Warm springs on land on the big island of Thera remind us that hot rocks are present at depth beneath the volcano and that it is still active. Three main hot springs are found on the main island of Thera: Plaka 34oC), Athermi Christu (56oC) and Vlihada hot springs (32oC).


On the young volcanic island of Nea Kameni in the Thera caldera, fumaroles and steam vents have temperatures up to 95oC, and thermal springs occur in shallow water around both the islands of Nea Kameni and Palea Kameni. Here the 38oC seawater is of deep reddish brown color because of the presence of metallic ores emitted by the submarine springs. The hydrothermal fluids rising within the volcanic system dissolve iron from the rocks at depth, and when the fluids emerge on the ocean floor they precipitate iron hydroxides upon reaction with the sea water.


Although the iron oxide deposits are most conspicuous around the shore of the young islands, we know from sediment cores taken from the sea floor that iron hydroxide deposits and metal-rich sediments of this type are present throughout the submarine caldera floor, forming an iron-rich layer up to 3 meters in thickness. It has been estimated that up to 350 thousand tons of iron and 19 thousand tons of manganese have been deposited from hydrothermal springs in the submarine caldera in the last 150 years alone. Other metals that occur in these hydrothermal sediments are copper, cobalt, chromium, nickel, lead and zinc. Thus submarine hydrothermal activity in the caldera is leading to the formation today of an ore deposit that may be of economic importance.