Dallol is a unique, terrestrial hydrothermal system in Ethiopia. It is known for its unearthly colors and mineral patterns, and the very acidic fluids that discharge from its hydrothermal springs.
Physical properties
Dallol lies in the evaporitic plain of the Danakil depression at the Afar Triangle, in the prolongation of the Erta Ale basaltic volcanic range. The intrusion of basaltic magma in the marine sedimentary sequence of Danakil resulted in the formation of a salt dome structure, where the hydrothermal system is hosted. The age of the hydrothermal system is unknown and the latest phreatic eruption that resulted in the formation of a diameter crater within the dome, took place in 1926. The wider area of Dallol is known as one of the driest and hottest places on the planet. It is also one of the lowest land points, lying below mean sea level. Other known hydrothermal features nearby Dallol are Yellow and Black Lakes. The hydrothermal springs of Dallol discharge anoxic, hyper-acidic, hyper-saline, high temperature brines that contain more than 26 g/L of iron. The main gas phases emitted from the springs and fumaroles are CO2, H2S, N2, SO2 and traces of H2, Ar, and O2. Although several other hyper-acidic volcanic systems exist, mainly found in crater lakes and hydrothermal sites, the pH values of Dallol decrease far below zero. The coexistence of such extreme physicochemical characteristics render Dallol one of the very few ‘poly-extreme’ sites on Earth. This is why Dallol is a key system for astrobiological studies investigating the limits of life. Parts of the region are nearly sterile, except for a diverse array of "ultrasmall" archaea. Dallol is highly dynamic; active springs go inactive and new springs emerge in new places in the range of days and this is also reflected in the colors of the site that change with time from white to green, lime, yellow, gold, orange, red, purple and ochre. In contrast to other hydrothermal systems known for their colorful pools, where the colors are generated by biological activity, the color palette of Dallol is produced by the inorganic oxidation of the abundant iron phases. Another fascinating feature of Dallol is the wide array of unusual mineral patterns, such as, salt-pillars, miniature geysers, water-lilies, flower-like crystals, egg-shaped crusts, and pearl-like spheres. The main mineral phases encountered at Dallol are halite, jarosite2, hematite, akaganeite and other Fe-oxyhydroxides, gypsum, anhydrite, sylvite and carnallite. Although Dallol is a natural system of exceptional beauty and aesthetic importance where unique geological processes are taking place, it is not yet included in the Unesco World Heritage list. Currently, hundreds of tourists visit the site annually, but Dallol is neither monitored nor protected, with danger lurking for both the site’s sustainable development and the tourists’ safety. In October 2019, a French-Spanish team of scientists published an article in Nature Ecology and Evolution that concludes there is no life in Dallol's multi-extreme ponds.
