Equilibrium

Question 1

is the state of graviational equilibrium between earth’s crust and mantle such that the crust “floats” at an elevation that depends on its thickness and density

Answer 1

Isostasy

Question 2

Developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth’s crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density such as mountain ranges, project higher above sea level than do those of greater density. The expanation for this, was that mountains resulted from the upward expansion of locally heated crustal material, which had a lower volume but a lower density after it had cooled.

Answer 2

Pratt’s Hypothesis

Question 3

Earth’s crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddel Airy, an english mathematician and astronomer, assumed that the crust has a uniform density througout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are bouyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater.

Answer 3

Airy’s hypothesis