BLACK LAVA isn’t what any reasonable person would expect to find inside their cocktail shaker. But when you are holding it, dangling over the edge of a crater in Tanzania, trying to study the world’s strangest volcano, it is exactly what you want to see.
Stealing lava isn’t usually so tricky. Sure, it is often many hundreds of degrees hotter than boiling water – not the kind of thing to dip your fingers into. But, armed with a decent shovel, you can dig into a lava flow and chuck some into a bucket of water, preserving its geological marvels. “Scooping it up is the easy bit,” says , a doctoral student of volcanology at University College London. Until, that is, the lava is 23 storeys below you, at the bottom of a steep crater.
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That crater belongs to Ol Doinyo Lengai, the only volcano in the solar system known to still erupt carbonatite lava, which has some odd properties. But there is more to Ol Doinyo Lengai than its uniqueness. The black lava it bleeds gives us a window into an otherwise inaccessible bit of our world.
Earth’s mantle is the hot, solid but plasticky part that makes up . It cooks up eruptive matter and moves the surface world to make mountain ranges, ocean basins and continents, and deliver the occasional volcanic cataclysm. Yet we know surprisingly little about it, from how it gobbles up dying tectonic plates to the origins of its furiously hot matter. Ol Doinyo Lengai’s black lava gives us a chance to find answers, but collecting it isn’t easy.
To refer to this volcano as atypical would be an understatement. The lava you and I – and most volcanologists – are familiar with comes in beautiful hues of yellow, burnt orange, amber and eye-popping vermilion. But if you clamber up Ol Doinyo Lengai’s steep summit, about 3 kilometres above sea level, and peer into its active crater, you will see the volcanic equivalent of a Rorschach test. Black fluids will fly into the air and splash across the crater floor, quickly cooling, mingling with the atmosphere and turning a silvery white.
Within the volcano’s ghostly active crater, you can find shadow-hued cones called hornitos. Sometimes 15 metres high, they squirt out grey, brown, beige and black lava in all directions from their multitude of tubes and holes. The reason for these differing colours is unknown, but this lava has other quirks aside from its colour. Molten rock can move at a range of speeds, but on a flat surface, you can generally walk faster than typical lava flows. Not so with Ol Doinyo Lengai’s black lava, which is more fluid than water, moving hurriedly as if late for an appointment.
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The reason for its unusual haste is what black lava contains. Most magma – the hot liquid rock found inside volcanoes – holds a fair amount of silica, a combination of one silicon atom and two oxygen atoms. Silica likes joining up into long chains that form a sort of skeleton for lava – magma that has been expelled from a volcano. The more extensive these chains are, the gloopier and more tar-like the lava is. Ol Doinyo Lengai’s black lava is a , which contains – among other things – a lot of calcium carbonate, the chalky stuff that builds up around your water tap, and sodium, but little silica. This means it lacks the skeletal strength of most other lavas, allowing it to flow absurdly quickly – fast enough, in fact, to occasionally catch up to an unlucky gazelle zooming down the volcano’s slopes.
It is freakishly cool, too. The basaltic lava shooting out of the Cumbre Vieja volcano on the Atlantic island of La Palma, for example, is around , fresh out of the pit, erupts at 500°C, only 400 degrees hotter than the water boiling in your kettle. At times, it cools so quickly that it freezes in mid-air, creating cascades seemingly stuck in time.
The reason this volcano is so unconventional is the same reason that volcanologists are so keen to collect samples of its lava – carbon from Earth’s mantle. A , led by , used an array of evidence to conjure up the volcano’s origin story. They found that the unusual long-term presence of mantle upwellings – superheated fountains that cook the base of tectonic plates, creating magma – have provided the volcano with an excess of carbon, allowing carbonatite magma to form. And a hefty supply of sodium from the deep, which bonds with the carbonatite, allows it to gush out as a lava. Without that crucial element, the carbonatite would break down and get belched out as carbon dioxide.
This bizarre chemistry means that getting samples of Ol Doinyo Lengai’s lava opens a window into a vast portion of the world that usually remains unreachable. It may be far below your feet, but “the mantle matters”, says , a planetary scientist at Washington University in St. Louis, Missouri. “It’s the single largest continuous structure on – well, in – Earth. It’s where at least half the heat from the interior comes from and where much of the volcanic material that shapes the surface originates.” And yet the finer details of its shape-shifting anatomy, including superheated plumes that can open oceans and tear continents apart, remain elusive. “If we’re to fully understand the geological character, evolution and even habitability of our own world, much less those properties of other planets, then we’d best not forget the mantle,” he says.
