Main Page
Welcome to the explain xkcd wiki!
We have an explanation for all 3180 xkcd comics, and only 52 (1.6%) are incomplete. Help us finish them!
Latest comic
| Geologic Core Sample |
 Title text: If you drill at the right angle and time things perfectly, your core sample can include a section of a rival team's coring equipment. |
Explanation
This image presents a core sample taken by a slightly chaotic team of geologists.
A core sample is a cylindrical piece of something, in this case the rock of Earth's crust, obtained with special drills in order to see the layers within. In typical xkcd fashion, the core sample depicted here contains a mix of real rocks found in core samples alongside many humorous or fictional additives. In addition it's shown that the coring drills have hit many, many obstacles on the way down they really shouldn't have impacted, culminating in a punchline the geologists have drilled straight through the Earth to the opposite hemisphere, far deeper than any core sample could be taken in reality.
- Topsoil
- Topsoil is the uppermost layer of the typical pedosphere, which often needs to be dug through before reaching actual rock. Whether the soil-core would actually be retained, and counted, is up to the nature of the study being made, but it will become clear that this core-sample wasn't obtained with much thought of finesse.
- Till
- Till is unsorted glacial sediment, which might underlie the soil layer and form the transition to the foundation rock below.
- Granite bedrock
- Bedrock is solid rock, and there may normally be nothing but more bedrock beneath it until the Earth's mantle. Granite is a very common igneous rock that could normally form such bedrock.
- There are suggestions, from what is seen beneath it, that this particular layer of bedrock (though being a natural material, at source) has been placed here as a construction base, infilled over with the above layers in this particular spot.
- Bottomsoil
- This is a presumably fictional counterpart of topsoil, on the basis that they are both soil somehow sandwiching the bedrock layer. Theoretically, however, the above granite (as a slab) could have been laid upon the lowest layers of the excavated area, later to be sampled by this corer as if a natural layer.
- Roof/Floor of subway car
- These two layers are indications that the drill has broken into a subway tunnel and through a subway train, possibly from amongst those shown in 1196: Subways, which will have been dug deep into the rock or perhaps cut'n'covered into the ground (hence the anomalous granite added above later). The drill has essentially compressed the 'void' that is the interior of the car and the rest of the tunnel, which may seem to be good luck (given a later layer), but this still doesn't bode well for the subway train that may have been trying to move when the drill started to pierce it.
- More granite
- Granite is a very common igneous rock.
- Municipal water main
- A pipe has been partly sliced through (enough to one side to not force the collapse of its void). Most water pipes of this size would not normally be forced through rock, only the loose material above it, relying upon pressure to carry water upwards, where necessary.
- However, drainage systems (that rely upon gravity for most of the route) may need at times to be dug deeper to maximize the natural flow. Some particularly large projects may be excavated deeply through rock, even below some subway lines, though they'll be tunnels/pipes with a far larger bore than seen here, for both construction and capacity reasons.
- Slightly different granite
- There are potentially many subtypes of granite, as well as being a lot of it.
- Piece of screaming spelunker's arm
- Cave systems exist underground in many places, though more usually within rock-types more likely to dissolve than granite layers are. The main exception might be from volcanic tunnels left in basalt, but that's technically still not granite, meaning that any cave system here would need explaining.
- Spelunkers (also known as cavers) explore caves, and one must have been in the wrong place when the corer passed through, being inflicted a clear injury possibly greater than any that the unknown (but not directly impacted) subway users might have already suffered. If the spelunker was not already screaming before the drill came through (perhaps for help, if they were stuck, the size of the cave is unknown with the open space closed up as with the subway), losing a chunk of arm will have definitely prompted screams.
- Cool crystals with no resale value
- There are many geological processes that can concentrate elements and compounds in a way that form crystalline minerals. Some are useful as ores, others as just the crystals themselves (for aesthetic reasons or otherwise).
- Whatever these crystals are, as a small seam within the granitic layer just below the spelunker's location, they look nice (or are otherwise interesting), but either have little further application or are just so common that there's no point trying to make use of this deposit. Even if they could perhaps be more 'easily' reached by any spelunker not put off by the threat of drillbits.
