The strange phenomenon of Earth's gravity anomalies

We don’t often think about gravity, the invisible force that anchors us to Earth, as it always seems constant and dependable. However, it will slightly vary, sometimes by enough to make you weigh slightly more or less, depending on where you stand. These variations are called gravity anomalies and come from differences in the density and structure of the materials that are beneath our feet.

Between 2006 and 2008, researchers discovered an anomaly in Earth’s gravitational field that was detected by satellites. According to an article in Live Science, this phenomenon occurred off the coast of Africa approximately 20 years ago. The unusual gravity signal lasted for about two years over the eastern Atlantic Ocean. Scientists say this gravitational anomaly happened during an abrupt change in the variation of Earth’s magnetic field, likely caused by a sudden redistribution of mass deep within the planet.

One of the most interesting examples of the gravity anomaly is in Hudson Bay, Canada. Here, gravity is measurably weaker than in most other parts of the world. The explanation may be due to the effects of the last Ice Age, when the massive Laurentide Ice Sheet covered much of North America. During that time, the enormous weight of the ice pressed the crust downward, displacing part of the thick layer of hot, solid rock below the crust that is called the mantle. Even though the ice melted thousands of years ago, scientists believe that the mantle is still slowly rebounding. There is still a lingering gravity deficit as part of the Earth’s mass is missing from deep within the crust. If you stepped on a scale in Hudson Bay, you would weigh a small fraction less than elsewhere.

By contrast, some locations will exhibit stronger gravity than normal. The Bangui anomaly in the Central African Republic is a region with a higher-than-average gravitational pull and magnetic signature. It’s still uncertain why this occurs, but some researchers believe it marks the scar of a massive, ancient meteorite impact. Others theorize that it’s a bulge of dense mantle rock pushing upward beneath the crust. Either way, it’s one of Earth’s most intriguing scientific mysteries.

Elevation and latitude can also influence gravitational strength. Because Earth bulges slightly at the equator and flattens at the poles due to the effect of its spin, gravity is slightly weaker near the equator. Combined with altitude, this creates extreme cases of gravitational anomalies. For example, the combination of high elevation and equatorial position produces the weakest gravitational acceleration at Mount Nevado Huascarán in Peru. At this spot, a person weighing 150 pounds at the North Pole would likely weigh about half a pound less at the summit of Huascarán.

At the other elevation extreme lies the Dead Sea, the lowest exposed land on Earth. This area is more than 1,300 feet below sea level. This low level is slightly closer to Earth’s core, which increases gravity’s pull just enough to register with sensitive instruments.

Gravity will also affect the passage of time through a phenomenon called gravitational time dilation. Predicted by Albert Einstein’s theory of general relativity prior to the days of atomic clocks, time passes more slowly in stronger gravitational fields. This means that clocks closer to a large mass, like Earth, will tick more slowly than clocks farther away. This concept has been proven by experiments and used as necessary corrections for technologies like GPS satellites. 

There are places famous for “mystery spots,” such as the one in Santa Cruz, Calif., that focus on illusions of gravity gone wild. This tourist area has tilted cabins, balls appearing to roll uphill and people appear to be leaning at impossible angles. But these effects are likely tricks of perspective, not gravitational anomalies. About five years ago, I visited this area and was quite fascinated by all of the unusual perspectives. I admit that I had a very unusual feeling while I was at the Mystery Spot. In fact, I was getting motion sickness while strolling around the area. It takes a lot for me to get motion sickness, but the experience was very interesting.

In terms of our local weather, more storm systems are expected to bring rain and snow showers to the Inland Northwest through at least the end of the month. Our normal November precipitation in Coeur d’Alene is 3.07 inches with an average snowfall of 8.7 inches. Conditions are looking favorable for a second month in a row with above-average precipitation. The long-range computer models are continuing to indicate a north-northwesterly flow developing over the Pacific Northwest toward the end of next week. This pattern would increase the chance of measurable snowfall in the lower elevations, but as usual, it’s always a temperature thing when it comes to snow in the Coeur d’Alene region. Stay tuned.

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Contact Randy Mann at [email protected].