2.8 Gravity and Isostasy Flashcards
Which two factors contribute to the value of gravitational attraction?
the mass of the objects
the distance between the objects
What are the units given for gravity surveys?
Gals
1 Gal = acceleration of 1 cm s-2
Variations in gravity are very small so usually milligal or mgal
Why can’t the value of gravity itself be used to identify a particular rock type?
there are too many variables that affect mass and therefore gravity
What does a positive gravity anomaly signify?
greater than expected gravity value
therefore more than average mass
therefore denser than average rock
What does a negative gravity anomaly signify?
less than expected gravity value
therefore less than average mass
therefore rock that is less dense than average
Why is a latitude correction applied to measured gravity values?
the Earth is slightly ‘flattened’ at the poles due to its spin, meaning the poles are slightly closer to the centre of mass
What is a free air gravity anomaly?
a correction for additional distance due to. altitude
if a location is at a height of 100m above sea level, it is 100m further away from the centre of the Earth
What is the Bouger gravity anomaly?
a correction for the additional mass due to altitude
if a location is at a height of 100m above sea level, there is 100m more rock
What is the meaning of the term isostasy?
the theoretical state of equilibrium between Earth’s lithosphere and asthenosphere, such that the lithosphere ‘floats’ at an elevation that depends on its thickness and density
What is the nature of the gravity anomaly over Scandinavia?
a negative gravity anomaly
The continental crust extends further into the mantle than expected as a result of the overloading of the area by thick continental ice sheets during the Ice Age.
What is meant by the term isostatic rebound?
the rising up of land masses that were once depressed by ice sheets
How does isostatic rebound provide evidence for the nature of the asthenosphere?
Vertical movement of land masses show that the mantle must be able to flow, as a rheid.
The rate of uplift allows us to calculate the rate of flow and therefore the viscosity of the mantle.