- Mangled fragments of drillbit from previous attempt
- When coring rock, it's possible for the tip of the coring drill to encounter problems (like particularly dense and hard rock) that damage it, perhaps by bending its track too much and shearing off the head.
- This latest attempt, probably sent down slightly to the side of the prior one (unless it had managed to gouge out just the remains of the previous drillpipe, and retain the rock/subway/spelunker layers previously cored out) has encountered the tip of the prior attempt.
- If there's one thing guaranteed to be as tough as a drill-bit, it's another drill-bit, which must necessarily be hard enough to cut through the expected rock-types. So it's lucky that the first one was clearly damaged enough, by its prior encounter, that it didn't thwart this next attempt and (perhaps literally) grind it to a halt. Nor, apparently, was there a repeat of whatever issue left that first drill like this.
- These bits being in the middle of the core it could be they are meant to be from another attempt to drill the diameter of the Earth from a complete different location. Assuming absurd precision all such drill holes would meet at the center of the Earth.
- Some boring intrusive rock that's basically granite but has a name like "diorite" or "andalite" that you always have to look up
- Again, it's perhaps one of those granites. Diorite is a real type of igneous rock, an intermediate between actual granite and gabbro, but an Andalite is an alien from the Animorphs book series, which Randall enjoys and has referenced before. Perhaps Randall is "misremembering" the name of andesite, another type of igneous rock, from his knowledge of the complex set of reality-inspired rock-types encountered in the game Dwarf Fortress. As the sample appears right before the Netherrack sample, it may also be referencing Minecraft as well, as granite, diorite, and andesite exist in-game as a mineable stone type, but all three types are often infamous for clogging up inventories whilst mining, due to their exclusively decorative use.
- Netherrack
- A dark red, and entirely fictional, stone appearing in Minecraft, with which Randall is also well acquainted. In Minecraft, Netherrack typically only appears naturally in The Nether, an alternate dimension resembling hell. In the overworld, where the core sample is presumably being taken, Netherrack only naturally forms in ruins of Nether Portals found on the surface, so for it to appear this deep in the sample, it would need to have been placed there by another person, likely while mining.
- Balrog wing
- The balrog is a creature in Lord of the Rings, found deep beneath the world, awakened when the dwarves delved too deep and too greedily, and previously encountered in 730: Circuit Diagram. The balrog's wings are often discussed upon, in the context of whether it had them, therefore whether they could or should have helped it escape the fall that was forced upon it in the books. At least one balrog, however, now appears to have at least one less wing than those it previously had, without us knowing if there was also any screaming involved.
- Granite
- This label is applied to rock that appears to cover both ends of a 'height' of rock-core that is simplified by a diagrammatic cut. From the context of later layers, this would include a very long length of drilled-material that passes into the Earth's mantle, and perhaps at least some of its core, before coming back up through the granite to be found somewhere on the other side of the planet.
- Topsoil
- Beyond the indeterminate length of granite, it transitions back into the loose upper layers, indicating that it the exploratory core is now being taken from ascening layers, albeit in a location lacking subways, etc, or just managing to miss everything originally seen.
- Cement
- This indicates and heralds the presence of a building, starting with its foundations.
- Floorboards
- Carpet
- These two layers are typical of a reasonably well-equipped residential building, probably the ground floor without any basement level. The core is coming up inside a furnished room.
- Possessions of a confused and angry homeowner in the other hemisphere
- The core sample has tunneled up into somebody's house, probably while they are there, and has traveled through some of the furniture, fixtures and/or fittings, to their clear unsettled annoyance.
Apart from the less expected elements to the core that was cut and retrieved (and the sheer impossibility of drilling the necessary several thousand miles 'down' through the Earth, and then drawing that sample back out again), the comic heavily plays upon the fact that someone with the ability and equipment to take this sample is yet not as sure about geology as they perhaps ought to be, with almost all rock just being considered 'granite', without any better (or more accurate) qualification.
