Caves in volcanic terrains in Costa Rica with the description of new volcano-speleogenetic mechanisms

Abstract

The continental territory in Costa Rica is composed in 50% of Cretaceous to Quaternary volcanic rocks. However, volcanic caves sensu strictu (formed only by volcanic processes) are very rare due to the geotectonic conditions of the Pre-Oligocene submarine volcanism, and rheological conditions of Neogene-Quaternary subalkaline lava flow in a subduction zone. A total of 90 volcanic caves are reported in the country, with a total length of passages of ~ 2.2 km. A few reported lava tubes (pyroducts) are related to Middle-Upper Pleistocene basaltic to basaltic-andesitic aa lava flows. However, there are an enormous number and variability of caves of different origin and hosted in volcanic terrains like mold caves in lahars, piping caves, erosion caves in fluvial settings (lateral erosion of a river or at the base of a waterfall) and littoral systems, including those in which the cave formed along fault zones or a stratigraphic contact in ignimbrites. Less common are dissolution caves in active hydrothermal systems, or caves in ignimbrites probably formed in a similar way as tumuli or pressure ridges in lava flows. Caves can form in fluvial systems when erosion happens at a stratigraphic contact within an ignimbrite package or when large blocks fall, and landslides occur in a narrow valley followed by further erosion at the base. Sometimes, the rock above the cave can be used as a natural bridge when the length is short. Several caves are the result of complex processes or are of mixed origin. A few other caves were apparently enlarged by anthropogenic interventions (or possibly by Plio-Pleistocene megamammals). More studies are required in order to better understand the diversity of the given volcano-speleogenetic processes.