The mantle is a layer inside a terrestrial planet and some other rocky planetary bodies. For a mantle to form, the body must be large enough to have undergone the process of planetary differentiation by density. The mantle surrounds the planetary core, the mantle is surrounded by the crust. The terrestrial planets, the Moon, two of Jupiters moons and the asteroid Vesta each have a made of silicate rock. Interpretation of spacecraft data suggests that at least two moons of Jupiter, as well as Titan and Triton each have a mantle made of ice or other solid volatile substances. The interior of Earth, similar to the terrestrial planets, is divided into layers of different composition. The mantle is a layer between the crust and the outer core, Earths mantle is a silicate rocky shell with an average thickness of 2,886 kilometres. The mantle makes up about 84% of Earths volume and it is predominantly solid but in geological time it behaves as a very viscous fluid. The mantle encloses the hot core rich in iron and nickel, past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface. A thin crust, the part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick.
In some places under the ocean the mantle is exposed on the surface of Earth. The mantle is divided into sections which are based upon results from seismology and these layers are the following, the upper mantle, the transition zone, the lower mantle, and anomalous core–mantle boundary with a variable thickness. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the shear velocity is reduced. Inge Lehmann discovered a seismic discontinuity at about 220 km depth, although this discontinuity has been found in other studies, the transition zone is an area of great complexity, it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous, the D layer at the core–mantle boundary separates the mantle from the core. A principal source of the heat that drives plate tectonics is the decay of uranium, thorium.
The mantle differs substantially from the crust in its properties as the direct consequence of the difference in composition
It includes all phenomena resulting from and causing magma within the crust or mantle of the body, to rise through the crust and form volcanic rocks on the surface. Magma from the mantle or lower crust rises through its crust towards the surface, if magma reaches the surface, its behavior depends on the viscosity of the molten constituent rock. Viscous magma produces volcanoes characterised by explosive eruptions, while non-viscous magma produce volcanoes characterised by effusive eruptions pouring large amounts of lava onto the surface, in some cases, rising magma can cool and solidify without reaching the surface. Instead, the cooled and solidified igneous mass crystallises within the crust to form an igneous intrusion, as magma cools the chemicals in the crystals formed are effectively removed from the main mix of the magma, so the chemical content of the remaining magma evolves as it solidifies slowly. Fresh unevolved magma injections can remobilise more evolved magmas, allowing eruptions from more viscous magmas, movement of molten rock in the mantle, caused by thermal convection currents, coupled with gravitational effects of changes on the earths surface drive plate tectonic motion and ultimately volcanism.
Volcanoes are places where magma reaches the earths surface, the type of volcano depends on the location of the eruption and the consistency of the magma. These are formed where magma pushes between existing rock, intrusions can be in the form of batholiths, sills, earthquakes are generally associated with plate tectonic activity, but some earthquakes are generated as a result of volcanic activity. These are formed where water interacts with volcanism and these include geysers, fumaroles and mudpots, they are often used as a source of geothermal energy. The amount of gas and ash emitted by volcanic eruptions has a significant effect on the Earths climate, large eruptions correlate well with some significant climate change events. When the magma cools it solidifies and forms rocks, the type of rock formed depends on the composition of the magma. Magma that reaches the surface to become lava cools rapidly resulting in rocks with small crystals such as basalt, some of this magma may cool extremely rapidly and will form volcanic glass such as obsidian.
Magma that remains trapped below ground in thin intrusions cools slower than magma exposed to the surface, magma that remains trapped in large quantities below ground cools most slowly resulting in rocks with larger crystals - such as granite and gabbro. Existing rocks that come into contact with magma may be melted and assimilated into the magma, other rocks adjacent to the magma may be altered by contact metamorphism or metasomatism as they are affected by the heat and escaping or externally circulating hydrothermal fluids. Volcanism is not confined only to Earth, but is thought to be found on any body having a solid crust, evidence of volcanism should still be found on any body that has had volcanism at some point in its history. It can be surmised that volcanism exists on planets and moons of this type in other systems as well. In 2014, scientists found 70 lava flows which formed on the Moon in the last 100 million years, bimodal volcanism Continental drift Hotspot Volcanic arc Glossary of Volcanic Terms. G. J.
