Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of
ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s during the
last glacial period, which had caused
isostatic depression
Isostatic depression is the sinking of large parts of the Earth's crust into the asthenosphere caused by a heavy weight placed on the Earth's surface, often glacial ice during continental glaciation. Isostatic depression and isostatic rebound o ...
. 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
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago ...
,
Northern America
Northern America is the northernmost subregion of North America. The boundaries may be drawn slightly differently. In one definition, it lies directly north of Middle America (including the Caribbean and Central America).Gonzalez, Joseph. 2 ...
,
Patagonia
Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...
, and
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. However, through the processes of ''ocean siphoning'' and ''continental levering'', the effects of post-glacial rebound on
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
are felt globally far from the locations of current and former ice sheets.
[Milne, G.A., and J.X. Mitrovica (2008) ''Searching for eustasy in deglacial sea-level histories.'' Quaternary Science Reviews. 27:2292–2302.]
Overview
During the
last glacial period, much of
northern Europe,
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
,
North America,
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
and
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
was covered by
ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s, which reached up to three kilometres thick during the
glacial maximum
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
about 20,000 years ago. The enormous weight of this ice caused the surface of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's
crust to deform and warp downward, forcing the
viscoelastic
In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly ...
mantle material to flow away from the loaded region. At the end of each
glacial period
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
when the
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
s retreated, the removal of this weight led to slow (and still ongoing) uplift or rebound of the land and the return flow of mantle material back under the
deglaciated area. Due to the extreme
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
of the mantle, it will take many thousands of years for the land to reach an
equilibrium level.
The uplift has taken place in two distinct stages. The initial uplift following deglaciation was almost immediate due to the
elastic response of the crust as the ice load was removed. After this elastic phase, uplift proceeded by slow viscous flow at an
exponentially
Exponential may refer to any of several mathematical topics related to exponentiation, including:
*Exponential function, also:
**Matrix exponential, the matrix analogue to the above
*Exponential decay, decrease at a rate proportional to value
*Expo ...
decreasing rate. Today, typical uplift rates are of the order of 1 cm/year or less. In northern Europe, this is clearly shown by the
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
data obtained by the BIFROST GPS network;
for example in
Finland
Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of B ...
, the total area of the country is growing by about seven square kilometers per year. Studies suggest that rebound will continue for at least another 10,000 years. The total uplift from the end of deglaciation depends on the local ice load and could be several hundred metres near the centre of rebound.
Recently, the term "post-glacial rebound" is gradually being replaced by the term "glacial isostatic adjustment". This is in recognition that the response of the Earth to glacial loading and unloading is not limited to the upward rebound movement, but also involves downward land movement, horizontal crustal motion,
changes in global sea levels
and the Earth's gravity field,
induced earthquakes,
and changes in the Earth's rotation.
Another alternate term is "glacial isostasy", because the uplift near the centre of rebound is due to the tendency towards the restoration of isostatic equilibrium (as in the case of isostasy of mountains). Unfortunately, that term gives the wrong impression that isostatic equilibrium is somehow reached, so by appending "adjustment" at the end, the motion of restoration is emphasized.
Effects
Post-glacial rebound produces measurable effects on vertical crustal motion, global sea levels, horizontal crustal motion, gravity field, Earth's rotation, crustal stress, and earthquakes. Studies of glacial rebound give us information about the flow law of mantle rocks, which is important to the study of mantle convection,
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
and the thermal evolution of the Earth. It also gives insight into past ice sheet history, which is important to
glaciology
Glaciology (; ) is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice.
Glaciology is an interdisciplinary Earth science that integrates geophysics, geology, physical geography, geomorphology, c ...
,
paleoclimate, and changes in global sea level. Understanding postglacial rebound is also important to our ability to monitor recent global change.
Vertical crustal motion
![GlacialRebound](https://upload.wikimedia.org/wikipedia/commons/9/9b/GlacialRebound.png)
Erratic
boulders,
U-shaped valley
U-shaped valleys, also called trough valleys or glacial troughs, are formed by the process of glaciation. They are characteristic of mountain glaciation in particular. They have a characteristic U shape in cross-section, with steep, straight s ...
s,
drumlin
A drumlin, from the Irish word ''droimnín'' ("littlest ridge"), first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated ...
s,
esker
An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Ame ...
s,
kettle lake
A kettle (also known as a kettle lake, kettle hole, or pothole) is a depression/hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating g ...
s,
bedrock striation
Striations means a series of ridges, furrows or linear marks, and is used in several ways:
* Glacial striation
* Striation (fatigue), in material
* Striation (geology), a ''striation'' as a result of a geological fault
* Striation Valley, in Ant ...
s are among the common signatures of the
Ice Age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
. In addition, post-glacial rebound has caused numerous significant changes to coastlines and landscapes over the last several thousand years, and the effects continue to be significant.
