Isostasy

Isostasy (Greek ísos "equal", stásis "standstill") is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

This concept is invoked to explain how different topographic heights can exist at Earth's surface. When a certain area of Earth's crust reaches the state of isostasy, it is said to be in isostatic equilibrium. Isostasy does not upset equilibrium but instead restores it (a negative feedback). It is generally accepted [1] that Earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement. Certain areas (such as the Himalayas) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, which are still rising, by proposing that their elevation is being supported by the force of the impacting Indian Plate; the Basin and Range Province of the Western US is another example of a region not in isostatic equilibrium.)

Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes such as the Hawaiian Islands.

In the simplest example, isostasy is the principle of buoyancy wherein an object immersed in a fluid is buoyed with a force equal to the weight of the displaced fluid. On a geological scale, isostasy can be observed where Earth's strong crust or lithosphere exerts stress on the weaker mantle or asthenosphere, which, over geological time, flows laterally such that the load is accommodated by height adjustments.

The general term 'isostasy' was coined in 1882 by the American geologist Clarence Dutton.[2][3][4]

Models

Three principal models of isostasy are used:

  1. The AiryHeiskanen model – where different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant density
  2. The PrattHayford model – where different topographic heights are accommodated by lateral changes in rock density.
  3. The Vening Meinesz, or flexural isostasy model – where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending.

Airy and Pratt isostasy are statements of buoyancy, whereas flexural isostasy is a statement of buoyancy when deflecting a sheet of finite elastic strength.

Airy

Airy Isostasy
Airy isostasy, in which a constant-density crust floats on a higher-density mantle, and topography is determined by the thickness of the crust.
Backstripping and eustasy correction
Airy isostasy applied to a real-case basin scenario, where the total load on the mantle is composed by a crustal basement, lower-density sediments and overlying marine water

The basis of the model is Pascal's law, and particularly its consequence that, within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation (surface of hydrostatic compensation). In other words:

h1⋅ρ1 = h2⋅ρ2 = h3⋅ρ3 = ... hn⋅ρn

For the simplified picture shown the depth of the mountain belt roots (b1) are:




where is the density of the mantle (ca. 3,300 kg m−3) and is the density of the crust (ca. 2,750 kg m−3). Thus, we may generally consider:


b1 ≅ 5⋅h1

In the case of negative topography (i.e., a marine basin), the balancing of lithospheric columns gives:




where is the density of the mantle (ca. 3,300 kg m−3), is the density of the crust (ca. 2,750 kg m−3) and is the density of the water (ca. 1,000 kg m−3). Thus, we may generally consider:


b2 ≅ 3.2⋅h2

Pratt

For the simplified model shown the new density is given by: , where is the height of the mountain and c the thickness of the crust.

Vening Meinesz / flexural

Local-regional isostasy - flexure, elastic thickness
Cartoon showing the isostatic vertical motions of the lithosphere (grey) in response to a vertical load (in green)

This hypothesis was suggested to explain how large topographic loads such as seamounts (e.g. Hawaiian Islands) could be compensated by regional rather than local displacement of the lithosphere. This is the more general solution for lithospheric flexure, as it approaches the locally compensated models above as the load becomes much larger than a flexural wavelength or the flexural rigidity of the lithosphere approaches zero.

Implications

Deposition and erosion

When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata eroded away and the lower layers rebounded upwards.

An analogy may be made with an iceberg—it always floats with a certain proportion of its mass below the surface of the water. If snow falls to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice melts off the top of the iceberg, the remaining iceberg will rise. Similarly, Earth's lithosphere "floats" in the asthenosphere.

Plate tectonics

When continents collide, the continental crust may thicken at their edges in the collision. If this happens, much of the thickened crust may move downwards rather than up as with the iceberg analogy. The idea of continental collisions building mountains "up" is therefore rather a simplification. Instead, the crust thickens and the upper part of the thickened crust may become a mountain range.

However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution for better understanding .

