Physical Environment of Hydrothermal Systems in Iceland and on Submerged Oceanic Ridges

Abstract

The details of magmatic and hydrothermal activity are much better known in Iceland than at submerged rift zones. Therefore, it is of interest to compare hydrothermal system in Iceland and on other parts of mid-ocean ridges. The effect of topography on hy­drothermal systems for submarine systems is opposite to that of systems discharging into air. The reason for this is the simple fact that these two kinds of hydrothermal systems discharge into different kind of fluids as compared to the hydrothermal fluid. The working pressure of submarine hydrothermal systems is usually much higher than for systems on land. This fact, together with the higher salinity of the submarine systems, is most likely the cause of the metallic depositions observed on the sea floor. Downward penetration of cold water into hot rock is the only known process which can explain the high heat flux density of some hydrothermal systems in Iceland and it seems natural to assume that this process is of major importance for submarine systems. A controlled cooling experiment of hot lava during the Heimaey eruption of 1973 supports further that this process is a natural phenomenon. By comparing the heat flux through Iceland with the energy released on submarine spreading axes, it is found that neither the volcanic nor the hydrothermal activity in Iceland is exceptionally high as compared to estimates for submerged ridges. Extrapolation of knowledge of hydrothermal systems is therefore found to be realistic. In Iceland the cooling of the crust by hydrothermal and volcanic activity is concentrated to a certain location, the central volcanoes. The spacing of cooling spots in the Icelandic rift zones is approximately 12–15 km. This distance is approximately the same as the estimated thickness of the crust. It is proposed that the cooling of the submerged rift zones might proceed in a similar way and that the spacing of cooling spots is related to the spreading rate.