In
Sweden, Lake
Mälaren was formerly an arm of the
Baltic Sea
The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain.
The sea stretches from 53°N to 66°N latitude and ...
, but uplift eventually cut it off and led to its becoming a
freshwater lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
in about the 12th century, at the time when
Stockholm was founded at
its outlet. Marine seashells found in
Lake Ontario
Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York. The Canada–United States border ...
sediments imply a similar event in prehistoric times. Other pronounced effects can be seen on the island of
Öland
Öland (, ; ; sometimes written ''Øland'' in other Scandinavian languages, and often ''Oland'' internationally; la, Oelandia) is the second-largest Swedish island and the smallest of the traditional provinces of Sweden. Öland has an area ...
, Sweden, which has little topographic relief due to the presence of the very level
Stora Alvaret
Stora Alvaret (; "the Great Alvar") is an alvar, a barren limestone terrace, in the southern half of the island of Öland, Sweden. Stora Alvaret is a dagger shaped expanse almost long and about at the widest north end. The area of this formation ...
. The rising land has caused the
Iron Age
The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age ( Paleolithic, Mesolithic, Neolithic) and the Bronze Age ( Chalcolithic). The concept has been mostl ...
settlement area to recede from the
Baltic Sea
The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain.
The sea stretches from 53°N to 66°N latitude and ...
, making the present day villages on the west coast set back unexpectedly far from the shore. These effects are quite dramatic at the village of
Alby, for example, where the
Iron Age
The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age ( Paleolithic, Mesolithic, Neolithic) and the Bronze Age ( Chalcolithic). The concept has been mostl ...
inhabitants were known to subsist on substantial coastal fishing.
As a result of post-glacial rebound, the
Gulf of Bothnia is predicted to eventually close up at
Kvarken
Kvarken ( sv, Kvarken, Norra Kvarken (as opposed to South Kvarken); ) is the narrow region of the Gulf of Bothnia separating the Bothnian Bay (the inner part of the gulf) from the Bothnian Sea. The distance from the Swedish mainland to the Finn ...
in more than 2,000 years. The
Kvarken
Kvarken ( sv, Kvarken, Norra Kvarken (as opposed to South Kvarken); ) is the narrow region of the Gulf of Bothnia separating the Bothnian Bay (the inner part of the gulf) from the Bothnian Sea. The distance from the Swedish mainland to the Finn ...
is a
UNESCO
The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. It ...
World Natural Heritage Site, selected as a "type area" illustrating the effects of post-glacial rebound and the
holocene glacial retreat
The Holocene glacial retreat is a geographical phenomenon that involved the global retreat of glaciers (deglaciation) that previously had advanced during the Last Glacial Maximum. Ice sheet retreat initiated ca. 19,000 years ago and accelerated ...
.
In several other
Nordic ports, like
Tornio
Tornio (; sv, Torneå; sme, Duortnus ; smn, Tuárnus) is a city and municipality in Lapland, Finland. The city forms a cross-border twin city together with Haparanda on the Swedish side. The municipality covers an area of , of which is ...
and
Pori (formerly at
Ulvila
Ulvila (; sv, Ulvsby) is a town and municipality of Finland. It is one of the six medieval cities of Finland, as well as the third oldest city in the country. Ulvila was granted charter as a town by King Albert of Sweden on 7 February 1365. How ...
), the harbour has had to be relocated several times. Place names in the coastal regions also illustrate the rising land: there are inland places named 'island', 'skerry', 'rock', 'point' and 'sound'. For example,
Oulunsalo
Oulunsalo ( sv, Oulunsalo, also formerly ) is former municipality in the region of Northern Ostrobothnia, in Finland. Along with Haukipudas, Kiiminki and Yli-Ii it lost its municipal status and was merged with the city of Oulu on 1 January 2013. ...
"island of
Oulujoki
Oulujoki is a river in Oulu province, Finland. Its name in Finnish means literally "Oulu River" ( sv, Ule älv), originally in old Northern Ostrobothnian dialect literally "Flood River". Its origin is Oulujärvi and its watershed area covers ...