Mantle convection

Note also that the perfect isostatic equilibrium is possible only if mantle material is in rest. However, thermal convection is present in the mantle. In such a case only the more general hypothesis of DDI (Deep Dynamic Isostasy) can be satisfied. [5]

Ice sheets

The formation of ice sheets can cause Earth's surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels. In this way, it is possible to find former sea cliffs and associated wave-cut platforms hundreds of metres above present-day sea level. The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing.

In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements. It can cause changes in Earth's gravitational field and rotation rate, polar wander, and earthquakes.

Lithosphere-asthenosphere boundary

The hypothesis of isostasy is used often to determine position of LAB (i.e., Lithosphere-Asthenosphere Boundary)[6]

Relative sea level change

Eustasy is another cause of relative sea level change quite different from isostatic causes. The term eustasy or eustatic refers to changes in the volume of water in the oceans, usually due to global climate change. When Earth's climate cools, a greater proportion of water is stored on land masses in the form of glaciers, snow, etc. This results in falling global sea levels (relative to a stable land mass). The refilling of ocean basins by glacial meltwater at the end of ice ages is an example of eustatic sea level rise.

A second significant cause of eustatic sea level rise is thermal expansion of sea water when Earth's mean temperature increases. Current estimates of global eustatic rise from tide gauge records and satellite altimetry is about +3 mm/a (see 2007 IPCC report). Global sea level is also affected by vertical crustal movements, changes in Earth's rotation rate, large-scale changes in continental margins and changes in the spreading rate of the ocean floor.

When the term relative is used in context with sea level change, the implication is that both eustasy and isostasy are at work, or that the author does not know which cause to invoke.

Post-glacial rebound can also be a cause of rising sea levels. When the sea floor rises, which it continues to do in parts of the northern hemisphere, water is displaced and has to go elsewhere.

See also

References

  1. ^ A.B. Watts, Isostasy and flexure of the lithosphere,Cambridge Univ. Press., 2001
  2. ^ Dutton, Clarence (1882). "Physics of the Earth's crust; discussion". American Journal of Science. 3. 23 (April): 283–290. doi:10.2475/ajs.s3-23.136.283.
  3. ^ Orme, Antony (2007). "Clarence Edward Dutton (1841–1912): soldier, polymath and aesthete". Geological Society, London, Special Publications. 287: 271–286. doi:10.1144/SP287.21.}
  4. ^ "Clarence Edward Dutton" (PDF). 1958. Retrieved 7 October 2014.
  5. ^ L. Czechowski PAGEOPH, https://link.springer.com/article/10.1007/s00024-019-02093-8
  6. ^ Grinc, M., Zeyen, H., Bielik, M., 2014. Contributions to Geophysics and Geodesy,Vol. 44/2, 115–131.

Further reading

  • Lisitzin, E. (1974) "Sea level changes". Elsevier Oceanography Series, 8
  • Watts, AB (2001). Isostasy and Flexure of the Lithosphere. Cambridge University Press. ISBN 0-521-00600-7. A very complete overview with much of the historical development.

External links

Anthony Brian Watts

Anthony Brian Watts FRS is a British marine geologist and geophysicist and Professor of Marine Geology and Geophysics in the Department of Earth Sciences, at the University of Oxford.

Clarence Dutton

Clarence Edward Dutton (May 15, 1841 – January 4, 1912) was an American geologist and US Army officer. Dutton was born in Wallingford, Connecticut on May 15, 1841. He graduated from Yale College in 1860 and took postgraduate courses there until 1862, when he enlisted in the 21st Connecticut Volunteer Infantry; he fought at Fredericksburg, Suffolk, Nashville and Petersburg.

In 1875, he began work as a geologist for the U.S. Geological Survey. Working chiefly in the Colorado Plateau region, he wrote several classic papers, including geological studies of the high plateaus of Utah (1879–80), the Cenozoic history of the Grand Canyon district (1882), and the Charleston, South Carolina, earthquake of 1886. As head of the division of volcanic geology at the USGS, he studied volcanism in Hawaii, California, and Oregon; his studies of basaltic lava flows in Hawaii led him to introduce the native Hawaiian language terms "ʻaʻā" and "pāhoehoe" for cool, clinkery lava flows and smooth, billowy lava flows respectively. He helped coordinate the scientific response to a large earthquake in the Mexican state of Sonora in 1887. He was elected a member of the National Academy of Sciences in 1884.

