3120: Geologic Periods
| Geologic Periods |
 Title text: Geologists claim it's because the earlier Cenozoic used to be called the Tertiary, but that's just a ruse to hide the secret third geologic period, between the Neogene and the Quaternary, that they won't tell us about. |
Explanation
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This is one of 52 incomplete explanations:
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This comic depicts a table representing planet Earth's geological time scale. For each period, Randall highlights his rather idiosyncratic likes and dislikes among their characteristics, instead of accompanying each geological name with facts pertinent to it, such as the duration of the period represented, the state of the Earth (e.g., glaciated), or the flora and fauna most common.
| Period and date (MYA) | Randall's comments | Explanation |
|---|---|---|
| Precambrian (4500–539) | Life develops | The Precambrian (italicized in the comic since it's not a geologic period) is the first 88% of Earth's history, including the time 4.1 to 3.4 billion years ago when life on Earth began. |
| Snowball Earth episodes | According to the Snowball Earth hypothesis, during some time spans in the past Earth became nearly or entirely frozen, with no liquid water on the surface. This is similar to the Icehouse Earth, including now, when the planet fluctuates between glacial and interglacial periods. | |
| Cambrian (539–487) | Trilobites! | Trilobites are related to present-day insects, crabs, and other arthropods, and appeared during the Cambrian. |
| Evolution could stand to calm down a little | The Cambrian explosion was a sudden radiation of complex life forms when nearly all important animal phyla, or precursors to them, appeared. Randall apparently thinks it all happened a little too fast. | |
| Ordovician (487–443) | Earth might have had rings | Due to the location of impact of one type of meteorite, they may have been part of a planetary ring system around Earth, formed by the destruction of a parent body that entered Eath's Roche limit, before colliding with it. |
| Scary volcanic eruption in North America | The volcanic eruptions that deposited layers of ash during the Late Ordovician were incredibly large. The volcanoes involved may have been formed during the mountain-building event in what is now north-eastern North America. | |
| Silurian (443–420) | First land animals | Green plants first became established on land during the Ordovician period, after having evolved ways to protect themselves from desiccation and ultraviolet light. During the Silurian, land animals (mostly arthropods resembling millipedes) followed the plants. |
| Earth's newfound mold problem | Mold soon evolved to attack them and decompose their remains. A "mold problem" often refers to mold growing in damp places in a building, causing unpleasant odors and various negative health effects. | |
| Devonian (420–359) | Big mountains in Boston | A section of the present-day Appalachian Range from the Canadian Maritimes to the Carolinas, including what is now the Boston area of Massachusetts, was created during this period. (At the time, Boston was in the tropics, just south of the equator.) |
| Yeah, sure, what those giant killer fish needed was armor | Placoderm fishes, which were common in the Devonian, had plates of dermal bone in the head and thoracic portions of the body. Not all placoderms were giants, or apex predators. These fishes likely had these bony plates because they helped protect them from predators. | |
| Carboniferous (359–299) | Cool forests | Forests in the Carboniferous lacked the flowering plants and conifers that are common in present-day forests. Instead, forests were dominated by giant versions of today's club mosses, horsetails, and ferns, as well as by several plant lineages that are now extinct. Artists' depictions of such forests are exotic-looking and considered "cool" by Randall. |
| Bugs too big | Carboniferous 'bugs' included the largest-ever known land invertebrate, a 2.6 m millipede-like animal; the largest-ever known flying insect, resembling a dragonfly with a wingspan of 75 cm; and a 70 cm scorpion. | |
| Permian (299–252) | Pangea | Pangaea was the most recent supercontinent containing nearly all of Earth's landmass. |
| Google "The Great Dying" | The Great Dying occurred at the end of the Permian and is the most severe of Earth's 'Big Five' mass extinction events. In it, 80% of marine species and 70% of terrestrial vertebrate species were wiped out. | |
