Earth's Internal Structure: Gravity Isostasy (2.3.2)

Question 1

Who was Pierre Bouguer?

Answer 1

• Plumb-line attached to surveyor’s level, provides simple portable geophysical instrument for measuring gravitational attraction.
• Bouguer conducted this in Ecuador at largest known mountain at the time.
• Found that deflection towards this volcanic peak much less than he estimated from mass of mountain.

Question 2

What was John Pratt’s explanation?

Answer 2

• “Mountains occur where mantle is less dense.”
• Error measured by Everest et al. was about 1/3 expected error for gravitational attraction of mass size of Himalaya.
• Plumb bob deflected less than expected as wasn’t as much mass expected.
• Suggested mountains supported by buoyancy of crust floating on mantle, mountains strand where underlying mantle density is low.

Question 3

What was George Biddell Airy’s explanation?

Answer 3

• Same Royal Society meeting - proposed crust is of uniform density by thicker under mountain ranges.
• “Mountains are supported by deep roots of buoyant material.”; roots of highest mountains could be <80km in depth.

Question 4

What was Archimedes’ Principle?

Answer 4

A floating object sinks into water (fluid medium) until it has displaced mass of water that equals object’s mass.

Question 5

What is isostasy?

Answer 5

• The gravitational equilibrium of topography (an object neither wants to sink deeper nor float higher).
• It’s the equilibrium achieved between Earth’s lithosphere and asthenosphere.
• Lithosphere ‘floats’ on asthenosphere, thus can undergo isostatic adjustments by buoyancy forces.

Question 6

What is isostatic uplift in mountain belts?

Answer 6

• E.g. height of mountain reflects mass of mountain and density of underlying material.
• Whereas uplift and crustal thickening in mountain belts initially generated by compression, folding, and thrusting.
• As mountain erodes, its mass decreases, mountain moved upwards as it strives towards reattaining isostatic equilibrium.

Question 7

What is glacial isostasy?

Answer 7

• Weight of ice causes regional flexure in lithosphere.
• Plastic asthenosphere moves outwards.
• Sea level determined by: depression of land surface, removal of liquid water into ice, rise of seafloor at margins of icecap, through outward asthenospheric flow.
• Ice melting causes reversal of process.

Question 8

What is glacial rebound - past loading

Answer 8

• Isostatic adjustment due to melting of ice cap after last glacial period.
• Last 8000 years.
• Rates up to 1m in 100 years.

Question 9

Glacial rebound across the world

Answer 9

• In UK, glacial ice extended down to mid-southern Wales during last glacial period.
• More substantial ice cap covered in Scotland and Northern England.
• Subsidence/uplift patterns reflect overall distribution.