In 1886, Dutton led a USGS party to Crater Lake, Oregon. His team carried a half-ton survey boat, the Cleetwood, up the steep mountain slope and lowered it 2,000 feet (610 m) into the lake. From the Cleetwood, Dutton used piano wire with lead weights to measure the depth of the lake at 168 different points. The survey team determined the lake was 1,996 feet (608 m) deep. The currently-accepted maximum depth figure, measured by sonar, is 1,943 feet (592 m).In a footnote to an 1882 review in the American Journal of Science, Dutton coined the term "isostasy". He later stated:

' ' In an unpublished paper I have used the terms isostatic and isostacy (sic) to express that condition of the terrestrial surface which would follow from the flotation of the crust upon a liquid or highly plastic substratum; — different portions of the crust being of unequal density' ' Thus he realised that there is a general balance within the Earth's crust, with lighter weight blocks coming to stand higher than adjacent blocks with higher density, an idea first expressed by Pratt and Airy in the 1850s. Dutton elaborated these ideas in his address to the Philosophical Society of Washington in 1889. When this was printed in 1892 the term isostasy was formally proposed, Dutton having, on the advice of Greek scholars, changed the ‘c’ to an ‘s’.

Dutton was a close associate of John Wesley Powell, G.K. Gilbert, and William Henry Holmes at the USGS. He was an energetic and effective field geologist: in 1875-1877 Dutton's field party mapped 12,000 square miles (31,000 km2) of the high plateaus of southern Utah, an area of rugged topography and poor access.

Dutton had a distinctive flair for literary description, and is best remembered today for his colorful (and sometimes flamboyant) descriptions of the geology and scenery of the Grand Canyon region of Arizona. "Dutton first taught the world to look at that country and see it as it was... Dutton is almost as much the genius loci of the Grand Canyon as Muir is of Yosemite" -- Wallace Stegner, Beyond the Hundredth Meridian.

In 1891 he retired from the USGS to serve as commander of the arsenal of San Antonio, Texas; then as ordnance officer of the department of Texas. After retiring from the Army in 1901, he returned to the study of geology. Dutton spent his last years at the home of his son in Englewood, New Jersey.

Depression (geology)

In geology, a depression is a landform sunken or depressed below the surrounding area. Depressions form by various mechanisms.

Erosion-related:

Blowout: a depression created by wind erosion typically in either a partially vegetated sand dune ecosystem or dry soils (such as a post-glacial loess environment).

Glacial valley: a depression carved by erosion by a glacier.

River valley: a depression carved by fluvial erosion by a river.

Area of subsidence caused by the collapse of an underlying structure such as sinkholes in karst terrain.

Sink: an endorheic depression generally containing a persistent or intermittent (seasonal) lake, a salt flat (playa) or dry lake, or an ephemeral lake.Collapse-related:

Sinkhole: a depression formed as a result of the collapse of rocks lying above a hollow. This is common in karst regions.

Kettle: a shallow, sediment-filled body of water formed by melting glacial remnants in terminal moraines.

Thermokarst hollow: caused by volume loss of the ground as the result of permafrost thawing.Impact-related:

Impact crater: a depression created by an impact such as a meteorite crater.Sedimentary-related:

Sedimentary basin: in sedimentology, an area thickly filled with sediment in which the weight of the sediment further depresses the floor of the basin.Structural or tectonic-related:

Structural basin: a syncline-like depression; a region of tectonic downwarping as a result of isostasy (the Hawaiian Trough is an example) or subduction (such as the Chilean Central Valley).

Graben or rift valley: fallen and typically linear depressions or basins created by rifting in a region under tensional tectonic forces.

