Puerto Rico gravity anomaly: What’s happening here?

(ORDO NEWS) — On the border of the Caribbean Sea and the Atlantic Ocean is a deep depression of Puerto Rico. A few kilometers above it, the surface of the ocean is slightly lowered due to a gravitational anomaly.

If you drop an object there, it will fall slightly faster than anywhere else on the planet or in the vicinity. Meanwhile, the anomaly could disable navigational equipment, resulting in false readings for sailors in the area.

When were gravitational anomalies first discovered?

In 1671, the astronomer Jean Richter traveled from Paris to Cayenne (South America). He took a clock with a pendulum with him.

While clocks were accurate in Paris, he noticed that they were slow in Cayenne, wasting as much as two and a half minutes each day.

No big deal, the pendulum has been shortened to make the clock accurate. However, when he returned to Paris, he found that the clock was running too fast, by two and a half minutes every day.

After hearing about the Richter clock, the mathematician Christian Huygens realized that it was experimental proof of the Earth‘s rotation.

Newton later showed, using data from similar pendulum clocks and Jupiter’s equatorial bulge, that the Earth is bulging at the equator due to the centrifugal force of its rotation, and estimated how much.

Gravity acts on you less near the equator than near the poles, because you are farther from the main mass of the Earth.

However, the gravity in the Puerto Rico Trench is different from the area around it. This, along with several other similar gravity anomalies on the planet.

What is a gravitational anomaly?

Gravity anomalies are when an object seen in free fall accelerates at a different speed than the gravity models predict for that location.

The Puerto Rico Trench had a gravity of -380 milliGal, making it the largest negative gravity anomaly on Earth. Elsewhere in the Indian Ocean, gravity was the lowest compared to what was expected.

What causes the gravity anomaly in the Puerto Rico Trench?

Puerto Rico Gravity Anomaly Whats Happening Here 2

In 1977, geophysicist Peter Molnar tried to figure out what was causing this unexpected downward force. He knew the likely source: something huge and dense must be under the surface.

In an article published in the Geophysical Journal International, he explained that previous models of gravity assumed that the thickness of the earth’s crust (lithosphere) was fairly uniform.

After surveying the area, he realized that this was not the case, and that the anomaly was probably caused by a large “hanging flap” of the Atlantic lithosphere.

“Thus, residual gravity anomalies are consistent with the existence of a subcrustal dense mass that could be a hanging plate of the lithosphere,” Molnar wrote in the study.

Although Molnar was not the first to explain the gravitational anomaly in this way, he was able to provide estimates of the mass and size of the object causing the gravitational anomaly.


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