Puerto Rico Gravity Anomaly: Unveiling the Mystery Behind the Deepest Trench in the Atlantic

Beneath the Atlantic Ocean and the Caribbean Sea lies the impressive geological feature of the Puerto Rico Trench. This underwater depression sits on the boundary of the two large bodies of water and is famous for its depth and complexity. Several kilometers above it, the surface of the ocean behaves strangely, believed to be due to an anomaly of Earth's gravity.

(Photo: Wikimedia Commons/ USGS)

Mystery of the Puerto Rico Trench

The anomaly was first discovered in 1671 when astronomer Jean Richter traveled from France to South America, taking with him a pendulum clock. The clock had been accurate in Paris, but it seemed to run slowly in Cayenne, French Guiana. Richter noticed that he and his crew lost a full two and a half minutes every day.

This was no big deal for them, as the astronomer decided to shorten the pendulum to make the clock accurate. Upon their return to Paris, however, Richter discovered that his clock was running too quickly by two and a half minutes each day.

After hearing the strange behavior of Richter's clock, mathematician Christiaan Huygens realized that it was experimental evidence that the Earth was rotating. Later, Isaac Newton used a similar pendulum clock and data on the equatorial bulge in Jupiter, which showed that our planet also bulged at the equator because of the centrifugal force of its rotation.

When a person jumps up and down in Brazil or Canada, it may feel the same for both locations, but the rate at which they fall is not uniform. This is because gravity acts less upon a person near the equator than it does near the north or south pole. In short, the effect of gravity differs as a person gets further away from the bulk of the Earth's mass.

However, the influence of gravity at the Puerto Rico Trench differs from the area surrounding it. If an object is dropped there, it will fall slightly faster than elsewhere on Earth or in the surrounding area. It also affects navigation equipment as it can cause false readings for sailors in the area.

What Causes the Puerto Rico Trench Gravity Anomaly?

Gravity anomaly refers to a phenomenon where an object in free fall accelerates at a rate different from the model of gravity predicted for a particular location. At the Puerto Rico Trench, gravity is only -380 milliGal, our planet's biggest negative gravity anomaly.

Here on Earth, the gravity anomaly is not exclusive to the Puerto Rico Trench alone since it can also be observed in other parts of the planet. For instance, the Indian Ocean is home to a spot where gravity is the lowest compared to what was expected.

To solve this mystery, geophysicist Peter Molnar tried to find the cause of this unexpected downward force. Just as there is slightly higher gravity at the poles from being closer to the bulk of Earth's mass, Molnar assumed that the likely source of an anomaly in Puerto Rico Trench is something huge and dense hiding under the surface.

In the 1977 paper "Gravity anomalies and the origin of the Puerto Rico Trench," Molnar discussed previous models of gravity that had shown that the thickness of the lithosphere was uniform. By inspecting the area, he realized that this was not the case and that the gravity anomaly was probably due to a huge "hanging flap" of the Earth's crust in the Atlantic. As Molar noted, the residual gravity anomalies are consistent with the presence of a subcrustal dense mass in the area.