3078: Anchor Bolts

Anchor Bolts

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Title text: The biggest expense was installing the mantle ducts to keep the carbonate-silicate cycle operating.

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Explanation

The comic displays a cross-section of a subduction zone, with an anchor bolt connecting the two tectonic plates. Sudden shifts in the positions of tectonic plates relative to each other is a major cause of earthquakes, especially the largest ones. Anchor bolts are used to secure an item in place, for instance to attach a building to its foundation. In earthquake prone areas, anchor bolts are often used to secure furniture so it won't slide or jump around during a tremor, potentially creating a hazard. The joke is that, instead of settling for an engineering solution that minimizes the consequences of an earthquake, geophysicists have applied the solution to prevent earthquakes from happening. The caption facetiously expresses frustration that the geophysicists have taken this long to address the earthquake problem at its source. The simple infeasibility of this project is another part of the joke.

Subduction is a geologic process in which two plates of planetary lithosphere converge, and one is dragged under the other. The Earth's lithosphere is divided into tectonic plates. They slowly move across the surface at a few centimeters per year, although the rate is nonuniform across plates. Where they collide, the denser plate gets dragged under the less dense plate, in a process called subduction. Earthquakes are common at subduction zones, and subduction can also lead to volcanic activity. An "anti-subduction anchor bolt" would aim to stop the process of subduction and the movement of plate tectonics as a whole.

A round head bolt is screwed in through both the oceanic lithosphere and the continental crust from the bottom up, with a plain washer on either side, and a wing nut tightened at the surface. Washers are present to prevent the bolt and the wing nut from sinking into the crust, by distributing the forces over larger areas. There are several concerns not addressed in the comic with such a design. The implication that the bolt is being screwed in from the mantle side would imply that a very large bolt head was operated from inside the mantle. (There are types of nut-and-bolt system that might be easier to deploy, such as toggle bolts and mollys. These would have the bolt head on the Earth's surface, rather than in the mantle, and use a spreading "nut" inside the Earth. They wouldn't require conducting enormous operations from below, "merely" a large hole bored from above.) As of the time of posting of the comic, humans have not drilled a hole through a continental crust, still less deployed large vehicles in the mantle. In addition, the presence of wing nuts, fasteners that are designed to be able to be screwed in by hand, implies work done by a larger being that has appendages able to use the wing nut. The bolt itself would be a technological challenge, as well. It would need to be made to withstand the temperature of Earth's mantle, around 1000°C near the surface. At these temperatures, most commercial stainless steel used to manufacture bolts would experience noticeable strength losses. The bolt would need be around 50 km long. Moreover, as subduction zones move parallel to each other, the construction would have to withstand high shear forces, something that a bolt is rather unsuited to compared to other tools, such as rivets. On top of that, ways to alleviate stress must be sought out as if the bolt fails, it could produce a highly amplified earthquake. On top of all this, having secured the bolt, the mantle team would have sealed off their most obvious route of exit back to the surface.

In the short term, earthquakes and volcanic eruptions are typically bad for those living nearby, and thus ways to prevent them happening might reduce economic risks in those areas. However, volcanic eruptions deposit nutrients in the surrounding area, enriching soils. Volcanos also release gases. The vents mentioned in the title text might replenish the nutrients and gases, replacing the benefits of eruptions. Earthquakes sometimes trigger tsunamis, which create or modify beaches, and redistribute nutrients from bays and estuaries across coastal plains. So, while the immediate effects of eruptions and earthquakes can be disruptive, they also enrich the environment. Areas at risk from these "disasters" are also attractive and enriched as a result of these same events.

When plates collide but do not subduct, they often uplift, thickening or raising the crust. The Himalaya mountains, are an example. Tectonic plates spread apart as new lithosphere is formed at ridges, most of which occur under oceans. If spreading continued, but subduction was prevented by the system of anchors pictured in this cartoon, there would likely be new areas of uplift. If positioned appropriately, the mantle ducts, mentioned in the title text, might slow or stop the spreading, reducing uplift.

