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“Post-glacial rebound (or Glacial Isostatic Adjustment) produces 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.
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 were exposed and many islands became connected with the continents through dry land. This was 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 allowed the migration of people and animals during last glacial maximum.
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 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.
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 be 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.
Earth’s Rotational Motion
Examination of ancient Chinese and Babylonian eclipse records reveals that the Earth’s rotation rate is not constant. For example, if the rotation rate were constant, then the shadow path of an ancient Babylonian eclipse would lie somewhere across western Europe and the ancient eclipse could not have been 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 extends her arms vertically over her head, her moment of inertia decreases and as a consequence, she spins faster. On the other hand, as she extends her arms horizontally, 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 closer to the poles, which is closer to the rotational axis. This causes the Moment of Inertia of the Earth-ice-water system to decrease and just like the rotating figure skater bringing her arms closer to her body, the earth should spin faster. During deglaciation, the melted ice water returns to the oceans – farther from the rotational axis – and thus causing the Earth’s spin to slow down. Also, the mantle rocks flow in a direction opposite to that of the water, but the rate is much slower. 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. This process would, in isolation, lead to an increase in the rotation speed of the Earth and therefore to a decrease of the length of day. Lambeck estimated that the isolated effect of post-glacial rebound on the length of the day would be a decrease of about 0.7 milliseconds per century. This process of nontidal acceleration of the rotation of the earth is corroborated by observations of the satellite LAGEOS and is generally attributed to glacial isostatic adjustment.
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 center of the ice masses at glacial maximum (Polar wander), thus it is moving towards eastern Canada at a rate of about 1 degree per million years.
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.
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 was 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. 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, 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 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. 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 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, GPS and GRACE 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.“
If you still do not believe in the melting of the ice at the north pole please resd this article:
Polar meltdown may open trade route short-cut: