Lake Abbe

Steaming chimneys where three tectonic plates pull apart

Lake Abbe, or Abhe Bad, is a salt lake on the border between Ethiopia and Djibouti and the end of the road for the Awash River, the last water of a chain of six connected lakes. It lies at the Afar triple junction, the meeting point of three pieces of the Earth's crust that are pulling away from one another, at different speeds, in the Afar Depression, and it is counted among the most inaccessible places on Earth. The dormant volcano Dama Ali rises 1068 m on the northwest shore, and salt flats 10 km wide run along the southwest and south.

The lake has no outlet. Water carrying mineral salts flows in and pure water evaporates off the top, leaving it hypersaline and alkaline, and because its level swings sharply with small changes in climate it is called an amplifier lake. Recent droughts, and water drawn from the Awash for irrigation, have shrunk it: by 1984 its surface was two thirds of what it had been in 1940, and some 11,500 hectares of new salt flat had formed to the southwest. Rain is under 172 mm a year, summer averages run from 30 to 40 °C, and flamingos still work the water. The Afar people have a settlement near the shore.

The chimneys bring the few who make it here. Carbonate towers stand scattered and in rows along the shore, steam spewing from some of them, and a study of their mineralogy and geochemistry gives them heights of up to 60 m. They are built of concentric layers of calcite and low-magnesium calcite with silica, and they formed underwater, when the lake stood higher than it does now, where hot spring fluid rising from below mixed with lake water and carbonate dropped out fast. In the chimney cores hydrothermal fluid made up about 45 percent of the mix, falling to about 4 percent in the outer layer. They may be younger than they look: uranium-thorium dating of the chimney the study sampled suggests it formed within the last 820 years or so.

The towers belong to the Lake Abhe Geothermal Field on the eastern shore, which sits on the Gob Aad graben. There, steam vents, hot springs and chimneys follow the main fault lines, with the strongest activity where faults cross, and the researchers who mapped it see promise there for geothermal power.