by Spanning the boundary between the Caribbean Sea and the Atlantic Ocean, the Puerto Rico Trench is the deepest trench in the Atlantic Ocean.
Interestingly, kilometers above this trench, the surface of the ocean dips slightly due to an anomaly in Earth’s gravity. Objects falling in this area fall slightly faster than anywhere else on the planet, and navigational equipment can degrade, producing false indications for mariners navigating the area.
The question then arises: what causes this gravity anomaly and are there similar anomalies elsewhere? To answer this, we must first delve into the history of the discovery of gravity anomalies.
In 1671, astronomer Jean Richter traveled from Paris to Cayenne, French Guiana, bringing with him a grandfather clock. While the clock was correct in Paris, Richter noticed that he lost two and a half minutes a day in Cayenne. After adjusting the pendulum to correct the problem, the clock advanced two and a half minutes after returning to Paris.
This observation led the mathematician Christiaan Huygens to realize that the rotation of the Earth was the underlying factor. Later, using data from grandfather clocks and Jupiter’s equatorial bulge, Newton showed that Earth’s equatorial bulge is due to the centrifugal force caused by its rotation.
Gravity is weaker near the equator than at the poles because the equator is farther from the Earth’s center of mass. However, the gravity in the Puerto Rico Trench is different from its surroundings, making it an anomaly. There are several of these anomalies all over the planet.
Gravity anomalies occur when a free-falling object accelerates at a rate different from the rate predicted based on gravity models for a given location. The Puerto Rico Trench has the largest negative gravity anomaly on Earth, with gravity measured at -380 milligals.
To understand the cause behind the gravity anomaly in the Puerto Rico Trench, geophysicist Peter Molnar conducted research in 1977. He hypothesized that a massive, dense object beneath the surface could be the source of the anomaly, similar to how it results from the higher gravity at the poles being closer to the Earth’s center of mass.
In an article published in the Geophysical Journal International, Molnar explained that previous models of gravity assumed uniform thickness of the Earth’s crust (lithosphere), which was not true in this case. He suggested that the anomaly was probably caused by a large “hanging fin” of Atlantic lithosphere.
Molnar’s study concluded that “residual gravity anomalies are consistent with the existence of a dense subcrustal mass, which could be the floating plate lithosphere.” The exact mass value depended on assumptions about the crustal mass in the Puerto Rico Trench and its landward wall, but if the other assumptions were correct, the dense mass would be enough to bend the surface down at trench.
Although Molnar was not the first to suggest a dense mass beneath the crust as a possible explanation, his research provided estimates of the mass and size of the object responsible for the gravitational anomaly in the Puerto Rico Trench.