The title text references the carbonate-silicate geochemical cycle. Briefly, subduction and subsequent heating of the global crust restores carbon dioxide and silicate rocks to the planetary surface, countering the effects of carbonate deposition and silicate rock weathering. Anchor bolts sufficient to stop plate tectonics would also stop the carbonate-silicate cycle, leading to unexpected, and likely unwelcome, changes in the surface geosphere and biosphere. (Arguably, if the carbonate cycle alone could be paused, it might be a means of reducing the amount of carbon dioxide in the biosphere.) To restore the cycle by an unknown mechanism, "mantle ducts" have been installed as part of the planet-wide plate anchoring system. It is stated that the mantle duct installation was the most expensive part of the project, implying greater intellectual and technical challenges than the already-massive ones associated with anchor-bolt design and deployment. It is unclear how these mantle ducts would operate in a way that does not replicate many of the issues that the anchor bolts were intended to solve.

This project would presumably render Beret Guy's Subduction License worthless.

Transcript

[Panel shows Randall's usual illustration of a subduction zone: a tectonic plate subducting from the left side of the panel with water above it, and a mountain range forming on the right side of the panel on the other tectonic plate. Beneath each tectonic plate is the asthenosphere. The main difference between this image and others like it is that there is a bolt shown attaching the plates together in the subduction zone. The head of the bolt is shown in the asthenosphere below the subducting plate. There are two washers displayed, one between the bolt head and the subducting plate and one above the other above the upper plate on the side of a smaller mountain. A wing nut is positioned above this washer, with part of the bolt sticking out above the nut, higher than the tallest mountains in the panel]

[Caption below the panel:]

Good news: Geophysicists are finally installing Earth's required anti-subduction anchor bolts.

Trivia

In the original version of the comic, the caption said "anti-subduction anchor screws" instead of "anti-subduction anchor bolts". The title of the comic was also changed, from "Anchor Screws" to "Anchor Bolts". The original comic image and title can be seen on an archived version of the xkcd.com site. The fastener illustrated is indeed a bolt (with a wing nut), not a screw. A screw has a pointed end and is drilled into a hole that is smaller than the diameter of the screw; the pressure caused by its thread and screw head binds two objects together. A screw does not need a nut to go with it.

A bolt has a flat end, and it goes into a hole that is larger than the diameter of the bolt; it needs a nut which, when fixed onto the bolt and tightened, together with the head creates the pressure that binds the two objects together. Because a nut is used to create pressure, "bolt" is a more correct term than "screw", although it is very common to talk of 'screws' for screw-headed bolts which attach panels (with non-threaded holes) against a substrate which incorporates a 'nut-like' threaded hole within it (or a nut encapsulated and held non-rotating in the backing plate's recess), even though they are also flat-faced at the thread-end and not self-tapping.

Or, for a differently worded definition, see here.

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Discussion

As a kid, I was ALWAYS worried about how plate tectonics would change the continent's layout in a few hundred million years' time, along with how the Sun will die (and maybe consume the Earth if we don't move it) in five billion years. Young me would be SO glad we are finally fixing the first issue. MinersHavenM43 (talk) 03:28, 19 April 2025 (UTC)

Was young you a Superman fan, and did you ever wonder what really happened on Krypton? Scientists and engineers, funded and enabled by a Trump-style politician and his promise to "Stop The Earthquakes NOW!", actually overcame (temporarily) the materials issues and solved the stress equations (see below), and installed a (temporarily) successful planetary plate-anchoring system. Jor-El objected to the project, he and any who supported him were de-funded as a result, and he spent his remaining time ensuring that he could get his son the [deleted] outa there before the accumulated strain ruptured the anchors and blew the planet apart. 162.158.41.45 04:09, 19 April 2025 (UTC)

Did you also worried about the collision with Andromeda galaxy? -- Hkmaly (talk) 05:15, 19 April 2025 (UTC)

I was more sad that I wouldn't be able to see it within my lifetime :( MinersHavenM43 (talk) 21:02, 19 April 2025 (UTC)

Would an anti-subduction screw really work? The tectonic plates are slow, but they are quite heavy, so they have a fair bit of momentum. Indeed, enough to overcome the not inconsiderable friction already present due to the weight of the uplifted portion of the upper plate. Such a screw would therefore need to exert quite a bit of additional pressure to bring the motion to a halt; Exactly how much I shall leave as an exercise for the reader (because I have no clue where to even start trying to work it out), but my guess is that you're gonna need some seriously high tensile strength material for these, even if they are placed at very short intervals along the plate boundary. 172.68.84.172 03:52, 19 April 2025 (UTC)

The screw material could easily be some sort of unobtanium, it would still not work. With the forces involved, the result would be the stone would break around the screws, IMHO. -- Hkmaly (talk) 05:16, 19 April 2025 (UTC)

