3243: Crystal Gazing

2026-05-28T01:55:27Z

108.44.153.86: the graph is deceptive in having only two clearly visible line ends, but there are apparently three lines intersecting on the graph, not two

{{comic
| number = 3243
| date = May 8, 2026
| title = Crystal Gazing
| image = crystal_gazing_2x.png
| imagesize = 281x397px
| noexpand = true
| titletext = 'Beyond that lies a vale of fire through which my vision cannot penetrate' is the kind of fun thing geologists, heliophysicists, and early universe cosmologists have a lot of opportunities to say.
}}

==Explanation==
A {{w|geochronology|geochronologist}} (someone who estimates the ages of rocks, fossils, etc.) dressed up as a wizard (assuming that isn't how they normally dress) announces — in archaic language befitting his costume — that he has determined the {{w|age of the Earth}} by analyzing {{w|zircon}}. Zircon is a crystal with the formula ZrSiO<sub>4</sub>. During its formation it can incorporate uranium instead of zirconium in its crystal lattice, but cannot incorporate lead. The uranium then decays (via several intermediates) into lead. Thus a sufficiently old zircon crystal will contain some lead, allowing geologists to {{w|Detrital_zircon_geochronology|calculate its age}}. This method is especially reliable, since uranium-238 decays into lead-206 with a half-life of 4.5 billion years, while uranium-235 decays into lead-207 with a half-life of 0.7 billion years, allowing geologists to determine the age even where some lead was lost from the crystal. The geochronologist says he predicted the age of the Earth by gazing into the [[2776|crystal]], similar to the traditional fortune tellers' method of making predictions by gazing into crystal balls.

A prophet of doom is someone who predicts impending disasters, particularly the end of the world (and there are lots of ridiculous ways that [https://what-if.xkcd.com/archive/ could happen]). These prophets and predictions are especially common in fantasy media. The joke in the caption is that someone who can determine when the world began is just doing this in reverse, and that's what geochronologists are doing when they calculate the age of the Earth. His last line, "the beginning was nigh", is a reversal of the phrase stereotypically used by prophets of doom: "the end is nigh". This is in keeping with the idea represented in the comic.

The title text points out, in similar flowery language (in keeping with the character being a wizard from a {{w|J. R. R. Tolkien|Tolkienesque}} or {{w|Dungeons & Dragons}}-type world, where [https://tolkiengateway.net/wiki/Vales_of_Anduin vales] are often part of the geographical language), that many areas of physical sciences have a limiting horizon to their ability to study their subject. {{w|Geologists}} are limited by the early molten history of the Earth, in that most of the crust has been recycled back into the molten mantle at some point, and the mantle acts as a physical 'vale of fire' for the core — each limiting what can be studied. Similarly, {{w|heliophysicists}} have difficulty knowing what happens within the Sun due to the physical 'vale of fire' that is the Sun's {{w|photosphere}}. Finally, {{w|cosmologists}} have difficulty studying the universe beyond the {{w|recombination (cosmology)|recombination epoch}}, beyond which the universe is shielded by a 'vale of fire' (the {{w|surface of last scattering }} from which photons of the {{w|cosmic microwave background}} escape) from electromagnetic observation — both in time (investigating the early universe) or in space (investigating beyond the event horizon of the visible universe).

[[:Category:Dungeons & Dragons|D&D]] is a recurring topic on xkcd.

==Transcript==

:[A character in wizard garb with long beard and hair talks to Cueball. To the left of the wizard is a poster with illegible text over a graph with three lines intersecting.]

:Wizard: By gazing into my crystals of zircon, I have divined the date of the hour of fire marking the limit of this world's existence.
:Wizard: 4.54 billion years ago, the beginning was nigh!

:[Caption below comic:]
:Geochronologists are just reverse prophets of doom.

{{comic discussion}}<noinclude>
[[Category:Geology]]
[[Category:Comics featuring Cueball]]

explain xkcd - User contributions [en]

3243: Crystal Gazing