Pull-apart basin caused by offset in a strike slip or transform fault (example: the Dead Sea area).

Oceanic trench: a deep linear depression on the ocean floor. Oceanic trenches are caused by subduction (when one tectonic plate is pushed underneath another) of oceanic crust beneath either oceanic crust or continental crust.

A basin formed by an ice sheet: an area depressed by the weight of the ice sheet resulting in post-glacial rebound after the ice melts (the area adjacent to the ice sheet may be pulled down to create a peripheral depression.)Volcanism-related:

Caldera: a volcanic depression resulting from collapse following a volcanic eruption.

Pit crater: a volcanic depression smaller than a caldera formed by a sinking, or caving in, of the ground surface lying over a void.

Maar: a depression resulting from phreatomagmatic eruption or diatreme explosion.

Earth's crust

The Earth's crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of lithosphere: a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of the Earth into space.

The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of peridotite and so is significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity.

The crust of the Earth is of two distinctive types:

Oceanic: 5 km (3 mi) to 10 km (6 mi) thick and composed primarily of denser, more mafic rocks, such as basalt, diabase, and gabbro.

Continental: 30 km (20 mi) to 50 km (30 mi) thick and mostly composed of less dense, more felsic rocks, such as granite.Because both continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. This is isostasy, and it's also one of the reasons continental crust is higher than oceanic: continental is less dense and so "floats" higher. As a result, water pools in above the oceanic crust, forming the oceans.

The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C (392 °F) to 400 °C (752 °F) at the boundary with the underlying mantle. The temperature increases by as much as 30 °C (54 °F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust.

Epeirogenic movement

In geology, epeirogenic movement (from Greek epeiros, land, and genesis, birth) is upheavals or depressions of land exhibiting long wavelengths and little folding apart from broad undulations. The broad central parts of continents are called cratons, and are subject to epeirogeny. The movement may be one of subsidence toward, or of uplift from, the centre of the Earth. The movement is caused by a set of forces acting along an Earth radius, such as those contributing to isostasy and faulting in the lithosphere.

Epeirogenic movement can be permanent or transient. Transient uplift can occur over a thermal anomaly due to convecting anomalously hot mantle, and disappears when convection wanes. Permanent uplift can occur when igneous material is injected into the crust, and circular or elliptical structural uplift (that is, without folding) over a large radius (tens to thousands of km) is one characteristic of a mantle plume.In contrast to epeirogenic movement, orogenic movement is a more complicated deformation of the Earth's crust, associated with crustal thickening, notably associated with the convergence of tectonic plates. Such plate convergence forms orogenic belts that are characterized by "the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism".Epeirogenic movements may divert rivers and create drainage divides by upwarping of the crust along axes. Example of this is the deflection of Eridanos River in the Pliocene Epoch by the uplift of the South Swedish Dome or the present-day drainage divides between Limpopo and Zambezi rivers in southern Africa.

Franz Kossmat

Franz Kossmat ( 22 August 1871 in Vienna – 1 December 1938 in Leipzig) was an Austrian-German geologist, for twenty years the director of the Geological Survey of Saxony under both the kingdom and the subsequent German Republic.

Kossmat was professor of Mineralogy and Geology at the Graz University of Technology. From 1913 to 1934 Kossmat was the director of the Geological Survey of Saxony and director of the Geological-Paleontological Institute of the University of Leipzig. In 1920 he presented the first gravity measures for middle Europe. It was published in 1921. In his life he published over twenty books himself, and collaborated on numerous others. He is most known for his work on isostasy and his opposition to Wegener's theories of continental drift.

Great Trigonometrical Survey

The Great Trigonometrical Survey was a project which aimed to measure the entire Indian subcontinent with scientific precision. It was begun in 1802 by the infantry officer William Lambton, under the auspices of the East India Company. Under the leadership of his successor, George Everest, the project was made a responsibility of the Survey of India. Everest was succeeded by Andrew Scott Waugh and after 1861 the project was led by James Walker, who saw the first completion of it in 1871.

