These are the largest things on the planet
January 13, 2020 11:04 PM   Subscribe

Deep inside the Earth lurk two gigantic blobs. One hunkers far below the Pacific Ocean, the other beneath Africa. Although they float way down at the boundary between the molten core and the semi-solid mantle, they may play a big role in events higher up in the crust, spawning some of our planet’s most spectacular volcanic features and triggering the occasional mass extinction. These enormous subterranean structures are called large low-shear-velocity provinces, or LLSVPs. While scientists ought to be ashamed of themselvers for tagging the monstrous anomalies with such an unremarkable moniker, they’ve more than made up for it by concocting some gripping origin stories and connecting them with more evocatively-named events such as ‘The Big Splat’ and ‘The Great Dying’.

The blobs appear to be hotter and denser than surrounding material, which is why they show up as distinct entities in seismic tomography imagery. This could indicate that they are made of different - and possibly very ancient - stuff.

The first theory holds that LLSVPs are regions known as ‘slab graveyards’ — the final resting places for oceanic plates that were subducted through tectonics aeons ago, back when the mantle was hotter and more uniform, allowing crustal material to sink much deeper than it does today by the same process. Now that the mantle has cooled somewhat and segmented into distinct layers, so the story goes, these ancient slabs have become entombed in the abyss.

A newer theory says they could be vestiges of our planet’s early development, when Earth’s slowly cooling molten interior may have formed extra-dense and persistently hot lumps. Mantle plumes that today bubble up from the depths to erupt at volcanic hotspots like Hawaii, the Galápagos, and Iceland might originate from these masses. This could explain why lava in such locations has unique chemical signatures, including high ratios of Helium-3, a primordial nuclide that predates the formation of the solar system.

Chunks of LLSVPs might also sometimes break off in the turbulence of mantle convection and form what are known as large igneous provinces, as described here in the Large Igneous Province of the Month newsletter for September 2016 (collect 'em all!). At the surface these can manifest as massive floods of basaltic lava pouring across the landscape, such as those at the Deccan Traps in India about 66 million years ago and the Siberian Traps in Russia about 252 million years ago. If those dates look familiar, it's because the Deccan Traps eruptions may have contributed to killing off the dinosaurs and the formation of the Siberian Traps has been linked to the Permian-Triassic Extinction Event (aka ‘The Great Dying’), when almost 90% of Earth's species were wiped out.

And perhaps the most novel - though untested - hypothesis is that the LLSVPs are actually fragments of an entirely different planet, buried inside ours like shrapnel after a cataclysmic impact. This event, known as 'The Big Splat', is also a major component of a leading theory to explain the formation of the moon. The idea here is that a collision between an astronomical body dubbed Theia [RIP, previously] and Earth ejected huge amounts of material into Earth orbit, which eventually coalesced to become our silvery companion.
posted by theory (33 comments total) 85 users marked this as a favorite
The visualization on that wikipedia page is very well done. It immediately makes clear what would otherwise be a fairly hard to picture.
posted by Spacelegoman at 11:43 PM on January 13, 2020 [9 favorites]

"A complete, largely unsens'd World, held within our own [...] waiting for some Summons to Light"

godammit, don't tell me Pynchon's going to be right about this, too!
posted by chavenet at 1:09 AM on January 14, 2020 [4 favorites]

forbidden fondue
posted by poffin boffin at 3:24 AM on January 14, 2020 [9 favorites]

Whatever they are, I think they resemble a colossal centrifuge, with the mass coalescing at two poles.
posted by I-Write-Essays at 3:47 AM on January 14, 2020

Wow, I am reading book three of NK Jamisin’s “Broken Earth” trilogy and let’s just say this is very relevant to my interests right now. Amazing books by the way if you weren’t already aware.
posted by soy bean at 4:34 AM on January 14, 2020 [14 favorites]

Inside of each of us, there are two gigantic blobs.
posted by thelonius at 5:14 AM on January 14, 2020 [13 favorites]

I, for one, welcome our new Fatberg Megazords
posted by oulipian at 6:13 AM on January 14, 2020 [2 favorites]

I think whatever name you come up with, it has to be The Elder [whatever boring name you want] Gods. For example, The Elder Large Low-Shear-Velocity Provinces Gods. There you go, much more interesting name.
posted by NoMich at 6:40 AM on January 14, 2020 [1 favorite]

