Editing 3078: Anchor Bolts

{{comic
| number    = 3078
| date      = April 18, 2025
| title     = Anchor Bolts
| image     = anchor_screws_2x.png
| imagesize = 381x326px
| noexpand  = true
| titletext = The biggest expense was installing the mantle ducts to keep the carbonate-silicate cycle operating.
}}

==Explanation==
{{incomplete|The comic was renamed! Finish the [[#Triva]] section, explaining why "screw" is less correct than "bolt".}}

The comic displays a subduction zone. Tectonic plates are plates that divide the Earth's crust. They slowly move across the Earth's surface at the rate of a few centimeters per year, although the rate is nonuniform across plates. When 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 trigger volcanic activity. Earthquakes and volcanic eruptions are typically bad for humanity, and thus ways to prevent them happening would reduce economic risks in those areas. An "anti-subduction anchor bolt" would effectively 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. 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. 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. The temperature of Earth's mantle is around 1000&deg;C near the surface, temperatures at which most commercial stainless steel used to manufacture bolts would experience noticeable strength losses. As such, materials capable of withstanding higher temperatures would need to be utilized. The bolt itself would need to have a length of around 50 km as well. Moreover, as subduction zones move parallel to each other, the construction would have to withstand high shear forces, something that the 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. 

The title text references the {{w|Carbonate–silicate_cycle|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. 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.

At the time this explanation page was set up, the title of the comic was "Anchor Screws". At some subsequent time, the title was changed to "Anchor Bolts". The fastener illustrated is indeed a {{w|Bolt_(fastener)|bolt}}, not a screw.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

==Trivia==
In the [https://www.explainxkcd.com/wiki/images/archive/0/00/20250419063115%21anchor_screws_2x.png 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 [https://web.archive.org/web/20250419024242/https://xkcd.com/3078/ here].

{{comic discussion}}

[[Category:Subduction]]
[[Category:Geology]]