The question of whether volcanoes exist in Puerto Rico is common, often prompted by the island’s location in a seismically active region. There are no active volcanoes on the island of Puerto Rico or its surrounding municipalities. This article explores the island’s ancient past and explains its current geological situation, which causes frequent earthquakes but no volcanic eruptions.
Active Volcanoes Are Not Present
Puerto Rico is currently considered geologically stable regarding active volcanism. An active volcano is defined as one currently erupting, showing signs of unrest, or having erupted within the last 10,000 years. The island’s mountains are not capable of producing the magma that fuels new eruptions. Therefore, residents do not face the hazards of an impending volcanic eruption.
The Island’s Ancient Volcanic Origins
While no volcanoes are currently active, the island itself is the product of an ancient volcanic island arc. The bedrock of Puerto Rico is primarily composed of extinct volcanic and plutonic rocks formed millions of years ago. This activity spanned from the Jurassic period through the Eocene epoch, roughly 190 to 40 million years ago. Volcanic flows and ash deposits accumulated during this time, forming the core of the landmass.
The central mountain chain, the Cordillera Central, consists largely of this ancient volcanic material. Large intrusive igneous bodies, known as plutons, were emplaced deep within the crust during the Late Cretaceous period. These rocks, such as the granodiorite of the San Lorenzo batholith, represent the cooled magma chambers of volcanoes that were active long ago. The volcanic action ceased approximately 40 million years ago.
Puerto Rico’s Current Tectonic Setting
Many people associate Puerto Rico with volcanic activity due to its position within a complex tectonic boundary zone. The island sits at the interface where the North American Plate meets the Caribbean Plate. This interaction is responsible for frequent seismic activity, including significant earthquakes and tsunamis.
The primary plate boundary lies north of the island at the Puerto Rico Trench, the deepest point in the Atlantic Ocean. Here, the North American Plate moves past the Caribbean Plate in a primarily sideways, or strike-slip, motion. This movement generates immense friction and stress, which releases as powerful earthquakes.
The lack of direct subduction beneath Puerto Rico explains why the island does not have active volcanoes; magma-generating material is not driven deep enough into the mantle. A second, less active boundary, the Muertos Trough, lies south of the island, contributing to seismicity but not producing a volcanic arc.
The Active Volcanic Arc Nearby
The subduction process absent beneath Puerto Rico occurs a few hundred miles to the east, creating a separate chain of volcanoes. The North American Plate subducts directly beneath the Caribbean Plate along the Lesser Antilles subduction zone. This is where the region’s volcanic activity is located.
This process created the Lesser Antilles Volcanic Arc, a chain of islands that includes Montserrat, St. Vincent, and Martinique. These islands feature numerous active volcanoes, such as the Soufrière Hills volcano. Puerto Rico is situated on the non-volcanic part of the plate boundary, serving as the western end of the older Greater Antilles arc. The proximity of this active arc often leads to the mistaken belief that Puerto Rico shares the same volcanic hazard.