Lithospheric flexure
The lithospheric flexure is the process by which the lithosphere bends under the action of forces such as the weight of a growing orogen or changes in ice thickness related to glaciations. The lithosphere is a thin, 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, 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 modeled adopting a pure elastic thin plate approach. The thickness of such plate that best fits the observed lithospheric bending is called the equivalent elastic thickness of the lithosphere, is related to the stiffness or rigidity of the lithosphere.
These lithospheric bending calculations are performed following the Euler-Bernoulli bending formulation, or alternatively the Lagrange equation
Iceberg
An iceberg is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating in open water. Another name for iceberg is "ice mountain". Small bits of disintegrating icebergs are called "growlers" or "bergy bits". Icebergs are possible on Earth because the oceans are filled with liquid water, a substance less dense when solid than liquid. Planets with oceans consisting of different substances like methane cannot have icebergs, as their chunks of frozen liquid would sink; because 90 percent of an iceberg is below the surface and not visible, icebergs have been considered a serious maritime hazard since the 1912 loss of the "unsinkable" RMS Titanic, leading to the formation of the International Ice Patrol in 1914. The expression "tip of the iceberg", illustrates a difficulty, only a small, visible part of a larger, complex problem; the largest iceberg reliably recorded was Iceberg B-15A which split off the Ross Ice Shelf in Antarctica in 2000. The word iceberg is a partial loan translation from the Dutch word ijsberg meaning ice mountain, cognate to Danish isbjerg, German Eisberg, Low Saxon Iesbarg and Swedish isberg.
Because the density of pure ice is about 920 kg/m3, that of seawater about 1025 kg/m3 about one-tenth of the volume of an iceberg is above water. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface; the visible "tips" of icebergs range from 1 to 75 metres above sea level and weigh 100,000 to 200,000 metric tons. The largest known iceberg in the North Atlantic was 168 metres above sea level, reported by the USCG icebreaker East Wind in 1958, making it the height of a 55-story building; these icebergs originate from the glaciers of western Greenland and may have interior temperatures of −15 to −20 °C. Winds and currents tend to move icebergs close to coastlines, where they can become frozen into pack ice, or drift into shallow waters, where they can come into contact with the seabed, a phenomenon called seabed gouging; the largest icebergs recorded have been calved, or broken off, from the Ross Ice Shelf of Antarctica. Iceberg B-15, photographed by satellite in 2000, measured 295 by 37 kilometres, with a surface area of 11,000 square kilometres.
The largest iceberg on record was an Antarctic tabular iceberg of over 31,000 square kilometres sighted 150 miles west of Scott Island, in the South Pacific Ocean, by the USS Glacier on November 12, 1956. This iceberg was larger than Belgium. A small iceberg less than 2 meters across that floats with less than 1 meter showing above water is called a growler, is smaller than a bergy bit, less than 5 meters in size. Both are spawned from disintegrating icebergs; as a piece of iceberg ice melts, it produces a fizzing sound called the "Bergie Seltzer". This sound results; as this happens, each bubble bursts. The bubbles contain air trapped in snow layers early in the history of the ice, that got buried to a given depth and pressurized as it transformed into firn to glacial ice. In addition to size classification, icebergs can be classified on the basis of their shape; the two basic types of iceberg forms are non-tabular. Tabular icebergs have steep sides and a flat top, much like a plateau, with a length-to-height ratio of more than 5:1.
This type of iceberg known as an ice island, can be quite large, as in the case of Pobeda Ice Island. Antarctic icebergs formed by breaking off from an ice shelf, such as the Ross Ice Shelf or Filchner-Ronne Ice Shelf, are tabular; the largest icebergs in the world are formed this way. Non-tabular icebergs include: Dome: An iceberg with a rounded top. Pinnacle: An iceberg with one or more spires. Wedge: An iceberg with a steep edge on one side and a slope on the opposite side. Dry-Dock: An iceberg that has eroded to form a slot or channel. Blocky: An iceberg with steep, vertical sides and a flat top, it differs from tabular icebergs in that its aspect ratio, the ratio between its width and height, is small, more like that of a block than a flat sheet. Before the early 1910s, although there had been many fatal sinkings of ships by icebergs, there was no system in place to track icebergs to guard ships against collisions. In 1907, SS Kronprinz Wilhelm, a German liner, had rammed an iceberg and suffered a crushed bow, but was still able to complete her voyage.