Aside from a handful of shards originating deep in the mantle that get coughed out of certain volcanic eruptions, and some incredibly rare spots on the surface where bits of ancient mantle have been exhumed by tectonic forces, this realm is impervious to human eyes and hands. Seismometers offer clues as to what it is like, because seismic waves from earthquakes can plunge into and ascend out of the mantle. But these signatures provide nothing unequivocal. Fortunately, samples of black lava can give us some concrete information about this mystifying swathe of Earth’s innards.
The first samples were collected in the 1960s. , which filled the crater with solidified lava, and a few more were collected. But an explosion in 2007 blasted it all away, leaving behind an incredibly steep wall. Now, the lava sits below a daunting drop of nearly 100 metres. No samples were gathered for 12 years, since it had stopped erupting, until Laxton tried in 2019.
“To understand the habitability of our own world, we’d best not forget the mantle”
Abseiling down would be too dangerous. A drone might be able to fly down and take a sample, but the wind at the summit is so strong that the drone could be blown into the crater and lost. “Technology always seems to crap out in front of me,” says Laxton. As a climber, she suspected there was a way to use a system of pulleys and ropes to safely reach the lava, but she would need the help of a pro.
Not long before her expedition, Laxton spotted someone dangling from a large building near her university’s campus in London. He turned out to be experienced climber Arno Van Zyl. They had coffee, she explained her mission, and by the end of it, they were planning their trip to Tanzania. With them came , a volcanologist at University College London, several seasoned Ol Doinyo Lengai climbers, and a team of Tanzanian porters bringing food and water. The heist was on.
Ol Doinyo Lengai’s arcane chemistry – first revealed through – has long suggested to observers that something otherworldly was going on far below the surface. Hints as to the source of this unusual alchemy can be found in the volcano’s vast neighbourhood. It is in a part of East Africa that is slowly but surely being torn asunder by titanic tectonic forces, sitting on a fissure that stretches thousands of kilometres from the Red Sea to Mozambique.
Here, the African tectonic plate is roughly split into two smaller plates: the Nubian plate to the north-west and the Somalian plate to the south-east. Both are moving in opposite directions. Known as the East African Rift, this is where . When this kind of rifting occurs, Earth’s crust thins out, and the planet’s mysterious mantle gets remarkably close to the skin of the world. That can unleash some exotic chemistry and produce bizarre volcanoes – including Ol Doinyo Lengai, the most extreme of outliers.
Once at the summit, it was time to deploy Laxton’scooping devices. It turns out there is nothing better to hold samples of black lava than stainless-steel cocktail shakers and wine measures that refuse to melt in molten rock.
Laxton and her team set up anchor points around the circumference of the crater rim. They linked two of them, creating a tightrope-like pulley system spanning the crater. After the first containers plunged into the lava, disaster struck. Laxton yanked on the pulley too vigorously, causing the rope to break and the cups to fall into the lava.
Geological jerky
Thankfully, this was Laxton’s only scoop failure. The team watched as six scoops containing fresh samples of black lava rose up in turn, air-dried like slices of geological jerky, preserving their minerals and textures.
Those lumpy strips of frozen lava are still undergoing careful analysis, so it is too early to know what they may reveal about the volcano and its enigmatic plumbing. But Laxton and her colleagues hope to use them to decode the volcano’s eruptive cycle. Its crater fills with lava over the course of several years before the summit is rocked by a huge explosion that ejects these molten contents, and then the slow eruption of black lava begins anew. The chemistry of the volcano’s lava, which has been sampled by various teams at different points during this cycle, should help explain why Ol Doinyo Lengai dramatically switches between calm effusion and explosive anger.
The mission to understand this volcano goes deeper. The sorts of samples of lava obtained by scientists like Laxton contain vital clues to the planet’s underworld. Chemical signatures can not only reveal where in the mantle the ingredients for this strange magma originally came from, but also a bit about the cooking process. Did those ingredients shoot straight up into the volcano, or did they get mixed with other matter in the crust? The answers will tell us why the volcano erupts in the way it does.
Now, we are closer to finding them. Partly thanks to our predilection for alcoholic beverages, but largely down to the ingenuity of Laxton and her team, valuable new samples of the rarest of volcanic matter have been obtained. It is a step forward for volcanologists hoping to decode the mantle magic that makes the most eccentric volcano known to science.
Article amended on 9 December 2021
We corrected the comparison between the temperature of the lava and boiling water
