In geology, a forebulge is a flexural bulge in front of a load on the lithosphere. The load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression under the load. The load and the resulting flexure place stress on the mantle, causing it to flow into the area around the loaded area. The subsidence of the area under the load and the uplift of the forebulge continue until the load is in isostatic equilibrium, a process that takes on the order of 10,000 to 20,000 years. Because of the coupling with the mantle, the rate of forebulge formation and collapse is controlled by mantle viscosity.
One cause for forebulge formation is loading of the continental lithosphere by ice sheets during continental glaciations. Because of the removal of the ice sheets, the formerly-glaciated areas are currently rising in a phenomenon known as post-glacial rebound. Because of the coupling of the mantle with the plates, data from post-glacial rebound are used as a direct probe of the viscosity of the upper mantle. As the ice melts and the land under it rises by isostatic recovery, the forebulge also subsides. Forebulge subsidence is the reason why the Netherlands and parts of southern England are still slowly sinking. One estimate is that the centre of the North Sea rose by about 170 m (558 ft) during the Ice Age because of forebulging.
The existence of a forebulge is predicted by the solution to the equations for the flexure of a thin elastic beam or plate (the lithosphere). The magnitude of the flexure goes as an exponentially decaying sinusoid. The lithosphere is therefore anomalously high at a flexural half-wavelength past the load; the flexural wavelength is determined in large part by the elastic thickness of the lithosphere.
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