Among the many accomplishments of the Survey were the demarcation of the British territories in India and the measurement of the height of the Himalayan giants: Everest, K2, and Kanchenjunga. The Survey had an enormous scientific impact as well, being responsible for one of the first accurate measurements of a section of an arc of longitude, and for measurements of the geodesic anomaly which led to the development of the theories of isostasy.

John Fillmore Hayford

John Fillmore Hayford (May 19, 1868 – March 10, 1925) was an eminent United States geodesist. His work involved the study of isostasy and the construction of a reference ellipsoid for approximating the figure of the Earth. The crater Hayford on the far side of the Moon is named after him. Mount Hayford, a 1,871 m mountain peak near Metlakatla, Alaska, United States, is named after him. A biography of Hayford may be found in the Biographical Memoirs of the National Academy of Sciences, 16 (5), 1935.

John Pratt

John Pratt may refer to:

John Pratt (judge) (1657–1725), Lord Chief Justice of England and interim Chancellor of the Exchequer

John Pratt (soldier) (1753–1824), United States Army officer

John Pratt, 1st Marquess Camden (1759–1840), British politician

John Pratt, 3rd Marquess Camden (1840–1872), British politician

John Pratt, 4th Marquess Camden (1872–1943), British peer

John Pratt (died 1835), hanged for sodomy

John Pratt (Archdeacon of Calcutta) (1809–1871), British clergyman and mathematician, developer of the theory of isostasy

John Pratt (cricketer) (1834–1886), English cricketer

John Teele Pratt (1873–1927), American corporate attorney, philanthropist, music impresario, and financier

John Pratt (Liberal politician) (1873–1953), Scottish Liberal politician

John Lee Pratt (1879–1975), American businessman who served on GM's board of directors

John Pratt (Canadian politician) (1894–1973), Manitoban politician

John H. Pratt (1910–1995), US Court of Appeals judge

John Pratt (Provost of Southwell) (1913–1992), Anglican archdeacon and provost

John Winton or John Pratt (1931–2001), English naval officer, author and obituarist

John W. Pratt (born 1931), professor of statistics, economics, and business at Harvard University

John Pratt (footballer) (born 1948), English footballer

John Bridge Pratt (1833–1870), husband of Anna Bronson Alcott Pratt, the elder sister of novelist Louisa May Alcott

John M. Pratt (1886–1954), tax resistance leader, activist, publicist and newspaper man

Lau, Gotland

Lau is a populated area, a socken (not to be confused with parish), on the Swedish island of Gotland. It comprises the same area as the administrative Lau District, established on 1 January 2016. Originally an island, it is now part of the main Gotland island due to the isostasy. It is mostly known for the good water from the spring Lau Käldu.

Lithospheric flexure

The lithospheric flexure (also called regional isostasy) is the process by which the lithosphere (rigid outer layer of the Earth) bends under the action of forces such as the weight of a growing orogen or changes in ice thickness related to (de)glaciations. The lithosphere is a thin, outer, rigid layer of the Earth resting on the asthenosphere, a viscous layer that in geological time scales behaves as a viscous fluid. Thus, when loaded, the lithosphere progressively reaches an isostatic equilibrium, which is the name of the Archimedes principle applied to these geological settings.This phenomenon was first described in the late 19th century to explain the shorelines uplifted in Scandinavia due to the removal of large ice massed during the last glaciation. G. K. Gilbert used it to explain the uplifted shorelines of Lake Bonneville.The geometry of the lithospheric bending is often modeled adopting a pure elastic thin plate approach (sometimes by fitting the gravity anomaly produced by that bending rather than more direct data of it). The thickness of such plate that best fits the observed lithospheric bending is called the equivalent elastic thickness of the lithosphere, and is related to the stiffness or rigidity of the lithosphere. These lithospheric bending calculations are typically performed following the Euler-Bernoulli bending formulation, or alternatively the Lagrange equation (Love-Kirchhoff).

