A Volcanic Rainbow in the Hottest Place on Earth

April 12, 2010 -- People who visit Dallol, Ethiopia certainly don't do it for the weather: for a time the mining ghost town held the record for the hottest inhabited place on the planet, with an average year-round temperature of 94 degrees Fahrenheit. Daily heat in the region, known as the Danakil Depression, regularly climbs past 115 degrees.

But for those who brave the searing equatorial desert, amazing visual wonders await. In 2002, I joined a group of scientists studying nearby Erta Ale volcano. An Ethiopian military helicopter brought us to Dallol at a time when reaching the area by land was considered exceedingly dangerous.

Dallol lies in northeastern Ethiopia close to the disputed Eritrean border. This, and the at times somewhat hostile Afar tribesmen, make the area somewhat unstable and several armed attacks on tourist convoys have occurred in recent years, somewhat hemming touristic development of the area.

Nevertheless, tourists increasingly are drawn to the springs to see the stunning yellow and red hydrothermal deposits. By 2008, when I made a second visit, small convoys of 4-wheel drive vehicles accompanied by armed guards were occasionally bringing visitors to the springs.

Dallol's hot springs are mostly located on a large mound that has formed due to magma pushing upward and locally lifting the over 1 kilometer (0.6 mile)-thick salt deposits. Heated by the molten rock, groundwater carries dissolved salts to the surface where the sun's relentless heat quickly does away with moisture.

Left behind are salt deposits sketched in brilliant hues of yellow, red, white, and green -- the varying colors come from potassium salts colored by sulphur and traces of ferric chloride, iron oxide, ferrous chloride and iron hydroxide. Fresh deposits are usually nearly white while older ones effectively rust and become reddish in color due to the gradual formation of iron oxides.

Taking a closer look at the deposits one can see fascinating patterns and variations in the shape of the salt crystals.

Dallol is also interesting for its mining history. Near the beginning of the last century, intrepid explorers discovered significant potash deposits at the site, which is used for fertilizer. An Italian company set up a small mining operation and the town went up, complete with buildings whose walls are made of the only building material for miles around: salt blocks.

Remnants of this, and of a narrow-gauge railway line connecting the site to the Eritrean port of Mersa Fatma remain, but much of the history of Dallol remains a mystery since hardly any records survived.