Imagine stepping on the scale and seeing a slightly lower reading than usual. It might be tempting to attribute it to a pre-breakfast miracle, but in certain parts of Canada, it could be due to something else: an interesting gravity anomaly.
We have recently written about the world’s largest gravity hole, but did you know that such gravity anomalies don’t only occur in the ocean, but also on land? Like in some parts of Canada, such as the Hudson Bay area and certain parts of Quebec, for example?
Gravity is proportional to mass, which means that the more mass an object has, the stronger its gravitational pull is. The Earth is not a perfect sphere, but rather a slightly flattened ball that bulges at the equator and gets flatter at the poles due to its rotation. The Earth’s mass is also not spread out proportionally, and it can shift position over time due to various factors. These factors can affect the local gravity of an area, making it higher or lower than the average. The resulting gravity map of Earth is called the Potsdam gravity potato.
One of the factors that can change the local gravity is convection in the Earth’s mantle. The mantle is a layer of molten rock called magma that exists between 60 and 124 miles (100 to 200 km) below the surface of the Earth. Magma, intensely hot and in constant motion, generates convection currents, which in turn pull down the Earth’s continental plates, reducing mass and gravity in the affected area.
Changes in local gravity can also be influenced by the presence or absence of ice sheets, thick layers of ice covering vast land areas like Antarctica or Greenland. These ice sheets exert significant weight on the Earth’s surface, causing a depression. When they melt, they release this weight, leading to a slow rebound effect similar to pressing on a springy bread. This rebound reduces mass in the area and subsequently decreases gravity.
Both convection and ice sheet rebound are responsible for the lower gravity in the Hudson Bay area and surrounding regions, including Quebec. This phenomenon was first identified in the 1960s when scientists were mapping the global gravity fields of the Earth. They found that gravity in this region was lower than expected, by about 0.3 milligals (a unit of gravity measurement). This difference may seem small, but it is significant enough to be detected by sensitive instruments.
For over four decades, however, researchers were unable to tell exactly what’s behind this gravitational anomaly in significant areas of Canada, particularly the Hudson Bay area. Only recently did they identify the Laurentide Ice Sheet, which covered much of present-day Canada and the northern United States during the last ice age, as the main cause of the anomaly.
The Laurentide Ice Sheet, reaching nearly 2 miles (3.2 km) thick in most parts and 2.3 miles (3.7 km) in specific areas of Hudson Bay, weighed approximately 10 trillion tons (9 trillion metric tons). Over 10,000 years ago, the ice sheet melted away, leaving a profound impression on the Earth’s surface. Presently, the Earth is still slowly rebounding from this event, at a rate of less than half an inch per year.
Using data collected by the Gravity Recovery and Climate Experiment (GRACE) satellites from April 2002 to April 2006, scientists at the Harvard-Smithsonian Center for Astrophysics assessed the impact of the Laurentide Ice Sheet. Positioned approximately 310 miles (500 km) above the Earth and spaced about 137 miles (220 km) apart, these advanced satellites can measure distances with remarkable precision, enabling the detection of minor gravitational fluctuations.
When the leading satellite passes over the Hudson Bay region, the reduction in gravity causes it to deviate slightly from its orbit, which is observed and utilized to calculate gravitational changes. This analysis, facilitated by GRACE data, produced topographical maps that approximate Hudson Bay’s appearance during the last ice age, revealing significant findings such as two prominent bulges on the bay’s western and eastern edges, indicating thicker ice. Consequently, gravity in these areas is lower compared to other regions within the bay affected by gravitational depletion.
A significant revelation from the GRACE data was that the ice sheet theory explains only a fraction, between 25 percent to 45 percent, of the gravitational fluctuations in Hudson Bay and its vicinity. After factoring out the “rebound effect,” scientists concluded that approximately 55 percent to 75 percent of the remaining gravitational variation is probably attributable to convection. Convection currents drag down the continental plates under Hudson Bay and Quebec, reducing their mass and gravity. The high density of molten rock under this area also plays a role, as it creates a downward pull on the surface.
The Hudson Bay area and some of the surrounding regions will continue to experience reduced gravity for an extended period to come. It’s projected that the Earth needs to rebound over 650 feet in this region to return to its initial state, a process expected to take another 5,000 years approximately. While global sea levels are on the rise, those along the Hudson Bay coast are receding due to the land’s ongoing recovery from the Laurentide Ice Sheet’s pressure.
The Hudson Bay area is not the only place where you can find gravity anomalies on Earth. There are many other regions where gravity is higher or lower than average due to various reasons, such as topography, geology, or tectonics.
The world’s largest gravity hole is in the Indian Ocean, at a spot called the Indian Ocean Geoid Low (IOGL). Due to the low gravity in this area, the sea level is up to 106 meters (348 ft) lower than the global average. Yes, lower, because higher gravity elsewhere basically pulls the water away from the IOGL, as we pointed out recently.
On land, Mount Chimborazo in Ecuador is among the places with the lowest gravity due to Earth’s equatorial bulge (because the Earth isn’t a perfect sphere, it bulges slightly at the equator). Since Mount Chimborazo is the farthest point on Earth from the Earth’s center of mass (due to the bulge), it experiences some of the weakest gravitational pull among natural locations (its summit experiences approximately one percent less gravity compared to the location with the strongest gravitational pull). However, owing to its elevation above the nearby landscape and localized gravitational irregularities, another Andes peak, the summit of Huascarán stands as the location on Earth with the least gravitational force.
Okay, so where is gravity the strongest on Earth’s surface, then? Well, the answer lies near the poles. The Earth’s oblate spheroid shape, with bulging at the equator, results in the poles being slightly closer to the center of gravity. Consequently, gravity’s intensity reaches its peak at the Earth’s surface near its two poles.
In conclusion, if you want to lose some weight without dieting or exercising, you might want to visit the Hudson Bay area or some other places where gravity is lower than average. But don’t expect too much: the difference will be very small – only about a tenth of an ounce (or 0.005%) less than you would weigh elsewhere – and you will gain it back as soon as you leave. So, the best way to maintain a healthy weight would still be to eat well and stay active.
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Not just Earth but also the moon has gravitation anomalies. The lunar gravity anomalies were not known during the 1960s Apollo moon landings. When the astronauts flew over a higher gravity area they needed to burn more fuel than expected. Apollo 11 was within 20 seconds of aborting the landing in 1969 to avoid crash landing because it was in a previously unknown high gravity zone!