[Tutorial] Using GPlates to model continental drift

[Charerg]

a. CREATING A CONTINENT

So, first off, the basics. Open GPlates, choose File->Save Project As and then create a new folder for your GPlates model, and save the project there. Assuming that you’ve familiarized yourself with the basic functions like moving the view (Drag Globe, hotkey D) and changing projections, we can get started with creating your first continent. To begin with open the “Digitisation tools” (hotkey 3). There are three options: digitise point, line and polygon geometry. For drawing the outline of a continent, we want to “Digitise new polygon geometry (hotkey G)”:



Go on drawing the outline point-by-point. Once you’ve finished click “Create feature”. Now a list will open where you’ll pick what type of feature you’re creating. Choose either ClosedContinentalBoundary or ContinentalCrust as the feature type and click next.



In the next window, we will name the feature we’ve created, define when it exists, and which Plate ID it belongs to. The Plate ID 0 represents the planet’s spin axis in GPlates (and should be kept immobile). Since we want our continent to move, pick a different Plate ID (100 in this example). For the “time of appearance”, pick a point far in the planet’s past (500 Mya in the example) and 0 Mya (present) for the “time of disappearance”.



Then click next. Another window will pop up, but we don’t need to touch anything there, so click Next. Then, create a new feature collection called “Continents” where you’ll save this feature (as well as other similar features) and save it in your project folder.



Btw, ignore that "Landmass" feature in the screenshot (it's an artifact from the tutorial writing, you shouldn't have it). You’ve now created your first continent, congratulations! Next, let’s give it some colour. First, download the following image and save it in your project folder.



Then, choose File->Import->Import raster and import the image you just loaded into GPlates. GPlates will ask about which longitudes and latitudes the raster covers. Since the above image already has the proportions of an equirectangular projection, the default settings are fine. GPlates will now generate a new feature collection from the loaded image in the folder where your image is located (which is why we saved it in the project folder to keep things organised). Now you should have the raster in your layer manager (Window->Show layers if you’ve closed it):



Open the raster, and under the “Inputs” tab, find “Reconstructed polygons” and choose “Continents”. Now your raster should only affect the features defined in the “Continents” feature collection.



Now you’ve created your continent and attached a raster image to it. This is very useful with existing maps, since you can import them as rasters and then draw the continent outlines over the image. If you tie the raster to a feature like we did here, it will also follow the movements of the continent. Speaking of said movements, the next step is to get the continent we just created moving. For that, we need a rotation file.



b. WRITING A ROTATION FILE

All motions in GPlates are defined as a rotation around an Euler pole, and relative to a conjugate plate. For that, you need to separately create a text file called the “rotation file”. I recommend using Notepad++ for this.
So, create a simple text file called “rotation” and copy the following there:

100 0.0 90.0 0.0 0.0 000 !Plate A
100 2000.0 90.0 0.0 0.0 000 !Plate A

All entries in the rotation file have this format. The different columns have the following meaning:

Plate ID	Time	Latitude	Longitude	Angle	Conjugate plate	Comment
100	0.0	90.0	0.0	0.0	000	Plate A

This row describes the position of plate 100 at 0.0 Mya. The latitude and longitude describe the position of the Euler pole, the angle describes the rotation around it, while the conjugate plate tells us that this rotation is relative to plate 000 (which is the planet’s spin axis). So, in this case the Euler pole is the North Pole. The rotations can also be described relative to other plates, but for now we’ll keep this simple and keep the rotations relative to the planet’s spin axis.

The good news is that you won’t actually have to calculate any of this stuff yourself. GPlates can automatically modify the rotation file once you’ve set it up. To do that, we just need to do what we just have: define the plate’s position at present and at some arbitrary point far away in the past. Ideally, both of these “boundary rotations” should be kept at [90, 0, 0] values (or [0, 0, 0], but in that case GPlates automatically rewrites it to [90, 0, 0] anyway).

So, for now all we need to do is to just copy the lines I gave into a text file. Make sure there is an empty line at the end of the file, otherwise it may not work properly. Then save it as “rotation.rot”. With Notepad++, you can do this by choosing “All types” and manually typing .rot as the file type.



