Puerto Rico is a large island situated in the northeastern Caribbean, a region known for intense geological activity. Its geography places it on the border between the Greater and Lesser Antilles island chains. Many people assume its location means it sits directly atop a currently active volcano, a common misconception given the region’s reputation. While ancient magmatic forces shaped Puerto Rico’s landscape, the island itself is not a site of modern volcanic activity. Its present-day geological story is defined by the stresses of two massive tectonic plates grinding past each other.
Is Puerto Rico an Active Volcano?
Puerto Rico is not on top of an active volcano. The island and its surrounding territories, including the U.S. and British Virgin Islands, have no active volcanoes and have not experienced an eruption in millions of years. This contrasts sharply with the nearby Lesser Antilles islands, such as Montserrat and Martinique, which host numerous active volcanic systems. The last major volcanic activity that built Puerto Rico ceased around the Eocene epoch, and the island is now considered geologically stable.
The closest active volcanoes are located hundreds of miles to the east and south, along the Lesser Antilles Volcanic Arc. These islands represent the modern expression of subduction-driven volcanism in the Caribbean. Puerto Rico’s geological position places it outside this active magmatic arc. The island’s mountainous terrain is instead a deeply eroded remnant of a much older period of intense geological activity.
The Ancient Volcanic Arc
The question of volcanism arises because Puerto Rico is the exposed part of an older, extinct volcanic island arc. The island’s formation began in the late Jurassic period, around 190 million years ago. The bulk of the volcanic activity occurred during the Cretaceous and Paleogene periods when the island was part of an active arc where subducting oceanic crust fueled magma generation. This process built up a massive accumulation of volcanic materials that now form the island’s core.
The geological makeup of the island’s interior, known as the Cordillera Central, consists largely of Cretaceous and Eocene volcanic and plutonic rocks. Volcanic rocks like andesite and volcaniclastic sediments, derived from explosive eruptions, make up the majority of the mountainous terrain. These materials were intruded by felsic plutonic rocks, which are cooled bodies of magma that never reached the surface. Today, these ancient structures are heavily eroded, and the volcanic processes that created them have been shut down for approximately 30 million years.
Subduction Zones and Seismic Risk
Although the threat of eruption is absent, Puerto Rico’s location along a major tectonic boundary means it faces significant seismic hazards. The island sits directly on the boundary between the eastward-moving Caribbean Plate and the North American Plate. To the north lies the Puerto Rico Trench, the deepest point in the Atlantic Ocean, which marks a complex and active subduction zone. Here, the North American Plate is being forced underneath the Caribbean Plate with a large component of sideways, strike-slip motion, a process known as oblique subduction. This constant movement and friction between the plates builds immense stress in the Earth’s crust.
This stress is released in the form of earthquakes, which pose the primary geological risk to the island. The subduction zone is considered capable of generating megathrust earthquakes of magnitude 8.0 or greater. Historical records confirm this danger, including a major earthquake in 1787 that caused widespread destruction across the northern coast.
The continuous tectonic activity also creates the risk of tsunamis, generated when a large section of the seafloor shifts suddenly during an earthquake. For example, the 1918 San Fermín earthquake near the Mona Passage triggered a destructive tsunami that affected the northwestern coast. While the island is not defined by volcanism, its proximity to the Puerto Rico Trench and ongoing plate convergence means that earthquakes and tsunamis are the most significant geological concerns.