A model of present-day mass change due to post-glacial rebound and the reloading of the ocean basins with seawater. Blue and purple areas indicate rising due to the removal of the ice sheets. Yellow and red areas indicate falling as mantle material moved away from these areas in order to supply the rising areas, and because of the collapse of the forebulges around the ice sheets.
Post-glacial rebound (sometimes called continental rebound, glacial isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheetsduring the last glacial period, through a process known as isostasy. It affects northern Europe (especially Scotland, Fennoscandia and northern Denmark), Siberia, Canada, the Great Lakesof Canada and the United States, the coastal region of the US state of Maine, parts ofPatagonia, and Antarctica.
Gravity field
Ice, water and mantle rocks have mass, and as they move around, they exert a gravitational pull on other masses towards them. Thus, the gravity field, 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 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 satellite motion.
§
Earth’s rotation
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. But taking into account the tidal interaction alone over-corrects the eclipse path which would lie east of Babylon. 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; by the conservation of angular momentum, the rotational motion must also change. This is illustrated by a rotating ice skater: as she extends her arms above her head, her moment of inertia decreases, and she spins faster. On the other hand, as she extends her arms horizontally, her moment of inertia increases and her spin slows.
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 – causing the Earth’s spin to slow down. 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 latitude, 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 ; 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.
HDTVNEWS. sept 12- 2012.
HDTVNEWS 