Great post! Now to entertain and inform myself by diving into the links...
posted by Agave at 6:44 AM on January 14, 2020

I hadn’t read much about this kind of things for years. They have much better models today than I’d seen before. Interesting—good post.
posted by Gilgamesh's Chauffeur at 7:27 AM on January 14, 2020

Thank you for posting this! I hadn't heard about this and wowie.
posted by LobsterMitten at 8:13 AM on January 14, 2020

This is really cool, thanks.
posted by PMdixon at 8:17 AM on January 14, 2020

Princess Leia’s Side Buns would be a pretty great name for a sockpuppet account.
posted by rhamphorhynchus at 8:23 AM on January 14, 2020 [2 favorites]

scientists ought to be ashamed of themselvers for tagging the monstrous anomalies with such an unremarkable moniker

That's the whole problem with science. You've got a bunch of empiricists trying to describe things of unimaginable wonder.

posted by Greg_Ace at 8:48 AM on January 14, 2020 [7 favorites]

A related large-scale structure is the Geoid, which is a depiction of the variation of density in the Earth's volume. Or the variation in local gravity, if you prefer. It's more or less the shape would be if there were enough water that the whole planet was underwater, and if there were no tides, etc. The resulting shape isn't a sphere, but a lumpy potato. It has practical significance because you need to know the local geoid correction to calculate GPS altitude (above sea level) correctly.

Given the LLSVPs are denser than the surrounding rock I thought they'd correlate to strong geoid corrections. But just eyeballing it they're different shapes, in different places. Why is that? They must be related somehow!

While looking for answers to this question online I ran in to an interesting paper Why are the Geoid and the Areoid so similar?. Areoid is Mars' version of the Geoid. They measure the Areoid directly, then infer the presence of Martian LLSVPs based on that, and then point out some interesting similarities in the geology of the two planets. They also throw out a hypothesis that Mars' LLSVPs were formed in a catastrophic collision.
posted by Nelson at 8:50 AM on January 14, 2020 [3 favorites]

ooooh this is an amazing post!!! I cannot wait to dive into these links after work today!

I consider myself a pretty well informed layperson when it comes to this sort of stuff (I obsessively read about it) and I had not heard of these LSBs before...very fascinating.
posted by supermedusa at 9:00 AM on January 14, 2020

Thanks to the first link for the word “blobettes”.
posted by freecellwizard at 9:03 AM on January 14, 2020 [1 favorite]

I'm not getting how blobs under the Pacific and Africa can be responsible for Iceland's volcanoes. But otherwise, very interesting.
posted by sjswitzer at 9:24 AM on January 14, 2020

And my understanding was that Iceland wasn't a hotspot, it's just above the spreading of the Mid-Atlantic ridge, aren't hotspots defined as isolated plumes?
posted by tavella at 10:24 AM on January 14, 2020

Iceland is both! It's a hotspot that coincides with the spreading plate boundary.
posted by LobsterMitten at 10:41 AM on January 14, 2020 [1 favorite]

Huh. I'd think that it's not exactly "coincides", especially since the map of hot spots in wikipedia has a whole line of them following the mid-Atlantic ridge. Is there any evidence that those hot spots move separately from the ridge? In the way that the hotspots in the Pacific move separately from the plate above them? That's how I've always thought of hotspots -- volcanic plumes that are entirely separate from the plates drifting above them. I'd expect volcanic plumes at spreading plate boundaries.

Either way, even the most northwestern blob of the African LLSVP seems well away from Iceland, so expansion on how the lava would be from there would be coming up in Iceland would be interesting.
posted by tavella at 10:59 AM on January 14, 2020

As Spacelegoman points out in the first comment, the wonderful Wikipedia animation (in the form of a gif) of the LLSVPs inside a spinning Earth is essential to get feel for this.

There are some really intriguing features of the LLSVPs visible in that gif.

