Post-glacial rebound

Post-glacial rebound (sometimes called continental rebound, isostatic rebound, isostatic adjustment or post-ice-age isostatic recovery) is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostatic depression. It affects northern Europe, especially Scotland, Fennoscandia and northern Denmark, Siberia, Canada, and the Great Lakes of Canada and the United States.

Overview

During the last glacial period, much of northern Europe, Asia, North America, Greenland and Antarctica were covered by ice sheets. The ice was as thick as three kilometres during the last glacial maximum about 20,000 years ago. The enormous weight of this ice caused the surface of the crust to deform and downwarp under the ice load, forcing the fluid mantle material to flow away from the loaded area. At the end of the ice age when the glaciers retreated, the removal of the weight from the depressed land led to uplift or rebound of the land and the return flow of mantle material back under the deglaciated area. Due to the extreme viscosity of the mantle, it will take many thousands of years for the land to reach an equilibrium level.

Studies have shown that the uplift has taken place in two distinct stages. The initial uplift following deglaciation was rapid (called "elastic"), and took place as the ice was being unloaded. After this "elastic" phase, uplift proceed by "slow viscous flow" so the rate decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm/year or less. In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST GPS network . Studies suggest that rebound will continue for about 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 replaced by the term glacial isostatic adjustment. This is in recognition that the response of the Earth to glacial loading and unloading is not just limited to the upward rebound movement, but involves downward land movement, horizontal crustal motion (Johansson et al., 2002; Sella et al. 2007), changes in global sea levels (Peltier 1998), the Earth's gravity field (Mitrovica & Peltier 1993), induce earthquakes (Wu & Johnston, 2000) and also changes in the rotational motion (Wu & Peltier 1984).

Effects

Post-glacial rebound (or Glacial Isostatic Adjustment) produce measurable effects on: (i) Vertical Crustal Motion, (ii) Global sea levels, (iii) Horizontal Crustal Motion, (iv) Gravity field, (v) Earth's rotational motion and (vi) State of stress and earthquakes. Studies of Glacial rebound give us information about the flow law of mantle rocks and also past ice sheet history. The former is important to the study of Mantle Convection, Plate Tectonics and the thermal evolution of the Earth. The latter is important to the study of Glaciology, Paleoclimate and changes in Global Sea Level. Understanding postglacial rebound is also important to our ability to monitor recent global change.

Vertical Crustal Motion

Erratic boulders, U-shaped valleys, drumlins, eskers, kettle lakes, bedrock striations are just some common signatures of the Ice Age. 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, but uplift eventually cut it off and led to it becoming a freshwater lake in about the 12th century, at the time when Stockholm was founded at its outlet. Marine seashells found in Lake Ontario sediments imply a similar event in prehistoric times. Other pronounced effects can be seen on the island of Öland, which has little topographic relief due to the presence of the very level Stora Alvaret. The rising land has caused the Iron Age settlement area to recede from the Baltic Sea, 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 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. The Kvarken is a UNESCO World Natural Heritage Site, selected as a "type area" illustrating the effects of post-glacial rebound and the holocene glacial retreat.

In several other Nordic ports, like Tornio and Pori (formerly at Ulvila), the harbour had to be relocated several times in the past centuries. Place names in the coastal regions also illustrate the rising land: there are places named 'island', 'skerry', 'rock', 'point' and 'sound' in the inland. For example, Oulunsalo "island of Oulujoki" is a peninsula, with names in the inland such as Koivukari "Birch Rock", Santaniemi "Sandy Cape", and Salmioja "the ditch of the Sound". (Compare and .)

In Great Britain, glaciation affected Scotland but not Southern England, and the post-glacial rebound of northern Great Britain is causing a corresponding downward movement of the southern half of the island . This is leading to an increased risk of floods, particularly in the areas surrounding the lower River Thames. Along with rising sea levels caused by global warming, the post-glacial sinking of southern England is likely to seriously compromise the effectiveness of the Thames Barrier, London's most important flood defence, after about 2030 .

