The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because the oceanic lithosphere of the South American Plate is cooler and denser than that of the Caribbean Plate. The subduction causes low density ocean floor sediment to be scraped off the surface of the South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a process called obduction. The line of subduction is marked by the deep sea Puerto Rico Trench.

As the South American Plate descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone.

Continued subduction of the South American Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting. This produces magma, which rises and may be erupted explosively as andesite at the surface.

Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000^oC. It is viscous, trapping gases as it rises. The water and gases in andesitic magma account for the explosive activity of andesitic volcanoes, which typically lie dormant for many hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows, as well as small amounts of andesitic lava.

The eruptions on Montserrat during the 1990s are a good example of this type of activity.

The Caribbean volcanic islands form a curved linear chain or ‘volcanic island arc’ parallel and to the west of the Puerto Rico Trench.