The Earth's Mantle
The name "Heuksando" (흑산도) literally means "Black Mountain Island," a direct reference to the dark, volcanic stone that forms its rugged cliffs and peaks. These rocks are predominantly a type of volcanic rock known as alkali basalt. While basalt is common worldwide, the chemistry of Heuksando's basalts shows an origin story distinct from other volcanic islands in the region, such as Jeju-do and Ulleung-do. The island is part of the Dadohaehaesang National Park, the largest national park in South Korea.
The volcanic activity that formed Heuksando occurred during the Cenozoic era, part of a period of intermittent volcanism across the Korean peninsula. The composition of the basalt here is characterized as Ocean Island Basalt (OIB). OIB-type rocks are chemically similar to those found in places like Hawaii or the Galápagos Islands. Their presence here suggests the magma did not originate from the melting of a subducting tectonic plate, which is a common source for volcanoes in the wider region of East Asia. Instead, the geochemical evidence suggests the magma source was a deep, enriched part of the Earth's upper mantle, potentially related to asthenosphere upwelling.