"
is a peninsula, with inland names such as ''Koivukari'' "Birch Rock", ''Santaniemi'' "Sandy Cape", and ''Salmioja'' "the brook of the Sound". (Compare
an
)
In
Great Britain
Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...
, glaciation affected
Scotland
Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
but not southern
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
, and the post-glacial rebound of northern Great Britain (up to 10 cm per century) is causing a corresponding downward movement of the southern half of the island (up to 5 cm per century). This will eventually lead to an increased risk of
flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
s in southern England and south-western Ireland.
Since the glacial isostatic adjustment process causes the land to move relative to the sea, ancient shorelines are found to lie above present day sea level in areas that were once glaciated. On the other hand, places in the peripheral bulge area which was uplifted during glaciation now begins to subside. Therefore, ancient beaches are found below present day sea level in the bulge area. The "relative sea level data", which consists of height and age measurements of the ancient beaches around the world, tells us that glacial isostatic adjustment proceeded at a higher rate near the end of deglaciation than today.
The present-day uplift motion in northern Europe is also monitored by a
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
network called BIFROST.
Results of GPS data show a peak rate of about 11 mm/year in the north part of the
Gulf of Bothnia, but this uplift rate decreases away and becomes negative outside the former ice margin.
In the near field outside the former ice margin, the land sinks relative to the sea. This is the case along the east coast of the United States, where ancient beaches are found submerged below present day sea level and Florida is expected to be submerged in the future.
GPS data in North America also confirms that land uplift becomes subsidence outside the former ice margin.
Global sea levels
To form the ice sheets of the last Ice Age, water from the oceans evaporated, condensed as snow and was deposited as ice in high latitudes. Thus global sea level fell during glaciation.
The ice sheets at the last
glacial maximum
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
were so massive that global sea level fell by about 120 metres. Thus continental shelves were exposed and many islands became connected with the continents through dry land. This was the case between the British Isles and Europe (
Doggerland
Doggerland was an area of land, now submerged beneath the North Sea, that connected Britain to continental Europe. It was flooded by rising sea levels around 6500–6200 BCE. The flooded land is known as the Dogger Littoral. Geological sur ...
), or between Taiwan, the Indonesian islands and Asia (
Sundaland). A
land bridge also existed between Siberia and Alaska that allowed the migration of people and animals during the last glacial maximum.
The fall in sea level also affects the circulation of ocean currents and thus has important impact on climate during the glacial maximum.
During deglaciation, the melted ice water returns to the oceans, thus
sea level in the ocean increases again. However, geological records of sea level changes show that the redistribution of the melted ice water is not the same everywhere in the oceans. In other words, depending upon the location, the rise in sea level at a certain site may be more than that at another site. This is due to the gravitational attraction between the mass of the melted water and the other masses, such as remaining ice sheets, glaciers, water masses and mantle rocks
and the changes in centrifugal potential due to Earth's variable rotation.
Horizontal crustal motion
Accompanying vertical motion is the horizontal motion of the crust. The BIFROST GPS network
shows that the motion diverges from the centre of rebound.
However, the largest horizontal velocity is found near the former ice margin.
The situation in North America is less certain; this is due to the sparse distribution of GPS stations in northern Canada, which is rather inaccessible.
Tilt
The combination of horizontal and vertical motion changes the tilt of the surface. That is, locations farther north rise faster, an effect that becomes apparent in lakes. The bottoms of the lakes gradually tilt away from the direction of the former ice maximum, such that lake shores on the side of the maximum (typically north) recede and the opposite (southern) shores sink. This causes the formation of new rapids and rivers. For example,
Lake Pielinen
Pielinen ( sv, Pielisjärvi) is the fourth largest lake of Finland, with a drainage basin area of equally distributed between eastern Finland and Russia. The creation of the lake and its outlet is attributed to a post-glacial isostatic rebound, w ...
in
Finland, which is large (90 x 30 km) and oriented perpendicularly to the former ice margin, originally drained through an outlet in the middle of the lake near Nunnanlahti to Lake
Höytiäinen
' is a lake in Finland. Due to a mismanaged drainage attempt during 1859, the lake was reduced by around one third of its size. It flows through a channel formed in the drainage attempt, called Höytiäisenkanava, into Lake Pyhäselkä in the p ...
