Hydrothermal sediments form in several places within the Santorini caldera. The sediments at the most active hot submarine spring (at SW Nea Kameni) consist of 40-48% Fe2O3 and 8-15% SiO2, but only little Al2O3 (0.5-1.6%), P (0.1-0.3%), Mn (100-450 ppm), Sr, Zr, Ba, Zn and V (∼30-200 ppm) and Cu, Ni and Y (∼ 1-30 ppm) are present. The sediments contain more Fe and P and less Si, Al, Mn and many trace elements than sediments formed at less active springs at Palaea Kameni; the differences probably being caused by faster deposition rates of Fe at Nea Kameni and less dilution by rock detritus, which delivers Al, Si and Mn. The Santorini sediments differ much from other exhalative sediments at present or old spreading centers, for example on the East Pacific Rise, in the Red Sea or at En Kafala, which are rich in Mn, Cu, Zn, Ni and Ba. Hydrothermal rock leaching probably occurs at much lower pH-values at Santorini than on the East Pacific Rise, causing a more complete breakdown of the leached rock at Santorini. The very low trace element to iron ratios in the rocks at Santorini, therefore, prevent the hydrothermal solutions there from being very rich in trace elements. The very acid leaching solutions at Santorini are created by extensive oxidation of hydrogen sulfide to sulfuric acid, a process that is readily possible due to the close proximity of the oxygen-rich atmosphere to the top of the magma chamber.