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Wednesday, February 3, 2016

Scientific fact: Less Gravity than the Rest of the Earth


There is an area in Canada with less gravity than the rest of the Earth!

In the 1960s, scientists began to chart Earth’s gravitational fields. You would expect that gravity be the same across the world. It’s not like you feel incredibly heavy in the United States while you practically float around in South America. You always feel the same gravitational pull.

However, they found that gravity actually did vary. In Canada, there are certain areas with less gravity than normal. The Hudson Bay, for example, is “missing” gravity.

There are two theories as to why this may be true. One theory cites convection as the cause. Convection pulls the Earth’s plates downward, which decreases the mass in that area and decreases the gravity. The other theory cites the Laurentide Ice Sheet as the cause. The Ice Sheet melted 10,000 years ago and left a huge indent in the Earth, which could mess with the gravity.

Magnetic Hill (India)

 Magnet Hill is a so-called "gravity hill" located near Leh in Ladakh, India.

None of the other responses seem to address the question, although they are very interesting explanations of how the data are obtained. 

I presume, by the way, that the vague term "gravity" actually means "the acceleration of gravity at a constant radial distance from the center of the Earth" (roughly the altitude of the satellite[s] that took the data). 

So... why near India

I don't know, and I wonder if anyone else does.  This is not my field, so I am free to make ignorant guesses:

First, I understand that India used to be a separate continental plate; it drifted North and rammed into Asia, thrusting up the Himalayas.  Perhaps this collision lowered the effective density in the neighborhood, although I can't see how. 

Second, maybe that's where the Moon was gouged out of the Earth long ago and it still hasn't recovered completely.  Somehow that seems unlikely too -- it's been quite a while!  Before Pangea, even, I think. 

The gravitational potential of Earth varies at different points on the planet. Gravitation is dependent on mass of the planet, mass of the object, and distance of the centers of gravity of each of these objects to each other. These things are obviously not constant. You change your distance to the center of earth whenever you change your elevation. Your gravitational potential in Death Valley, CA (elev. 86 meters below sea level) will be different than your weight on K2(elev. 8,611 meters above sea level). The mass of the Earth is also not evenly distributed. If the density of the amount of earth between its center and your center is very high (like if there was only a thin layer of continental crust, but a thick layer of liquid outer core) there will be more gravitation than if the density is very low. So a seemingly contradictory effect of this, is that on a really tall mountain, you are farther away from the Earth’s center, resulting in a lower gravity. You are also standing on a relatively thicker layer of crust, meaning that, all else being equal, there is more mass of dirt between you and the center, resulting in a higher gravity. The potato-earth attempts to represent these gravitational fluctuations in a visual way. The red areas are spots where gravity is relatively high, and the blue areas where it is relatively low, in effect showing what your relative weight might be at a particular location.

Most people think of the Earth as being a sphere. For most purposes that’s close enough, but its actually a spheroid, something close to but not precisely a perfect sphere. It bulges in the middle due to its spin, the Moon’s gravity warps it, the continents and oceans distort the shape. And the surface gravity changes with all this too; it’s different on top of the highest mountain, for example, compared to its strength in Death Valley.
This is why the gravity at some places is stronger than other places.