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Post-glacial rebound

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Changes in the elevation of Lake Superior due to glaciation and
   post-glacial rebound
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   Changes in the elevation of Lake Superior due to glaciation and
   post-glacial rebound

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

Overview

   By the end of the last ice age about 11,000 years ago, much of northern
   Europe and North America was covered by ice sheets up to 3 km thick.
   The enormous weight of this ice caused the crust to sink into the fluid
   mantle. At the end of the ice age when the glaciers retreated, the
   removal of the weight from the depressed land led to a rapid uplift due
   to the buoyancy of crustal material relative to the mantle. 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 was rapid, proceeding at about 7.5 cm/year.
   This phase lasted for about 2000 years, and took place as the ice was
   being unloaded. Once de-glaciation was complete, uplift slowed to about
   2.5 cm/year, and decreased exponentially after that. Today, typical
   uplift rates are of the order of 1 cm/year or less, and studies suggest
   that rebound will continue for about another 10,000 years. The total
   uplift from the end of deglaciation will be about 400 m.

Effects

   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 then Stockholm was founded at its outlet. Marine seashells found
   in Lake Ontario sediments imply a similar event in prehistoric times.
   In several Nordic ports, like Tornio, the harbour had to be relocated
   several times in the past centuries.

   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.

   In Sweden some of the most pronounced effects can be seen on the island
   of Oland, 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.

   Globally, post-glacial rebound was tending to make the earth more
   spherical over time. However, this effect has been reversed by other
   factors such as large scale movements of ocean water caused by El Niño
   and related effects.

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.

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