Hudak, University of Wisconsin Oshkosh,2001, crumpler, L. S. and Lucas, S. G. Volcanoes of New Mexico, An Abbreviated Guide For Non-Specialists. New Mexico Museum of Natural History and Science Bulletin, archived from the original on 2007-03-21
The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major plates, where the plates meet, their relative motion determines the type of boundary, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually, tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading.
In this way, the surface of the lithosphere remains the same. This prediction of plate tectonics is referred to as the conveyor belt principle, earlier theories, since disproven, proposed gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere transfers heat by convection and has a nearly adiabatic temperature gradient.
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can be considered a subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea. North America covers an area of about 24,709,000 square kilometers, about 16. 5% of the land area. North America is the third largest continent by area, following Asia and Africa, and the fourth by population after Asia and Europe. In 2013, its population was estimated at nearly 565 million people in 23 independent states, or about 7. 5% of the worlds population, North America was reached by its first human populations during the last glacial period, via crossing the Bering land bridge. The so-called Paleo-Indian period is taken to have lasted until about 10,000 years ago, the Classic stage spans roughly the 6th to 13th centuries. The Pre-Columbian era ended with the migrations and the arrival of European settlers during the Age of Discovery.
Present-day cultural and ethnic patterns reflect different kind of interactions between European colonists, indigenous peoples, African slaves and their descendants, European influences are strongest in the northern parts of the continent while indigenous and African influences are relatively stronger in the south. Because of the history of colonialism, most North Americans speak English, Spanish or French, the Americas are usually accepted as having been named after the Italian explorer Amerigo Vespucci by the German cartographers Martin Waldseemüller and Matthias Ringmann. Vespucci, who explored South America between 1497 and 1502, was the first European to suggest that the Americas were not the East Indies, but a different landmass previously unknown by Europeans. In 1507, Waldseemüller produced a map, in which he placed the word America on the continent of South America. He explained the rationale for the name in the accompanying book Cosmographiae Introductio, for Waldseemüller, no one should object to the naming of the land after its discoverer.
He used the Latinized version of Vespuccis name, but in its feminine form America, following the examples of Europa and Africa. Later, other mapmakers extended the name America to the continent, In 1538. Some argue that the convention is to use the surname for naming discoveries except in the case of royalty, a minutely explored belief that has been advanced is that America was named for a Spanish sailor bearing the ancient Visigothic name of Amairick. Another is that the name is rooted in a Native American language, the term North America maintains various definitions in accordance with location and context. In Canadian English, North America may be used to refer to the United States, usage sometimes includes Greenland and Mexico, as well as offshore islands
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can refer generally to the study of the features of any terrestrial planet. Geology gives insight into the history of the Earth by providing the evidence for plate tectonics, the evolutionary history of life. Geology plays a role in engineering and is a major academic discipline. The majority of data comes from research on solid Earth materials. These typically fall into one of two categories and unconsolidated material, the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three types of rock, igneous and metamorphic. The rock cycle is an important concept in geology which illustrates the relationships between three types of rock, and magma. When a rock crystallizes from melt, it is an igneous rock, the sedimentary rock can be subsequently turned into a metamorphic rock due to heat and pressure and is weathered, eroded and lithified, ultimately becoming a sedimentary rock.
Sedimentary rock may be re-eroded and redeposited, and metamorphic rock may undergo additional metamorphism, all three types of rocks may be re-melted, when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Geologists study unlithified material which typically comes from more recent deposits and these materials are superficial deposits which lie above the bedrock. Because of this, the study of material is often known as Quaternary geology. This includes the study of sediment and soils, including studies in geomorphology and this theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity. This coupling between rigid plates moving on the surface of the Earth and the mantle is called plate tectonics. The development of plate tectonics provided a basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries, mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries, where two plates move apart.
