isostasy

• Continental lithosphere has a low density (2.7g/cm3).

  Mantle has a density of 3.3g/cm3. 

   
• The root displaces the dense mantle, because the root is less dense there is a negative gravity anomaly. 
• Mountains balance like icebergs on water. The root sinks into the mantle until equilibrium is reached, this is isostacy. 
• If balance is not achieved gravity anomalies occur in unusual places. 
• When Scandinavia was covered in ice, it was pushed downwards by the ice. When the ice retreated the continental lithosphere slowly rebounded, called isostatic rebound, this is why there is a negative anomaly. 
• The evidence for isostatic rebound is raised beaches, supporting the idea of a rheid underneath the crust which is the rate of uplift that links to the viscosity of the mantle.