The Geological Time Scale - an introduction

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Prologue

I have been a Teaching Assistant for the course ES412: Sedimentary Processes and Stratigraphic Principles this past semester, and I have enjoyed the experience tremendously. It was an undergraduate course designed to give a foundational understanding of sedimentology and stratigraphy to the students through both lectures and labs. For me, it was also a chance to revisit how a student begins to think like a geologist. I think perhaps this is where undergraduate teaching gets tricky - it is both an advantage to find a blank slate, and a responsibility to fill it appropriately. No single course can fill the slate completely, but one does hope to lay the proper framework and instigate the student's curiosity so he/she searches for the details themselves.

One of the topics that must be invariably introduced through a course on Stratigraphy is the concept of the Geological Time Scale (GTS) - the big chart that condenses all we know about the Earth's past. A first glance at the GTS is quite daunting, there are rows and columns and symbols and dates and an inherent hierarchy of terms. It's all very convenient to have everything in one place - for geologists. For a beginner, however, it may be too much to take in (certainly was so for me!). A simpler way of understanding and remembering things is described here. It is a rather crude way of looking at the GTS and some of it will distinctly resemble mugging up facts, but I think it helps. I also intertwine the whole post with etymological trivia because I think it helps develops the "story" (and words rock ๐Ÿ˜„).

Please note that the full version of the GTS is deliberately not inserted at this point. I hope to demonstrate that it's more fun to build it up by oneself. ๐Ÿ˜Š

The Divisions

Just like our dates are expressed in a hierarchical manner - day, month, year, which can be extended to century and millennium - geological time is also divided using various hierarchical terms. It is useful to remember the sequence E - E - P - E - A which stands for Eons - Eras - Periods - Epochs - Ages. The Eons are the largest divisions of geological time, which are then divided into Eras, which are further subdivided into Periods, and so on. It is important to remember this hierarchy.

The Eons

We begin with the name of the 4 eons - IN ORDER (this is important to note). The oldest time is described under the Hadean, and the most recent under the Phanerozoic.


  1. The term "Hadean" is supposed to refer to the "hellish" conditions on Earth during that eon (from "Hades", the Greek God of the underworld). The Earth was in a nascent stage of formation and conditions were very harsh - high temperatures, no atmosphere and frequent bombardment by other bodies precluded any possibility of life.
  2. The term "Archean" has a Greek root which means "beginning". The Archean was recognized before the Hadean, and when the term was first coined, this eon was naturally thought to be the oldest.
  3. "Proterozoic" derives from protero (former, earlier) + zoic (life). Rocks of this eon show evidences of primitive life.
  4. "Phanerozoic" derives from phanero (visible, manifest) + zoic (life). Rocks of this eon show fossils from complex life-forms.

The Eras

The subdivisions of the Eons into the Eras are based on very simple nomenclature using Greek roots - eo (earliest), paleo (old, ancient), meso (middle), neo (new) and ceno (from kainos, recent).


Hence, our scale now develops into:

From an introductory point of view, the Archean and Proterozoic eras aren't as important as are the ones from the Phanerozoic. However, they're very easy to remember and also reinforce the simplicity of the nomenclature based on the prefixes.

The Periods

We focus only on the subdivisions of the Phanerozoic periods for now. These can be remembered using the following sequences:


The sequences look formidable, but the stories are very interesting!
  1. The Paleozoic Era has 6 subdivisions which can be remembered with C - O - S - D - C - P.
    • Cambrian - "Cambria" is another name for the country Wales, which houses prominent exposures of rocks of this age.
    • Ordovician - The Ordovician is named after the land of the "Ordovices", a tribe that lived in Wales.
    • Silurian - Like its predecessor, the Silurian is also named after the region occupied by a Welsh tribe called "Silures".
    • Devonian - The Devonian rocks are called so because they were first studied from the British region called "Devon" or "Devonshire".
    • Carboniferous - This name is a merger of carbo (coal) and fero (bearing) and reflects the fact that many coal beds were formed globally during this time.
    • Permian - This name is based on the Russian city of Perm.

      [ At this point, it is easy to notice that a lot of research into Paleozoic rocks must have been carried out in Britain, since 4 out of the 6 names are based on British localities. ]
  2. The Mesozoic Era has 3 subdivisions which can be remembered using T - J - C.
    • Triassic - A series of three distinct rocks - red beds, marine limestone, and terrestrial mud and sandstone can be seen in Germany and NW Europe. This series is called "the Trias" and is the basis of the name of the Triassic Era.
    • Jurassic - The Jurassic is named after the Jura Mountains within the European Alps.
    • Cretaceous - The Cretaceous is derived from the Latin root creta which means chalk, and is named so because extensive beds of calcium carbonate or chalk were deposited in this period.
  3. The Cenozoic Era has 3 subdivisions which can be remembered with P - N - Q.
    • Paleogene - This name derives from paleo (old) + gene (to arise / something produced).
    • Neogene - This name derives from neo (new) + gene.
    • Quaternary - An older classification system based on rocks in the Alps used terms like Primary (crystalline rocks), Secondary (rocks of the Mesozoic), Tertiary (which has now been replaced by Paleogene + Neogene) and Quaternary. Only the last term is formally recognized today.

Hence, the timescale now looks a bit like this:

The Epochs and the Ages

It's a bit unrealistic to attempt to learn all the epochs and ages in the GTS. Geologists tend to be best acquainted with the finer subdivisions according to the time period they study, and I find it best to not choose some eras / periods to focus more on.

At this stage, we're ready to tackle the full GTS, but keeping one key thing in mind -- we've been looking at names in a chronological sequence but that's not how they appear in the rock record. When a geologist goes to the field, he/she sees the oldest rocks at the bottom and the youngest at the top. To mimic this natural order, the GTS also shows the oldest time periods at the bottom and the younger ones on top. (So everything may seem "inverted" at first glance!)

The best place to download the GTS is from the website of the International Commission on Stratigraphy.

Source
Some key points to note:
  • The oldest time periods are at the bottom right, the youngest at the top left.
  • The GTS keeps getting updated as and when different working groups find more detailed information about the geological past. The version is indicated on the top right of the image.
  • The yellow pin-like symbols denote GSSPs, implying boundaries are defined in rock successions. The clock symbols (upto the Meso-proterozoic) denote GSSAs, implying boundaries are defined at fixed time points.
  • There is a LOT of information on the GTS, but as a minimum, I recommend everyone remembers the following dates: (this is subjective, opinions may vary)

Epilogue

I hope this post helps beginners to familiarize themselves with the GTS without getting intimidated at any point. To me, the GTS represents a beautiful compilation of some of the most impressive feats of geologists and is no doubt, a very handy tool to study and use.

My apologies for the rudimentary figures. Almost all of the facts in this article are based on Wikipedia articles and the Online Etymology Dictionary.
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