Places with more mass, such as mountains, have stronger gravitational forces. Places with less mass underground, such as valleys and deep ocean trenches, have weaker gravitational forces. Image Credit: Washington Post/Instagram

As the story goes, mathematician Sir Isaac Newton was sitting in an orchard after supper when he saw an apple fall from a tree. He wondered why apples always fall straight to the ground, instead of sideways or even up. He would later develop the law of universal gravitation.

But gravity - that invisible force that draws objects to the center of the Earth - is not uniform across the planet, data shows.

Newton found that gravity is partly dependent on mass; objects with more mass experience a stronger gravitational pull. On Earth, that generally means the strength of gravity's pull on an object may be stronger or weaker at different locations, depending on Earth's inner structure and topography. Places with more mass, such as mountains, have stronger gravitational forces. Places with less mass underground, such as valleys and deep ocean trenches, have weaker gravitational forces.

"Mass creates gravity," said John Ries, a senior research scientist at the University of Texas at Austin. "If you see a change in gravity, you see a change in mass."

You can also think of the gravity changes in terms of acceleration. On average, the acceleration of an object falling to Earth due to gravity is around 9.8 meters per second squared. But in places with more or less gravity, that acceleration may be slightly different.

Ries said people aren't able to notice these very minor variations, but advanced scientific instruments can measure the small abnormalities. He and his colleagues work with a NASA satellite mission known as Gravity Recovery and Climate Experiment (GRACE), which provides global snapshots of Earth's gravity field. Scientists can use this information to track mass changes in polar ice and water reservoirs and help make sense of how the processes below Earth's surface affect those above ground.


The largest gravity anomalies stem from plate tectonic movements, as large slabs crash or pull away from each other. Changes in water content on Earth, such as a drought or persistent rains, can also drive changes in gravitational pull, though to a smaller degree.

"The big issue is to try to understand how the oceans, the atmosphere and the land areas are interacting," said Byron Tapley, a geophysicist at the University of Texas at Austin. "They're all coupled together essentially in the Earth's system and trying to understand those interactions, how what happens to one influences another."

Here's where gravity varies the most across the Earth.

Stronger gravity in the Pacific Ocean

We often draw Earth as a smooth sphere, but our planet is lumpy and bumpy, an irregular shape called the geoid. Using satellite-based data, scientists can study anomalies in gravity across Earth by comparing the difference between the actual gravitational pull on this planet to a hypothetical uniformly smooth Earth.

Some of the strongest gravitational forces on Earth are located in the Pacific near Australia and Indonesia due to plate tectonic movements.

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. The growth in mass increased the gravitational force along the volcanic chain. Other volcanic chains, such as around Hawaii, also show stronger gravity.

In fact, plate tectonic movements are the driving force for nearly all the features we see on Earth's surface from mountains to trenches. These plate movements are driven by convection in our mantle, which transports heat from deep within Earth to the surface.

"The friction of the crust, as the mantle goes through it, either drives [plates] together or drives them apart," said Ries, who studies Earth's shape, rotation and gravity using GRACE satellite data. Gravity maps allow scientists to decipher these movements below our crust.

In this region, Ries explained that the anomaly occurred due to the collision of two plates, where ocean crust was thrust underneath the continental plate. The ocean crust, he explained, is older and denser and sinks below the lighter continental plate to form a trench. Trenches along the Pacific plate appear along the Aleutian Islands, Japan and Tonga, where data show weaker gravitational forces.

As the ocean crust is submerged, he said it "sweats" water and pressure increases, which forces magma up and causes the crust to lift and form volcanoes. The growth in mass increased the gravitational force along the volcanic chain. Other volcanic chains, such as around Hawaii, also show stronger gravity.

This plate tectonic behavior is similar to what we see in mountains as well. For mountains, two continental plates can crash into one another and cause uplift. For instance, the Himalayas were created when two equally dense continental plates converged and pushed up. Data show stronger gravitational forces across the Himalaya Tibetan plateau. Ries said the plates are still active, and Mount Everest is still slowly growing in height.

Gravity depression from the North American ice sheet

Not all gravitational anomalies are linked to continental plates crashing into one another. Changes on the surface can also affect gravity.

About 2.6 million years ago, the Laurentide Ice Sheet blanketed much of Canada and the northern United States. The weight of the massive ice sheet pushed the land down, creating a depression. Data shows that gravity is slightly weaker in the region.

Since then, the ice sheet has completely melted and the land is rebounding - but very slowly. (Think of putting your finger in dough and then lifting your finger; the dough eventually returns to its original shape.) Ries said the land may not level out for another 1,000 years or more.

Unusual gravity hole in the Indian Ocean

One of the weakest gravity signals on Earth appears on a patch of ocean south of the Indian peninsula, covering 1.2 square million miles. But the phenomenon has long puzzled scientists.

The gravity hole's "existence is one of the outstanding problems in the field of geodynamics," said Debanjan Pal, a geologist at the Indian Institute of Science. Because the weak spot was not associated with a trench or ice sheet depression, the cause of the weak gravitational force remained a mystery - until now.

In a recent study, Pal and his colleague Attreyee Ghosh showed that the anomaly is linked to plumes of hot, low-density rocks from the mantle coming to the surface around 20 million years ago.

Using computer models, they simulated Earth's plate tectonic movements from 140 million years ago to today. They found that as the ancient supercontinent Gondwana started to break apart, the Indian plate started to move north. When the Indian plate moved, the adjacent ancient Tethys Ocean started to close and sank into the mantle, perturbing denser material sitting above the boundary of our core and mantle. Mantle plumes formed and brought lower density material to the surface, which helped form the gravity low.

The researchers say it's hard to know if the gravity hole will eventually disappear or move.

Melting mass from the polar ice caps

While many of the largest gravitational anomalies are traced to ancient and natural processes, humans are also affecting gravity through the movement of water.

Across seasons, researchers observe changes in rivers, lakes and other bodies of water that occur naturally through the water cycle. But as our atmosphere warms due to increases in greenhouse gases, that has impacted water reservoirs and polar ice caps.

"The effect on Greenland is a very large effect," Tapley said. "There's a large amount of mass that's been dropped there, and there's a noticeable gravitational signal associated with that change."

Tapley said the ice melt over the surface lowers the water-induced gravity signal over the land and the subsequent runoff adds to the gravity signal in the ocean.

But the melt doesn't mean the signal over Greenland is overall lower. As the weight of the water is reduced, tectonic plates adjust and allow material from the mantle to move in, adding mass, Tapley said. Additionally, Greenland is still rebounding and lifting from the last Ice Age (much like the process with the Laurentide Ice Sheet but on a faster time scale).

Outside of the polar ice caps, groundwater pumping and droughts can also remove water from the surface and cause small changes in gravity. These deviations are much smaller than those induced by underground magma plumes or plate tectonic movements, but they change much more quickly on smaller time scales - and will have major implications for people as global temperatures continue to increase. These measurements are critical for scientists to track water supplies for general consumption, agriculture, underground aquifers and more.

"This whole thing we're looking at is the water cycling and changes in the water cycle," Tapley said.