. The change of tilt caused Pielinen to burst through the Uimaharju
esker
An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Ame ...
at the southwestern end of the lake, creating a new river (
Pielisjoki
Pielisjoki (River Pielinen) is a long river in Eastern Finland. It is the sixth most fast-flowing river in Finland and the largest river draining into Lake Saimaa. The river connects the two largest lakes in North Karelia, Pielinen and Pyhäsel ...
) that runs to the sea via
Lake Pyhäselkä to
Lake Saimaa
Saimaa ( , ; sv, Saimen) is a lake located in the Finnish Lakeland area in southeastern Finland. At approximately , it is the largest lake in Finland, and the fourth largest natural freshwater lake in Europe.
The name Saimaa likely comes from a ...
. The effects are similar to that concerning seashores, but occur above sea level. Tilting of land will also affect the flow of water in lakes and rivers in the future, and thus is important for water resource management planning.
In Sweden
Lake Sommen's outlet in the northwest has a rebound of is 2.36 mm/a while in the eastern Svanaviken it is 2.05 mm/a. This means the lake is being slowly tilted and the southeastern shores drowned.
Gravity field
Ice, water and mantle rocks have
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
, and as they move around, they exert a gravitational pull on other masses towards them. Thus, the
gravity field
In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
, which is sensitive to all mass on the surface and within the Earth, is affected by the redistribution of ice/melted water on the surface of the Earth and the flow of mantle rocks within.
Today, more than 6000 years after the last deglaciation terminated, the flow of
mantle material back to the glaciated area causes the overall shape of the Earth to become less
oblate. This change in the topography of Earth's surface affects the long-wavelength components of the gravity field.
The changing gravity field can be detected by repeated land measurements with absolute gravimeters and recently by the
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
satellite mission.
The change in long-wavelength components of Earth's gravity field also perturbs the orbital motion of satellites and has been detected by
LAGEOS
LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. Each satel ...
satellite motion.
Vertical datum
The ''
vertical datum
In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built str ...
'' is a reference surface for altitude measurement and plays vital roles in many human activities, including land surveying and construction of buildings and bridges. Since postglacial rebound continuously deforms the crustal surface and the gravitational field, the vertical datum needs to be redefined repeatedly through time.
State of stress, intraplate earthquakes and volcanism
According to the theory of
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
, plate-plate interaction results in earthquakes near plate boundaries. However, large earthquakes are found in intraplate environment like eastern Canada (up to M7) and northern Europe (up to M5) which are far away from present-day plate boundaries. An important intraplate earthquake was the magnitude 8
New Madrid earthquake that occurred in mid-continental US in the year 1811.
Glacial loads provided more than 30 MPa of vertical stress in northern Canada and more than 20 MPa in northern Europe during glacial maximum. This vertical stress is supported by the mantle and the flexure of the
lithosphere. Since the mantle and the lithosphere continuously respond to the changing ice and water loads, the state of stress at any location continuously changes in time. The changes in the orientation of the state of stress is recorded in the
postglacial faults in southeastern Canada.
When the postglacial faults formed at the end of deglaciation 9000 years ago, the horizontal principal stress orientation was almost perpendicular to the former ice margin, but today the orientation is in the northeast–southwest, along the direction of
seafloor spreading
Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
History of study
Earlier theories by Alfred Wegener a ...
at the
Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North ...
. This shows that the stress due to postglacial rebound had played an important role at deglacial time, but has gradually relaxed so that tectonic stress has become more dominant today.
According to the
Mohr–Coulomb theory of rock failure, large glacial loads generally suppress earthquakes, but rapid deglaciation promotes earthquakes. According to Wu & Hasagawa, the rebound stress that is available to trigger earthquakes today is of the order of 1 MPa.
This stress level is not large enough to rupture intact rocks but is large enough to reactivate pre-existing faults that are close to failure. Thus, both postglacial rebound and past tectonics play important roles in today's intraplate earthquakes in eastern Canada and southeast US. Generally postglacial rebound stress could have triggered the intraplate earthquakes in eastern Canada and may have played some role in triggering earthquakes in the eastern US including the
New Madrid earthquakes of 1811.
The situation in northern Europe today is complicated by the current tectonic activities nearby and by coastal loading and weakening.
Increasing pressure due to the weight of the ice during glaciation may have suppressed melt generation and volcanic activities below Iceland and Greenland. On the other hand, decreasing pressure due to deglaciation can increase the melt production and volcanic activities by 20-30 times.
Recent global warming
Recent
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
has caused mountain glaciers and the ice sheets in Greenland and Antarctica to melt and global sea level to rise. Therefore, monitoring
sea level rise
Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...
and the mass balance of ice sheets and glaciers allows people to understand more about global warming.
