EXPLORING CONTROLS ON ROCK TYPE-RELATED DIFFERENCES IN MOUNTAIN RELIEF IN THE SUBTROPICS, PUERTO RICO

Topographic relief varies with rock type on the island of Puerto Rico. Intermediate-to-ultramafic volcaniclastic (VC) and serpentinite (SP) units form steep, high-relief mountains, while the more felsic granodiorite (GD) units comprise gentler hills. We explore two hypotheses for these distinct topographic outcomes: (H1) rock strength is higher in the VC and SP rocks, and (H2) VC and SP units produce courser bedload relative to GD rocks, elevating incision thresholds. To test these hypotheses, we collected rock strength proxies (Schmidt hammer and facture density) from unweathered bedrock channels and grain size measurements from 60 drainage basins (1.12-97.45 km²), predominantly draining one lithology (>80% coverage). Rock strength proxies are comparable in VC and GD, but lower in SP, suggesting rock strength cannot explain the observed relief patterns. GD exhibits the largest grain sizes (D50 = 59.38, D84 = 224.78), followed by SP (D50 = 52.43, D84 = 165.71), with VC having the smallest (D50 = 50.88, D84 = 148.07), also suggesting that changes in incision thresholds might not explain the rock-type-related relief structure in Puerto Rico. Interestingly, all rock types show moderate positive relationships between grain size and topographic steepness. The variability observed in the characteristics and potential outliers underscores the complexity of these systems and highlights the need for further data collection to test these competing hypotheses rigorously. Future research aims to understand the role of channel hydraulics and flood distributions, in concert with topographically modulated grain size distributions, in possibly explaining rock-type related relief changes in Puerto Rico. Ultimately, this study will enhance our understanding of the interconnectedness of Earth's surface processes, climate, and tectonics with their influence on the evolution of topographic forms and landscapes, especially in the understudied tropical and sub-tropical regions.