Posts Tagged With: Geological Time Scale

Historical Geology Timeline Activity

Some time ago I dreamed about the perfect tool for teaching historical geology. It would be an interactive timeline that students could update with events, and it would even have the ability to allow students to quiz themselves.

While that tool has yet to materialize, I did come up with a low-tech solution that permits the plotting of geological events that occur over widely varying timescales. I created four timeline forms to fit 8.5” x 11” paper. The forms cover different intervals and scales of geological time. They are:

  • Timeline 1: The Precambrian (4.5 billion years to 0.5 billion years ago
  • Timeline 2: The Paleozoic Era (540 million years to 250 million years ago)
  • Timeline 3: The Mesozoic and Cenozoic Eras (250 million years ago to present)
  • Timeline 4: The Cenozoic Era (last 10 million years of the Cenozoic Era)

The forms have divisions of the geological timescale along the bottom, and a numerical timescale across the top.


Click here to download timeline forms.

So here you go, but you should probably know…

Plotting geological events on the forms can be tricky. Some events have well-known dates, but some don’t. Events may occur in a geological instant, or take hundreds of millions of years. I provided a sample timeline to give students an idea of what to do in different cases, and how to express uncertainty.


An example of how to plot events

Even with those considerations handled, the students who used this exercise were looking up dates for themselves, and that added another layer of complexity. I prepared a handout to guide them through some additional challenges.

Challenge 1: Different sources or different pages in the textbook give more than one date for an event.

Expect that this will happen. Sometimes a source will be general (akin to saying World War 2 happened in the twentieth century), and sometimes it will have more specific information (it began in 1939). Ideally, the interval on the timeline should reflect the most specific information available (i.e., the most specific date or the narrowest range of dates). This makes it much easier to see how the timing of one event compares to the timing of another. We could plot the appearance of both the Ford Model A and the Ford Mustang as single bars covering the twentieth century. But if someone who had never seen or heard of an automobile before looked at our timeline, they would lack the context to understand that some evolution had taken place.

Keep in mind that geologists themselves may not know the exact dates of a particular event. The boundaries of the timescale can also shift if new information comes to light, so an older source might put the same event at a slightly different time than a new one.

In the end, students shouldn’t get hung up on finding exact dates (because mostly they won’t be able to), but they should be trying to get the dates in the right ballpark, and trying to make that ballpark as small as possible.

Challenge 2: The textbook or other source mentions a geological time interval that isn’t at the bottom of the form.

The forms have increments of the geologic timescale (eras, periods, and epochs in some cases) at the bottom, but these intervals are in fact subdivided into smaller ones. For example, the Devonian period is divided into Early, Middle, and Late epochs. The Late Devonian is divided into the Frasnian and Famennian ages. To figure out where a division belongs, consult the Geological Society of America’s Geologic Time Scale.

Challenge 3: The source says an event happened in the “upper” or “lower” of some interval.

When talking about the timescale, periods are sometimes divided into Early, Middle, and Late. These are official designations for the timescale. In contrast, when talking about the rocks themselves, a layer may be referred to as “Upper Devonian.” “Upper” and “Lower” refer to the position of a bed in a stack of rock layers. Older beds are lower down, and younger beds are higher up. While careful decisions have been made about the ways in which uppers and lowers fit into the timescale, a rough approximation for this exercise would be to treat “Upper” the same  as “Late,” and “Lower” the same way as “Early.” A less-rough approximation would be to look up the ages of the units in question.


Ways to use the timelines

My students used the timelines to plot events listed in assignments. Some students printed the sheets and wrote directly on them. Some students added to the timelines using drawing software, or using the mark-up tools within Adobe Acrobat. But if you wanted to go all out, you could print the forms out in mega-huge format, post them on the classroom wall, and stick on images or text to mark events.

Or you could turn them into PowerPoint backgrounds and have students build presentations on top of them.

Or you could make them SUPER-mega-huge in drawing software, decorate them all up with whats and whens, then print a poster.

Whatever you do, send pictures!

Categories: Assessment, Challenges, Science and such, Teaching strategies | Tags: , , , , | Leave a comment

How to make sense of historical geology

Imagine that someone changed the clock on you, breaking the day into irregular blocks, and giving the blocks names and symbols in no systematic way. Now imagine that you are given a list of events to memorize- activities of people you don’t know at places with which you are unfamiliar:

Fantasy clock with radiolarians

No, they’re not aliens. They’re radiolarians. And aren’t you glad you don’t use this to tell time?

During the Early Fizz, Pierre Bezukhov and Cthulu squared off on Callisto. By Middle to Late Fizz, Pierre Bezukhov had the advantage, so Cthulu migrated to Kore. At the Fizz-Zoot boundary, Dmitry Dokhturov and a shoggoth appeared on Europa, but both went extinct by the end of the Zoot, likely due to a lack of habitat. Beginning in the Flap, Callisto, Europa, and Taygete began a collision that culminated in their merger by mid-Flap. Land bridges that formed allowed the migration of Anna Mikhaylovna Drubetskaya from her original habitat on Taygete, leaving a niche open, and allowing Nyarlathotep to diversify.

Now stuff that in your head so I can ask you about it on an exam, in no particular order.

If you are familiar with the moons of Jupiter, the characters of War and Peace, or the fiction of H. P. Lovecraft, then you might have a chance at remembering some of the names, but their relationships would probably be new to you. This is the scenario faced by students taking introductory historical geology.