Arcs of volcanoes and earthquakes were explained as convergent boundaries, where one plate subducts under another, transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics provided a mechanism for Alfred Wegeners theory of continental drift and they provided a driving force for crustal deformation, and a new setting for the observations of structural geology
A compression fossil is a fossil preserved in sedimentary rock that has undergone physical compression. While it is uncommon to find animals preserved as compression fossils. The reason for this is that physical compression of the rock often leads to distortion of the fossil, the best fossils of leaves are found preserved in fine layers of sediment that have been compressed in a direction perpendicular to the plane of the deposited sediment. Since leaves are flat, the resulting distortion is minimal. Plant stems and other plant structures do not preserve as well under compression. Typically, only the outline and surface features are preserved in compression fossils. These fossils may be studied while still partially entombed in the rock matrix where they are preserved. Compression fossils are formed most commonly in environments where fine sediment is deposited, such as in river deltas, along rivers, the best rocks in which to find these fossils preserved are clay and shale, although volcanic ash may sometimes preserve plant fossils as well.
When excavated the matrix may be split along the grain or cleavage of the rock. A fossil embedded in the sediment may also split down the middle, comparing slab and counter slab has led to the exposure of a number of fossil forgeries. Chinese palaeontologist Xu Xing came into possession of the counter slab through a fossil hunter, on comparing his fossil with images of Archaeoraptor it became evident that it was a composite fake. His note to National Geographic led to consternation and embarrassment, a certain Lewis Simons investigated the matter on behalf of National Geographic. In October 2000 he reported what he termed, a reptile fossil found in Liaoning Province was described and named Sinohydrosaurus in 1999 by the Beijing Natural History Museum. Hyphalosaurus is now the accepted name
A lithosphere is the rigid, outermost shell of a terrestrial-type planet or natural satellite that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a planet, the crust, is defined on the basis of its chemistry. Earths lithosphere includes the crust and the uppermost mantle, which constitute the hard, the lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere and biosphere through the forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, the study of past and current formations of landscapes is called geomorphology. The concept of the lithosphere as Earth’s strong outer layer was described by A. E. H, love in his 1911 monograph Some problems of Geodynamics and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term lithosphere.
The concept was based on the presence of significant gravity anomalies over continental crust and these ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work Strength and Structure of the Earth and have been broadly accepted by geologists and geophysicists. The temperature at which olivine begins to deform viscously is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle, the mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the mantle by the change in chemical composition that takes place at the Moho discontinuity. Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle and is denser than continental lithosphere, oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick, the thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.
H ∼2 κ t Here, h is the thickness of the mantle lithosphere, κ is the thermal diffusivity for silicate rocks. The age is equal to L/V, where L is the distance from the spreading centre of mid-oceanic ridge. Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years and this is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones, geoscientists can directly study the nature of the subcontinental mantle by examining mantle xenoliths brought up in kimberlite and other volcanic pipes. The histories of these xenoliths have been investigated by many methods, such studies have confirmed that mantle lithospheres below some cratons have persisted for periods in excess of 3 billion years, despite the mantle flow that accompanies plate tectonics.
Cryosphere Geosphere Kola Superdeep Borehole Plate tectonics Solid Earth Chernicoff, Whitney, earths Crust and Asthenosphere Crust and Lithosphere
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps, subducting oceanic slabs carry large amounts of water, this water lowers the melting temperature of the above mantle wedge. Melting of the wedge can be contributed to depressurization due to the flow in the wedge. This melt gives rise to associated volcanism on the earth’s surface and this volcanism can be seen around the world in places such as Japan and Indonesia. Magmas produced in subduction zone regions have high volatile contents and this water is derived from the breakdown of hydrous minerals in the subducting slab, as well as water in the oceanic plate from percolation of seawater. This water rises from the slab to the overriding mantle wedge. The water lowers the temperature of the wedge and leaves behind melt inclusions that can be measured in the associated arc volcanic rocks.