The advent of steel ship construction led designers to declare their ships "unsinkable". The April 1912 sinking of the Titanic, which killed 1,518 of its 2,223 passengers and crew, changed all that. For the remainder of the ice season of that year, the United States Navy patrolled the waters and monitored ice flow. In November 1913, the International Conference on the Safety of Life at Sea met in London to devise a more permanent system of observing icebergs. Within three months the participating maritime nations had formed the International Ice Patrol; the goal of the IIP was to collect data on meteorology and oceanography to measure currents, ice-flow, ocean temperature, salinity levels. They monitored iceberg dangers near the Grand Banks of Newfoundland and provided the "limits of all known ice" in that vicinity to the maritime community; the IIP published their first records in 1921, which allowed for a year-by-year comparison of iceberg movement. Aerial surveillance of the seas in the early
Sea level rise
Since at least the start of the 20th century, the average global sea level has been rising. Between 1900 and 2016, the sea level rose by 16–21 cm. More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm from 1993 to 2017, a trend of 30 cm per century. This acceleration is due to human-caused global warming, driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers. Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise. Climate scientists expect the rate to further accelerate during the 21st century. Projecting future sea level is challenging, due to the complexity of many aspects of the climate system; as climate research into past and present sea levels leads to improved computer models, projections have increased. For example, in 2007 the Intergovernmental Panel on Climate Change projected a high end estimate of 60 cm through 2099, but their 2014 report raised the high-end estimate to about 90 cm.
A number of studies have concluded that a global sea level rise of 200 to 270 cm this century is "physically plausible". A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of 2.3 metres over a period of two millennia: an example of climate inertia. The sea level will not rise uniformly everywhere on Earth, it will drop in some locations. Local factors include tectonic effects and subsidence of the land, tides and storms. Sea level rises can influence human populations in coastal and island regions. Widespread coastal flooding is expected with several degrees of warming sustained for millennia. Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations and degradation of agricultural land and damage in cities. Natural environments like marine ecosystems are affected, with fish and plants losing parts of their habitat. Societies can respond to sea level rise in three different ways: to retreat, to accommodate and to protect.
Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies. Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or man-made barriers. Understanding past sea level is important for the analysis of current and future changes. In the recent geological past, changes in land ice and thermal expansion from increased temperatures are the dominant reasons of sea level rise; the last time the Earth was 2 °C warmer than pre-industrial temperatures, sea levels were at least 5 metres higher than now: this was when warming because of changes in the amount of sunlight due to slow changes in the Earth's orbit caused the last interglacial. The warming was sustained over a period of thousands of years and the magnitude of the rise in sea level implies a large contribution from the Antarctic and Greenland ice sheets. Since the last glacial maximum about 20,000 years ago, the sea level has risen by more than 125 metres, with rates varying from less than a mm/year to 40+ mm/year, as a result of melting ice sheets over Canada and Eurasia.
Rapid disintegration of ice sheets led to so called'meltwater pulses', periods during which sea level rose rapidly. The rate of rise started to slow down about 8,200 years before present. Sea level changes can be driven either by variations in the amount of water in the oceans, the volume of the ocean or by changes of the land compared to the sea surface; the different techniques used to measure changes in sea level do not measure the same. Tide gauges can only measure relative sea level, whilst satellites can measure absolute sea level changes. To get precise measurements for sea level, researchers studying the ice and the oceans on our planet factor in ongoing deformations of the solid Earth, in particular due to landmasses still rising from past ice masses retreating, the Earth's gravity and rotation. Since the 1992 launch of TOPEX/Poseidon, altimetric satellites have been recording the change in sea level; those satellites can measure the hills and valleys in the sea caused by currents and detect trends in their height.
To measure the distance to the sea surface, the satellite sends a microwave pulse to the ocean's surface and records the time it takes to return. A microwave radiometer corrects any delay. Combining this data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters. Current rates of sea level rise from satellite altimetry have been estimated to be 3.0 ± 0.4 millimetres per year for the period 1993–2017. Earlier satellite measurements were at odds with tide gauge measurements. A small calibration error for the Topex/Poseidon satellite discovered in 2015 was identified as the cause of this mismatch, it had caused a slight overestimation of the 1992–2005 sea levels, which masked the ongoing sea level rise acceleration. Satellites are useful for measuring regional variations in sea level, such as the substantial rise between 1993 and 2012 in the western tropical Pacific; this sharp rise has been linked to increasing trade winds, which occur when the Pacific Decadal Oscillation and the El Niño–Southern Oscillation change from one state to t
Wave-cut platform
A wave-cut platform, shore platform, coastal bench, or wave-cut cliff is the narrow flat area found at the base of a sea cliff or along the shoreline of a lake, bay, or sea, created by erosion. Wave-cut platforms are most obvious at low tide when they become visible as huge areas of flat rock. Sometimes the landward side of the platform is covered by sand, forming the beach, the platform can only be identified at low tides or when storms move the sand. Wave-cut platforms form when destructive waves hit against the cliff face, causing an undercut between the high and low water marks as a result of abrasion and hydraulic action, creating a wave-cut notch; this notch enlarges into a cave. The waves undermine this portion until the roof of the cave cannot hold due to the pressure and freeze-thaw or biological weathering acting on it, collapses, resulting in the cliff retreating landward; the base of the cave forms the wave-cut platform as attrition causes the collapsed material to be broken down into smaller pieces, while some cliff material may be washed into the sea.
