Working with the Rotations File in GPlates

I’m going to start out with a quick introduction to the rotation file.

First thing, let’s go over the basic data line in the rotation file. This describes a point in time and space.

100 0.0 0.0 0.0 0.0 000 !1

The first column is the PlateID that is being referenced.

The second column is the date for which this data is valid. In this case, this can be considered the start point.

Next, the third, fourth, and fifth columns describe the Euler rotation for this plate. For our purposes, it will suffice to know that this describes the way in which the feature is moved from its starting point. In this case, three zeroes means that there is no displacement or rotation of the feature or features from their state as defined in the input file. For what we’re doing, it will suffice to always enter zeroes, as any deformation of position will be done graphically in gplates. If you intend to do this on a more real-world basis, you should be able to find plenty of help on the internet. Here, would be a good starting point.

The sixth column is your conjugate plateID. All movements of this plate will be made relative to the conjugate plate, which can be considered “stationary.” In this case, the value of zero means we are basing the movements of this plate on the default reference coordinate system. More on this later.

The seventh column consists of an exclamation mark(!) followed by descriptive comments of some sort. Perhaps the name of the continent. This information is not processed by gplates and exists primarily for the user’s benefit.

Now this isn’t quite sufficient. gplates tries to animate by interpolating between points in time. It can’t extrapolate. This means we have to add a row describing end conditions. Like so…

100 0.0 0.0 0.0 0.0 000 !1

100 150.0 0.0 0.0 0.0 000 !1

As you see we’ve added a new line. The only difference is that the time is now 150.0, rather than 0.0. Now if you were to set the date in gplates to 75.0 and use the modify reconstruction pole to move a feature with a plateID of 100, gplates would automatically add another row between the two we defined with date 75.0 and whatever rotations required to put the feature where we placed it graphically. No muss, no fuss!

Now if you move the time forward, the feature will move back toward its starting point. This might not be desirable, you might want to know how you are moving the feature relative to its last position, not its initial position. In that case open the rotation file in your trusty text editor after saving it in gplates and replace the three columns defining the Euler rotation for time 150.0 with the ones that have been created for 75.0. Changing…

100  0.0   0.0    0.0    0.0  000 !1

100 75.0 -37.12  -11.67  -60.04  000 !Calculated interactively by GPlates

100 150.0   0.0    0.0    0.0  000 !1

Into…

100  0.0   0.0    0.0    0.0  000 !1

100 75.0 -37.12  -11.67  -60.04  000 !Calculated interactively by GPlates

100 150.0   -37.12  -11.67  -60.04  000 !1

A simple copy and paste. Now the plates will remain in the last defined position. Wash, rinse and repeat as you make changes…

Let’s say we want to extrapolate to 200.0 Ma. In that case we simply add another row with the time value set to 200.0. Thus…

100  0.0   0.0    0.0    0.0  000 !1

100 75.0 -37.12  -11.67  -60.04  000 !Calculated interactively by GPlates

100 150.0   -37.12  -11.67  -60.04  000 !1

100 200.0   -37.12  -11.67  -60.04  000 !1

And keep on rockin’!

Next monday I’ll post a silly “toy” example, demonstrating the use of the conjugate or “fixed” plate.

Hopefully this was helpful,
The Astrographer

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