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.