Why gravity on earth is very strong on the north atlantic ocean?

[menniandscience]

why gravity on Earth is very strong on the north atlantic ocean? ok, obviously where there is thick crust like himalaya (everest), huge land mass - more gravity, but the ocean?! where the crust is thin?!
source http://en.wikipedia.org/wiki/File:GRACE_globe_animation.gif

 

[mfb]

In general, the crust thickness and density is different for different places - oceanic crusts tend to have a higher density (that's why they do not float up!).
You do see mountain ranges (as they just have mass where other parts of the surface do not), but you also see density differences below the sea level.

 

[Chronos]

Density gradients are responsible for gravitational anomalies.

 

[D H]

why gravity on Earth is very strong on the north atlantic ocean? ok, obviously where there is thick crust like himalaya (everest), huge land mass - more gravity, but the ocean?! where the crust is thin?!
source http://en.wikipedia.org/wiki/File:GRACE_globe_animation.gif

Not my favorite picture. A picture nominally is worth 1000 words, but not if it needs thousands of words to explain what is being depicted.

I'll start with a simple picture. Imagine a picture of the globe colored to indicate the gravitational acceleration, with 0 m/s² being blue and 10 m/s² being red. The whole globe will be almost the exact same color, red. If you look closely you'll see that the poles are a tiny bit redder than is the equator. Gravitational acceleration varies by latitude and altitude, from a low value of 9.764 m/s² atop mountains near the equator to a high value of 9.8337 m/s² at sea level at the North Pole.

Let's magnify this tiny difference (it's less than 1%) by subtracting 9.80665 m/s² from the local gravitational acceleration. Now we're looking at a range of -0.04275 m/s² to +0.02705 m/s². It might help to multiply those small numbers by 100, yielding a range of -4.275 cm/s² to 2.705 cm/s².

Another name (deprecated) for cm/s² is the galileo. We'll see that latitudinal variation in gravitation pop right out if we change our color scheme so that -4.3 gal is blue, +2.8 gal is red. The equator will be bluish, the poles reddish (and the North Pole will be considerably redder than the South Pole). You might see some slight variations in color locally, but it's not going to be strong.

The reason gravitation is strong at the poles than the equator is because the Earth is rotating. The rotation has a direct and indirect effect at the equator. The direct effect is that the rotation decreases gravitation. The indirect effect is the equatorial bulge that is a consequence of the rotation.

The next step is to remove these rotational effects as well. That's what this image is showing. The deep blue represents a deviation of -50 mgal (-50/1000 of a galileo, or -5×10^-4 m/s²) from the local gravitational acceleration on featureless but spinning Earth while the red represents a deviation of +50 mgal. It's tiny, tiny, tiny.

That image does not show what you think it shows. That's why it's not my favorite image. It's neat if you know what it's showing, but it's not so neat when you have to explain the misconceptions that that image generates.

 

[menniandscience]

ok thanks everyone :)

 

[litup]

Not my favorite picture. A picture nominally is worth 1000 words, but not if it needs thousands of words to explain what is being depicted.

I'll start with a simple picture. Imagine a picture of the globe colored to indicate the gravitational acceleration, with 0 m/s² being blue and 10 m/s² being red. The whole globe will be almost the exact same color, red. If you look closely you'll see that the poles are a tiny bit redder than is the equator. Gravitational acceleration varies by latitude and altitude, from a low value of 9.764 m/s² atop mountains near the equator to a high value of 9.8337 m/s² at sea level at the North Pole.

Let's magnify this tiny difference (it's less than 1%) by subtracting 9.80665 m/s² from the local gravitational acceleration. Now we're looking at a range of -0.04275 m/s² to +0.02705 m/s². It might help to multiply those small numbers by 100, yielding a range of -4.275 cm/s² to 2.705 cm/s².

Another name (deprecated) for cm/s² is the galileo. We'll see that latitudinal variation in gravitation pop right out if we change our color scheme so that -4.3 gal is blue, +2.8 gal is red. The equator will be bluish, the poles reddish (and the North Pole will be considerably redder than the South Pole). You might see some slight variations in color locally, but it's not going to be strong.

The reason gravitation is strong at the poles than the equator is because the Earth is rotating. The rotation has a direct and indirect effect at the equator. The direct effect is that the rotation decreases gravitation. The indirect effect is the equatorial bulge that is a consequence of the rotation.

The next step is to remove these rotational effects as well. That's what this image is showing. The deep blue represents a deviation of -50 mgal (-50/1000 of a galileo, or -5×10^-4 m/s²) from the local gravitational acceleration on featureless but spinning Earth while the red represents a deviation of +50 mgal. It's tiny, tiny, tiny.

That image does not show what you think it shows. That's why it's not my favorite image. It's neat if you know what it's showing, but it's not so neat when you have to explain the misconceptions that that image generates.

But still, with 1% difference, I weigh 220 pounds but 1% less and I have lost 2.2 pounds. At least a scale could measure it. My personal gravity field would not have changed, however. Light would bend around me just the same at the equator or the poles:)

 

[olivermsun]

In general, the crust thickness and density is different for different places - oceanic crusts tend to have a higher density (that's why they do not float up!).
You do see mountain ranges (as they just have mass where other parts of the surface do not), but you also see density differences below the sea level.

Just to add to this, NASA's publication Studying Earth's Gravity Field from Space says:

"The North Atlantic Mid-Atlantic ridge is an active spreading center where new crust is being created, and shows up as a strong positive anomaly in the middle of the Atlantic."

 

[jim hardy]

Just to add to this, NASA's publication Studying Earth's Gravity Field from Space says:

"The North Atlantic Mid-Atlantic ridge is an active spreading center where new crust is being created, and shows up as a strong positive anomaly in the middle of the Atlantic."

Interesting how much of the red is near earthquake regions.