Now that you have your rotation file, open your GPlates project and choose File->Open feature collection and open the rotation file you just saved. The rotation file should now be in your feature manager (File->Manage feature collections to check), and you’re all set for getting your continent moving!



c. ANIMATING THE CONTINENTAL DRIFT

Now we’re ready to get your continent moving. First off, pick a point in the past (type on the Time window), say 150 Mya. Then open the “Pole manipulation” tools (hotkey 5). “Choose feature (hotkey F)” and pick the continent you just created. You’ll note that it’s still in its present-day location. Next, pick “Move pole (hotkey O)”, and check “Enable pole”. There should now be an arrow representing a pole at the location of the north pole of the planet.



This is the pole that you'll rotate your continent around when you define its new location. The Move pole tool allows you to drag it around. Once you are happy with where the pole is located, pick “Modify reconstruction pole (hotkey P)”. Now you can drag the outline of the continent around the pole.



Fiddle around with the location of the continent (you can move the pole around too), and once you’re happy, click Apply. This will create a new reconstructed rotation for the continent in the rotation file at 150 Mya. However, it still needs to be saved, so open your Feature manager (File->Manage feature collections) and save the rotation file. Now you can click “Play” and see how your continent moves from the location you just defined into its present-day location!

However, your continent looks a bit lonely right now. To give it a pal, let’s try that other method of drawing a continent. Download the following picture and import it into GPlates as a raster, just like you did in the earlier parts of this section.



Depending on how the layers are ordered in the layer manager, your planet should look something like this:



If you need to, you can hide some layers by clicking the eye icon in the layer manager. Set your “Time” back to 0.0 Mya and draw the outline of a new continent from the imported raster, just like you did previously. Then create it as a new feature (as before). Remember to give it a different Plate ID than Plate A (200 in this example), and save it in a new feature collection (say, Continents II), because we want to use a separate raster image for it than continent A.



Again, open your layers manager, open the layer corresponding to the “Continent B” raster, then under input click reconstructed polygons and pick “Continents II” (or whatever name you gave to the new feature collection). Your planet should now look like this:



Now, you need to set up your rotation file to include a separate plate for the continent we just created. First, File->Manage feature collection and make sure you’ve saved the rotations you made previously in GPlates. Then, open the rotation file in a text editor, and “set up” plate B, just like you did previously. Your rotation file should look something like this:



Now that you’ve modified your rotation file, you need to reload it into GPlates. Open your “Manage feature collections” window again, and click the action “Reload” for the rotation file (the recycle icon).



Now, we can create our ancient supercontinent. Move back to 150 Mya in time and arrange continents A and B into a single continent. The result might look something like this:



As you can see, the fit isn’t quite perfect. However, we can fix that. Let’s say you’re really happy with continent B and don’t want to change the shape, so we’ll redraw continent A to fit the model. Basically, just redraw continent A like you’ve done previously, only now do so in 150 Mya. Remember to save the newly created continent A with the Plate ID 100 and save it in the right feature collection ("Continents").



The new outline of the continent should now appear, and you can delete the old one. So, your supercontinent might look something like this:



Then you can play the animation and see the continents breaking apart. A few example pics from this:

75 Mya:


0 Mya:


As you can see, these movements are a bit clunky and overlap a little. You can keep adding new reconstruction poles to new dates and so forth to get the movements right. You could also change the location of the continents in the present day by giving them “non-zero” present-day rotations. However, this should be avoided since it can mess up some of the more advanced features of GPlates (we’ll get to those later). So, that’s it for the first section of this tutorial! You’ve now learned how to create and move continents in GPlates. Give yourself a pat in the back and prepare for more to come, since in the next section we'll proceed into more advanced stuff: modelling the location of mid-oceanic ridges and the creation of new oceanic crust.

This my first tutorial here at the Guild, but I hope everything was clear and understandable. If you have any comments, questions, or ideas about the tutorial, feel free to post them. I'll try to keep track of this thread and answer them as time permits. Oh, and the 2nd section should be up "soonish" so stay tuned !