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ISOSTASY 
Presentation 
Engr. Faisal 13-CE-26 
& 
Group
Definition of Isostasy 
• Huge plates of crustal and upper mantle 
material (lithosphere) “float” on more dense, 
plastica...
Contd. 
• The taller a block of crust is (such as a 
mountainous region), the deeper it penetrates 
into the mantle becaus...
Isostatic models 
• Three principal models of isostasy are used: 
• The Airy-Heiskanen Model - where different 
topographi...
Important to Note 
1. ISOSTATIC EFFECTS OF DEPOSITION AND 
EROSION 
2. ISOSTATIC EFFECTS OF PLATE 
TECTONICS 
3. ISOSTATIC...
ISOSTATIC EFFECTS OF DEPOSITION AND EROSION 
• When large amounts of sediment are deposited on a particular 
region, the i...
ISOSTATIC EFFECTS OF PLATE TECTONICS 
• When continents collide, the continental crust may thicken at their edges in the 
...
ISOSTATIC EFFECTS OF ICE SHEETS 
• The formation of ice sheets can cause the Earth's surface 
to sink. Conversely, isostat...
Isostasy- Engineering Geology
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Isostasy- Engineering Geology

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Engineering Geology

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Isostasy- Engineering Geology

  1. 1. ISOSTASY Presentation Engr. Faisal 13-CE-26 & Group
  2. 2. Definition of Isostasy • Huge plates of crustal and upper mantle material (lithosphere) “float” on more dense, plastically flowing rocks of the asthenosphere. The “depth” to which a plate, or block of crust, sinks is a function of its weight and varies as the weight changes. This equilibrium, or balance, between blocks of crust and the underlying mantle is called isostasy.
  3. 3. Contd. • The taller a block of crust is (such as a mountainous region), the deeper it penetrates into the mantle because of its greater mass and weight. Isostasy occurs when each block settles into an equilibrium with the underlying mantle. Blocks of crust that are separated by faults will “settle” at different elevations according to their relative mass
  4. 4. Isostatic models • Three principal models of isostasy are used: • The Airy-Heiskanen Model - where different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant density • The Pratt-Hayford Model - where different topographic heights are accommodated by lateral changes in rock density. • The Vening Meinesz, or flexural isostasy model - where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending. • Airy and Pratt isostasy are statements of buoyancy, while flexural isostasy is a statement of buoyancy while deflecting a sheet of finite elastic strength.
  5. 5. Important to Note 1. ISOSTATIC EFFECTS OF DEPOSITION AND EROSION 2. ISOSTATIC EFFECTS OF PLATE TECTONICS 3. ISOSTATIC EFFECTS OF ICE SHEETS
  6. 6. ISOSTATIC EFFECTS OF DEPOSITION AND EROSION • When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again. • An analogy may be made with an iceberg - it always floats with a certain proportion of its mass below the surface of the water. If more ice is added to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. Similarly, the Earth's lithosphere "floats" in the asthenosphere.
  7. 7. ISOSTATIC EFFECTS OF PLATE TECTONICS • When continents collide, the continental crust may thicken at their edges in the collision. If this happens, much of the thickened crust may move downwards rather than up as with the iceberg analogy. The idea of continental collisions building mountains "up" is therefore rather a simplification. Instead, the crust thickens and the upper part of the thickened crust may become a mountain range.[citation needed] • However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution.[citation needed] • The formation of ice sheets can cause the Earth's surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels. In this way, it is possible to find former sea cliffs and associated wave-cut platforms hundreds of metres above present-day sea level. The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing. • In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements. It can cause changes in the gravitational field and rotation rate of the Earth, polar wander, and earthquakes.
  8. 8. ISOSTATIC EFFECTS OF ICE SHEETS • The formation of ice sheets can cause the Earth's surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels. In this way, it is possible to find former sea cliffs and associated wave-cut platforms hundreds of metres above present-day sea level. The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing. • In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements. It can cause changes in the gravitational field and rotation rate of the Earth, polar wander, and earthquakes.

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