For example, there's a strikingly circular structure sitting on the core at the Equator in the mid-Pacific which looks for all the world like a popped bubble, sitting right next to something which could be a bubble which hasn't popped yet, which is in turn next to a circular arc at an edge that looks like the remains of another popped bubble — all of which, together with the surprisingly ramified and ragged surfaces of the LLSVPs in general, gives the impression that we're seeing a freeze frame of a viscous liquid at full boil. Which would raise so many interesting questions.
posted by jamjam at 11:15 AM on January 14, 2020 [2 favorites]

The header of the Quanta article actually has a better gif, larger and the colors are less murky.
posted by tavella at 11:22 AM on January 14, 2020

Let me modify that somewhat, the LLSVPs look to me a lot like the forms you get when you pour a liquid into a larger volume of another liquid when the temperature of the first liquid is above the boiling point of the second liquid (or possibly some component of the second liquid), such as pouring water into liquid nitrogen.
posted by jamjam at 11:49 AM on January 14, 2020

This post says so much about why I love Metafilter. Thank you theory!
posted by churl at 12:57 PM on January 14, 2020 [1 favorite]

That gets to something I've sometimes pondered. Is a rock more of a "thing" than a cloud or a candle flame? It's really all a question of timescales. Here we're seeing something that's richly dynamic, but at imponderable timescales. It's quite fascinating.

(But to the extent that these things might create plumes that occasionally erupt and produce ridiculous numbers of cubic miles of flood basalts... also sometimes in very ponderable timescales.)
posted by sjswitzer at 1:17 PM on January 14, 2020

>Either way, even the most northwestern blob of the African LLSVP seems well away from Iceland, so expansion on how the lava would be from there would be coming up in Iceland would be interesting.

tavella: I didn't want to overburden the post with links, but that's a question I had too. Here's a paper that goes some way towards addressing it [pdf]. It seems that the African LLSVP is indeed considered the likely origin of the deep mantle plume that is expressed at the surface as the Iceland hotspot, and that most deep mantle plumes are believed to originate from LLSVPs. From the paper:
Over the last 300 Myr the majority of plumes appear to have originated at the edges of two pronounced LLSVPs beneath Africa and the Pacific Ocean (Doubrovine et al., 2016; Burke et al., 2008). The root of the Iceland plume lies near the tip of the African LLSVP
It turns out that deep mantle plumes can migrate due to convective currents in the mantle. In the case of the Iceland plume:
The models show that over the last ∼ 100 Myr a remarkably stable pattern of convergent flow has prevailed in the lowermost mantle near the tip of the African Large Low-Shear Velocity Province (LLSVP), making it an ideal plume nucleation site.
posted by theory at 2:27 PM on January 14, 2020 [1 favorite]

Interesting fact I found while poking around trying to understand hotspots/plumes: not only do the LLSVPs produce nearly all of the flood basalt areas, most kimberlite pipes come from them too. So periodically it cracks the skin of the earth with vast seas of magma, and shoots out supersonic pipes of kimberlite eruptions at the crust. I'm getting a picture of a dead zombie Theia still trying to claw its way back out of the earth...
posted by tavella at 3:06 PM on January 14, 2020 [1 favorite]

My roommate in grad school was a seismologist. I bet she is geeking the fuck out over this!
posted by Kitchen Witch at 5:45 PM on January 14, 2020 [1 favorite]

It boggles my mind that people have developed techniques to image structures and currents thousands of kilometers below the crust. If you'd told a late-20cen geologist that what would their laugh have sounded like?
posted by away for regrooving at 11:05 PM on January 14, 2020 [1 favorite]

(Now I read the article and see it was imaged in 1984. Well.)
posted by away for regrooving at 11:09 PM on January 14, 2020 [1 favorite]

If you'd told a late-20cen geologist that what would their laugh have sounded like?

Reflection seismology was invented over 100 years ago. It was first used for seismic prospecting, mostly to detect underground structures related to petroleum.

Whole earth studies really took off in the 1960s as a side effect of the Cold War. During discussions of strategic arms limitation treaties it was realized that the USSR and US needed methods to determine if anyone was cheating on restrictions to atom bomb tests. So billions of dollars were given to geo scientists to put in a worldwide network of seismometers to detect nuclear blasts. This network from the 1960s provided a wealth of data about the deep 3D structure of the earth using earthquake data.
posted by JackFlash at 8:18 AM on January 15, 2020 [5 favorites]

Seconding nomich. How did scientists pass up the opportunity to name these the "Elder Blobs"?
posted by lollusc at 2:30 PM on January 19, 2020 [1 favorite]

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