other plutonic rocks that are intruded by dikes as well as granitic plutons from the Nevadan Orogeny . Terrane There is also an older usage of the term terrane , which described a series of related rock formations or an area with a preponderance of a particular rock or rock group. A tectonostratigraphic terrane did not necessarily originate as an independent microplate , since it may not contain

754-521: The short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the Alps ). For this reason

783-494: The side of the continent as a result of plate tectonic movements. Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be

812-425: The upper volcanic unit are made of a siliceous rock type and have sandstones and conglomerate tops. Next is the lower volcanic unit, which is mostly pillow lava, breccia , and intrusive rocks. Metasediments of mostly mafic and intermediate tuffaceous composition are cut by dikes (intrusive rock). Below the lower volcanic unit is the dike complex. The dike complex has dacite and rhyolite dikes that cut into

841-506: Was soon determined that these exotic crustal slices had in fact originated as "suspect terranes" in regions at some considerable remove, frequently thousands of kilometers, from the orogenic belt where they had eventually ended up. It followed that the present orogenic belt was itself an accretionary collage, composed of numerous terranes derived from around the circum- Pacific region and now sutured together along major faults. These concepts were soon applied to other, older orogenic belts, e.g.

#476523