Earth's Internal Structure: Gravity and the Geoid (2.3.1)

Question 1

What did most people in Aristotle era believe was the shape of the Earth?

Answer 1

A sphere.

Question 2

What is the actual shape of the Earth?

Answer 2

Oblate Spheroid.
- Flattened sphere.
- Polar axis around 21km shorter than equatorial axis.
- Earth flattened at poles, bulges at equator.

Question 3

What is the mass of Earth?

Answer 3

5.977 x 10^24 kg

Question 4

What is the density of Earth?

Answer 4

Earth denser towards centre, mean density greater than crustal density. Earth composition must change with depth. Mean density = 5.58g cm^-3

Density (p = M/V)

Question 5

What is gravitational potential energy?

Answer 5

When gravity acts on an object but cannot move it.

Question 6

What is equipotential surface?

Answer 6

Surface which all points have some potential energy. Reacting to sum of all gravitational forces acting upon it.

Question 7

Does potential energy increase or decrease as you go up a slope?

Answer 7

Increases.

Question 8

Spatial variation in earth gravity

Answer 8

• Gravity controlled by sum of all masses.
• If gravity depends on density, and density depends on composition, temperature, and pressure, then gravity on Earth’s surface must vary with composition and temperature. Shouldn’t be same everywhere.

Question 9

Variations in Earth gravity

Answer 9

• Earth’s mean gravity ~9.8 s^-2.
• Variation = 9.80665 m s^-2 +/- 0.00050 m s^-2.
• Amplitude of surface gravity variation is ~50 millionths of Earth’s total gravity field.

Question 10

What are gravity anomalies?

Answer 10

• Difference between observed acceleration of Earth’s gravity and value predicted by model.

Question 11

Deviation of observed geoid from reference geoid.

Answer 11

Positive anomaly = stronger gravitational pull.
Negative anomaly = weaker gravitational pull.

Question 12

Anomalies from irregular mass distribution within Earth

Answer 12

Positive anomaly = more local heavy material e.g. mantle density (hot/cold), crustal thickness (rock type).

Question 13

What is the Geoid?

Answer 13

• Equipotential surface corresponding to mean sea level.
• On land: corresponds to level water would reach in canals connecting seas.
• Geoid ‘height’ varies as gravity differs from place to place due to variable mass and density distribution in crust or mantle.

Question 14

What is Geodesy and Satellite-Derived Ocean Bathymetry?

Answer 14

• Local regional topography measured by radar altimeter aboard satellite (follows geoid).
• Ocean surface bulges outwards and inwards, mimics shape of ocean floor.
• Satellites invert m-scale variations in sea level to infer km-scale seafloor bathymetry (submarine topography).
• Geodetic surveying from satellites provides detailed dataset of bathymetry of oceans (<10% ocean floor been surveyed directly).