Geological time scale, proportionally represented as a log-spiral with some major events in Earth’s history by Jarred C Lloyd for Wikipedia “Geologic Time Scale” licensed under https://creativecommons.org/licenses/by/4.0/deed.en
TOP 10 GEOLOGICAL TIME PERIODS (SPECIAL MENTION)
But wait – there’s more!
I’ve counted out my Top 10 Geological Time Period but there’s clearly more than enough for special mentions with 22 periods formally defined as such, not to mention the longer 4 eons and 10 eras as well as shorter 37 epochs and 96 ages.
Not that I’m going to go through each eon, era, period, epoch or age of geological time. Indeed, my first two special mentions aren’t even geological time periods as such but are intimately caught up with them.
Speaking of which…
(1) EXTINCTION EVENTS
Yes, extinction events aren’t geological time periods as such but are intimately caught up with them and are indeed named for them.
There’s the so-called “Big Five”, of which the most famous (but not the deadliest) is the extinction of the dinosaurs in the Cretaceous-Paleogene (or K-Pg) extinction event – and to which is proposed a sixth big extinction event for our own Holocene Epoch…by us. All the corresponding periods for the “Big Five” extinction events feature as entries in my top ten.
However, there are more theorized extinction events – “estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty” – so more than enough for their own top ten.
(2) CONTINENTAL DRIFT – SUPER-CONTINENTS
Again, continental drift and super-continents are clearly not geological time periods as such but are intimately connected to them and indeed take geological time periods to manifest, given that continental drift averages to something similar to the growth of human hair or nails (1.5-10 cms or 0.6-4 inches per year).
The most well known is Pangaea. While there are only four supercontinents by strict definition of most or all continents as one landmass, by broader definitions there’s enough supercontinents for their own top ten.
(3) PRE-CAMBRIAN (4.567 BILLION – 538.8 MILLION YEARS AGO)
Informally, the time before the Cambrian Period is known as the Pre-Cambrian (Supereon).
I for one endorse this informal usage because let’s face it, there’s nothing much of interest before the Cambrian.
(4) EON – HADEAN (4.567 – 4.031 BILLION YEARS AGO)
Eons are the longest period of geological time, ranging from 536.3 million years (the Hadean Eon) to 1.9612 billion years (the Protozeroic Eon).
The Hadean Eon is the shortest eon of geological time – and the oldest, commencing with the planet’s formation.
Not to mention hellish, consistent with its name. Essentially, the planet had to cool for the conditions for life to emerge. There was also the little matter of the interplanetary impact (with an object theorized as the size of Mars) that produced the Moon and resulted in a magma ocean. Magma ocean!
The average atmospheric temperature was 230 degrees Celsius or 446 degrees temperature – “thanks to asteroids bombarding the planet, a thick atmosphere trapping the heat, volcanic eruptions, constant radiation, and lava as a floor”. Also no oxygen in atmosphere – and atmospheric pressure was 27 atmospheres.
On the bright side, at that atmospheric pressure, water remains liquid even at that temperature, such that it is theorized that eventually a superocean was formed, covering nearly all the planet and turning Earth into an ocean planet.
It is theorized that primitive life emerged from the middle to the end of the Hadean Eon, although If if did, it copped constant extinction events from the bombardment of asteroids every thousand to million years in the Late Heavy Bombardment – with some of them theorized to be bigger than the one that killed the dinosaurs
(5) EON – PHANEROZOIC (538.8 MILLION YEARS – PRESENT)
The second shortest eon and our present one, starting with the Cambrian Period – so essentially the converse of the informal Precambrian. Let’s face it – life on Earth only really gets interesting from the Cambrian Period onwards. That’s reflected in the name of the eon itself – Phanerozoic from the Greek for abundant life.
And yes, I’ve skipped the two intervening eons between the Hadean and the Phanerozoic – the Archean and Proterozoic, even if they are just under 3.5 billion years between them, because they’re mostly boring.
(6) ERA – CENOZOIC (66 MILLION YEARS AGO – PRESENT)
Eras are the second longest geological time period. There’s ten of them – three in the Phanerozoic Eon, three in the Proterozoic Eon, and four in the Archean Eon (none in the Hadean Eon because what’s the point?) – so there’s enough for their own top ten, but only a few are distinctive enough to score special mention, foremost among them our own present era (and the shortest at 66 million years ago to the present), the Cenozoic Era.
The Age of Mammals, yeah! The era of rapid evolution and diversification of mammals and birds after the extinction of the dinosaurs.
