It’s a funny thing, travelling the world as a human being. It’s easy to trick yourself into thinking you’re seeing all there is to see, as a creature of the land making your ways across the sea and through the sky – and yet by volume, you’re missing almost all of the planet we live on. Even if you expanded your awareness into the abyssal oceanic depths and consider the existence of the longest mountain-range on Earth – well, you’re still missing basically all of the stuff of our planet.
Consider this: the surface of our world is around 197 million square miles – while everything underneath it comes to around 260 billion cubic miles. We hardly ever think about that staggering difference in scales, except when some titanic subterranean movement sets the ground shaking beneath us. The Earth below is almost entirely inaccessible, out of sight and therefore out of mind – and it’s so easy to feel it’s hardly part of ‘the world’ at all.
But a few handfuls of curious scientists have been looking, and they’ve been surprised to learn that there are vast blobs of something down there – and they look a bit like trees?
You probably grew up with the classic gobstopper model of our planet in cross-section, layer upon layer. It’s pretty easy to get your head around: we live on the relatively cool outer ring, and the inner parts are 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. Then right in the middle is the core. Simple!
But then there’s the problem of scale. In every cutaway picture you’ve ever seen, the Earth’s crust is thick enough to be visible. In reality, if the planet was reduced to the size of an apple, the crust would be far thinner than an apple’s skin – it’s just 40km of the entire 6,400km to the centre of the planet.
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!
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.
Underneath Hawaii is an enormous stationary upwelling of hot material, cooking the crust from below, and dumping energy into the island chain’s feisty volcanoes.
This has been going on for millions of years – and during that time, the Pacific tectonic plate on the surface has moved along, while this hot, deep “plume” has stayed put and kept blasting its heat vertically upwards. The result can be seen on the surface today – the 6,000-km-long chain of ancient volcanoes, like a series of heat-induced blisters on the ocean floor, with the islands of Hawaii 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 firsthand 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.”)
There’s no manmade probe that can compare to the raw power of an earthquake. 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.”
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.”
Elsewhere in the world, more evidence is piling up for the existence of these kinds 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.”
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 as long as the distance from New York to Chicago?
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 are, quite literally, otherworldly.
So next time you look up into the night sky and wonder what it’d be like to set foot on another planet, it could be that you already have!