Recent rise in sea levels has been monitored by tide gauges and satellite altimetry (e.g.
TOPEX/Poseidon
TOPEX/Poseidon was a joint satellite altimeter mission between NASA, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. ...
). As well as the addition of melted ice water from glaciers and ice sheets, recent sea level changes are affected by the thermal expansion of sea water due to global warming, sea level change due to deglaciation of the last glacial maximum (postglacial sea level change), deformation of the land and ocean floor and other factors. Thus, to understand global warming from sea level change, one must be able to separate all these factors, especially postglacial rebound, since it is one of the leading factors.
Mass changes of ice sheets can be monitored by measuring changes in the ice surface height, the deformation of the ground below and the changes in the gravity field over the ice sheet. Thus
ICESat
ICESat (Ice, Cloud, and land Elevation Satellite) was a NASA satellite mission for measuring ice sheet mass balance, cloud and aerosol heights, as well as land topography and vegetation characteristics. It operated as part of NASA's Earth Obs ...
,
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
and
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
satellite mission are useful for such purpose.
However, glacial isostatic adjustment of the ice sheets affect ground deformation and the gravity field today. Thus understanding glacial isostatic adjustment is important in monitoring recent global warming.
One of the possible impacts of global warming-triggered rebound may be more volcanic activity in previously ice-capped areas such as Iceland and Greenland. It may also trigger intraplate earthquakes near the ice margins of Greenland and Antarctica. Unusually rapid (up to 4.1 cm/year) present glacial isostatic rebound due to recent ice mass losses in the Amundsen Sea embayment region of Antarctica coupled with low regional mantle viscosity is predicted to provide a modest stabilizing influence on
marine ice sheet instability in West Antarctica, but likely not to a sufficient degree to arrest it.
Applications
The speed and amount of postglacial rebound is determined by two factors: the
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
or
rheology
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with Plasticity (physics), plastic flow rather than deforming Elasticity (phy ...
(i.e., the flow) of the mantle, and the ice loading and unloading histories on the surface of Earth.
The viscosity of the
mantle is important in understanding
mantle convection
Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carrying heat from the interior to the planet's surface.
The Earth's surface lithosphere rides atop the asthenosphere and the two for ...
,
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
, dynamical processes in Earth, the thermal state and thermal evolution of Earth. However viscosity is difficult to observe because
creep experiments of mantle rocks at natural strain rates would take thousands of years to observe and the ambient temperature and pressure conditions are not easy to attain for a long enough time. Thus, the observations of postglacial rebound provide a natural experiment to measure mantle rheology. Modelling of glacial isostatic adjustment addresses the question of how viscosity changes in the radial
and lateral directions
and whether the flow law is linear, nonlinear,
or composite rheology.
Mantle viscosity may additionally be estimated using
seismic tomography, where seismic velocity is used as a proxy observable
Ice thickness histories are useful in the study of
paleoclimatology
Paleoclimatology (American and British English spelling differences, British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the ...
,
glaciology
Glaciology (; ) is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice.
Glaciology is an interdisciplinary Earth science that integrates geophysics, geology, physical geography, geomorphology, c ...
and paleo-oceanography. Ice thickness histories are traditionally deduced from the three types of information: First, the sea level data at stable sites far away from the centers of
deglaciation give an estimate of how much water entered the oceans or equivalently how much ice was locked up at glacial maximum. Secondly, the location and dates of
terminal moraines tell us the areal extent and retreat of past ice sheets. Physics of glaciers gives us the theoretical profile of ice sheets at equilibrium, it also says that the thickness and horizontal extent of
equilibrium ice sheets are closely related to the basal condition of the ice sheets. Thus the volume of ice locked up is proportional to their instantaneous area. Finally, the heights of ancient beaches in the sea level data and observed land uplift rates (e.g. from
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
or
VLBI
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...
) can be used to
constrain local ice thickness. A popular ice model deduced this way is the ICE5G model.
Because the response of the Earth to changes in ice height is slow, it cannot record rapid fluctuation or surges of ice sheets, thus the ice sheet profiles deduced this way only gives the "average height" over a thousand years or so.
Glacial
isostatic adjustment also plays an important role in understanding recent global warming and climate change.