Migmatite

Migmatite is a composite rock found in medium and high-grade metamorphic environments. It consists of two, or more constituents often layered repetitively; one layer was formerly paleosome, a metamorphic rock that was reconstituted subsequently by partial melting; the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance. Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains, commonly within Precambrian cratonic blocks,

Migmatites form under extreme temperature and pressure conditions during prograde metamorphism, when partial melting occurs in metamorphic paleosome. Components exsolved by partial melting are called neosome (meaning ‘new rock’), which may or may not be heterogeneous at the microscopic to macroscopic scale. Migmatites often appear as tightly, incoherently folded veins (ptygmatic folds). These form segregations of leucosome, light-colored granitic components exsolved within melanosome, a dark colored amphibole and biotite-rich setting. If present, a mesosome, intermediate in color between a leucosome and melanosome, forms a more or less unmodified remnant of the metamorphic parent rock paleosome. The light-colored components often give the appearance of having been molten and mobilized.

Outer trench swell

The outer trench swell, outer trench high, or outer rise is a subtle ridge on the seafloor near an oceanic trench, where a descending plate begins to flex and fault in preparation for its descent into the mantle at a subduction zone. The lithosphere is bent upwards by plate stresses, and is not in isostatic equilibrium (distinguish from the "outer ridge" of a forearc).

Typically, the gravity field over the outer swell is about 50 mGal (0.5 mm/s²) higher than expected from isostasy, while gravity over the trench is about 200 mGal (2 mm/s²) less than that expected from isostatic considerations.

The bending of the plate is associated with tension in the upper 20 km, and shallow earthquakes, caused by tensional failure induced by the downward bending of the oceanic plate are common; about 20 extensional outer rise earthquakes with magnitude 5 or greater occur annually. Most tension axes are perpendicular to the trench, independent of the direction of relative motion between the two plates, indicating that failure is controlled by bending stresses in the plate. Plate bending also causes deeper (down to 50 km) earthquakes due to compression.

The wavelength and amplitude of this flexure can be used to constrain the state of stress across the plate boundary. The width of the outer rise is directly related to the flexural rigidity of the lithosphere. The thickness of the elastic lithosphere varies between 20 and 30 km for most trench profiles. Faulting related to plate bending and stair-stepping of the descending slab into the trench may allow seawater to infiltrate deep into the crust and perhaps upper mantle. This may lead to large scale formation of serpentinite in the upper mantle of the downgoing plate (Ranero et al., 2003).

Faulting of the downgoing plate results in a horst and graben structure that allows sediment that reaches the trench to be deposited in graben and carried downward. This faulting also breaks up seamounts as they approach the trench. The principal mechanism of frontal erosion may reflect combined effects of seamount tunneling, mass wasting and transport to the trench, deposition in a graben on the downgoing plate, and descent into the mantle.

Outer trench swells are geoscientific frontiers and much remains to be learned about them. Recent volcanoes have been discovered on ~135-million-year-old Pacific Plate east of Japan (Hirano et al., 2006). These small alkalic volcanoes are small percent melts of asthenosphere that exploit bending-related lithospheric faults to reach the seafloor. Hirano et al., (2006) proposed that these small volcanoes erupted along lithospheric fractures in response to plate flexure during subduction. If bending-related faulting and serpentinization is an important process beneath outer trench swells, there are probably also abundant low-temperature hydrothermal vents on the swells, similar to those of the Lost City (hydrothermal field).

Post-glacial rebound

Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the lifting of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound and isostatic depression are phases of glacial isostasy (glacial isostatic adjustment, glacioisostasy), the deformation of the Earth's crust in response to changes in ice mass distribution. The direct raising effects of post-glacial rebound are readily apparent in parts of Northern Eurasia, Northern America, Patagonia, and Antarctica. However, through the processes of ocean siphoning and continental levering, the effects of post-glacial rebound on sea level are felt globally far from the locations of current and former ice sheets.