The title text humorously refers to a "rival team" and their coring equipment — implying that (with the correct angle) you can meddle with their own coring experiment. This is, outside of cold war-type pettiness, not considered a constructive approach to science.[citation needed]
Only about 15% of the earth's land surface is directly antipodal to other land, which would making this sample less than "typical" if it was aimed directly down through the exact centre of the Earth and back up again (a distance of almost 8,000 miles or more than 12,000 km). If one were to do this, from a random spot of land, one would be much more likely to have the sample terminate in an ocean and the chances of ending up in given house would be even lower. Though considering that the title text mentions drilling at an angle, the 'other hemisphere' point might be not necessarily be at the antipodal point, and there also seems to be the capacity to aim at a more desirable target. In which case, this is a "typical" core under deliberately chosen circumstances. The exact nature of reaching "the other hemisphere" is not expanded upon, it could be as simple as drilling (mostly sideways) a short distance across the equator, or prime meridian, or have to go at least a quarter of the way under the planet's surface (slightly over 1.4 times the Earth's radius, by the most direct route), in any direction, such that the two ends cannot be counted as being in any single arbitrary hemisphere.
This comic comes not long after 3162: Heart Mountain, which involved strange stratification, so may be part of the same thought process about the nature, and occasional oddities, of the geologic column.
Transcript
- [This shows a drill sample with various labels, in order from the top of the panel toward the bottom.]
- [Against a short section of core:] Topsoil
- [A more obviously granular shorter section with a diagonal transition:] Till
- [A light and lightly-marked phase:] Granite bedrock
- [Repeating the Topsoil appearance:] Bottomsoil
- [A short/squat and possibly squeezed 'lump':] Roof of subway car
- [A similar squeezed-out lump:] Floor of subway car
- [A longer length of the 'granite' texture, within which...:] More granite
- [Not quite half of a pipe-width, cut out as a gap perpendicular and not quite all the way across the core:] Municipal water main
- [Slightly more grainy version of the 'granite':] Slightly different granite
- [In a junction between 'granites', an squat, unidentifiable lump:] Piece of screaming spelunker's arm
- [Within a longer granite layer, a short stretch of spiky/crystalline features:] Cool crystals with no resale value
- [The same granite, an intrusion of mechanical-looking junk:] Mangled fragments of drillbit from previous attempt
- [As per granite, but slightly more grainy:] Some boring intrusive rock that's basically granite but has a name like "diorite" or "andalite" that you always have to look up
- [Dark, cobbly textured stone:] Netherrack
- [Within a stretch of granite, a short, dark but otherwise unidentifiable lump:] Balrog wing
- [At this point, there is a discontinuity indicating that an arbitrary length has been omitted. The sample then resumes:]
- [Still 'granite': Granite
- [Dark soil texture:] Topsoil
- [Light, fine and sparse 'grains':] Cement
- [Two very short cross-sections, each with contrasting wood-grain stripes:] Floorboards
- [Two very short sections with a textile-base plus piles appearance:] Carpet
- [A mish-mash of 'stuff', possibly including cloth, metal components, grainy wood and 'topped' at the lowst end by something equally puzzling at an angle:] Possesions of a confused and angry homeowner in the other hemisphere
View comic discussionNew here?
Last 7 days (Top 10) |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
You can read a brief introduction about this wiki at explain xkcd. Feel free to create an account and contribute to the wiki! We need explanations for xkcd comics, characters, What If? articles, and everything in between. If it is referenced in an xkcd comic, it should be here.
- If you're new to wiki editing, see the explain xkcd:Editor FAQ for a specific guidance to this wiki and the more general help on how to edit wiki pages. There's also a handy wikicode cheatsheet.
- Discussion about the wiki itself happens at the Community portal.
- You can browse the comics from the list of all comics or by navigating the category tree at Category:Comics.
- The incomplete explanations are listed here. Feel free to help out by expanding them!
Rules
Don't be a jerk!
There are a lot of comics that don't have set-in-stone explanations; feel free to put multiple interpretations in the wiki page for each comic.
If you want to talk about a specific comic, use its discussion page.
Please only submit material directly related to xkcd and, of course, only submit material that can legally be posted and freely edited. Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.
If you need assistance from an admin, post a message to the Admin requests board.