Looks like somebody got to Randall M. and pointed out that what he drew is a bolt, not a screw. The title and caption of the comic have been edited accordingly. 162.158.41.3 05:38, 19 April 2025 (UTC)

The item depicted is a 'machine screw'; A bolt has a portion of the shaft un-threaded. An actual bolt would likely be more suitable for this application, but it's not uncommon for machine screws to be used instead.172.69.23.21 09:21, 19 April 2025 (UTC)

This appears to be incorrect, as least in the USNA (United States in North America), fans of spurious renaming of political geography. "The [sigh] American Society of Mechanical Engineers (ASME) defines machine screws as featuring a diameter of up to 0.75 inches. While machine-screw diameters can be smaller than this, they can't be any larger, which means machine screws are typically smaller than most other screws." "Machine screws are used with a threaded hole to join two components together, sometimes requiring a nut. Bolts rely on nuts and are fitted through a clearance hole to secure parts together." On this evidence, the illustrated fastener is a bolt. 172.71.142.16 14:34, 19 April 2025 (UTC)

It's hard to be pedantic about this, because nobody agrees on the distinction between a screw a bolt.172.68.245.131 13:24, 21 April 2025 (UTC)

Kinda figures. 'Cause, it seems, no matter how we try, or don't try, to make this bolt fit, we're screwed. [Runs.] 172.71.147.211 14:20, 21 April 2025 (UTC)

This is silly. The global cooperation and engineering required to make this work (I'm assuming unobtanium as a given) far surpasses that required to decarbonise commerce and fix climate change, which project is not going well, to say the least. Neil UK (talk) 08:32, 19 April 2025 (UTC)

I love how we'll give the narrative enough suspension of disbelief to allow for the bolognium we'd need to make this physically work, but actual humans, working together on a global scale project? That's way too unbelievable.172.70.126.168 22:19, 20 April 2025 (UTC)

Yes, but that's the entire reason it's funny. Log of n (talk) 12:03, 21 April 2025 (UTC)

Added an Actual Citation Needed to vulcanism being bad for humanity. Without it, life may not have started in the first place. And fertile volcanic soils would not have sustained us/our predecessors if such life had started. Minor issues like localised danger are surely a blip in time compared to that, and even now there's only a danger to some humans (less than, say, yellowstone erupting, which isn't something a bolt could stop... in fact, drilling the bolthole sounds like it could cause a Yellowstone, if done wrongly (if, in fact, there is a 'right' way)).
...other than that, yeah, go ahead! 172.70.91.181 09:49, 19 April 2025 (UTC)

Sure, but that's not the right way to get that issue fixed. Feel free to rephrase it, or ask people to rephrase it for you in the incomplete tag. --FaviFake (talk) 11:21, 19 April 2025 (UTC)

Did #2 --FaviFake (talk) 11:22, 19 April 2025 (UTC)

Solved the phrasing? I think I can remove the request from the Incomplete, but would want to retain the other expected bit (at least until the whole Incomplete gets removed, which would traditionally be no earlier than some time next week). Looks like there's arguments about this, though. 172.70.58.97 14:20, 19 April 2025 (UTC)

To avoid having to travel through the mantle to insert the bolt from the bottom, wouldn't it be better to use something like a spring toggle bolt in this case? Seems like sort of a drywall situation to me. 172.69.58.9 14:22, 19 April 2025 (UTC)

Or a molly-bolt (which is dangerously close to using a wall plug/rawlplug with a screw... as originally alluded). 172.70.58.156 14:30, 19 April 2025 (UTC)

So what would be the result of this for the oceans (and connected systems)? Assuming that the bolts worked, and that the spreading at mid-ocean ridges was halted (by the vent system installed). The sea floor would not be renewed. Sediment would accumulate on the sea floor, and not be swepet under the rug. What would the long term consequences be? Would we have shallower oceans with less rocky bottoms? What would that do? (To currents, to climate, to marine life, ...) Would sedementation disrupt the circulation of ocean water through the lithosphere, messing with such things as the CO2 balance? 108.162.246.57 20:35, 19 April 2025 (UTC)