The clock they have to work with is the Geological Time Scale– a way geologists have of carving up Earth’s 4.5 billion years of history into chunks that reflect key events or phases. The chunks are not the same size, and there are chunks within chunks. The exact dates when each chunk (or sub-chunk) starts and ends are moving every few years as geologists get better information about the timing of key events that define the boundaries. There is no system- you just have to memorize it, and you’d better do it in a hurry, because everything you will learn about the Earth’s history will be described in terms of the Geological Time Scale.

Aside from learning this new clock, students must also learn the names of extinct and extant organisms, the names and histories of various continents and oceans, extant or otherwise, and the geological processes that have influenced those organisms, continents, and oceans. Did I mention that students usually have to learn a range of dates, because we can’t be sure of the actual date, and/or because the event happened over millions or hundreds of millions of years? Oh, and one more thing- the dates of different events will overlap to varying degrees, and the story lines will be almost impossible to disentangle from each other. But don’t worry- the exam is multiple choice.

The obvious way to organize all of this information is a timeline, and most textbooks have a version of the Geological Time Scale with some dates and key events marked on it. The problem is that to construct a timeline with all of the information that students need, you would have to devote a book to that alone, so most of these are just Geological Time Scales with some pretty pictures attached. The Geologic Time Spiral (below), showing Earth history spiraling away from the beginning of time, is a classic, and fascinating to look at, but of limited use to my students.  The durations of the events pictured are gross approximations, there is no description of those events, and there is no sense of the spatial changes that occurred. The timeline also glosses over the multiple story lines in Earth history, and the complex interconnections between story lines.

A spiral diagram illustrating the evolution of life on Earth through geological time

Geological Time Spiral: The names of units within the Geological Time Scale are written along the edges.

How to fix it

Make it adapt to scaling

What’s needed is a timeline in electronic format, but not just any timeline- it should be a scalable timeline. Users must be able to zoom out to see big-picture, long-term history, or zoom in to see the finer details. It would be the temporal analog to Google Maps, where the details which appear, including the divisions of the Geological Time Scale itself, depend on the scale. This would solve the problem of the necessarily limited amount of information in current timelines, but it would also do something more important. Users would be able to easily go back and forth between scales to understand how events are situated in a broader context. This is what you do every time you are planning a route to a new address- look at the larger map of the city to see the main thoroughfares, then zoom in to the streets within a particular neighbourhood, then zoom out again to remind yourself where the neighbourhood is relative to the freeway. Then you might zoom in again to the exact address, and depending on the tool you are using, you might look at a picture of the building that you are headed to.

Show cause and effect relationships

In Google Maps, you can see how streets are connected to each other. In Earth history, individual story lines are interconnected in the same way, and the complexity of city streets is probably not a bad analogy for the complexity of these interconnections. The scalable timeline would also show branches that link one story line to other stories, so a user could follow a single timeline, or choose to follow a branch and see how another series of events was impacted by the first story line. Because of how complex the interactions are, these branches would also have to appear or disappear depending on the scale, and depending on which timeline is being viewed.

Add multimedia

Like Google Maps, where resources like photographs, or information like phone numbers are linked to particular points in space, the timeline would have resources linked to a particular point in time, to a broader range of events, or to branches that connect related events. There could be pictures of the organisms that existed, or videos to explain a concept or expand on the details of an event. This would replace the limited images in the timelines that exist at present.

Add a responsive map

The scalable timeline should have an easy way to view the geographic location of a particular event, if it happens to occur in a specific place. This would require an omnipresent world map that lights up in the right spots to correspond to a particular event, but which also changes to reflect the shifting positions of the continents. The map would show where an event happened, but also where climate zones are, where glaciers are present, and where other key contemporaneous events occurred.

Get hypothetical

Hypothetical timelines could be introduced to consider alternative histories. For example, what would have happened if Earth had never been hit by an extraterrestrial object 65 million years ago? Would we even exist if mammals hadn’t been able to take over niches left open by the extinction of the dinosaurs? Or would the dinosaurs have gone extinct anyway for some other reason? Hypothetical timelines could be places to host discussions.

No more Brontosauruses

Brontosaurus illustration from 1896

Brontosaurus, redlined. Skeleton illustration appeared in “The Dinosaurs of North America” by O. C. Marsh (1896)

A timeline of this nature would be much easier to update as new data become available, or as the thinking about Earth history changes. In the popular Golden Guide to Fossils, which some of my students use, there still exists an entry for  Brontosaurus. Brontosauruses were invented by mistake in 1879 because Othniel Charles Marsh was in a rush to publish and didn’t realize that his new dinosaur find was just an adult version of a juvenile dinosaur he had already documented, called Apatosaurus.  The  iconic dinosaur that came to be known as Brontosaurus was actually Apatosaurus with the wrong head attached. The Brontosaurus story could be corrected with a few keystrokes and turned into a teachable moment about the challenges of interpreting paleontological data.

Why would it work?

It would work because narratives are better than lists. The standard timeline offers a way to summarize some of the events in Earth’s history, and to express temporal relationships as spatial ones, but it doesn’t go far enough to make the events into a meaningful whole. A list of seemingly isolated events is just that- a list. It takes context and meaning to make it a story, and stories are things we can remember and understand. There’s a reason you need to write down your grocery list to remember it, but you don’t need notes to be able to relate a relatively trivial story about what your dog did the other day. Whether you get all of the groceries you need or not will likely have a bigger impact on your life than if you can remember your dog story, but if you remember the dog story, it’s because it means something to you. A scalable timeline is a dog story rather than a grocery list because it will make it easy to examine the relationships between events in Earth history, and to synthesize essential details into a meaningful whole.


Categories: Learning technologies, Teaching strategies | Tags: , , , , | 2 Comments

Blog at