Iōjima is of volcanic origin, and is the northern edge of the Kikai Caldera, a stratovolcano rising from the ocean floor to above sea level at its highest peak (Mount Iōdake). Kikai Caldera is ranked class A for volcanic activity. It is constantly erupting, emitting massive amounts of sulfur dioxide which causes damage to agricultural products. Hot springs high in iron concentration from the port bottom gush and due to contact with oxygen, the port waters change to a reddish-brown color. Due to the sulfur, the sea area around the island is yellow in color. This gave rise to the name Sulfur Island (Iōjima).