Abstract
Contextual early observations on volcano genesis are valuable but scarce. Resembling a shield volcano, the Dallol dome is a complex 40 m-high geological structure on the Danakil depression, a North-South-elongated salt plain lying 120 m below sea level in the North Afar (Ethiopia). Dallol has become a tourist destination famous for its colorful hydrothermal features and raised scientific interest due to its life-challenging polyextreme conditions. Although some general models for its genesis exist, little is known about the origin and temporal evolution of both, the dome and its geothermal activity resulting in hyperacidic and halite-oversaturated brines. In this study, we combine published information with data obtained from our three multidisciplinary field campaigns (January 2016, 2017, and 2019) to refine the geological mapping of the North Danakil and the Dallol dome. The analysis of stratigraphic, geomorphological, geochemical, and hydrogeochemical data as well as satellite, drone and infrared aerial images allows us to shed light in its complex temporal evolution. Our results suggest that the recorded history of the dome began when at least one deep magmatic basalt intrusion occurred later than 6000 years ago, forcing the uplifting of the lacustrine deposits of that age covering the west side of the dome. The interaction of the magma with the buried salt deposit resulted in a halo-volcanic activity with, likely, several melted-salt effusion events. Substrate accommodation after effusion led to the current collapsing crater on the dome top and the geothermal still-ongoing degassing. An important hydrothermal reactivation took place after a dyke intrusion event in October–November 2004. It triggered the appearance of new fractures on the dome top and the northward migration of the hydrothermal activity, as we inferred from the analysis of historical aerial images combined with high-definition visible and infrared images taken from a drone during our field campaigns. Based on our observations, we present an updated hydrogeothermal conceptual model linking deep magmatic activity with halokinetic processes and geothermal fluids to explain the origin and evolution of the Dallol halo-volcanic complex. These geothermal manifestations may potentially inform about rarely documented premises of a volcano’s birth
Highlights
Direct observations of volcano formation including very early contextual genetic steps are extremely rare
We carried out additional x-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of selected anhydritegypsum layers of this unit to compare it with similar lacustrine deposits of the southern Afrera Formation outcropping around the Bakili and Afrera lakes (Figure 6)
The flat bottom of the depression is covered by a salt crust (Figure 3E) formed by the evaporation of a temporal lake that repeatedly forms after periodic flooding from both meteoric and/or outflowing of hydrothermal groundwater
Summary
Direct observations of volcano formation including very early contextual genetic steps are extremely rare. The salt plain formed when the northern extremity of an ancient Red Sea arm was closed at the Gulf of Zula (Eritrea) due to uplift and doming preceding the Alid volcano Quaternary magma flows (Lalou et al, 1970; Bonatti et al, 1971; Mitchell et al, 1992; Corti et al, 2015) This closure possibly happened during the Pleistocene and left marine reefs at different altitudes (−30 to +90 m), marking ancient shorelines (Barberi and Varet, 1970; Bonatti et al, 1971; Hutchinson and Engels, 1972; Mitchell et al, 1992; Talbot, 2008). Subsequent seawater evaporation led to the deposition of a ∼2 km thick salt sequence over several thousand years (Bonatti et al, 1971)
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