Investigating Weak Gravitational Field in India/Indian Ocean

[curiousphoton]

Anyone know what causes the weak gravitation field at the bottom of India / Indian Ocean (see link below)?

Thanks

http://news.bbc.co.uk/2/hi/science/nature/8767763.stm

 

[Archosaur]

Well, you can rule out crustal thickness.
http://earthquake.usgs.gov/research/structure/crust/images/topo.jpg" of crustal thickness shows that Indonesia (which has a high geoid level) and the Indian Ocean (which has the mysteriously low geoid level) have similar crustal thicknesses.

Tectonics must have something to do with it. The Indian-Eurasian plate convergence is unique in a lot of ways. It has given rise to the Himalayas, and is now apparently home to a gravitational anomaly. The areas with a low geoid level seem to all be on actively subducting plates. The high geoid levels tend to be near spreading centers. Hmm... I fail to see a connection, though...

 

[billiards]

Curiosphoton, careful with your interpretation of the map there. If you look at the scale bar you will see that it has units of meters. But gravity has units of acceleration. So what is that map really showing?

That map is showing the level of the geoid (the surface defined as having equal gravitational potential) relative to a datum, a reference ellipsoid.

We can say that each point at any given latitude on the reference ellispoid is the same distance from the center of the Earth as any other point on that latitude. And if we had a layered Earth with a perfectly symmetric density structure (about the pole-axis) then the gravitational potential would sit exactly flat on the reference ellipsoid. But we see that at India the geoid dips down closer to the centre of the Earth. This does not mean incidentally that gravity is stronger or weaker at that location (although it would tend to make me think it is stronger there), because g is a measure of the local gradient of the potential field which we cannot measure given only one equipotential surface. But it does mean that something in Earth's density structure is anomalously dense below India.

 

[Andre]

But it does mean that something in Earth's density structure is anomalously dense below India.

Would you expect a higher or lower density below India? If I understand it correctly, the graph depicts the theoretical shape of the earth, when there was no vertical structure, say when the Earth was fluid.

Now. if there is a stronger gravity anomaly somewhere, you'd expect that fluid to move towards that anomaly causing a higher surface locally, an idea of Nils Axel Morner. So, as an analogy, I'd expect the fluid to flow away from a weaker local gravity, causing a lower lever locally.

 

[curiousphoton]

Curiosphoton, careful with your interpretation of the map there. If you look at the scale bar you will see that it has units of meters. But gravity has units of acceleration. So what is that map really showing?

That map is showing the level of the geoid (the surface defined as having equal gravitational potential) relative to a datum, a reference ellipsoid.

We can say that each point at any given latitude on the reference ellispoid is the same distance from the center of the Earth as any other point on that latitude. And if we had a layered Earth with a perfectly symmetric density structure (about the pole-axis) then the gravitational potential would sit exactly flat on the reference ellipsoid. But we see that at India the geoid dips down closer to the centre of the Earth. This does not mean incidentally that gravity is stronger or weaker at that location (although it would tend to make me think it is stronger there), because g is a measure of the local gradient of the potential field which we cannot measure given only one equipotential surface. But it does mean that something in Earth's density structure is anomalously dense below India.

I disagree. If you look at the colored map up top, the meters refers to the distance between Earth's flattened sphere (see note 1 on the diagram 'mapping the different effects of gravity') and the geoid (note 3). The smaller the distance, the weaker the gravitational field...hence my original question.

 

[Xnn]

The elevations over the oceans are reflective of winds and currents.

 

[curiousphoton]

The elevations over the oceans are reflective of winds and currents.

Um doesn't the article say that this is what the ocean would look like with no winds and currents and only the effect of gravity. See the main diagram in the middle of the article for more...

 

[billiards]

Would you expect a higher or lower density below India? If I understand it correctly, the graph depicts the theoretical shape of the earth, when there was no vertical structure, say when the Earth was fluid.

Now. if there is a stronger gravity anomaly somewhere, you'd expect that fluid to move towards that anomaly causing a higher surface locally, an idea of Nils Axel Morner. So, as an analogy, I'd expect the fluid to flow away from a weaker local gravity, causing a lower lever locally.

Agreed. The implication of that is that g is betrayed (at least in its relative "anomaly" sense) by the shape of the equipotential surface, which is a consequence I haven't quite got my head around yet. Back to the books for me! I guess if mass goes from high gravitational potential to low gravitational potential and at some level beneath India the equipotential surface smooths out then g (being the gradient of potential) must be weaker where the surface was at a mimimum higher up??

 

[Andre]

Maybe an important factor is that the (local) surface of a fluid (assuming equilibrium) is perpendicular to the (local) gravity vector, regardless of the value of that vector.