Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Wednesday, October 30, 2024

The oddest volcano on Earth

As far as we've progressed in our understanding of science, we still have a long way to go.

This is true in every scientific endeavor, but I was thinking about it apropos of geology while reading volcanologist Tamsin Mather's wonderful recent book Adventures in Volcanoland: What Volcanoes Tell Us About the World and Ourselves.  Mather's fascinating and often lyrical narrative takes us all over the world, describing her studies of volcanoes in Nicaragua, Greece, Hawaii, Iceland, Japan, Indonesia, Wyoming, Sicily, Ethiopia, and Tanzania.  It was this last-mentioned that got me pondering the gaps in our understanding, because she described seeing the bizarre Ol Doinyo Lengai volcano in the northern part of the country.

Ol Doinyo Lengai is strange by any standards.  It sits in the middle of the East African Rift Zone, which extends from Ethiopia to Mozambique, and is one of the Earth's only above-water divergent zones, places where two pieces of crust are moving apart.  The Rift being on land won't last forever, of course; ultimately a crescent-shaped chunk of East Africa will cleave off from the rest of the continent, the ocean will flood in, and afterward the rift will (like most of the others) lie at the bottom of the sea floor.  The separated piece will then creep off to the east, becoming an island -- or, depending on how you define it, a new continent.

The violent geology of the region has created a topography that in a post last year I referred to as "a beautiful hellscape."  The Dallol Depression is already 48 meters below sea level; the only thing keeping the water out is the Afar Highlands to the east acting as a barrier.  It's not only filled with bubbling mud pots and hot springs, but its position near the equator and the surrounding mountains creating a rain shadow make it blisteringly hot -- think Death Valley in midsummer -- so despite the otherworldly beauty of the brilliantly-colored rocks, it's not a place most people would ever think of going.

But even by comparison to the strange landscape that surrounds it, Ol Doinyo Lengai is peculiar.  It is the only active volcano on Earth that produces carbonatite lava, which (as you might surmise) is rich in carbonate minerals like calcite, dolomite, sodalite, apatite, and ancylite.  The magma is cool by volcanic standards, at only around 500-600 C, and yet produces some of the most fluid, low-viscosity lava flows known, moving at around five meters per second -- so about a typical human running speed.

Staying out of the way when Ol Doinyo Lengai erupts is a really good idea.

The lava comes out dark brown or black, but once it freezes and is exposed to the air for a few days, carbonate minerals crystallize on the surface and turn it a frosty white.

[Image licensed under the Creative Commons Thomas Kraft, Kufstein, Lava lengai, CC BY-SA 3.0]

So the entire cone has a snow-covered appearance, but there's no snow there -- it's all mineral deposits.

The weirdest thing is that we don't really understand where all this carbonatite magma is coming from.  My first guess was that somehow the plumbing of the volcano was moving up through a limestone or marble deposit, and picking up the carbonates as it went, but geochemical analysis of the rocks produced from it seems to have ruled that out.  Right now, it's thought to be some kind of weird fractionation -- the source magma deep underground is separating into high-carbon and low-carbon bits, and the high-carbon bits are the ones feeding this particular volcano -- but the fact is, this is a guess, and the word you see most often attached to its mineralogy and chemistry is "peculiar."

So what's happening here in northern Tanzania -- and even more apposite, why it isn't happening anywhere else on Earth -- is a mystery.  The strangeness, though, only increases its fascination, and there are geologists who are devoting a lot of time to figuring out what is going on in this odd and inhospitable place.  Strange, too, that the East African Rift Valley is where humanity got its start; Oluduvai Gorge, where some of the best-preserved hominin fossils were found, is part of the Rift complex. 

Perhaps there's a reason we're drawn to this mysterious spot.  Our roots are here -- in one of the most tectonically-active places on the planet.  That it still leaves us with unanswered questions only makes the draw stronger, bringing us back to the place our distant kin left a hundred thousand years ago, to use the tools of science to finally understand our own ancestral home.

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