This era includes our present Holocene Epoch and the preceding Pleistocene Epoch, as well as quite a few others, from most to least recent – Pliocene (more about that later), Miocene, Oligocene, Eocene, and Paleocene. While there are obviously distinctions between them – most notably the continents drifting towards their modern positions – it’s mammals all the way down, once that asteroid got rid of those pesky dinosaurs.
(7) ERA – MESOZOIC & PALEOZOIC
Eras are the second longest geological time period. There’s ten of them – three in the Phanerozoic Eon, three in the Proterozoic Eon, and four in the Archean Eon (none in the Hadean Eon because what’s the point?) – so there’s enough for their own top ten, but only our present era (and the shortest at 66 million years ago to the present), the Cenozoic Era, and two preceding eras, score special mention.
Otherwise, the Mesozoic Era earns special mention as effectively the era of dinosaurs (251.9 million to 66 million years ago) and the Paleozoic Era as everything before that from the Cambrian onwards (538.8 million to 251.9 million years ago). All the other eras – I mean, who cares, really?
(8) SIDERIAN PERIOD (2.5 BILLION – 2.3 BILLION YEARS AGO)
So now we come to the units of geologic time that are most interesting – the periods formally defined as such. There’s no periods for the Hadean and Archean Eons because who cares, really? They only commence from the Protorezoic Eon onwards.
And they commence with the Siderian Period, the first period in the geologic time scale and of the Proterzoic Period. I mean, still not too interesting but for the Great Oxygenation Event, which saw the irreversible rise of oxygen in the atmosphere, usually inferred to be caused by microbial photosynthesis.
(9) RHYACIAN & OROSIRIAN PERIODS (2.3 – 2.05 & 2.05-1.8 BILLION YEARS AGO)
The second and third periods after the Siderian as part of the Proterozoic Eon.
More of the same from the Siderian Period, really – although the Rhyacian Period may have seen the first macroscopic life while the Orosirian Period saw two of the largest asteroid impacts in Earth’s history.
(10) BORING BILLION (1.8 BILLION – 720 MILLION YEARS AGO)
Yes, the “Boring Billion” (or Earth’s Middle Ages) is a nickname for the Mid-Proterozoic – the Stratherian Period (1.8 – 1.6 billion years ago), the Calymmian Period (1.6 – 1.4 billion years ago), the Ectasian Period (1.4 – 1.2 billion years ago), the Stenian Period (1.2 billion – 1 billion years ago), and the Tonian Period (1 billion – 720 billion years ago).
I’ll give a pass for the Ectasian Period, because the name is reminiscent of ecstasy and prompts images of microbial rave parties. I’ll also give a pass to the Stenian Period for the first fossilized evidence of sexual reproduction.
The Boring Billion is “known for geological and biological stability, characterized by low oxygen, slow evolution (mostly microbes), and calm climate, contrasting sharply with the dynamic events before (Great Oxidation Event) and after (Snowball Earth, Cambrian Explosion).”
However, “recent research challenges this “boring” label, suggesting dynamic tectonic shifts, like supercontinent breakup, actually created crucial conditions for complex life, making it a pivotal time for evolutionary pathways, not a stagnant era.”
(11) CRYOGENIAN PERIOD (720 – 635 MILLION YEARS AGO)
Snowball Earth!
No, seriously – this period is identified as having two glaciations, that may well have extended to the entire planet, although a compromise Slushball Earth (with a band of open sea at the equator) is proposed.
(12) EDIACARAN PERIOD (635 – 541 MILLION YEARS AGO)
Named for the Ediacara Hills in South Australia, where significant fossils were found – of Ediacaran Biota, the first large complex soft-bodied multicellular organisms like jellyfish or segmented sea worms, representing “a crucial transition from simple microbial life to the emergence of animal life (metazoans)”.
The period of life bouncing back from the Cryogenian (huddled around geothermic vents) but before the Cambrian Explosion.
(13) CARBONIFEROUS PERIOD (359 – 299 MILLION YEARS AGO)
The Age of Coal!
Well, not quite – more the age of forests that became coal, hence the name for the period. It’s the intervening period between the Devonian and Permian Periods.
It wasn’t just forests, however – “The Carboniferous is the period during which both terrestrial animal and land plant life was well established. The period is sometimes called the Age of Amphibians”. It also saw the appearance of amniotes – the forerunners of reptiles, birds and mammals. Insects, particularly flying insects, “also underwent a major evolutionary radiation”.
Oh – and everyone’s favorite supercontinent, Pangaea, formed.
