Geology of Ol-doinyo Lengai Volcano (Tanzania)

Geology of Ol-doinyo Lengai Volcano (Tanzania)
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The Mountain with the Coolest Lava on Earth

black carbonatite lava flow

Ol doinyo Lengai, (which means “Mountain of God” in the local Maasai language), is an active volcano located on southern shore of Lake Natron in the greater Arusha Region. The mountain is a stratovolcano standing at 2870m above sea level, built up by several layers of hardened lava shaped nearly to a perfect cone.  Apart from its regular eruptions, the volcano is famous and unique since it produces natrocarbonatite lava, making it an active volcanic carbonatite. The volcano is estimated to have started developing 370,000 years ago in the Great Rift Valley, with the cone we see today forming about 15,000 years ago following a series of explosive eruptions of tuffs and pumices and effusive eruptions of lava.

Most lavas are usually rich in silicate minerals but the Ol Doinyo Lengai lava is a carbonatite, containing about 50% sodium and potassium carbonates that causes it to erupt at relatively lower temperatures of approximately 510°c compared to basalt based lavas that erupt red-hot lava at around 1100 degrees Celsius. This temperature is so low that the molten lava appears black in sunlight, rather than having the red glow common to most lavas. It is also much more fluid than silicate lavas, often less viscous than water. The sodium and potassium carbonate minerals of the erupted lava are usually unstable at the Earth’s surface and prone to rapid weathering that causes it to rapidly turn color from black to grey. The grey-white colour of the lava creates the illusion that the top of the volcano is covered with snow. The resulting volcanic landscape is different from any other in the world. Once in contact with rain, the decomposed lava rocks turn brittle and crumble easily.

The local Masai community respects it as holy Mountain of God and home to their God Enkai, while geologists study the mountain for its unique lava and tourists, travellers and hikers attempt to capture its magical aura in photographs.

Aerial and crater view of the volcano

During eruptions, the volcano usually grumbles, sending tremors across the lands all the way to Kenya. The latest recorded eruptions include:

  • Eruptions are dated from 1883, with lava flows between 1904 and 1910 and between 1913 and 1915.
  • In June 1917 there was a major eruption that deposited volcanic ash close to 50kms away.
  • Eruptions were also recorded severally in 1926.
  • In late 1940 another major eruption deposited ash nearly 100kms away. Other minor eruptions were observed in 1954, 1955, and 1958 and early 1960s.
  • From 12th to 18th July 2007, volcanic activity in the mountain caused daily earth tremors in Kenya and Tanzania (the strongest tremor measured 6.0 on the Richter scale) before it erupted on 4th September 2007, sending a plume of ash and steam at least 18kms.





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