Weathering and Erosion of Flood Basalt Provinces

Abstract

The weathering and erosion of flood basalt provinces are phenomena not directly related to the volcanism itself, but they together give rise to the landscapes and appearance of flood basalt provinces as observed today. In this way, this final chapter “completes the cycle” and brings the reader back to Chap. 1.Weathering products of flood basalts include clays and clayey “bole” beds (most commonly red, but also brown, green, and even black), laterites (more appropriately called ferricretes), and bauxites. Ferricretes and bauxites are academically important, in discussions of geomorphology, climate, hydrogeology and tectonics. They are also commercially important, for building stone and aluminium ore, and similarly in civil engineering (regarding landslides and associated hazards, particularly in flood basalts like the Deccan which receive extensive rainfall). Ferricretes capping the Deccan highlands of the Western Ghats have been interpreted as the erosional remnants of a once-continuous weathering blanket (e.g., Widdowson 1997) and alternatively as formed in a palaeoriver valley system now exposed in inverted relief (Ollier and Sheth 2008). In Iceland, alteration of glassy basalt produces palagonite. Terracettes (produced by small mass movements) and spheroidal weathering (a type of exfoliation, formed due to volume expansion on ingress of meteoric water) are universal features of global flood basalts and their intrusive equivalents. Wave erosion at rocky beaches produces pedestal rocks, whereas natural bridges and imposing pillars are left by advanced erosion of flood basalts like the Deccan.Whereas very advanced erosion and planation will produce a flat terrain out of anything, leaving no surface irregularities (except a few monadnocks), moderately high degrees of erosion tend to emphasize and bring out the internal rock structure and rock fabric. This is what explains the continued persistence of even the smallest outcrop-scale features in older flood basalts, such as pipe vesicles, chiesel marks on joint columns, and delicate but beautifully preserved ropy pāhoehoe surfaces, illustrated in this book. The record of small-scale magmatic processes that ocurred hundreds of millions of years before human beings is still not completely erased from the rocks. This record is available for anyone going to the field to read and marvel at. Whatever be the fate of the human race, flood basalts will continue to form, to affect the lithosphere, hydrosphere, atmosphere and biosphere, and to leave a geological record of all this far into the Earth’s future.