Isostasy
This concept is invoked to explain how different topographic heights can exist at Earth's surface.
Although Earth is a dynamic system that responds to loads in many different ways,[3] isostasy describes the important limiting case in which crust and mantle are in static equilibrium.
The general term isostasy was coined in 1882 by the American geologist Clarence Dutton.
A party working in Ecuador was aware that its plumb lines, used to determine the vertical direction, would be deflected by the gravitational attraction of the nearby Andes Mountains.
Similar observations in the 19th century by British surveyors in India showed that this was a widespread phenomenon in mountainous areas.
It was later found that the difference between the measured local gravitational field and what was expected for the altitude and local terrain (the Bouguer anomaly) is positive over ocean basins and negative over high continental areas.
[7] The American geologist Clarence Dutton use the word 'isostasy' in 1889 to describe this general phenomenon.
[4][5][6] However, two hypotheses to explain the phenomenon had by then already been proposed, in 1855, one by George Airy and the other by John Henry Pratt.
[3] Both the Airy-Heiskanen and Pratt-Hayford hypotheses assume that isostacy reflects a local hydrostatic balance.
[9] Lithospheric flexure was first invoked in the late 19th century to explain the shorelines uplifted in Scandinavia following the melting of continental glaciers at the end of the last glaciation.
It was likewise used by American geologist G. K. Gilbert to explain the uplifted shorelines of Lake Bonneville.
At the center of a level plateau, it is approximately equal to the free air anomaly.
[14] The basis of the model is Pascal's law, and particularly its consequence that, within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation (surface of hydrostatic compensation):[3][8] h1⋅ρ1 = h2⋅ρ2 = h3⋅ρ3 = ... hn⋅ρn For the simplified picture shown, the depth of the mountain belt roots (b1) is calculated as follows: where
In the case of negative topography (a marine basin), the balancing of lithospheric columns gives: where
[3][15] This hypothesis was suggested to explain how large topographic loads such as seamounts (e.g. Hawaiian Islands) could be compensated by regional rather than local displacement of the lithosphere.
[3][9] For example, the vertical displacement z of a region of ocean crust would be described by the differential equation where
Solutions to this equation have a characteristic wave number As the rigid layer becomes weaker,
Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate.
[18] An analogy may be made with an iceberg, which always floats with a certain proportion of its mass below the surface of the water.
However, convergent plate margins are tectonically highly active, and their surface features are partially supported by dynamic horizontal stresses, so that they are not in complete isostatic equilibrium.
[22] Mid-ocean ridges are explained by the Pratt hypothesis as overlying regions of unusually low density in the upper mantle.
This provides evidence (via the Pratt hypothesis) that the upper mantle in this region is inhomogeneous, with significant lateral variations in density.
Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea[24] and Hudson Bay.
[25] As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels.
The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing.
[26] It can cause changes in Earth's gravitational field[27] and rotation rate, polar wander,[28] and earthquakes.
