attraction of gravity increases at the poles because the centrifugal acceleration due to the earth's rotation
decreases to zero and the mass attraction is greater since it is closer to the center of the earth where the
mass is considered concentrated. The attraction of gravity at the equator decreases since the distance to
the center of the earth's mass is greater. Additionally, the component of centrifugal force opposing
gravity is greater at the equator.
h. Additional variations in gravity can be caused by elevation, surface density, and topography.
Oceans, plains, and mountains distort the surface, and the density (the amount of mass in a given
volume) and elevation of these topographic features create regional differences in gravitational
attraction. Furthermore, buried masses lying below the earth's crust exert additional gravitational pull
on surface objects.
i. The variations in gravity, inferred at first from surveying discrepancies and later determined by
actual observations, led to the concept of isostasy. The term isostasy, meaning equal pressure, was first
proposed by C. E. Dutton in a paper written in 1889.
(1) Isostasy is a condition of approximate equilibrium in the outer part of the earth, such that the
gravitational effect of masses extending above the surface of the geoid in continental areas nearly
counterbalanced by a deficiency of density in the material beneath those masses, while the effect of
deficiency of density in ocean waters is counterbalanced by an excess of density in the material under
the oceans.
(2) The basic principle of isostasy is that the masses of prismatic columns of the outer part of the
earth, extending to some constant depth below the surface of the geoid, are proportional to the areas of
their sea-level sections regardless of their surface elevations. The depth below sea level to which these
hypothetical columns extend is known as the depth of isostatic compensation and is somewhere between
96.56 and 112.65 kilometers. The area of the sea-level section of a unit hypothetical column for which
isostatic compensation is ordinarily complete has not been determined; it may be uniform for all parts of
the earth, or it may vary with the character of the relief in the same continental region.
(3) While isostasy is generally accepted as a proven principle, there are several theories as to the
relative distribution of the matter producing this condition of equilibrium. The two principal theories are
those of Pratt and Airy. The fundamental difference between the two theories is that Pratt postulated
uniform depth with varying density, while Airy postulated uniform density with varying depth.
The Pratt theory, announced by J. H. Pratt in 1855, assumed that the continents and
islands project above the average elevation of the solid surface of the earth because of
the material of less density beneath them--the higher the surface, the less the density
below it. Under the Pratt theory, complete equilibrium exists at some uniform depth
below sea level--the same depth for ocean areas as for landmasses.
1-19
EN0593
Previous Page
