Please enable JavaScript.
Coggle requires JavaScript to display documents.
The Lithosphere (Isostasy (Strong negative gravity anomalies across…
The Lithosphere
Isostasy
Strong negative gravity anomalies across mountain ranges indicate that rocks below them appear to be lighter than expected
In 1855 Sir George Airy suggested that mountains, made of less dense continental crust, were rather like icebergs, sinking down into the mantle until they were in a state of balance, supported by the denser rocks of the mantle
This is a result of the overloading of the area by thick continental ice sheets during the ice age. The ice melted relatively quickly, leaving the curst to rise slowly back to its original level by isostatic rebound
Isostasy is most interesting when the balance is not achieved, as shown by gravity anomalies
Evidence of this uplift is shown by raised beaches which are now perched above present sea level, seen around the coast of Scotland
There is a negative gravity over Scandinavia where the continental crust extends further into the mantle than expected
These vertical movements of land masses show that the mantle must be able to flow. The rates of uplift allows us to calculate the rate of flow and therefore the viscosity of the mantle
-
Gravity anomalies
-
It is more useful to plot the variations in the values from place to place. These values which are different from the average are called gravity anomalies
Gravity anomalies are used to investigate the interior of the Earth and also in the search for ores and geological structures which may hold hydrocarbons. Typical variations in the value of gravity are 20 milligals, although over the deep ocean trenches they exceed 300 milligals
-
The value of gravity itself cannot be used to identify a particular rock type because too many variables are involved