Aspronisi was formed through the gradual volcanic activity in the Santorini caldera, which itself is composed of four distinct shield volcanoes. The island was formed along with the current Santori caldera during a massive eruption in the 2nd millennium BC that expelled 14 cubic miles (60 km3 ) of dense rock. Aspronisi was briefly a submarine volcano before the force of the eruption pushed the islet above the surface, after which dense layers of pumice and tephra ejected from the volcano covered the island. These deposits eventually solidified into the pale white deposits that give Aspronisi its moniker of the "White Isle". The base of the island is composed of black lava rock, and the island is connected to the larger islands of Therasia and Santorini by the submerged outer edge of the Santorini caldera.