Geologic setting
The island of Puerto Rico is the smallest and easternmost of the Greater
Antilles (Fig. 1) and the current land mass was formed during submarine
and subaerial volcanism that ended approximately 30 million years ago
(MYA). Uplift at approximately 5 MYA resulted in the current
configuration of the island (Brocard et al. 2016). Puerto Rico has never
been connected to a continental land mass and sits on the edge of the
Caribbean Plate adjacent to the Puerto Rico trench, second only to the
Marianas trench in depth. Lithology in the Luquillo Mountains is
primarily volcaniclastic, but a granitoid pluton underlies some of the
Mountain massif (Figure 2), providing a second major lithology in the
Luquillo Mountains as well as contacts where the pluton metamorphosed
volcaniclastic materials into more erosion-resistant hornfels that is
found on many of the mountain peaks and high elevation ridges (Seiders
1971). The two lithologies result in very different weathering regimes
and stark contrasts in stream channel grain size and morphology (Fig.
2).