A geophysical multi-parametric analysis of hydrothermal activity at Dallol, Ethiopia

Abstract

During December 2003, three seismic stations were installed close to the hornitos of the hydrothermal system at Dallol, complemented by radiometer and infrasonic measurements. A combined geophysical data set was collected for about three days. During this period thermal, seismic and acoustic records indicate the presence of two regimes characterized by a different energy distribution in frequency. Few volcano-tectonic events appear superimposed to the continuous hydrothermal tremor. The continuous data indicate variable shallow processes most likely related with variations in temperature and degassing processes within the shallow geothermal system. This alternation of low and high regimes shows significant similarities with other volcanic systems of different nature, although at Dallol the transition is more evident in the thermal than in the seismic and acoustic data.

Introduction

Hydrothermal systems are zones where significant heat and mass redistribution between a magma reservoir and the ground-surface takes place (e.g. Norton, 1984, Igebritsen and Sanford, 1998, Simonelli, 2002). These complex and dynamic systems are characterized by different geophysical and geochemical parameters (e.g. a wide range of temperature and pressure conditions). Hydrothermal systems usually appear on active volcanoes in response to the hydrothermal fluid circulation (i.e. magmatic gases, groundwater, meteoric waters and seawater) (Todesco et al., 2006). Quantification of processes and rates in these systems may involve a better investigation of heat and fluid circulation using geophysical monitoring techniques because hydrothermal systems could produce a variety of quantifiable geophysical signals in response to changing internal and surface conditions (Finizola et al., 2003, Todesco et al., 2006). Hence, hydrothermal systems may represent an important key to understand and evaluate volcanic activity.

The study area that we discuss in this paper is Dallol (14.24°N, 40.30°E, at about 120 m below sea level) located in the Danakil depression, in the Afar province, Ethiopia (Fig. 1a). It is one of the hottest area on earth, with an average annual temperature of 34.4 °C recorded between 1960 and 1966 (Young and McWirther, 1999). Dallol has a superficial volcanic system associated to post-eruptive activity; its last recorded eruption was in 1926 (GVN 2008). The majority of this area was occupied by the Red Sea waters and later was substituted by a thick succession of flood basalts (Beyene and Abdelsalam, 2005, Bosworth et al., 2005). The interior sea remained trapped and processes of evaporation left a sequence of sediments, mainly evaporites, that covers an area of 1.150 km². Under the salt lake an important source of volcanic heat survives, causing the ascent of hot water through the layers of halite and anhydrite. The minerals are dissolved and deposited over the basaltic lava flows, originating fumaroles and formations called hornitos – which are notably not the classical hornitos related to a lava flow, although they resemble their shape – and crystal concentrations, micro-terraces and colourful structures of sulphur, halite, sylvite and potassium, all of which are clear superficial evidences of the underlying hydrothermal system. Several photos and videos depicting these phenomena can be seen at Alean et al. (2009).

In Fig. 2a and b, two hypothetical 3D block diagrams of the volcanic system at Dallol (SW–NE) are presented. At depth, a system of fractures permits the rise of fluids through layers of basaltic lava flows and evaporites due to the stagnation of the water of the Red Sea and the salt lake. This water, together with the meteoric water (blue¹ arrows) forms the fluids that enter in the system (Fig. 2a). Fig. 2b is a sketch of the superficial hydrothermal system structures, including hornitos and fumaroles. The yellow colour indicates hydrothermal mineralization and hydrothermal alteration.

In the following we present the first integrated multi-parameter data set collected close to the superficial hornitos of Dallol and we discuss the use of this data set for understanding the underlying hydrothermal system. The combined analysis of this multi-parametric data allows us to identify at a short-term time scale the presence of low and high activity regimes, which are determined by shallow processes and are therefore mostly independent for each of the superficial structure within the hydrothermal system.