2013: Rock

Explain xkcd: It's 'cause you're dumb.
Jump to: navigation, search
It traveled so far to reach me. I owed it my best.
Title text: It traveled so far to reach me. I owed it my best.


Megan either knows enough about geology to tell on sight how this particular rock formed, or has brought this rock from a collection. Alternatively she’s simply guessing. Despite admiring its formation, all she wants is to use it as a skipping stone to give it "a weird day in its life" (similar to 325: A-Minus-Minus), and possibly confuse future geologists.

Megan provides three pieces of information about the rock: It formed at the south pole, during an ice age, just before multicellular life developed. Unfortunately, due to disagreements among geologists and palæontologists about when exactly the first multicellular life emerged it is unclear which time Megan refers to - and consequently where she is and what kind of rock she is holding. There are two possibilities:

  1. The Francevillian biota, living about 2.1 billion years ago, has been proposed as the first multicellular life. If Megan subscribes to this theory, then the Ice age just before would be the Huronian glaciation which extended from 2.4 to 2.1 billion years ago. The land which was at the South pole at that time would eventually become part of Africa.
  2. However, not all scientists accept the Francevillian biota as the first multicellular life. If Megan shares this view the first fossils multicellular life would be only 600 million years old (e.g. in the Doushantuo Formation). In this case the ice age "just before" would be the Cryogenian lasting from 720 million to 635 million years ago. The land occupying the South Pole at the time became present-day Scandinavia and Baltic sea.

Thus — assuming that Megan has accurately identified the stone — the stone is either from Western Africa or Northern Europe and has "travelled" from there to get to her.

Stone skipping is the art of throwing a flat stone across water in such a way that it bounces off the surface. Despite there being many factors attributed to successfully skipping a stone (including the attributes of the stone itself), Cueball and Megan are in agreement that skipping this particular stone five times is an above-average throw. (It is, however, far short of the world record of 88 skips set by Kurt Steiner in 2013).

This comic is one of many that look at everyday things from a new, philosophical perspective.


[Cueball and Megan are looking at a rock that Megan is holding up in one hand.]
Megan: This rock erupted from a volcano near the South Pole when the world was frozen over, just before multicellular life arose.
[Zoom out reveals that Cueball and Megan are standing on the beach of a bay with hills in the background. The water surface is quite flat without any waves. Megan throws the rock which skips 5 times across the water before it sinks.]
Stone: Skip Skip Skip Skip Skip Plunk
[Zoom back on Cueball and Megan who are still looking in the direction she threw the stone.]
Megan: Now it'll be covered in sediment that becomes a new rock layer. It will likely stay buried until it melts down, erodes away, or the earth is consumed by the sun.
[Cueball and Megan still looking the same way.]
Cueball: Today was a weird day in its incredibly long life.
Megan: Five brief skips, then eons of darkness.
Cueball: Five is a lot, though!
Megan: It was a good throw.


Megan throws the rock with her left hand, which supposes that she may be left-handed.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


Wow, 3 paragraphs and still created by a “BOT”. Good self control today, explainers! 😂. If someone does change it, may I humbly suggest it be created by a “rock”? PotatoGod (talk) 19:40, 29 June 2018 (UTC)

Note: this explanation was written by someone who's not very good at skipping stones, if they think 1-2 skips is typical. A single skip is about the same as just throwing a rock in the water! (Just kidding around because it's Friday) Ianrbibtitlht (talk) 20:39, 29 June 2018 (UTC)

Actually, just throwing a rock in the water would be zero skips by Randall's reckoning. The five skips in this comic are followed by a "plunk", indicating Randall does not count the rock's final entry into the water as a skip. Though you did say "ABOUT the same", so technically you're still right. 8-) Redbelly98 (talk) 02:23, 16 July 2018 (UTC)

Anyone know if there could even be a rock that came from a volcano near the south pole when the world was frozen over before multicellular life began, and if so, when would that have occurred? Ianrbibtitlht (talk) 20:40, 29 June 2018 (UTC)

Looking at Geologic temperature record, I suspect even the "world was frozen over before multicellular life began" is pretty bold statement not supported by actual evidence. Existence of specific volcano is likely even less supported. On the other hand, we can't disprove it either. -- Hkmaly (talk) 22:21, 29 June 2018 (UTC)
Likely, she is holding some sort of igneous rock found at one of the locations mentioned above. How she knows the age of the rock is the real mystery. Did she pick it up just now and she is guessing? Was it catalogued in a geoarchaeologic dig site? 03:48, 2 July 2018 (UTC)
Perhaps she knows which formation it is from through colouration / texture / present-day context, and has read about the formation. 23:30, 5 January 2021 (UTC)

"philosophical perspective". Does mean that this is the philosophers' stone?

Only outside the US. These Are Not The Comments You Are Looking For (talk) 12:46, 1 July 2018 (UTC)

I don't think the assertion that the stone travelled from North Europe or South Africa is correct. I read it as Megan is at one of those locations, the stone's journey was from its birth at the south pole to meet her millions of years later and thousands of miles away. 16:58, 2 July 2018 (UTC)

It's also unclear if she means where the south pole is now (that is, the land now called Antarctica, formerly existing at lower latitudes) or where it was then (now existing at lower latitudes). But yeah, the means of mass transport aren't elucidated, where plate tectonics, surface flow such as erosion or orogeny, or human activity like sample shipping -- but it's probably all of the above. All we know is what's happening to the stone at the moments the comic depicts, which is definitely a short-range motion. 23:30, 5 January 2021 (UTC)

He had seen things people wouldn't believe. Attack ships on fire off the shoulder of Orion. He watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments will be lost in time, like skip, skip, skip, skip, skip, plunk! Time to die.

So it's a replicant stone! 10:40, 3 July 2018 (UTC)
Or a finity stone. 23:30, 5 January 2021 (UTC)

Very similar to an old Peanuts cartoon. https://i0.wp.com/www.overheavenshill.com/wp-content/uploads/2016/06/charlie-brown.jpg (deadlink)

How can a stone remain buried until it erodes away? Remaining buried until the Earth is swallowed by the Sun is at least theoretically possible, though highly unlikely given my probably-flawed understanding of geological and astronomical time scales, and remaining buried until it melts down into the mantle seems plausible. But if the stone erodes away, won’t it need to be unburied first? 11:52, 28 November 2018 (UTC)

I think the operative commonality between the three of the possible fates mentioned is that the rock has lost identity as an individual stone -- instead, it is merely part of a sedimentary composite. Now, I suspect a volcanic (or metamorphic if the rock was significantly altered since eruption, which seems likely given the time scales) stone would be harder than the matrix in which it is embedded, and so erosion around it would tend to reveal it before it is eroded away itself; but if it was, for the sake of supposition, about as hard -- then part of it may be surfaced, but the whole stone will not be exposed until the last bits of it have worn to sediment. That is, it would never again be picked up and held or indeed skipped by a creature analogous to Megan. Of course, that's likely for reasons unrelated to spans of time, also -- consider the mass of all biota over the course of geologic time compared to the overall mass of the crust, let alone mantle material! Most rock material is never touched by life, which I think adds to the marvel of how the planet has been transformed by life. 23:30, 5 January 2021 (UTC)