Hudson Bay Has Low Gravity?

Question

Is there lower gravity at the Hudson Bay? How does this happen? And where exactly is this because I might want to go.

Answer 1

Gravitational acceleration does vary. It tends to be strongest at the poles and weakest at the equator, this is due to the oblate shape of the Earth which means that at the Equator you are further from the centre, and gravity decreases the further you are.

Also the spinning of the Earth makes gravity seem less at the Equator. The combination of these effects causes felt gravity to vary from about 9.77 $\frac{N}{kg}$ at the Equator to 9.83 $\frac{N}{kg}$ as you travel from the equator to the pole (a difference of about 0.6% in weight).

On top of this there are local variations in gravity, which are called gravitational anomalies. They are caused by the structure of the rocks in a particular location. There is a large scale anomaly over the Hudson Bay region, and so gravity there is slightly weaker than you would expect at its latitude. Gravitation anomalies are generally due some combination of two mechanisms:

1. The massive ice sheet that covered Canada during the Ice Age pushed mass down and south. Now that the ice has gone, the rocks are rebounding back but the mass has still been pushed aside. Less mass means less gravity.
2. There are mantle plumes - hot spots like Yellowstone and cold spots where the mantle sinks. Hot spots tend to have high gravity and cold ones tend to have lower gravity.

These two effects combine in the Hudson Bay region to give a distinct gravitational anomaly. However the difference is tiny. There is still lower gravity in Florida than in Hudson, simply because Florida is closer to the Equator (and thus further from the Earth's center). And you still wouldn't notice the difference between the strongest gravity and the weakest. It is much too small to feel (though pole vaulters still could have a small gain [1 or 2 cm] if they compete in countries close to the Equator). It is absolutely not the case that there is any part of Caanda where you can float or experience "half the strength of gravity".

Answer 2

The link you give seems largely plausible about the cause of the anomaly in that area:

a 2007 study found that now-melted glaciers were to blame. The ice that once covered the area during the last ice age has long since melted, but the Earth hasn't entirely snapped back from the burden. Since gravity over an area is proportional to the mass on top of that region, and the glacier's imprint pushed aside some of the Earth's mass, gravity is a bit less strong in the ice sheet's imprint. The slight deformation of the crust explains the unusually low gravity in the area.

However it is massively wrong about the scale of it. This site does indeed show a negative anomaly in that area, about 50 milliGals, or 0.05 $cm/s^2$ or about 50 parts per million of the Earths gravitational field.I wonder if someone confused 50 ppm with 50%?

Answer 3

The anomaly at Hudson's Bay is actually about 0.005% - or 1/200th of a percent. That tiny difference is likely caused by both the additional convection of molten rock underneath the area and the "crushing" of land due to the higher weight of a glacier presence in the depression back in the last ice age - the latter a temporary issue from which it has yet to rebound, but will in ~5000 years.

Credit: Mark Simons / California Institute of Technology, 1997 / Science Photo Library

Answer 4

In the Nature Letter Localization of the gravity field and the signature of glacial rebound by Mark Simons & Bradford H. Hager, Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 4 December, 1997:

The negative free-air gravity anomaly centred on Hudson Bay, Canada, shows a remarkable correlation with the location of the Laurentide ice sheet, suggesting that this gravity anomaly is the result of incomplete post-glacial rebound1–3. This region, however, is also underlain by higher-than-average mantle seismic velocities, suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings4–7. Here we analyse the global gravity field as a simultaneous function of geographic location and spectral content. We find that the Hudson Bay gravity low is unique, with anomalously high amplitude in the spectral band where the power from the Laurentide ice load is greatest2 and the relaxation times predicted for viable models of viscous relaxation are longest8 . We estimate that about half of the Hudson Bay gravity anomaly is the result of incomplete post-glacial rebound, and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central Laurentide and Fennoscandian regions6 . This model has a jump in viscosity at 670 km depth, comparable to that in dynamic models of the geoid highs over subducted slabs4,9, but lacks a low-viscosity asthenosphere, consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions.