Episode 12 – The Geologic Time Scale

Listen to Episode 12 on PodBean, iTunes, or Stitcher!

This episode, we discuss the history of our planet, and how scientists have converted the complex rock record into the iconic Geologic Time Scale.

In the News:
The Shapes of Bird Eggs
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Evolution of the Strangest Amphibians

Two fossils from Colorado reveal the ancient ancestry – and surprising relationships – of the world’s most mysterious group of amphibians, the legless caecilians. [Report]
More Dinosaurs With Sensitive Faces
It turns out spinosaurs and tyrannosaurs aren’t the only ones with super-sensitive snouts. This feature seems to be common in dinosaurs, and there may be many reasons for it. [Blog Post]
Hippo History
Fossil teeth from Ethiopia reveal details of hippos’ rise to dominance in Africa, including a newly-discovered transitional species and a tight connection with the rise of modern grasses. [Report]

The Geologic Time Scale

Earth has a long and detailed history, and just as with the long and detailed Tree of Life, we humans need a way to organize all that information so that we can discuss and study it.

And so, scientists developed the Geologic Time Scale.

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The Geologic Time Scale in all its glory. Image: USGS

The time scale is an organized timeline of Earth history. The divisions of the timeline (Eons, Eras, Periods, Epochs) represent sections of time with specific geological or biological conditions. The boundaries between time periods are usually associated with major changes, such as mass extinctions or global climate shifts.

The Rock Record

The Geologic Time Scale is a human invention – we made up the names and we chose where to place the boundaries – but it is a visual representation of the real-life timeline recorded in the rock layers of our planet.

Picture1
The Grand Canyon is a great example of extensive stratigraphy. From bottom to top, the layers follow a long (slightly interrupted) stretch of prehistory. Photo by Luca Galuzzi, Diagram: USGS

Every layer of ancient sediment represents an environment from the past. If you know how to look for it, you can find clues about ancient life, climate, landscape, continental movement, and more. Up and down the rock record, the layers change, revealing changing conditions over time. By comparing and correlating rock layers from around the world, geologists can put together a full understanding of the order of events from the beginning of our planet’s history until the present.

Once we’ve lined them up, the stratigraphic layers of Earth’s crust read like the pages in a book, recording the originations and extinctions of various groups of life, the movement of continents, the rise and fall of mountains, the opening and closing of oceans, the warming and cooling of climate, all documented hundreds of times over in sedimentary pages all over the globe.

The end result of all of this is an intensely-detailed story of the history of our planet – and the details are always being refined! Dive into Earth history with this interactive Time Scale, or this one!

Also, you can listen to the Ages of Rock song that Will mentioned!

How Old Is It?

Whenever a new fossil or unique rock layer is uncovered, we want to put it into context: where does it fall on the Time Scale? There are two broad methods of finding the answer.

Relative Dating means finding where our new discovery falls within the order of events in the past. Do the events of this new rock layer occur before, during, or after particular events in the past? (See, we’re figuring out its age relative to other events).
Biostratigraphy is the practice of using fossils for dating. Certain species or groups of life only existed during certain times in history – you never find humans alongside trilobites, for example – so the fossils in a particular rock layer can narrow down the possible age of the layer.
Magnetostratigraphy is the science of reading the prehistoric record of changes to the Earth’s magnetic field. As time has gone on, varying conditions of our magnetic field have corresponded with particular time periods.
By lining up these and other approaches, we can say, for example, that our new rock layer happens before this mass extinction but after this tectonic shift, and during this magnetic anomaly, and this lets us place the layer in its rightful period on the Time Scale.

Index_fossils_2
Index fossils – from widespread but short-lived species – are often very specific to certain time periods. Image: USGS

Absolute Dating is the process that lets us place numbers on our Time Scale. This usually means finding a natural process that occurs at a predictable rate, and calculating how long it has been since a rock or fossil was formed.
Radiometric dating is the most famous form of absolute dating. Certain elements break down (decay) over time at a known rate. If we measure how much break-down has occurred within a rock or fossil, and we know how long that break-down takes, we can tell the age! Carbon dating is a well-known form of radiometric dating, but there are many others as well.
-Elemental decay isn’t the only chemical process we can use in this way. Various techniques can look at the side effects of radiation, changes to electron activity, or the changing shapes of DNA molecules to calculate age in a similar way.
-In some cases, reading age is as simple as counting layers, such as the annual growth rings in a tree (this is called dendrochronology or tree-ring dating) or seasonal sedimentary layers under a lake.

Every dating approach has its strengths and limits, but the most important feature they all share is that they support each other. Most of the time when scientists are trying to date a new discovery, they will use as many dating approaches as possible.

If the fossils of your layer line up with fossils found in the Albian stage of the Cretaceous Period, and Potassium-dating identifies an age around 110 million years ago (the age of dated Albian rocks elsewhere in the world), and the rocks reveal a reversed magnetic field (which is known to have occurred during the Albian), then you have great confidence in your age: Three lines of evidence corroborating each other!

There are many, many dating techniques. Wikipedia has a few good lists of them here, here, and here.

Patron Question

At the end of the episode, we took a question from one of our Patrons, Sam! He asked about air sacs in dinosaurs. For some cool images and diagrams of how air sacs work in birds, look here.

For more from us, follow Common Descent on Twitter, Facebook, and Patreon!

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