(14) PALEOGENE PERIOD (66 – 23 MILLION YEARS AGO)
The period on the other side of the Cretaceous-Paleogene extinction event that killed the dinosaurs and hence characterized by the rise of mammals to top evolutionary spot. Informally known by its former title of the Tertiary Period (which also included my next special mention entry), hence the former abbreviation of K-T extinction event (K for Cretaceous and T for Tertiary), as opposed to the current usage of K-Pg.
(15) NEOGENE PERIOD (23 – 2.6 MILLION YEARS AGO)
As its name indicates, the period after the Paleogene (and before our own period) and pretty much more of the same – “During this period, mammals and birds continued to evolve into modern forms, while other groups of life remained relatively unchanged. The first humans (Homo habilis) appeared in Africa near the end of the period”.
(16) QUARTERNARY PERIOD (2.6 MILLION YEARS AGO – PRESENT)
Our present period of geological time, as well as the shortest, consisting only of the Pleistocene Epoch and our present Holocene Epoch – those two epochs effectively took the place of this period in my top ten (in the two top spots). It largely coincides with the Paleolithic period of prehistory, although the latter is somewhat longer.
(17) EPOCH – PLIOCENE (5.33 – 2.38 MILLION YEARS AGO)
Epochs are sub-divisions of geologic periods from the Cambrian Period onwards, because why bother before then? Apart from our present Holocene Epoch of only 11,700 years, they range from 2.5683 million years (the Pleistocene) to 42.6 million years (the Lower Cretaceous).
As I said in the introduction to my Top 10 Geological Periods of Time, epochs also tend not to be particularly distinctive – with the recurring convention of being the “upper”, “middle” and “lower” parts of their period, for example the Upper Jurassic, Middle Jurassic, and Lower Jurassic.
The distinctive epochs are those of our present Cenezoic Era – indeed, I gave the two top spots of my top ten to our present epoch, the Holocene Epoch, and the preceding Pleistocene Epoch or Ice Age.
However, the Cenezoic Era includes quite a few others prior to the Pleistocene, all distinctive for mammalian evolution, from most to least recent – Pliocene, Miocene, Oligocene, Eocene, and Paleocene.
So why does the Pliocene score special mention above these others?
Firstly, for human evolution – “The Pliocene is bookended by two significant events in the evolution of human ancestors. The first is the appearance of the hominin Australophithecus anamensis in the early Pliocene, around 4.2 million years ago. The second is the appearance of Homo, the genus that includes modern humans and their closest extinct relatives, near the end of the Pliocene at 2.6 million years ago. Key traits that evolved among hominins during the Pliocene include terrestrial bipedality and, by the end of the Pliocene…brains with a large neocortex relative to body mass and stone tool manufacture.”
Secondly, for the setting of the Pliocene Exile Saga SF book series by Julian May – it might not have the fame of Jurassic Park but I like it
(18) AGE – MEGHALAYAN (4,200 YEARS AGO – PRESENT)
Yes – there are ages as units of geological time. They are the shortest units of geological time, of which there are 96 formal ages, ranging from thousands of years to millions of years. Like the epochs of which they are sub-divisions, they are only used from the Cambrian Period onwards
But who cares, really? No one seems to refer to them, other than specialists.
However, because they are a unit of geological time, I have to give them special mention, represented here by our own present age.
(19) ANTHROPOCENE EPOCH
An informal term for our present epoch, whether coinciding with the Holocene Epoch or succeeding it at some point – usually that of the Industrial Revolution or 1780 onwards – to represent the period of time in which humanity has become a planetary force of change.
It’s been rejected as a formal unit of the Geologic Time Scale but has seen wide popular usage – and even when rejecting it as a formal unit, the International Union of Geological Sciences stated that “it will remain an invaluable descriptor of human impact on the Earth system”.
(20) EVOLUTION OF S€XUAL REPRODUCTION
Wait – what? That’s not a geological time period!
I like to reserve my twentieth special mention for a kinky (or kinkier) entry, where the subject permits, so I took that personally – as a challenge where the subject was geological time units.
After all, I had special mentions for the Anthropocene and Boring Billion, but also extinction events and continental drift or super-continents as caught up with geological time periods.
Which led to me to…the evolution of s€xual reproduction, which indeed took place over geological time periods, as far back in the fossil record as 2 billion years ago in the Proterozoic Eon, although the later date of 1.2 billion years ago has also been presented as its origin.
Don’t get too excited though – I think it was algae or something? Phwoah!
What amuses me is the thought of people looking for fossilized sexual reproduction (and what exactly would be fossilized sexual reproduction), which strikes me as being of the same nature as looking through the dictionary for dirty words.