The Great Lakes of North America lie approximately on the 'pivot' line between rising and sinking land. Lake Superior was formerly part of a much larger lake together with Lake Michigan and Lake Huron, but post-glacial rebound raised land dividing the three lakes about 2100 years ago. Today, southern shorelines of the lakes continue to experience rising water levels while northern shorelines see falling levels.

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 was 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 network called BIFROST (Johansson et al. 2002). Results of GPS data shows that a peak rate of about 11 mm/year exist in the north part of the Gulf of Bothnia, but this uplift rate decreases away and become 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 (Peltier 1998). GPS data in North America also confirms that land uplift becomes subsidence outside the former ice margin (Sella et al. 2007).

Global Sea Levels

To form the ice sheets of the last Ice Age, water is taken from the oceans through evaporation, condensation as snow and then deposited as ice in high latitudes. Thus global sea level would fall during glaciation.

The ice sheets at the last Glacial Maximum were so massive that global sea level fell by about 120 metres. Thus continental shelves become exposed and many islands became connected with the continents through dry land. This is the case between the British Isles and Europe, or between Taiwan, the Indonesian islands and Asia. Most important is the existence of a land-bridge between Siberia and Alaska that allow the migration of people and animals during last glacial maximum (Peltier 1998).

The fall in sea level also affects the circulation of ocean currents and thus has important impact on climate during the Ice Age.

During deglaciation, the melted ice water return to the oceans thus sea level in the ocean increases again. However, geological records of sea level changes shows 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. (Peltier 1998) and the changes in centrifugal potential due to Earth's variable rotation (Mitrovica, Milne & Davis 2001)

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 (Johansson et al 2002). 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 (Sella et al. 2007).

Gravity field

Ice, water and mantle rocks have mass, and as they move around, they exert a gravitational pull of other masses towards them. Thus, the gravity field, which is sensitive to all mass on the surface and within the Earth, will become 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 satellite mission . The changing long wavelength components of Earth's gravity field also perturbs the orbital motion of satellites and has been detected by LAGEOS satellite motion (Yoder et al. 1983).

Vertical Datum

The Vertical Datum is a theoretical reference surface for altitude measurement and plays vital roles in many human activities - including land surveying, construction of buildings, bridges etc.. Since postglacial rebound continuously deforms the crustal surface and the gravitational field, the vertical datum needs to be redefined repeatedly through time.

Earth's Rotational Motion

Examination of ancient Chinese and Babylonian eclipse records reveal that the Earth’s rotation rate is not constant. For example, if the rotation rate is constant, then the shadow path of an ancient Babylonian eclipse would lie somewhere across western Europe and the ancient eclipse could not be observed at the recorded time in Babylon. It is well known that tidal interaction between Earth and the Moon ( Tidal Friction or Tidal Dissipation) causes the Earth’s rotation to slow down. But taking into account the tidal interaction alone over-corrects the eclipse path which would lie east of Babylon. In order to have the shadow path pass through Babylon at the recorded time, we need to take into account the effect of Glacial Isostatic Adjustment on Earth’s rotational motion.

To understand how Glacial Isostatic Adjustment affects Earth’s rotation rate, we note that the movement of mass on and beneath the Earth's surface affects the Moment of Inertia of the Earth, and by the Conservation of Angular Momentum, the rotational motion must also change. This is illustrated in the case of a rotating ice skater: as she pulls in her arms, her moment of inertia decreases and as a consequence, she spins faster. On the other hand, as she extends her arms, her moment of inertia increases and her spin slows down.

During glaciation, water is taken from the oceans, whose average position is nearer the equator, and deposited as ice over the higher latitudes. This causes the Moment of Inertia of the Earth-ice-water system to decrease and just like the rotating figure skater pulling in her arms, the earth should spin faster. During deglaciation, the melted ice water returns to the oceans and thus causing the Earth’s spin to slow down. In reality, one also has to take into account the flow of mantle rocks whose direction is opposite to that of the water, but whose rate is much slower. Anyhow, after the end of deglaciation, the dominant mass movement is from the return flow of the mantle rocks back to the glaciated areas at high latititude, making the shape of the Earth less oblate, thus the rotation rate of the Earth increases today.

