isostatic equilibrium?

Question

how does isostacy lead to the deepening of the ocean as the lithosphere ages?
it's for an oceanography lab.

Answer 1

Here's good summary of isostacy and it's effect on relative elevation of continental and oceanic crust from this site:
http://www.geology.ufl.edu/Class_Notes/gly2010/earths_interior.html

I would add though (and I think it's the most relavent to the focus of ypour question) that as the oceanic crust cools over time it's density increases and this further drives isostatic adjustment.

Isostacy-


Observation of average elevation shows the following distribution:


- Average elevation of continental crust is 840 m above sealevel

- Average depth of oceanic crust is 3800 m below sealevel


This difference in elevation is caused by differences in the density of rocks of the oceanic and continental crust. The difference in elevation is controlled by the principal of isostacy.


The principal of isostacy = The mass of a floating body equals the mass of the material that it displaces.


An alternate way to think about it is that if you have two materials with different density floating in a third material, the less dense material will float higher (displace less material) than the more dense material.


This is why boats made out of steel or concrete can float- they are filled up with air, so they displace a large amount of water. In fact, the weight of the water they displace is equal to the weight of the boat.



Example for crustal rocks:


Continental crust density is ~2.7 g/cm3

Oceanic crust is ~3 g/cm3

Mantle is ~3.3 g/cm3


Assume you have 1 cm3 of continental and oceanic crust. Converting from volume to mass, you would have 2.7 g of continental crust, but 3 g of oceanic crust.


Assume your two little pieces of the crust is "floating" in the mantle and you want to know how much of the crust is displaced.


By the principal of isostacy, you would displace 2.7 g of mantle with the continental crust, but you would displace 3.0 g of mantle with the oceanic crust. How much volume is that?


for the continental crust = 2.7 g divided by 3.3 g/cc = 0.81 cc of mantle is displaced


for the oceanic crust = 3.0 g divided by 3.3 g/cc = 0.91 cc of the mantle is displaced


In other words, more mantle is "pushed" away by the more dense oceanic crust and the crust sinks lower.



Examples of isostacy caused by continental glaciation



~18,000 years ago there was ~2 to 3 km of ice (the glaciers) on the continents. This added much mass and caused the continental crust to "sink". After the ice melted, the depressions filled with water- Hudsons Bay, and the Baltic Sea. The areas are actually moving upward


Also remember that the center of Greenland now is below sealevel, which is caused by isostacy.


These are small examples but they are exactly why the cotinental crust is at a higher elevation than the oceanic crust.





This one is good too. It discusses deepening of the seabed related to erosion from highlands. Erosion 'lightens' the mountains and the material shed into the oceans causes them to then be relatively heavier. The mountains rise and the seafloor sinks to compensate. This mechanism doesn't really address the aging of the lithosphere directly though, so I think you're after the density increase as the crust cools as the driver for deepening the ocean.

http://www.tectonic-forces.org/pt08.htm


Hope this helps.