Introduction to Isostasy
Isostasy is a principle of buoyancy and a fundamental concept in the Earth Sciences. Isostasy is a word derived from the Greek words “iso” and “stasis” meaning “equal standing”. The idea of isostasy is based on light crust floating on the denser underlying mantle. This is used to describe a condition to which the Earth’s crust and mantle tend, in the absence of disturbing forces. Materials transporting over the Earth’s surface such as glaciers, volcanism, and sedimentation can all be factors that disturb isostasy. As the crust and mantle react to these disturbances it allows us to understand more about mountain building and other phenomena (Watts, 2001).
Discovery of Isostasy
Like all theories of science, isostasy went through phases of being accepted and also rejected by others. Geologist C.E. Dutton, in 1889, used the word isostasy for the first time (Watts, 2001). However, in the early 1800’s, British engineers attempted to make a map of India near the Himalayas and discovered that their plumb bobs did not hang straight down but instead were deflected towards the Himalaya mountain range (Watts, 2001).
Airy Model
G.B. Airy was a British scientist who came up with the theory that Earth’s crust is floating on a dense, plastic substratum and that mountain elevations are supported by a root of low-density rocks (Oreskes, 1999). In figure 1, Airy’s model shows that across the lithosphere, the rock densities are about the same, but the crustal blocks have different thicknesses which mountains that have a higher elevation also extend deeper roots into denser material below(Pan, 2007).
Pratt Model
J.H. Pratt was a British physicist who proposed the theory that the crust is in a state of flotational balance, but suggesting that topography is supported by a crust having a uniform thickness below sea-level with different densities (Oreskes, 1999). In figure 1, Pratts model shows that the less dense crustal blocks float higher to make mountains and the more dense blocks form basins and lowlands (Pan, 2007).

Pratt's Model of Isostasy (On the Left) Airy's Model of Isostasy (On the Right)

Isostasy can account for the formation of many of earth's landscapes including the Rocky Mountains, rebounding glacial area's, and mid-ocean ridges. Isostasy is the combination of factors such as buoyancy, lithosphere and asthenosphere, and erosion. The models of isostasy.

Ross Hokett, Brian Gabel, Grant Miller, Jay Thomas, Derek Cavinder, Kari Lanphier