ISOSTASY

1. ISOSTASY Lisa Lau Amanda Evan

2. Buoyancy • The forces of buoyancy and gravity are equal and opposite. • The buoyant force is equal to the weight of the fluid displaced by the object. • Objects denser than the fluid will sink. • Objects less dense than the fluid float and are only partially submerged.

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. • 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 and the crust rises. • The process 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. • See an animation of isostasy: http://www.discoverourearth.org/student/isostasy.html

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.