BIGGER isn鈥檛 always better when it comes to volcanoes. Mount Vesuvius in Italy and Mount St Helens in Washington state are hardly giants, but for the scale and destructive power of their eruptions they are hard to beat. Ol Doinyo Lengai, some 120 kilometres north-west of Arusha, Tanzania, in east Africa鈥檚 Rift Valley can pack a punch too. Three times during the last century it propelled boulders and dense columns of ash several kilometres into the air.
But this is not what gets geologists most excited. Ol Doinyo Lengai (the name translates as Mountain of God) is the only volcano on Earth with alkali-rich carbonatite lava. Unlike the basalt lavas of other volcanoes, this is rich in sodium and potassium carbonates and low in silicon. With a temperature of around 550 掳C, it is also relatively cool 鈥� a good 500 掳C cooler than basalt lavas and not hot enough to glow in daylight. And it鈥檚 as fluid as oil.
The carbonatite emerges looking like black or dark brown mud, but turns quickly to grey and then white as it reacts with moisture in the atmosphere. The slopes of Ol Doinyo Lengai look a bit like dirty snowfields streaked with oil slicks. And since 1983, it has been in a state of near-constant eruption.
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鈥淭he volcano could offer a hint about the surface of Venus鈥�
The volcano is not short of scientific visitors. The low temperatures mean researchers can get close to the lava flows, channels, vents and lakes without protective equipment. And the low viscosity of the lava appears to make it particularly sensitive to changing atmospheric conditions. A recent study by Josh Gordon and Fred Belton at Middle Tennessee State University in Murfreesboro found that Ol Doinyo Lengai鈥檚 activity increased when atmospheric pressure was falling or at a minimum. The researchers suggest the falling pressure might trigger an eruption by causing a slight expansion of the gases in the lava. They also noted more eruptions when the moon was closest to the Earth, possibly because of the stronger tidal pull. But they stress they need a lot more data to support these theories.
There is another reason why researchers get excited about the Mountain of God: it could offer a hint about what happens on the surface of Venus. Our sunward neighbour has several lava channels on its surface, some up to 7000 kilometres long. Only a low-melting-point magma like carbonatite could have stayed liquid long enough to flow such a long way.
Fancy a visit to Ol Doinyo Lengai? First check whether the volcano is in a particularly eruptive phase. It can turn explosive without warning, as it did in 1917, 1940 and 1966, when the whole summit was destroyed. 鈥淥ne should always be prepared to flee,鈥� advises Charles Frankel, whose book Worlds on Fire, describing the solar system鈥檚 most spectacular volcanoes, is published this week by Cambridge University Press. 鈥淐amping at the summit is not a good idea,鈥� he warns.
