Chapter 9 (Large-scale Gravity and Isostasy)




Isostasy is the ability of layer in the Earth to yield under the weight of large masses (e.g., mountains; large basins)


*Mountains have roots!


Isostasy follows Archimedes’ Principle, which states, “The decrease of weight [of a body] equals the weight of liquid displaced by the part of the body under the liquid surface.”  In other words, “the denser the liquid, the less far the block sinks before it floats.”  This is the way in which an iceberg floats.  The variations of mass that account for changes in size or weight is isostatic compensation.


Isostasy on Earth:


Ø      The lithosphere “floats” on the asthenosphere (the layer that will yield to the weight).

Ø      Over geologic time scales the asthenosphere and mantle behave like liquids.

Ø      If blocks of lithosphere are fully “floating” they are in equilibrium.


Simple isostatic corrections


Add up columns:      






Airy Model


Ø      Lithospheric blocks all have the same density but different depths (thickness)


Ø      The “root” under a mountain




Pratt Model


Ø      Lithospheric blocks have same depth but different densities



These are not the only models, and both work in different areas.




Regional Compensations


Lithosphere has lateral strength and cannot be defined just as lithospheric blocks.


Ø      Small loads are supported without much effect on plates

Ø      Larger loads will show some effect or bending (more severe nearest the load and becoming broader further away from the load)


Isostatic Anomalies: subtracted from the free-air anomaly (also removes edge anomalies)


Evidence for Isostasy: evidence that support the concept of isostasy has come from looking at the gravity signature across Hawaii, Greenland, and the Himalayas and comparing it to Bathymetry and/or topography.


Isostatic Rebound:  The ability for the lithosphere to regain its shape once a load is released or imposed (e.g., Glaciation).


The Mantle: acts like a solid and liquid (slushy material that undergoes solid-state creep).  Its behavior (solid or liquid) depends on the amount and rate of imposed stresses.


Lithosphere/Asthenosphere boundary: Compositional boundary that is not well defined.  The boundary is typically thick under cold craton (~200 km) and thinner under young crust (~120 km).


Plate Tectonics: Induces additional forces on the lithosphere (e.g., subduction zones)




The Shape of the Earth:


Reference spheroid: approximates the sea-level surface

Geoid: actual mean sea-level


*The largest difference between them is 80 m.