This may be deposited at the end of the platform. Because of the continual wave action, a wave-cut platform represents an hostile environment and only the toughest of organisms can utilize such a niche. Abrasion and Hydraulic Action are the most common reason behind cliff erosion. Ancient wave-cut platforms provide evidence of past lake levels. Raised and abandoned platforms, sometimes found behind modern beaches, are evidence of higher sea levels in the geological past, have been used to identify areas of isostatic adjustment. By using scientific dating methods, or examination of marine fossils found on the platform, it is possible to work out when the platform was formed, thus giving geographers and geologists information about sea levels at known times in the past; this has been used in the United Kingdom and other glaciated areas to calculate the rate at which land is rising now that it is no longer covered in ice. Where the coastline itself is changing due to seismic action, there may be a series of platforms showing earlier sea levels and indicating the amount of uplift caused by various earthquakes.
According to Trenhaile and Massalink and Hughes, the term'wave-cut platform' should no longer be used as it assumes that shore platforms are the result of wave action, not always true. Shore platforms, like comparable river and lake platforms, are erosional features that develop when removal of saprock and other debris by waves and currents leaves behind a bedrock surface below the water table. Beach Bench Machair Marine terrace Raised beach Raised shorelines Strandflat Terrace
Density
The density, or more the volumetric mass density, of a substance is its mass per unit volume. The symbol most used for density is ρ, although the Latin letter D can be used. Mathematically, density is defined as mass divided by volume: ρ = m V where ρ is the density, m is the mass, V is the volume. In some cases, density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials have different densities, density may be relevant to buoyancy and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material water.
Thus a relative density less than one means. The density of a material varies with pressure; this variation is small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid; this causes it to rise relative to more dense unheated material. The reciprocal of the density of a substance is called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density. In a well-known but apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.
Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated and compared with the mass. Baffled, Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water. Upon this discovery, he leapt from his bath and ran naked through the streets shouting, "Eureka! Eureka!". As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment; the story first appeared in written form in Vitruvius' books of architecture, two centuries after it took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time. From the equation for density, mass density has units of mass divided by volume; as there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use.
The SI unit of kilogram per cubic metre and the cgs unit of gram per cubic centimetre are the most used units for density. One g/cm3 is equal to one thousand kg/m3. One cubic centimetre is equal to one millilitre. In industry, other larger or smaller units of mass and or volume are more practical and US customary units may be used. See below for a list of some of the most common units of density. A number of techniques as well as standards exist for the measurement of density of materials; such techniques include the use of a hydrometer, Hydrostatic balance, immersed body method, air comparison pycnometer, oscillating densitometer, as well as pour and tap. However, each individual method or technique measures different types of density, therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question; the density at all points of a homogeneous object equals its total mass divided by its total volume. The mass is measured with a scale or balance.
To determine the density of a liquid or a gas, a hydrometer, a dasymeter or a Coriolis flow meter may be used, respectively. Hydrostatic weighing uses the displacement of water due to a submerged object to determine the density of the object. If the body is not homogeneous its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: ρ = d m / d V, where d V is an elementary volume at position r; the mass of the body t
Continental crust
Continental crust is the layer of igneous and metamorphic rocks that forms the 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 silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima, richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, more mafic in character; the continental crust consists of various layers, with a bulk composition, intermediate. The average density of continental crust is about 2.83 g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3. Continental crust is less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is thicker than oceanic crust, which has an average thickness of around 7–10 km.
