Oh the buzzin' of the bees in the cigarette trees
Near the soda water fountain
At the lemonade springs where the bluebird sings
On the big rock candy mountain
Do you remember the late Burl Ives singing that old 1930s hobo song, dreaming of a mountain made of sugar, where life was easy and work unknown? There's a real mountain as outlandish as that, but it's not for lazy folks at all.
We all know about volcanoes—they erupt red-hot lava. But one volcano does not do that. Oldoinyo Lengai makes black lava, as liquid as fresh roofing tar and not really much hotter. It is Earth's only volcano erupting a carbonate lava instead of a silicate one. The more you know about geology, the more mind-boggling Oldoinyo Lengai is.
The Carbonatite Puzzle
Carbonates are a group of minerals that, 99 percent of the time, form in the ocean, coming gently out of solution like sugar crystals in old syrup. Calcite is a carbonate; limestone and marble are composed of it. Seashells are made of another carbonate mineral, aragonite. After a year in a rock lab, geology students develop a gut notion of the carbonate minerals as peaceful things that form at the Earth's surface in places like the Bahamas. So the idea of a carbonate lava—carbonatite—doesn't feel right. And carbonatite is rare enough for the average geologist to ignore.
But over the whole world, there are about 500 known occurrences of carbonatite. That's enough so that theories of geochemistry have to account for it. And what a geochemist notices first about Oldoinyo Lengai is that its lavas have almost no silicon in them—none of the familiar silicate minerals found in ordinary lava, like olivine, like pyroxene, like feldspar. On this planet, which is silicates all way down to the iron core, finding an igneous (that is, erupted) rock without silicon in it is as freaky as taking a water sample from the ocean that has no salt.
An obvious explanation would be that carbonatite must be some sort of melted limestone, made as a body of magma rises into a buried carbonate deposit, but it isn't. We know that from the geochemical details, notably high concentrations of rare-earth elements and other constituents, such as niobium, that have no part in true limestone.
But in fact, you can take fresh water out of the salty sea easily—wherever you find sea ice. As seawater freezes, it rejects its salt, leaving pure water ice behind. Carbonatite somehow separates out of silicate magmas in a similar way. Current theory has it that under the right conditions of temperature, pressure, and chemistry, carbonatite comes out of the silicate melt by differentiation, like cream from milk. Those conditions are most often met where an old, thick continent begins to crack apart and the long-suffering rocks beneath it, after stewing in their juices for millions of years, begin to yield melt. Such places include the African Rift Valley and parts of the extensional terrane of Nevada.
Recent research appears to show that the rocks in the mantle beneath Oldoinyo Lengai are no different from mantle rocks elsewhere. Apparently what matters is that only a tiny amount of partial melting is allowed there, so only the most highly volatile bits are able to become magma. And even then, the first product tends to be a low-silica magma rich in nepheline. It would take another round of melt extraction, maybe more, to yield a decent carbonatite.
When carbonatite erupts, it doesn't last. It has a very low melting point, low enough that you could melt it with a cigarette lighter. And the carbonate minerals tend to react quickly with moisture and wash away. So a volcano like Oldoinyo Lengai is, geologically speaking, a very temporary thing. Its lavas come out black, not red, and within hours their surfaces start turning to white powder and begin to weather away.
Oldoinyo Lengai is in the African Rift Valley, deep in the Maasai country of Tanzania, within the great Ngorongoro wildlife preserve. It's a harsh desert there, and visiting the mountain is recommended only for fit and determined people willing to risk being waylaid by bandits. The nearest water is the bitter Soda Lake, tainted with the highly mineralized streamflows off the volcano. Yet Oldoinyo Lengai is scientifically precious, a geochemical shrine. And its name means "Mountain of God" in the language of the Maasai people.
There are several Web sites about this unique place (the best are Celia Nyamweru's at St. Lawrence University and a picture page at Stromboli Online), but the people behind them don't seem to have deep affection for the volcano. Some of the pictures of it give me the creeps, frankly. Yet the more I learn about this bizarre mountain the more I hear that chorus about the hobo's dream . . . only the singer sounds rather devilish.