Cylindrical concentric jointing in lava flows: Description and preliminary ideas on its origin

Abstract

Studying structures and joint systems in viscous lava flows is important to constrain emplacement and cooling conditions and to decipher the thermal mechanisms acting during solidification of thick felsic lavas. Columnar jointing in basalt lava flows has been extensively studied and the variety of structures and joint systems found is ample. For more viscous flows such as andesites and dacites, however, there are cases that deviate from the classic jointing patterns seen in basaltic flows. Concentric cylindrical and platy jointing in intermediate and silicic lavas is one of such cases that is rarely observed but needs detailed description to draw attention of volcanologists and prompt further research. Based on field data and image analysis, we present a description of three sites, one in Colombia—Machines Quarry, and two in Costa Rica—Virilla River and Zanja Tapada Creek, where intermediate to felsic lavas show unusual jointing systems characterized by pervasive arching joints that form curved slabs surrounding a cylindrical or spindle-shaped massive core with a nearly-circular cross section characterized by concentric joints showing an increase in spacing inwards. Among the three sites we find similarities in tectonic environment (continental arc), age (Lower Pliocene to Middle Pleistocene), lithology (andesites to dacites), dimension (∼5–12 m for the massive core) and jointing styles suggesting comparable sources and emplacement/cooling mechanisms. By comparing the structures and joint systems at the three sites with structures in volcanic rocks elsewhere we discard alternative explanations and conclude that the massive cylindrical or spindle-shaped core surrounded by curved slabs is consistent with either known cleft lavas or the internal structure of ogives, large curved surface ridges common in intermediate to silicic lava flows.