About 40% of Earth's surface is occupied by continental crust. It makes up about 70% of the volume of Earth's crust; because the surface of continental crust lies above sea level, its existence allowed land life to evolve from marine life. Its existence 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 is now called the Cambrian explosion. All continental crust is derived from mantle-derived melts through fractional differentiation of basaltic melt and the assimilation 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 at magmatic arcs associated with subduction. There 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 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. In contrast to the persistence of continental crust, the size and number of continents are changing through geologic time. Different tracts rift apart and recoalesce as part of a grand supercontinent cycle. There are about 7 billion cubic kilometers of continental crust, but this quantity varies because of the nature of the forces involved; the relative permanence of continental crust contrasts with 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 subducted back into the mantle. Continental crust is subducted. For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in recycled oceanic crust. 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 related to the thickness of crust. This results from the isostasy associated with orogeny; the crust is thickened by the compressive forces related to continental collision. The buoyancy of 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, where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and severed, replaced by oceanic crust; the edges of continental fragments formed. The high temperatures and pressures at depth 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 be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer beneath it. Continental crust is produced and destroyed by plate tectonic processes 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. Material can be accreted horizontally when volcanic island arcs, seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is lost through erosion and sediment subduction, tectonic erosion of forearcs 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, over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction, it is a mat
Ice sheet
An ice sheet known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2. The only current ice sheets are in Greenland. Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an ice cap. An ice cap will feed a series of glaciers around its periphery. Although the surface is cold, the base of an ice sheet is warmer due to geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly; this process produces fast-flowing channels in the ice sheet — these are ice streams. The present-day polar ice sheets are young in geological terms; the Antarctic Ice Sheet first formed as a small ice cap in the early Oligocene, but retreating and advancing many times until the Pliocene, when it came to occupy all of Antarctica. The Greenland ice sheet did not develop at all until the late Pliocene, but developed rapidly with the first continental glaciation.
This had the unusual effect of allowing fossils of plants that once grew on present-day Greenland to be much better preserved than with the forming Antarctic ice sheet. The Antarctic ice sheet is the largest single mass of ice on Earth, it covers an area of 14 million km2 and contains 30 million km3 of ice. Around 90% of the Earth's ice mass is in Antarctica, which, if melted, would cause sea levels to rise by 58 meters; the continent-wide average surface temperature trend of Antarctica is positive and significant at >0.05 °C/decade since 1957. The Antarctic ice sheet is divided by the Transantarctic Mountains into two unequal sections called the East Antarctic ice sheet and the smaller West Antarctic Ice Sheet; the EAIS rests on a major land mass but the bed of the WAIS is, in places, more than 2,500 metres below sea level. It would be seabed; the WAIS is classified as a marine-based ice sheet, meaning that its bed lies below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, outlet glaciers that drain into the Amundsen Sea.
The Greenland ice sheet occupies about 82% of the surface of Greenland, if melted would cause sea levels to rise by 7.2 metres. Estimated changes in the mass of Greenland's ice sheet suggest it is melting at a rate of about 239 cubic kilometres per year; these measurements came from NASA's Gravity Recovery and Climate Experiment satellite, launched in 2002, as reported by BBC News in August 2006. Ice movement is dominated by the motion of glaciers, whose activity is determined by a number of processes, their motion is the result of cyclic surges interspersed with longer periods of inactivity, on both hourly and centennial time scales. The Greenland, the Antarctic, ice sheets have been losing mass because losses by ablation including outlet glaciers exceed accumulation of snowfall. According to the Intergovernmental Panel on Climate Change, loss of Antarctic and Greenland ice sheet mass contributed about 0.21 ± 0.35 and 0.21 ± 0.07 mm/year to sea level rise between 1993 and 2003. The IPCC projects that ice mass loss from melting of the Greenland ice sheet will continue to outpace accumulation of snowfall.
Accumulation of snowfall on the Antarctic ice sheet is projected to outpace losses from melting. However, in the words of the IPCC, "Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude." More research work is therefore required to improve the reliability of predictions of ice-sheet response on global warming. In 2018, scientists discovered channels between the East and West Antarctic ice sheets that may allow melted ice to flow more to the sea; the effects on ice sheets due to increasing temperature may accelerate, but as documented by the IPCC the effects are not projected and in the case of the Antarctic, may trigger an accumulation of additional ice mass. If an ice sheet were ablated down to bare ground, less light from the sun would be reflected back into space and more would be absorbed by the land.
The Greenland Ice Sheet covers 84% of the island and the Antarctic Ice Sheet covers 98% of the continent. Due to the significant thickness of these ice sheets, global warming analysis focuses on the loss of ice mass from the ice sheets increasing sea level rise, not on a reduction in the surface area of the ice sheets. Müller, Jonas. Ice Sheets: Dynamics and Environmental Concerns. Hauppauge, New York: Nova Science. ISBN 978-1-61942-367-1. United Nations Environment Programme: Global Outlook for Ice and Snow http://www.nasa.gov/vision/earth/environment/ice_sheets.html
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Oldham, Richard Dixon (1922). . Encyclopædia Britannica (12th ed.).
