In Search Of The Great Tree Of Fire

"A blob of nightmarishly gargantuan proportions" and other wonders of modern geology.

Hello again! This is Everything Is Amazing, a newsletter about stoking your curiosity, ignoring clickbait, that time when the Earth rang like a bell and magical-looking but wholly natural pillars of light that also aren’t really there?

(Or putting it another way: a lot. It’s a newsletter about a lot. But all grounded in credible science, I promise.)

Right now I’m feverishly working away on something big that I’m launching next week…

…so it’s time for a rerun of something I originally published at the start of 2023 just for paid subscribers, on a topic that gave me something of an imagination hangover after I’d finished researching it.

(Seriously on this: I clearly remember the afternoon I finished this story, and the walk I went on afterwards, during which I sat on the beach, and then laid back flat, my mind expanding until I seemed to feel the whole planet under me with its vast and mysterious structures that scientists are just starting to learn about, as you’ll read below. It made me dizzy enough to dig my fingers into the sand, as if I could prevent myself being flung away into space. I guess all this is a safety disclaimer, but it probably had more to do with the kebab I ate the night before, which didn’t taste right. Not right at all. Anyway, maybe that’s you warned?)

Today, as part of this season-within-a-season for paid subscribers, we’re taking another look under the surface of our planet, at recent discoveries in cutting-edge geology. And really, nothing in the world gets bigger than today’s topic.

It turns out there’s a vast blob of something down there - and it looks a bit like a tree?

When I was growing up in rural Yorkshire, the world looked like this:

You probably grew up with this gobstopper model of our planet as well. (If you want to get the attention of schoolkids, visualising ideas in the form of sweets/candy seems a pretty smart way to go.)

It’s pretty easy to get your head around: we live on the relatively cool outer bit, and the inner bit is incredibly hot and you’d be squashed into a flaming dot in the blink of an eye by all that superheated rock pressing down on you. Simple!

(Also, horrifying! Kids love a bit of horror.)

But then there’s the problem of scale. In this picture, the Earth’s crust is thick enough to see. In reality, if the planet was reduced to the size of an apple, the crust would be thinner than an apple’s skin - it’s just 40km of the entire 6,400km distance to the centre of the planet.

(Scale seems like a huge challenge for scientific illustrators: that tricky balancing act between what would capture someone’s imagination and the frequent nothingness that you’d actually see because things are either too big, or too small, or some appalling combination of both. Case in point: mountains on maps are almost always vertically exaggerated, because otherwise they’d disappear.

But when it’s done to the correct scale, it can be so powerful: I remember a diagram of the true depth of the Titanic in Rob Ballard’s National Geographic piece in 1985, and it looked something like this, which, um - brrrrr.)

Our world plunges to a mighty depth - and to date, humanity’s only drilled through around 12km of it. That’s over a quarter of the way through the crust, which seems (and is!) a lot - but that’s also a bit less than 0.2% of the whole way down. Under one-five-hundredth. Good grief.

It’s not surprising that this is such a lively field of startling breakthroughs and continent-sized question marks. There is still so much that’s unclear and so much being discovered every year…

And nothing illustrates this better than the science of mantle plumes.

The gobstopper model serves its purpose for absolute beginners, but this illustration is more befitting the liquid-like nature of our world.

Noting again the capitalised “NOT TO SCALE” in the bottom right, this is what’s believed to be going on under Hawai’i: an enormous upwelling of hot material cooking the crust from below, and dumping energy into all those feisty volcanoes.

This has been going on for millions of years - and during that time, the Pacific plate has moved along (from right to left in this illustration) while the plume has stayed put and kept blasting its heat upwards. The result can be seen on the surface today - the 6,000-km-long chain of ancient volcanoes, with the islands of Hawai’i at the most recently active end.

Of course, a plausible model is one thing - but finding proof for all this has been incredibly hard. Nothing below 12km deep has ever been seen first-hand by human or electronic eyes.

Beyond this point, it’s all about seismology, the study of earthquakes and the waves of force they send rippling through the planet. (Think “geological ultrasound.”)

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(Nowadays, my go-to experts on this topic are Judith Hubbard & Kyle Bradley of Earthquake Insights. Go sign up if you want regular doses of awe about the geological power of our planet.)

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There’s no manmade probe that can compare to the raw power of an earthquake. As I write this, a 5.6 earthquake has just struck Nepal’s Kathmandu - and surface tremors were felt a thousand kilometres west in New Delhi. These staggeringly powerful vibrations also travel downwards, into the Earth, and can be picked up as they emerge elsewhere. The form they take, and the amount they’ve been disrupted by whatever is in the way, can give seismologists a lot of information about the contents of the crust, the mantle and even the Earth’s core.

In theory, anyway. It’s a field of research grappling with a lot of ambiguities, especially those coming from relying on a small sample set where comparisons are problematic. What’s needed for a really clear reading is a massive cluster of earthquakes all happening pretty much at the same time...

And between November 2018 and April 2022, the motherlode of earthquake-storms struck far below the Hawai’ian town of Pāhala. Over 192,000 of them in total.