The increase in Earth's rotation rate causes the length of day (LOD) to decrease. Lambeck (1977) estimated that the Length of the Day decreases by about 0.7 milli-seconds per century. Because the Moment of Inertia that determines rotation rate is closely related to the Earth's oblateness, whose rate of change is related to the observed anomalous motion of artificial Earth satellites such as LAGEOS, the increase in the Earth's rotation rate is confirmed by the observed motion of LAGEOS (Yoder et al. 1983) and is generally attributed to Glacial Isostatic Adjustment (Wu & Peltier 1984).

In addition to the changes in the Earth's rotation rate, the changes in the Moment of Inertia due to Glacial Isostatic Adjustment also cause the rotational axis to move from the current position near the North Pole towards the centre of the ice masses at glacial maximum (True Polar Wander), thus it is moving towards eastern Canada at a rate of about 1 degree per million year (Vicente & Yumi 1969, Wu & Peltier 1984).

This drift of the Earth's rotational axis in turn affects the centrifugal potential on the surface of the earth and thus also affects sea levels (Mitrovica, Milne & Davis 2001).

State of Stress and Intraplate Earthquakes

According to the theory of Plate Tectonics, 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 is the magnitude 8 New Madrid earthquakes that occurred in mid-continental USA in the year 1811.

Glacial loads provide 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 (Wu 1996). When the postglacial faults formed at the end of deglaciation 9000 year 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 spreading at the Mid-Atlantic Ridge. 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 (1996), 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 reactive 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 souteast USA. Generally postglacial rebound stress could have triggered the intraplate earthquakes in eastern Canada and may have played some role in triggering earthquakes in eastern USA including the New Madrid earthquakes of 1811 (Wu & Johnston 2000). The situation in northern Europe today is complicated by the active tectonic activities nearby and by coastal loading and weakening.

Recent Global Warming

Recent Global warming 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 and the mass balance of ice sheets and glaciers allow us to understand more about global warming.

Recent rise in sea levels has been monitored by tide gauges and Satellite Altimetry (e.g. TOPEX/Poseidon). In addition to the addition of melted ice water from glaciers and ice sheets, recent sea level changes are also affected by the thermal expansion of sea water due to global warming, sea level change due to deglaciation of the last Ice Age (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 remove these other 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, GPS and GRACE satellite mission are useful for such purpose (Wahr, Wingham & Bentley 2000). 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.

Applications

How fast and how much (i.e. the amplitude) postglacial rebound proceeds is determined by two factors: (i) the viscosity or rheology (flow law) in the mantle, and (ii) the ice loading and unloading histories on the surface of Earth.

Viscosity or rheology of the mantle is important in understanding mantle convection, plate tectonics, dynamical processes in Earth, the thermal state and thermal evolution of Earth. However viscosity is difficult to observe because creep experiments of mantle rocks 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 (Vermeersen et al. 1998, Peltier 1998, Kaufmann & Lambeck 2002) and lateral directions (Wang & Wu 2006) and whether the flow law is linear or nonlinear (Karato & Wu 1993).

Ice thickness histories are useful in the study of paleoclimatology, glaciology 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 eastimate 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 or VLBI) can be used to constrain local ice thickness. A popular ice model deduced this way is the ICE5G model Peltier (2004). 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 (Hughes 1998).

Glacial isostatic adjustment also play an important role in understanding recent global warming and climate change.

Exploration history

Before the 18th century the belief in Sweden was that the sea levels were sinking. On the initiative of Anders Celsius 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 is connected with the ice age that first had been discovered in 1837. The theory was accepted after investigations by Gerard De Geer of old shorelines in Scandinavia published in 1890.

Legal status

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, paradoxically, if the owner of the land wishes to build a pier over the "new land", he needs the permission of the owner of the water area.