| Triassic (252–201) | Tanystropheus | The Tanystropheus was an Archosauromorph with a proportionally unusually long neck (as depicted in the comic). |
| Damage to Canada still visible from space at Manicouagan | Manicouagan Reservoir is a ring-shaped lake, the remains of the crater caused by a 5 km (3 mi) asteroid hitting Quebec. | |
| Jurassic (201–143) | Birds | The ancestors of modern birds emerged during the Jurassic. Randall has repeatedly found it cool that birds are modern-day dinosaurs. |
| Parasitoid wasps | Parasitoid wasps reproduce by implanting their eggs into still-living animals, whose bodies are then eaten from the inside out by the wasp's larva. It is such a grisly process that it caused a crisis of faith among 19th-century European scholars. | |
| Cretaceous (143–66) | Raptors | Raptors, especially Velociraptors, are a trope within xkcd, especially in its early years. They were popularized by their appearance in the Jurassic Park film series. In the films, velociraptors are depicted as small (shorter than adult humans) bipedal scaled dinosaurs which frequently attacked and killed humans. Velociraptors and the irrational fear of being attacked by them in the modern world are a subject of several xkcd strips. Thus, "raptors" appears in both the "Favorites" and "Complaints" columns of the table. |
| Raptors | ||
| Paleogene (66–23) | Pretty horseys!!! | Fossils of members of the horse family first appear during this period. Horses, fossil and extant, are prime examples of charismatic megafauna ("Pretty horseys!"). The rapid diversification of horses from a presumed single common ancestor is an oft-cited example of mammalian adaptive radiation. |
| Paleocene-eocene thermal maximum | The Paleocene–Eocene Thermal Maximum was a time when the global average temperature rose by 5-8 °C in a relatively short period of time. It can be viewed as a slower version of climate change, which Randall has repeatedly complained about. | |
| Neogene (23–2.6) | Forests of Dracaena dragonblood trees | Dracaena draco and Dracaena cinnabari trees are a source of dragon's blood, a naturally occurring bright red resin used as a varnish and a dye. |
| Zanclean flood | The Zanclean flood is theorized to be the flood that refilled the Mediterranean Sea. | |
| Quaternary (2.6–present) | Burrito invented | Randall jokes that, in the last 2.6 million years, his favorite moment was the invention of the burrito, rather than many other, much more significant discoveries. The precise origin of the burrito is not known, but the Maya civilization made food resembling burritos as early as 1500 BC. |
| Whoever picked the name for the third period of the Cenozoic | The third period of the Cenozoic Era is the Quaternary ("Fourth"), named by Jules Desnoyers in 1829. Randall is riffing on the cognitive disconnect between "third" and "fourth", for which the current geological naming conventions offer no justification. |
The title text expands on the complaint about the "third" v "fourth" discrepancy in regard to the Quaternary Period by postulating the existence of an unnamed geologic period within the Cenozoic Era "that geologists won't tell us about". In fact, the use of "Quaternary" (and "Tertiary") in recent/current geological nomenclature is a relic of four centuries of the history of geological studies in Western Europe, complicated by the religiously-inspired acceptance until the beginning of the 19th century, among European scholars, of an Earth that was only 6000 years old. "Primary" rocks were those considered to have been present in mountains before the "Great Deluge" of Noah (the Genesis flood narrative), while "Secondary" rocks represented the rubble from the Flood. Igneous and metamorphic rocks came to be understood as "primary", and eldest (within the context of a 6000-year-old Earth), and sedimentary rocks as "secondary". More recent geological formations came to be known as "tertiary" (relatively newer) and "quaternary" (even more recent). As the idea of Earth being billions of years old gained acceptance and we invented tools for dating rocks, "primary" and "secondary" fell away as descriptors for both rock types and rock ages, replaced by terms that convey information about the rocks more accurately. "Tertiary" was applied to "Cenozoic minus Quaternary", and survived in formal nomenclature into the 21st century. No alternative for "Quaternary" has yet been accepted, so the name persists as fodder for cartoonists who wonder, not without cause, how a "third" element in Earth history could be labeled a "fourth" element.
