On Surtsey a large amount of richly colored encrustations formed around the craters, both during the eruption and for a few years after it had ended. These surface encrustations have now disappeared due to weathering. There are numerous caves on the island, most notably lava tubes, but they have not been studied in depth. In some of the caves, there can be found a large quantity of mineral encrustations that formed out of hot gases of 30-300°C. Halite, thenardite, gypsum, calcite and opal are common there. A total of 18 different kinds of mineral encrustations have been found on the lava and in lava tubes on Surtsey. Some of these are quite rare and have not been found elsewhere in Iceland. Two of the minerals are likely to be new to the world.