Discovery
Before the eighteenth century, it was thought, in
Sweden, that sea levels were falling. On the initiative of
Anders Celsius
Anders Celsius (; 27 November 170125 April 1744) was a Swedish astronomer, physicist and mathematician. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Germ ...
a number of marks were made in rock on different locations along the Swedish coast. In 1765 it was possible to conclude that it was not a lowering of sea levels but an uneven rise of land. In 1865
Thomas Jamieson came up with a theory that the rise of land was connected with the ice age that had been first discovered in 1837. The theory was accepted after investigations by
Gerard De Geer
Baron Gerard Jacob De Geer (20 November 1858 – 24 July 1943) was a Swedish geologist who made significant contributions to Quaternary geology, particularly geomorphology and geochronology. De Geer is best known for his work on varves. In 1890 D ...
of old shorelines in Scandinavia published in 1890.
Legal implications
In areas where the rising of land is seen, it is necessary to define the exact limits of property. In Finland, the "new land" is legally the property of the owner of the water area, not any land owners on the shore. Therefore, if the owner of the land wishes to build a pier over the "new land", they need the permission of the owner of the (former) water area. The landowner of the shore may redeem the new land at market price.
Usually the owner of the water area is the
partition unit of the landowners of the shores, a collective holding corporation.
Formulation: sea-level equation
The sea-level equation (SLE) is a linear
integral equation
In mathematics, integral equations are equations in which an unknown function appears under an integral sign. In mathematical notation, integral equations may thus be expressed as being of the form: f(x_1,x_2,x_3,...,x_n ; u(x_1,x_2,x_3,...,x_n) ...
that describes the
sea-level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised ...
variations associated with the PGR.
The basic idea of the SLE dates back to 1888, when Woodward published his pioneering work on the form and position of mean
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
,
[Woodward, R. S., 1888. On the form and position of mean sea level. United States Geol. Survey Bull., 48, 87170.] and only later has been refined by Platzman
[Platzman , G. W., 1971. Ocean tides. In Lectures in Applied Mathematics, 14, part 2, pp. 239292, American Mathematical Society, Providence, RI.] and Farrell
[Farrell, W. E., 1973. Earth tides, ocean tides and tidal loading. Phil. Trans. R. Soc. Lond. A, 274, 253259.] in the context of the study of the ocean tides. In the words of Wu and Peltier,
[Wu, P., and W. R. Peltier. Glacial isostatic adjustment and the free–air gravity anomaly as a constraint on deep mantle viscosity. Geophys. J. R. Astron. Soc., 74, 377449, 1983.] the solution of the SLE yields the space– and time–dependent change of ocean
bathymetry
Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...
which is required to keep the
gravitational potential of the sea surface constant for a specific
deglaciation chronology and
viscoelastic
In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly ...
earth model. The SLE theory was then developed by other authors as Mitrovica & Peltier,
[Mitrovica, J. X. & Peltier, W. R., 1991. On postglacial geoid subsidence over the equatorial ocean. J. geophys. Res., 96, 20,05320,071.] Mitrovica et al.
[Mitrovica, J. X., Davis, J. L. & Shapiro, I. I., 1994. A spectral formal- ism for computing three–dimensional deformations due to surface loads. J. geophys. Res., 99, 70577073.] and Spada & Stocchi.
[Spada G. & Stocchi, P., 2006. The Sea Level Equation, Theory and Numerical Examples. , 96 pp., Aracne, Roma.] In its simplest form, the SLE reads
:
where
is the sea–level change,
is the sea surface variation as seen from Earth's center of mass, and
is vertical displacement.
In a more explicit form the SLE can be written as follow:
:
where
is
colatitude and
is
longitude
Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
,
is time,
and
are the densities of ice and water, respectively,
is the reference surface gravity,
is the sea–level Green's function (dependent upon the
and
viscoelastic load–deformation coefficients - LDCs),
is the ice thickness variation,
represents the
eustatic term (i.e. the ocean–averaged value of
),
and
denote spatio-temporal convolutions over the ice- and ocean-covered regions, and the overbar indicates an average over the surface of the oceans that ensures mass conservation.
See also
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Isostatic depression
Isostatic depression is the sinking of large parts of the Earth's crust into the asthenosphere caused by a heavy weight placed on the Earth's surface, often glacial ice during continental glaciation. Isostatic depression and isostatic rebound o ...
- The opposite of isostatic rebound
References
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Further reading
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As Alaska Glaciers Melt, It’s Land That’s Rising' May 17, 2009
New York Times
''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid ...
External links
Glacial ReboundNASA