By feeding the resulting seismic data into their models, a team of CalTech researchers found, in the words of science journalist Robin George Andrews, “a collection of pulsing magmatic structures — the beating volcanic heart of southern Hawai’i.”

(This topic seems to be Robin’s chosen beat - as indicated by his Twitter handle “SquigglyVolcano.” He’s a former volcanologist, a reliable source of mind-blowing stories and well worth a follow.)

The heart metaphor here is amazingly appropriate: these structures seem to be vast, miles-wide horizonal chambers that respond to the volcanic cycle by either filling up or emptying of magma, analogous to blood pumping through the chambers of a heart:

“This new 3D map of a key segment of the Hawaiian circulatory system “is extraordinary,” said Jackie Caplan-Auerbach, a volcano seismologist at Western Washington University who wasn’t involved with the new study. It’s “awfully cool,” she said, that scientists cannot only see this previously hidden heart, but also can perceive the convulsions of the ventricles within.”

- “‘Mind-blowing’ network of magma chambers found under Hawaii’s volcanoes”, Robin George Andrews, Washington Post

(A part of this I found particularly awe-inspiring: we’re all used to thinking of earthquakes being caused by sudden violent movements of solid rock - but they’re also caused by fluid movements, by pulses of immense energy as liquid rock “sloshes” between regions deep underground. It’s like thinking about the power released by the thousand-mile-long underwater Storegga landslide, as it shoved all that water to one side. Never underestimate the violence of a liquid in full flow. [Update: here’s a more recent example.])

Elsewhere in the world, more evidence is piling up for the existence of plumes. In a landmark study in 2012, scientists focused on the floor of the Indian Ocean, to attempt to map the projected plume emerging underneath the volcanically hyperactive French-owned island of Réunion. By scattering 57 seismometers across the sea bed, supported by 37 seismic stations in nearby islands including Madagascar, the study created a 2,000-by-2,000-kilometer array.

The subsequent report in Nature Geoscience noted that the plume didn’t look like a column after all.

“…a titanic mantle plume “tree” rises from the fringes of the planet’s molten heart, with superheated branchlike structures appearing to grow diagonally out of it. As these branches approach the crust, they seem to sprout smaller, vertically rising branches — super hot plumes that underlie known volcanic hot spots at the surface.”

(That’s Robin George Andrews again there, writing it up for Quartz. I told you this was his thing.)

The origin of many of these plumes appears to be - and I swear this is the word currently being used in the literature - two enormous blobs. There’s one 2,900km under Africa, and there’s another far under the Pacific.

Exactly what these are made of it unclear - but tendrils of them are welling upwards, forming trunk-like mantle plumes that throw off branches that themselves branch off - until in places they reach the surface, like at Réunion, or Hawai’i.

The base of these blobs appears to be sitting at the boundary between the Earth’s mantle and its core. In the memorable words of one writer, they “straddle the core like a pair of headphones.”

Yes, absolutely, if you can imagine headphones covering the distance from New York to Chicago?

(Or maybe you could just have a lie down, or shriek uncontrollably into the middle distance for a while. Whatever works for you.)

So what are these things? Why are they different? Why are they so active?

One bewitching if currently highly speculative theory: they’re fragments of the corpse of Theia, a Mars-sized protoplanet that’s proposed to have struck Earth some 4.5 billion years ago on our planet’s undisputed Worst Day Ever.

In other words, the reason these blobs aren’t behaving like normal Earth material is that they aren’t. They’re from somewhere else. They’re alien.

(Dear fantasy novelists looking for an explanation for why all your exotic magical weapons have ‘unearthly’ powers: behold some hard science to play with!)

As a final mind-exploding twist, these blobs may also explain why ancient Gondwana peeled away from its surrounding continents and maybe even helped create the foundations of the International Appalachian Trail, which I wrote about a few weeks ago:

“Some scientists suspect that plumes from the African giant blob spent at least 120 million years tearing the ancient supercontinent of Gondwana into shards. As the plumes rose into its base, they heated it and weakened it; like moles making hills, they caused the land atop these plumes to dome upward, then slide downhill. Australia was unzipped from India and Antarctica, Madagascar from Africa, and the Seychelles microcontinent from India — an act of destruction that made the Indian Ocean.

Should the plume or plumes beneath East Africa sustain their onslaught, they will contribute to the future disintegration of the African continent: specifically, the breakup of East Africa and the creation of a new microcontinent floating beside the world’s youngest ocean.

But that future tectonic divorce seems insignificant when you consider the catastrophe that may befall the continent’s southern tip. The team estimates that, in tens of millions of years, a blob of nightmarishly gargantuan proportions will pinch off from the central cusp and rise to meet what is now South Africa’s foundations. This…would produce cataclysmic eruptions. The Deccan Traps were caused by what we would think of as a solitary mantle plume. This future mega-blob, though, would be capable of producing volcanism so prolific and extensive that the Deccan Traps would be a firecracker in comparison.”

So that’s a lovely thought to help us all sleep tonight. Thank you, big geology. You’re a real tonic.

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Images: Ben Stanley; macrovector; Joel E. Robinson, USGS; Cedric Letsch; NASA/JPL; Alain Bonnardeaux; Solen Feyissa.