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ISOSTASY

ISOSTASY. Lisa Lau Amanda Evan. Buoyancy. For any object in a fluid, gravity and buoyancy act on it in opposite directions. Buoyancy uplifts an object with a force equal to the weight of the displaced fluid.

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ISOSTASY

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  1. ISOSTASY Lisa Lau Amanda Evan

  2. Buoyancy • For any object in a fluid, gravity and buoyancy act on it in opposite directions. • Buoyancy uplifts an object with a force equal to the weight of the displaced fluid. • If an object is denser than the fluid, then even the buoyant force on the entire volume of the object will not counteract its weight, and the object will sink. • If an object is less dense than the fluid, then it will be partially submerged, according to the volume of displaced fluid necessary to make the buoyant force equal to the weight of the object.

  3. Equilibrium between Buoyancy and Gravity

  4. Buoyancy in Earth’s mantle • Continental crust and oceanic crust float on the asthenosphere due to their lower densities. • Density of mantle: 3.3 g/cc • Density of oceanic crust: 2.9 g/cc • Density of continental crust: 2.8 g/cc • Equilibrium between the weight of crust and the buoyant force in the mantle requires only a portion of the volume of the crust to be submerged in the asthenosphere. • The equilibrium between the weight of crust and the buoyant force of the mantle is called isostasy.

  5. Continental Crust and Isostasy • Because continental crust is less dense than oceanic crust, it rises higher in the asthenosphere. • The size of the portion of continental crust submerged in the asthenosphere depends on its mass. • Therefore, mountains, which add a great amount of mass to the crust, cause the formation of large “roots” in the mantle to create a large enough buoyant force to counter the mountains’ weight.

  6. Isostasy and Eroding Mountain Ranges

  7. Effects of Erosion on Isostasy • Conversely, as erosion takes mass away from crust, the weight of the crust decreases. • A smaller buoyant force is needed to counter the weight, causing the volume of displaced fluid to decrease, i.e., the crust to rise. • The process by which crust rises as mass is taken away is called isostatic rebound. • Isostatic rebound explains why the Appalachian Mountains, which at their present rate of erosion should have disappeared long ago, still exist today.

  8. Oceanic crust and Isostasy • Roots can also form under oceanic crust, for example under seamounts that form over hot spots in the mantle. • However, oceanic crust does not form as large of roots because it is not as thick. • See an animation of isostasy: http://www.discoverourearth.org/student/isostasy.html

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