History of Istocacy
In the mid 1800s, British surveyors discovered a discrepancy of 150 m in horizontal distance between 2 stations in India 625 km apart, as measured by surveying and as calculated from astronomical readings.
J.H. Pratt, a British physicist, explained the discrepancy by arguing that the mass of the Himalaya Mountains would deflect the plumb line (a way of measuring vertical) northward at each station, but more at the station closer to the Himalayas. However, Pratt's estimate of how much the mass of the Himalayas should deflect the plumb line was 3 times the amount actually measured.
G.B. Airy, another British scientist, explained the difference by theory of isostasy. He proposed that the Earth's crust is floating on a dense, plastic substratum and that the extreme elevation of the Himalayas is supported by a root of low-density rocks, much as an iceberg is supposed by its underwater mass. Pratt later agreed that the crust is in a state of flotational balance, but hypothesized that topography is supported by a crust having a uniform thickness below sea-level, but varying density (e.g., a balsa wood plank will float higher than an oak plank).
Evidence supporting the idea of isostasy comes from Scandanavia and particularly, the Hudson Bay lowlands of North America, where the region around Hudson Bay has rebounded by as much as 330 m. During the Pleistocene ice ages, the land there was once covered by a sheet of ice 3 kilometers thick, weighing down the crust. The ice melted some tens of thousands of years ago, and the crust is still rising slowly because isostatic balance has not yet been attained.
Worldwide gravity measurements indicate that some form of isostatic compensation exists over most of the Earth's surface, although the density distribution within the crust is far more complicated than suggested by either the Airy or Pratt models.
Bryson
Burke Diamond Corporation
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Explaining Isostacy
Compressional orogens on the continents result from excess friction along subduction zones, collision and accretion of microcontinents like island arcs, and continent-continent collisions. Collisions result in thickening of the crust - building of a mountain belt. This orogenic belt will typically include a belt of folded and thrust faulted sedimentary strata, a belt of metamorphic rocks uplifted by the collision and also rocks that are originally metamorphosed in the core of the mountain belt that are later exposed by erosion. As the mountains are eroded, the eroded materials are deposited in a clastic wedge that is thickest and contains the coarsest sediments nearest to the eroding mountain front.
Foreland basins subside as a result of the load. The crust beneath the thrust load is depressed as a result of isostacy and the adjacent crust is depressed via flexure since it is attached.
Orogenic belts
continue to be uplifted long after erosion has removed the original mountains.
The mountains were originally supported by very deep roots. As the mountains are
eroded the deep crustal roots undergo isostatic rebound.
Findiung Burried Anomalies in Mexico which Changed the Earth
Study of gravity and magnetic fields in the Yucatan Peninsula, Mexico has helped to locate a large crater caused by the impact of a comet or meteorite at the end of the Cretaceous Period about 65 million years ago. Some scientists postulate that the effect of this extraterrestrial object colliding with the Earth is responsible for the extermination of many life forms on Earth, including the dinosaurs. Although today the crater does not have an obvious topographic expression (because of the Earth's active weathering and erosional processes), the gravity and magnetic data allow us to "see" through a kilometer of younger sedimentary rocks to image a buried, 200-km wide crater ( Chicxulub Crater) which was excavated by an extraterrestrial body that may have killed off the dinosaurs.
Andean Isostacy
Gravity profiles (Lyon-Caen et al., 1985) and crustal-thickness studies (Beck et al., 1996) of the Central Andes suggest that, although Airy compensation is operative across much of the orogenic belt, the surface load may be in part supported by a flexural isostatic mechanism. Flexural isostacy must at least play a role in the structural development of the foreland basin system. Watts et al. (1995) suggest that the observed curvature of the Andean forebulge is the result of a smooth, eastward increase in the effective elastic thickness of the South American lithosphere.
A program to calculate the three-dimensional flexure of the elastic lithosphere under topographic loading has been developed and used to estimate the lithospheric deflection associated with the Andes. The geometry of the foreland basin system adjacent to the Central Andes is reasonably approximated by a flexural model assuming a load density of 2800 kg/m3 and a constant effective elastic thickness of 60 km. The model correctly predicts the position and curvature of the forebulge axis throughout the Central Andean region and yields accurate estimates of the foredeep thickness (~3 km) and width (~200 km) and forebulge width (~350 km) along a profile through 18°S. This suggests that the geometry of the observed foreland basin system need not be the product of lateral variations in effective elastic thickness. However, the observed forebulge amplitude exceeds current model predictions. This may imply that uplift is occurring by a locally nonelastic mechanism in the forebulge region (Horton and DeCelles, 1997).
