Puerto Rico is not currently home to any active volcanoes, distinguishing it from many of its Caribbean neighbors. The island’s geological story is instead defined by intense tectonic activity that manifests as frequent and sometimes powerful earthquakes. While the region is part of a dynamic plate boundary, the specific nature of this interaction means the island faces a hazard far different from volcanic eruptions.
The Geological Status of Puerto Rico
Puerto Rico’s foundation is built upon the remnants of an ancient volcanic arc that was active tens of millions of years ago. The island’s central mountain range, the Cordillera Central, contains a core of volcanic and plutonic rocks dating back to the Cretaceous and Eocene periods (roughly 120 to 40 million years ago). These rocks are evidence of a time when the region was a chain of active volcanoes.
Any volcanic material found today, such as the hard rock forming El Yunque peak, is the cooled, solidified product of this long-extinct activity. The island’s development involved the piling up of this material, which was later overlain by younger sedimentary layers. The presence of these ancient features does not indicate any potential for future eruptions, as the geological process that fueled them has long ceased beneath the island.
Tectonic Setting: The Puerto Rico Trench
Puerto Rico sits within a broad deformation zone where the massive North American Plate meets the Caribbean Plate. This boundary is marked by the Puerto Rico Trench, the deepest point in the Atlantic Ocean, reaching depths over 27,000 feet. The trench is not a simple subduction zone where one plate plunges beneath the other, which is the mechanism that generates volcanic arcs.
Instead, the North American Plate primarily slides past the Caribbean Plate in a left-lateral strike-slip motion, with only a small component of oblique subduction. This sliding motion, where the plates scrape against each other, is the primary reason magma is not generated to feed active volcanoes beneath Puerto Rico. The island itself rests on the Puerto Rico–Virgin Islands microplate, which is continuously deformed by the stresses of the surrounding major plates.
The Volcanic Neighbors of the Caribbean
The active volcanic islands of the Caribbean, such as Montserrat and St. Vincent, belong to the Lesser Antilles Volcanic Arc, which lies further to the southeast. These islands experience regular eruptions because they are located along a different section of the plate boundary. In the Lesser Antilles, the Atlantic plate is undergoing a direct and steep subduction beneath the Caribbean Plate.
This classic subduction process drives the mantle material below the Caribbean Plate to melt, producing magma that rises to the surface and creates active volcanoes. This mechanism is distinct from the oblique, strike-slip-dominated boundary defining the Puerto Rico Trench. The difference in plate interaction explains why the Lesser Antilles have a high volcanic risk while Puerto Rico is non-volcanic.
Earthquakes: Puerto Rico’s Primary Seismic Risk
The complex, grinding motion of the tectonic plates near the Puerto Rico Trench does not produce volcanoes, but it creates a significant seismic hazard. The island is traversed and surrounded by numerous active faults, including the Muertos Trough to the south and the Mona Rift to the west. Earthquakes are concentrated in these offshore zones, where the North American and Caribbean plates are constantly building and releasing stress.
The region has a long history of powerful seismic events, including a magnitude 7.2 earthquake in the Mona Passage in 1918 and a magnitude 6.4 event in 2020 off the southwest coast. These movements along the plate boundary and fault systems generate frequent tremors and pose a threat of large-magnitude earthquakes, potentially greater than 7.0. Furthermore, large-scale offshore earthquakes have the potential to trigger destructive tsunamis, as occurred following the 1918 event.