Geothermal Resource Assessment of

Alid volcanic center

In January and February of 1996, a five-person team from the USGS traveled to Eritrea to help perform a geothermal assessment of Alid volcanic center, about 30 km south of the Gulf of Zula in the Danakil (Afar) Rift. The expedition was funded mostly by the US Agency for International Development (USAID) with the aim that the study would provide background information useful to the Eritrean government and geothermal developers interested in the site. The group of five U.S. and five Eritrean geoscientists spent three weeks in the field collecting water, steam, gas, and rock samples and preparing a geologic map of the volcanic center. They have since prepared a report of their findings and recommendations to the Eritrean Government and USAID a USGS Open-File Report and several papers for the scientific literature.

Eritrea, which gained full independence from Ethiopia in 1993, after conducting a national referendum, is a rapidly developing democracy with a secular government, a peaceful domestic situation and relatively good relations with its neighbors. Currently, nearly all its energy needs are met through imported oil and diesel fuels. As of 1992, the annual electrical use per person was about 40 kWh, for a country of 3 million people. Future development and improvements in infrastructure cannot proceed rapidly without significant increases in the country's ability to generate power.

The Alid volcanic center is about 90 km south of Massawa, Eritrea's dominant port city. The road is unpaved but flat. Alid is an elliptical, 5x7 km, 900-m-tall mountain with its long axis parallel to the direction of spreading of the surrounding Danakil Rift. Though the floor of the rift valley lies near sea level, the adjacent Precambrian highlands of the Eritrean Plateau and Danakil Horst reach 3000 m and 600 m respectively. Nearly all of the rifting and subsidence have occurred since the Oligocene and much of it near Alid has occurred within the last 1 million years.

The volcanic center consists of marine sediments, pillow lavas and subaerial volcanic units that were uplifted up to 1 km or more by intrusion of a pyroxene-bearing magma, which began around 35 ka. This uplift created a structural dome with dips that radiate away from the geographic center of the structure. Uplift caused deformation, landsliding and consequent collapse of the top of the edifice. Subsequently, the summit basin was further excavated when pyroxene rhyolite was erupted 23.5 ka as pumice fallout and pyroclastic flows that originally covered much of the mountain.

Fumaroles and thermal pools are found in at least 11 small (~1-2 ha) sites over about 10 square km in the depressed summit region and northern flank of Alid. Most of these zones are clay-altered and are covered by sublimates of various NH4-, Ca- and K- sulfates. The thermal areas vent through a variety of lithologies, including rhyolites, siltstones and a small block of Precambrian mica schist that crops out in a deep canyon cut into the central part of the uplift. There are no obvious structural controls on the distribution of fumaroles. Thermal pools contain mixtures of shallow groundwater and fumarolic condensate. Hot or cold springs unrelated to fumaroles were not found.

Fumarolic steam and gases vent at the boiling temperature of water for their elevation (~95-98°C). Non-condensable gases (ncg) are dominated by CO2 (95.5-98.9%) with an isotopic composition (-3 to -5 per mil VPDB) similar to that of magmatic CO2 possibly mixed with some marine carbonate. Hydrogen is generally the next most abundant gas (0.5 - 2.5%) and a variety of gas geothermometers all yield high parent reservoir temperatures, up to and over 250°C.

Isotopic and ncg ratios suggest that the steam is derived from a reservoir of air-saturated groundwater derived from winter rains in the lowland, possibly mixed with some fossil Red Sea water left over from earlier periods of marine incursions. Shallow wells in the alluvial fans to the immediate west of Alid yield waters with isotopic compositions consistent with some recharge from the Eritrean Plateau.

 


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