Gravity - Part 3

Question 1

What effect does truncating an infinite slab into a semi-infinite slab have? 3 things

Answer 1

1. Essentially no gravity effect in regions far from the slab
2. An increase or decrease in gravity crossing the edge of the slab
3. Full gravity effect in regions over the slab but far from the edge

Question 2

How does gravity effect of the semi-infinite slab change relative to the slab edge? (4 things)

Answer 2

1. Far away from slab contribution is zero
2. Above the edge of the slab the contribution is exactly 50% of the maximum value
3. Over the slab but far from the edge the gravity anomaly is the same as for an infinite slab
4. rate of change in gravity depends on the depth of the slab

Question 3

What are the two fundamental properties of gravity anomalies illustrated by the semi-infinite slab?

Answer 3

1. Amplitude of the anomaly reflects the mass excess or deficit. The mass excess or deficit depends on the product of the density contrast and thickness of the anomalous body
2. The gradient (rate of change) of the anomaly reflects the depth of the excess or deficient mass below the surface (z). the depth this determines how abruptly the gravity anomaly changes from zero to almost full.

Question 4

How would a body near the surface and a body deep within the Earth vary in gradient of the gravity anomaly as illustrated by semi infinite slab?

Answer 4

Near surface results in gravity change with steep gradient

Deep within the earth the same body would produce a more gentle gradient

Question 5

What kind of regions can the semi-infinite slab model be used to approximate contributions to the free air gravity anomaly?

Answer 5

Regions in isostatic equilibrium

Question 6

How does the Bougeur anomaly for the simple passive margin occur?

Answer 6

Results from correcting the mass deficit of the water to approximate that of the upper crust, the passive margin model thus illustrates general form of the Bougeur anomaly for a region in local isostatic equilibrium

Question 7

What does the Bougeur anomaly in a simple passive margin show? (3 things)

Answer 7

1. Values are near zero over normal continental crust
2. Bougeur anomaly mimics the Moto increasing to large positive values as the mantle shallows beneath the ocean
3. The form of the Bougeur anomaly is a mirror image of topography; the increase in anomaly correlates with deepening of water

Question 8

What three things does the free air gravity anomaly for a mountain range illustrate about regions in local isostatic equilibrium? (mountain range)

Answer 8

1. Values are near zero because mass excess of the topography equals mass deficit of crustal root
2. Significant edge effect occur because shallow and deep contributions have different gradients
3. The area under the curve of the anomaly sums to zero

Question 9

What 3 key attributes does the Bougeur gravity anomaly for the mountain range show?

Answer 9

1. Values near zero over continental crust of normal thickness
2. Form of the Bouygues mimics root contribution, anomaly decreases as Moto depends beneath the mountains
3. Form of the Bougeur anomaly is almost mirror image of topography, the anomaly decreases where the topography of the mountain rises

Question 10

In tectonic settings what do the gross forms of the free air and Bougeur gravity anomalies reflect? (two)

Answer 10

1. Density distribution of Earth materials in a region

2. Flexural strength of the materials

Question 11

What are the two boundaries that the Airy model suggests causes isostatic boundaries?

Answer 11

Topography/bathymetry and crust/mantle boundary (Moho)

In some regions a third boundary, lithosphere/asthenosphere