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).