St. Mary's Islands

St. Mary's Islands are a small group of islands off Malpe in the Udupi district of Karnataka, India, known for exposed columnar basaltic lava with hexagonal and other polygonal columns forming a horizontal mosaic, the tallest columns standing about 6 meters high. The northernmost island, Coconut Island, is roughly 500 meters long and 100 meters wide, reaching a maximum elevation of about 10 meters. The basalt formed from subvolcanic activity rather than a broad flood-basalt eruption, and its magmatism has been linked to the initial break-up between India and Madagascar during the Late Cretaceous; dating methods give somewhat different ages, with potassium-argon dating around 93.1 million years and the generally preferred argon-argon dating around 85.6 million years, both placing the rock in the Upper Cretaceous. Alongside the basalt, the islands also contain more silica-rich igneous rock, including dacite, rhyodacite, rhyolite and granophyre. This entity documents St. Mary's Islands specifically as a geological formation defined by columnar basalt volcanism tied to continental rifting.