Transcript
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This is one of 27 incomplete transcripts: Don't remove this notice too soon. If you can fix this issue, edit the page! |
- [A table with 3 columns, labelled: "Period", "My favorite part" and "My biggest complaint". There are 13 rows below the labels]
- [Row 1: Period:] Precambrian [My favorite part:] Life develops [My biggest complaint:] Snowball Earth episodes
- [Row 2: Period:] Cambrian [My favorite part:] Trilobites! [My biggest complaint:] Evolution could stand to calm down a little
- [Row 3: Period:] Ordovician [My favorite part:] Earth might have had rings [My biggest complaint:] Scary volcanic eruption in North America
- [Row 4: Period:] Silurian [My favorite part:] First land animals [My biggest complaint:] Earth's newfound mold problem
- [Row 5: Period:] Devonian [My favorite part:] Big mountains in Boston [My biggest complaint:] Yeah, sure, what those giant killer fish needed was armor
- [Row 6: Period:] Carboniferous [My favorite part:] Cool forests [My biggest complaint:] Bugs too big
- [Row 7: Period:] Permian [My favorite part:] Pangea [My biggest complaint:] Google "The Great Dying"
- [Row 8: Period:] Triassic [My favorite part:] Tanystropheus [accompanying the text in this cell is an image of a Tanystropheus and its characteristic elongated neck, with Cueball standing next to it for scale] [My biggest complaint:] Damage to Canada still visible from space at Manicouagan
- [Row 9: Period:] Jurassic [My favorite part:] Birds [My biggest complaint:] Parasitoid wasps
- [Row 10: Period:] Cretaceous [My favorite part:] Raptors [My biggest complaint:] Raptors
- [Row 11: Period:] Paleogene [My favorite part:] Pretty horseys!!! [My biggest complaint:] Paleocene-eocene thermal maximum
- [Row 12: Period:] Neogene [My favorite part:] Forests of Dracaena dragonblood trees [My biggest complaint:] Zanclean flood
- [Row 13: Period:] Quaternary [My favorite part:] Burrito invented [My biggest complaint:] Whoever picked the name for the third period of the Cenozoic
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Discovered this explanation fresh off the griddle. The transcript doesn't even exist yet wow. Also, hi! This is my first time commenting! Did I do it right? Giraffequeries (talk) 22:54, 25 July 2025 (UTC)
- (Yes, looks like you did.)
- A couple of hours on, and nobody's attempted the Transcript yet. If you're still around right now-ish and you've got more time than everyone elses seems to have (including me, sorry), that could be your next thing.
- Check prior Transcripts for the right kind of way (and a few wrongs, but hey?), and imagine the words+'markup' being read through the hypothetical screen-readers. That might not know how to 'audible' a table, may at best shout/stress bold-strong/italics-emphasis, but perhaps not correctly.
- But just getting the words down helps the next soul with a few more minutes at hand. Any normal weekend, I'd be happy to do it right now, but I've got to be up in five hours, and I mildly regret just checking right now to see if I might have missed the latest comic popping up when it was a bit earlier and I was prepping my weekend bags. :) 82.132.236.123 01:49, 26 July 2025 (UTC)
- Darn, forgot to say my intended actual personal comment I was just going to add. i.e.: Looks like Randall hasn't forgotten about Raptors! Anyway, goodnight/early-morning (my time).... 82.132.236.123 01:53, 26 July 2025 (UTC)
I've done my part. Also, Raptors TPS (talk) 02:20, 26 July 2025 (UTC)
Did anyone else google "The Great Dying"? How about "Manicouagan"? How about "Picture of a dinosaur eating a burrito" (just to prove Randall wrong)?
