Gravity - Part 2

Question 1

What did the Pratt local isostasy model assume?

Answer 1

Earth crust is composed of blocks of different density, lighter blocks extend further in the air than thinner heavier blocks. Also assumed that there is a flat crust/mantle boundary

Question 2

What does Airy model assume?

Answer 2

Crustal blocks have equal density but they float on higher density material (mantle).
Base of crust is hence an exaggerated mirror image of topography, areas of high elevation have low density ‘crystal roots’ supporting their weight

Question 3

What is lithospheric pressure?

Answer 3

Pressure exerted on a point at a given depth within the Earth

Question 4

How does the equation for local isostasy look?

Answer 4

P = pgh
Where:
P - pressure on crystal block
p - density of crystal block
h - thickness of crystal block

Question 5

What two equations hold true at depth of compensation beneath each region of the Airy Isostatic model?

Answer 5

1. Total pressure, P, exerted by each vertical column, divided by gravitational acceleration, g, is constant
2. Total thickness of each vertical column is constant

Question 6

What is the lithospheric flexure controlled (regional isostasy)?

Answer 6

Load in centre causes flexure to form at edges

Question 7

What is the difference between small and large flexural rigidity?

Answer 7

Small flexural rigidity - great bend near load, short

Large flexural rigidity - smaller bend, appears more rigid but over greater distance

Question 8

What is the flexural rigidity?

Answer 8

Resistance to bending

Question 9

How does regional isostasy appear in subduction zones? (load, flexure)

Answer 9

Like bending on edge of diving board, load is primarily topography of accretionary wedge and volcanic arc on overriding plate, flexure of downgoing plate results in trench and farther out to sea a bulge will develop

Question 10

How does regional isostasy appear in mountain ranges?

Answer 10

Mass of mountains puts a load on plate seen with depressions between mountains and flexural bulges in both directions, depressions can fill up with sediment fast (forehand basin)

Question 11

What does the buried sphere model illustrate about gravity anomalies?

Answer 11

1. Mass excess causes an increase in gravity, while mass deficit results in a gravity decrease
2. The more massive the sphere the greater the amplitude of the gravity anomaly
3. The anomaly is attenuated as the sphere is buried more deeply within the Earth
4. Width of the gravity anomaly increases as the sphere is buried more deeply (points 3 and 4 as buried deeper has shallower effect on greater area)