Summit accordance

A summit accordance exists when hill and mountaintops tops, and eventually also plateaux, have such disposition that they form a geometric plane that may be either horizontal or tilted. Summit accordances can be the vestiges of former continuous erosion surfaces that were uplifted and eroded. Other proposed explanations include:

the possibility that erosion becomes more effective at height, tearing down mountains that stand out

that isostasy regulates the height of individual mountain masses meaning that small mountains might be uplifted and large mountains dragged down

that landscape dissection by uniformly spaced streams eventually reach a state in which summits attain similar heights

that summit accordance is derivative of structural planes exposed by erosion

Thermal subsidence

In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes more dense than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a sedimentary basin.

Thomas Jamieson

Thomas Francis Jamieson (1829-1913) was a Scottish scientist most associated with his studies of sea level and glacial isostasy during the Quaternary.Born the son of a jeweller, Jamieson was raised in Aberdeen and educated at Aberdeen Grammar School and the University of Aberdeen, at which he was appointed Fordyce Lecturer in Agriculture in 1862, a post he held for 15 years. He was later employed as the factor managing the estate lands of Ellon Castle in Aberdeenshire.Interested in geology from an early age, Jamieson corresponded widely with other scientists, including Charles Lyell and Charles Darwin. After early research on petrology, Jamieson studied the glaciated rocks of Scotland, providing evidence for the then-fledgling theory of ice ages. Later work on marine sediments found above sea level in the Forth Valley convinced Jamieson that the area had once been beneath sea level, and that this was caused by the weight of glaciers depressing the land.

While these views brought Jamieson into conflict with the prevailing orthodoxy of the Geological Survey of Scotland (now the British Geological Survey), he continued to elaborate them, identifying raised shorelines around Scotland at a series of elevations (7.6, 15.0 or 30.5 metres). Despite these efforts, and his election to the Geological Society of London in 1862, his views on the geological history of Scotland only gained full acceptance in the late 20th century.

Timeline of the development of tectonophysics (before 1954)

The evolution of tectonophysics is closely linked to the history of the continental drift and plate tectonics hypotheses. The continental drift/ Airy-Heiskanen isostasy hypothesis had many flaws and scarce data. The fixist/ Pratt-Hayford isostasy, the contracting Earth and the expanding Earth concepts had many flaws as well.

The idea of continents with a permanent location, the geosyncline theory, the Pratt-Hayford isostasy, the extrapolation of the age of the Earth by Lord Kelvin as a black body cooling down, the contracting Earth, the Earth as a solid and crystalline body, is one school of thought. A lithosphere creeping over the asthenosphere is a logical consequence of an Earth with internal heat by radioactivity decay, the Airy-Heiskanen isostasy, thrust faults and Niskanen's mantle viscosity determinations.

Uwharrie Mountains

The Uwharrie Mountains () are a mountain range in North Carolina spanning the counties of Randolph, Montgomery, Stanly, and Davidson. The range's foothills stretch into Cabarrus, Anson and Union counties, and terminate in the hills of Person County.

The Uwharries were once a coastal mountain range; isostasy has slowly raised the eastern seabed until today they lie in the Piedmont of North Carolina over 150 miles from the coast. Formed approximately 500 million years ago by accretion along the Gondwanan tectonic plate, they are thought to have once peaked at some 20,000 feet, before eroding to a maximum of just over 1,100 ft. The range's high point is High Rock Mountain (1,188 feet/362 meters as measured by the NC Geodetic Survey), in southwestern Davidson County.

The Uwharries lie within the Southeastern mixed forests ecoregion. They give their name to the Uwharrie National Forest. Once entirely cleared for timber and farmland, the mountains were designated a U.S. National Forest in 1961 by President John F. Kennedy. The woodlands have since returned, providing a haven for a diversity of wildlife, recreational facilities, and numerous Native American archeological sites.

In 1799, the discovery of gold at the nearby Reed Gold Mine in Cabarrus County led to America's first gold rush.

The North Carolina Zoo, America's first state-supported zoo, is located in the Uwharries region.

The Caraway Mountains, a segment of the Uwharries, are located in western Randolph County, west of Asheboro.

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