- Ask and you shall receive (shitty AI pic made in five seconds): https://imgur.com/a/4lVKoqD 2A02:2455:1960:4000:1972:32FB:7958:52D3 18:30, 26 July 2025 (UTC)
Who wrote this? Tanystropheus wasn't a dinosaur! 70.115.234.146 03:28, 26 July 2025 (UTC)
Given that this wiki doesn't really like tables, and this one is formatted rather simply, maybe we should just transfer its content to the transcript and retain only explanations in the main part, as separate paragraphs? The Rooster (talk) 08:41, 26 July 2025 (UTC)
- What are you talking about. This wiki loves tables! And it is used extensively. --Kynde (talk) 09:07, 28 July 2025 (UTC)
Yeah I'm not sure what to do for transcript, comic 2627 is in similar style but not sure if we necessarily want it like that.--Darth Vader (talk) 09:18, 26 July 2025 (UTC)
Just a small explanation to the Quarternary/Tertiary naming issue because this has become rather obscure and is seldomly spelled out in newer geology textbooks: When the first geologists came up with a table of geologic epochs it consisted of four parts: "Primary", "Secondary", "Tertiary" and "Quarternary". Only the last two names have survived into our time, because the first two parts became split up into the systems that we still use today rather quickly. This is also the main reason that most stratigraphers want to get rid of the terms Tertiary and Quarternary and why Paleogene/Neogene were invented instead. 2003:DD:472A:5500:35F8:1CD8:D274:E286 14:46, 26 July 2025 (UTC)
Burrito? Surely not, everyone knows its "soft toilet tissue" 2A00:23C8:252D:A301:B573:A9F2:E80C:711B 10:08, 27 July 2025 (UTC)
I don't know why he doesn't like the Zanclean flood. It would have been a spectacular sight had anyone been around to see it.2A02:8388:1701:E100:60D1:5BC3:D420:5528 16:44, 27 July 2025 (UTC)
Should the explanation of the Zanclean flood have some sort of reference to 1190: Time? Morgan Wick (talk) 00:38, 28 July 2025 (UTC)
- That would be obvious and could explain his dislike mentioned above. Lots of animals would have died during this event. --Kynde (talk) 09:07, 28 July 2025 (UTC)
I have deleted my previous comment, as it was in response to text in the Explanation that no longer exists. These Are Not The Comments You Are Looking For (talk) 03:00, 30 July 2025 (UTC)
Randall is wrong - parisitoid wasps are very cool. 82.13.184.33 08:14, 28 July 2025 (UTC)
The Quaternary is obviously so called because it's the period of Quatermass. 82.13.184.33 09:46, 28 July 2025 (UTC)
If y'all disagree with my docutainment interpretation, feel free to erase it again of course, but the common denominator in Randall's table here really is each period's entertainment value as a focal point. Also, I'd like to add that it's not a bad thing at all for a documentary to be entertaining; quite the opposite. For example, I might never have studied geography if it wasn't for films about interesting foreign places and cool-looking books about volcanoes and dinosaurs and weather, all of which I devoured in my childhood. PaulEberhardt (talk) 20:54, 28 July 2025 (UTC)
- Whatever the inspiration, Randall has been using the table as a comic device for a powerful long time, see the category Charts on this wiki, and i.a. the comics 181: Interblag (20061108) and 394: Kilobyte (20080310). 2605:59C8:160:DB08:8552:7338:3C0A:5AFC 06:34, 29 July 2025 (UTC)
And thus Velociraptors return after (according to the velociraptor category) 12 years of silence, all hail the great Deinonychus! Xkcdjerry (talk) 12:13, 20 August 2025 (UTC)
Isn't the Quaternary also called "Anthropocene"? It's a really anthropocentric name, but the definition of the period is "since the advent of man". You can't get more anthropocentric than the Quaternary, or Anthropocene! -- 2a04:cec0:1207:653f:1ce3:a4ff:feb2:a5fe (talk) 16:31, 22 August 2025 (please sign your comments with ~~~~)
- The Quaternary is a geologic period, from 2.58 million years ago to the present. The Anthropocene has been proposed (but never yet officially adopted) as an epoch being from various points 'post-Holocene' (an epoch that started after the start of the greater quaternary period), the change-over being as late as the Trinity nuclear test (as the moment at which technogenic radionuclides started to be added to the environment). We are currently in the quaternary period and the subordinate epoch of your choice. Either still the holocene, or maybe at some point (best left to be argued with far more hindsight, presuming anyone cares to) we entered the anthropocene...
- Or, by the time anyone can be sure, they will be far more sure that every anthropocentric idea of dividing up geologic time is critically wrong and there's a much better alternative method of dividing it all up, with completely different names. ;) 20:44, 22 August 2025 (UTC)
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