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In January and February of 1996, a five-person team from the USGS travelled 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 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, 5 x 7 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 Pre-Cambrian highlands of the Eritrean Plateau and Danakil Horst reach 3000 m and 600 m, respectively. Nearly all of the rifting and subsidence has occurred since the Oligocene.

The volcanic center consists of marine sediments, pillow lavas and sub-aerial volcanic units that were uplifted up to 1 km or more by intrusion of a pyroxene-bearing magma. 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 as pumice fallout and pyroclastic flows that originally covered much of the mountain.

The age of the structure is unknown, though geologic and geomorphic relations suggest that uplift and rhyolite eruption occurred between about 50 and 200 ka. Argon geochronology will be completed by late 1996 and should provide detailed information to determine the timing of important events in the history of the volcanic center.

Fumaroles and thermal pools are found in at least 11 small (about 1-2 ha) sites over about 10 km ² 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 went through a variety of lithologies, including rhyolites, siltstones and a small block of Pre-Cambrian 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 CO ₂ (95.5-98.9%) with an isotopic composition (-3 to -5 per mil VPDB) similar to that of magmatic CO ₂ possibly mixed with some marine carbonate. Hydrogen is generally the next most abundant gas (0.5 - 2.5%). 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.

More information can be obtained through the USGS( jlwnstrn@usgs.gov) or Eritrean Ministry of Energy, Mines and Water Resources.

Taken from:J. Lowenstern, "USGS/Eritrean Team assessment of geothermal potential of Eritrean volcanic center", IGA News, 1996, n.25, pp.6-7.

A Hypertext Document by Li Bellucci, Marnell Dickson, and Mario Fanelli