Isostasy

• Both continental and oceanic crustal plates make up the lithosphere, which itself floats on a viscous layer called the aethenosphere. Crustal plates vary in mass and density, and isostasy explains the gravitational forces exerted on these plates that allow them to remain balanced and afloat in the aethenosphere.
  
  

Example of Isostasy: Mountain Ranges

• In mountain ranges, the majority of land mass exists below sea level and stretches into the molten aethenosphere where it remains buoyant. Maintenance of this buoyancy is known as the isostatic effect, and depends on the size of the mountain above ground, its density and the distance between the mountain peak and the Earth's surface. A comparison can be made to large icebergs, where a greater proportion of the iceberg lies underwater.
  
  

Erosion

• Erosion of mountain rock causes a reduction in land mass and a shift in crustal equilibrium. As a result, land rises from within the aethenosphere to replace what was eroded and thus maintain buoyancy. This is called an isostatic uplift. Also, during orogenic mountain-building processes, where eroded materials form deposits on mountains causing an increase in mass, equilibrium is again rebalanced as land sinks deeper into the aethenosphere.
  
  

Causes of Erosion

• Most mountain erosion occurs at the surface due to the action of river flow. Areas of higher flow rates experience greater erosion. This is exacerbated in mountains with little vegetation due to infrequent rainfall or destruction by fire. Extreme weather conditions such as heat and cold also cause rock to become brittle by repeated expansion and contraction, increasing susceptibility to erosion. In some cases the impact of magma from volcanic activity also plays a role in erosion.