Seismic wave travel in which density Seismic wave travel in which density increases monotonouslyincreases monotonously
Hypsographic CurveHypsographic Curve
Extreme elevations are rare
Two common elevations Ocean basins
(abyssal plains)
Continental interiors
Why??
The weight of an Object is determined The weight of an Object is determined by its mass and the force of gravityby its mass and the force of gravity
F = the force of gravity (weight)F = the force of gravity (weight)
g = the gravitational constant (6.672 x 10g = the gravitational constant (6.672 x 10-11-11 N N · m· m22 · kg · kg-2-2))
mm11 = the mass of the object = the mass of the object
mm22 = the mass of the Earth = the mass of the Earth
r = the distance between the object and the center of the Earthr = the distance between the object and the center of the Earth
1 22
m mF g
r
Weight varies due to three factorsWeight varies due to three factors
1. Altitude1. Altitude
As altitude increases r increases, so F decreasesAs altitude increases r increases, so F decreases
1 22
m mF g
r
Weight varies due to three factorsWeight varies due to three factors
3. Local variations in rock density3. Local variations in rock density
Geographic Survey of IndiaGeographic Survey of India
Mid-19Mid-19thth century century
Sir George EverestSir George Everest
Triangulation systematically Triangulation systematically disagreed with astronomical disagreed with astronomical near the Himalayasnear the Himalayas
Why? Why?
Archdeacon Pratt’s theory of IsostasyArchdeacon Pratt’s theory of Isostasy
Water density 1.0
0.5 0.50.3 0.30.2 0.2
density= 0.1
Each block weighs 2 g
Astronomer Royal Airy’s theory of IsostasyAstronomer Royal Airy’s theory of Isostasy
Water density 1.0
0.5 0.50.3 0.30.2 0.2
density= 0.1
Each block weighs 2 g
Water density 1.0
Wood density 0.5
2 g 2 g3 g 3 g5 g 5 g
8 g
Uncorrected gravity profile at 20,000 ft.Uncorrected gravity profile at 20,000 ft.
Free-Air correction to 0 ft.Free-Air correction to 0 ft.
Bouguer Corrected profileBouguer Corrected profile
1 22
m mF g
r
Free-air and Bouguer gravity Free-air and Bouguer gravity anomaly map of the Alpsanomaly map of the Alps