Changed "crust" to "lithosphere" in a few places to avoid the popular misconception that the plates consist only of crust, rather than crust plus the upper rigid layer of mantle (upper mantle lithosphere). This misconception might be in the comic too, as the scaling of the bolted 'plates' looks more appropriate to the crust (typically ~10 km thick on the ocean side and ~35 km thick in a continent) rather than the real thickness of the plates of lithosphere (typically ~100 km).Waldronjwf (talk) 16:40, 20 April 2025 (UTC)

It's odd to me that the discussion of anchor bolts talks about holding buildings to foundations, and furniture to the floor, but not the geotechnical engineering application to prevent falling rocks/erosion. An anchor bolt in that case is generally cemented into a hole in the rock, and then tensioned against the face of the hill/cliff to stabilize it.172.70.38.246 13:36, 21 April 2025 (UTC)

And all the state's boltings, and all the state's men,/Couldn't put Humpty together again. Vanity of vanities ... 172.68.22.41 14:32, 21 April 2025 (UTC)

This is basically the plot of the Mike McQuay book Richter 10, except for the book using "spot welding" with buried nukes. 172.71.178.58 10:03, 22 April 2025 (UTC)

Trivia (bolt vs screw)

The Trivia description of screws vs. bolts isn't entirely correct: (1) At the very least, screws aren't drilled in to holes; they're screwed in. They may have a pilot hole drilled before the screw is screwed in (to prevent the screw from splitting the material being fastened), which is what this section appears to be trying to describe, but for most common applications they don't (instead making their own holes as they're screwed in, with no drilling taking place). (2) A bigger issue is the description of screws as necessarily being pointed, and the implication (final sentence of this section) that they're necessarily self-tapping. Many types of screws (eg. machine screws, socket screws, set screws, grub screws, etc. etc.) are neither pointed nor self-tapping. Rather, the difference between screws and bolts is that screws are screwed in (meaning that torque must be applied along a lever radial to the direction of drive in order to drive the screw into the material), whereas bolts are driven in (meaning pressure is applied in the direction of drive in order to drive the bolt into the material) and then torque is applied to a nut to make fast. As has been mentioned both in this discussion page and at the citation in the Trivia section, bolts often have an un-threaded length of shank but screws (usually) don't: this is a *consequence* of the difference between them (viz. that one is screwed in and the other is driven in), not the difference between them itself (it can't be, since bolts don't always have an un-threaded length of shank and some rare types of screws do have one). If "screwed in" is too circular a way to define screws, "threaded-in", "rotated-in", or "torqued-in" seem like reasonable synonyms, and if a succinct defining criterion is needed I suggest "A screw either makes its own thread or makes use of a captive thread and is torqued-in to the material being fastened, whereas a bolt is driven in to the material straight, without being rotated, and then a nut is torqued onto the end of it." (3) Since presumably Randall's anchor bolt was driven in straight, he is correct to refer to it as a bolt rather than a screw. 172.68.186.51 15:21, 23 April 2025 (UTC)

Broadly correct, but fully a quarter of representstive screws on this product category page are unthreaded at the top half of the shank (and some of the others do funny things towards 'the pointy end'), and my experience is that many of the (self-tapping, household) screws I use are like that. For whatever reason. Though I haven't done a fuller survey of what I have, or what proportion of all sub-types do or do not have that. Anyway, obviously, still a valid point in line of how you state it (more so!), but couldn't let that pass. ;) 162.158.216.114 16:20, 23 April 2025 (UTC)

The project could be conducted entirely from the surface, without complicated sub-surface operations, by drilling a hole the diameter of the bolt head - not the bolt shank - and filling it with some high-density liquid (mercury? lava? trans-uranic elements?) to prevent pressure from closing the hole immediately, then inserting the bolt head-first, then extracting the remaining liquid (or possibly even solidifying it, perhaps through cooling or polymerisation?), causing the hole to collapse around the bolt thus securing the head to prevent the bolt from rotating (I suspect the pressure of the rock on the shank would probably accomplish this even if the bolt didn't have a head) and finally the nut could be threaded onto the protruding shank-end and torqued-down. If the shear strength of the bolt is the most likely cause of failure, a considerable torque would need to be applied to increase the friction between the surfaces being mated so as to use the bolt's tensile-strength advantage to resist the shear load and oh goodness I've just been nerd-sniped....) 172.68.186.51 15:21, 23 April 2025 (UTC)

I found out the joke, I think. Anchor bolts protect against tornado damage, not earthquake damage.

MisterAnchorBolt (talk) 01:09, 26 April 2025 (UTC)
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