The forearc mantle extends from where the subducting slab meets the cold nose of the mantle wedge, low seismic attenuation, and high seismic velocities characterize this region. There is a boundary between this low attenuation region and a high attenuation region on the side of the arc volcanoes. To image the mantle wedge region below volcanic arcs P-wave, S-wave and these tomographic images show a low velocity, high attenuation region above the subducting slab. The slowest velocities in these volcanic arc regions are Vp=7.4 km·s−1, mantle wedge regions that do not have associated arc volcanism do not show such low velocities. This can be attributed to the production in the mantle wedge. Flow in mantle wedges has important effects on the structure, overall mantle circulation. Minerals are anisotropic and have the ability to align themselves within the mantle when exposed to strain and these mineral alignments can be seen using seismic imaging, as waves will travel through different orientations of a mineral at different speeds.
Shear strain associated with mantle flow will align the fast direction of pyroxene and olivine grains in the direction of flow and this is the most common theory on flow within the mantle, although opposing theories do exist. Flow within the wedge is parallel to the crust until it reaches the relatively cooler nose of the wedge. The nose of the wedge is generally isolated from the mantle flow. Studies have shown that magmas that produce island arcs are more oxidized than the magmas that are produced at mid-ocean ridges and this relative degree of oxidation has been determined by the iron oxidation state of fluid inclusions in glassy volcanic rocks
The Farallon Plate was an ancient oceanic plate that began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart during the Jurassic period. It is named for the Farallon Islands, which are located just west of San Francisco, over time, the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. These fragments from elsewhere are called terranes, much of western North America is composed of these accreted terranes. The understanding of the Farallon Plate is rapidly evolving as details from seismic tomography provide improved details of the submerged remnants, since the North American west coast shows a convoluted structure, significant work has been required to resolve the complexity. In 2013 a new and more nuanced explanation emerged, proposing two additional now-subducted plates which would account for some of the complexity. As data accumulated, a view developed that one large oceanic plate, the Farallon plate, acted as a conveyor belt, conveying terranes to North Americas west coast.
As the continent overran the subducting Farallon plate, the plate became subducted into the mantle below the continent. When the plates converged, the oceanic plate sank into the mantle to form a slab below the lighter continent. However this simple model was unable to explain many terrane complexities, one such large slab wall runs from north-west Canada to the eastern U. S. and extends to Central America, this slab wall had traditionally been associated with the subducting Farallon plate. Sigloch and Mihalynuk proposed that the Farallon should be partitioned into Northern Farallon, Mezcalera, the overridden segment is replaced by an incipient South Farallon trench. 160–155 Myr ago the Rocky Mountain deformation begins, recorded by a synorogenic clastic wedge, the Franciscan subduction complex on the South Farallon plate begins. 125 Myr ago the collision of the North America margin with an archipelago of terranes begins and this broad expanse causes strong deformations and creates the Sevier Mountains and the Canadian Rocky Mountains.
124–90 Myr ago the Omenica magmatic belts are formed in the Pacific Northwest along with a gradual override of the Mezcalera promontory by the Pacific Northwest,85 Myr ago the South Farallon trench moves westward after accretion of the Shatsky Rise Conjugate plateau. Sonora volcanism results from the slab sinking, the Tarahumara ignimbrite province is formed. 85–55 Myr ago Strong transpressive coupling of Farallon plate to terranes produces the buoyant Shatsky Rise, the Laramide orogeny results from basement uplift more than 1,000 km inland. 72–69 Myr ago the Angayucham arc, is overridden by North America, northward shuffle of Insular terrane, Intermontane terrane, and Angayucham terranes along margin. 55–50 Myr ago saw the override of the Cascadia Root arc by the Pacific Northwest along with accretion of the Siletzia and Pacific Rim terranes