Shield Volcanoes and Calderas

Olympus Mons on Mars. A gigantic and striking example of a shield volcano with calderas.

Olympus Mons on Mars. A gigantic and striking example of a shield volcano with calderas.

Trying to come up with coherent advice for this fellow trying to create an island over on the Cartographer’s Guild, I ran across a sometimes successful method for creating a shield volcano with one or more calderas. This should be good for making your own private Mons Olympus or Hawaii. A variant on this might prove useful in creating impact craters, but I’m not really going to delve into that. Focus!

I’m going to make a quick build of this using nothing but the Mound filter on Wilbur. The effect, I’m sure, could be improved with additional noise and erosion effects. Again, focus!

I’ll start with a 1024×1024 blank map. To keep things sane, I’m just going to have this thing pop up out of a flat plane. No worries about developing the rest of the island or other landmass upon which this volcano may be growing.

Figure 1. Shield Selection

Figure 1. Shield Selection

First, using the freehand selection tool with feather set to 1.0, select a roughly circular base for your volcano.

Next, we’ll invoke the Mound filter(ctrl-M or Filter>Fill>Mound).

FirstMound_Settings

Figure 2. Shield Settings

Because I want to give this shield a bit of a lip above the surrounding plains like the edge of Mons Olympus on Mars, I will set the minimum height to 300 meters. I arbitrarily choose a maximum height of 15000 meters, making this lower than Olympus Mons, but definitely more of a Martian volcano than Terrestrial. For a shield volcano on an earthlike planet 5 or 6 kilometers above sea level would be a really tall shield. Set the Operation to Add and Noise to zero.

Now click the Edit Profile button. Your goal here will be to create a fairly steep-sided flat-topped profile. I usually start by entering a value like 0.9 in the Non-Linearity and hit Apply several times till I like the shape. Next hand

Figure 2. Shield Profile

Figure 3. Shield Profile

draw a flatter top to the curve, trying to keep it continuously rounded and monotonically increasing to the right. Repeated applications of Smooth, Non-Linearity and Normalize will help. With luck and a bit of work you’ll end up with a curve similar to the one below. With more luck, experience and effort you could get better results. This is pretty quickly thrown together.

Figure 3. First Caldera Selection

Figure 4. First Caldera Selection

Next, we want to create a caldera. Select a large round area near the top of the previous mound. Invoke the Mound tool with the Minimum Height set to zero and the Maximum Height set to a negative value equal to the desired depth of your caldera. In this case, -3000. Most of the time you’ll want a much steeper dropoff around the edge of the caldera than the edge of the shield. A few more applications of Non-Linearity, perhaps flattening off the top again, followed by more Smoothing and Normalize should do for that.

Figure 5. First Caldera Settings.

Figure 5. First Caldera Settings.

Figure 6. Caldera Profile.

Figure 6. Caldera Profile.

To give an overview of the problem cases of making subsidiary calderas, I’ll go through the process of making a caldera completely within the larger caldera, and one that lies across the edge of the larger caldera. As we go along, it should become clearer what I’m talking about.

First we’ll use the freehand selection tool to select a roughly circular area completely within the larger caldera.

InnerCaldera_Selection

I’m going to make this one a bit shallower than the larger caldera, so let’s use a maximum height of -800. Otherwise, use the same settings as before. For more interest, you could use a different Profile for each caldera, some could be steep, while others are gently sloping. For this exercise, the previously generated Profile will do just fine.

For this exercise, let’s make a larger subsidiary caldera that crosses the edge of

Figure 8. Edge Caldera Selection with Inner Caldera visible to left.

Figure 8. Edge Caldera Selection with Inner Caldera visible to left.

the first caldera. It’ll still be smaller than the main pit, but bigger than the other sub-caldera.

Before I do anything else, I’ll Select>Feather the selection twice with a Sigma of 1.0. After I feather the selection I’m going to Filter>Blur>Gaussian Blur the area pretty radically. Perhaps three times with a Sigma of 8. This will tend to

Figure 9. Edge Caldera blurred.

Figure 9. Edge Caldera blurred.

make the existing edge a lot less visible in the final new caldera. You can definitely blur it more for better results, but for now… Hurry, hurry!

Before making our Mound(hole), we want to harden the selection back up. Select>Modify>Binarize with a Threshold of 128, then Select>Feather with a Sigma of 1.0 to bring its hardness back in line with what we’ve done before.

I’m going to make this caldera deeper than the main one, so set the Maximum

Figure 10. Result with Edge Caldera in place.

Figure 10. Result with Edge Caldera in place.

Height to -5000. You might have noticed that the maximum and minimum values have been reversed. They should probably have been something like Outer Height and Inner Height, but the programmer seems to have intended this as a tool for making hills not holes. I’m just being contrary here.

Figure 11. Final Result with embellishments, not all successful.

Figure 11. Final Result with embellishments, not all successful.

 

After doing all this, I tried to embellish here and there, adding inner subsidence to calderas and additional calderas. I even tried to add a smaller cinder cone to the side. When this failed miserably, I made a try at creating a landslip on the side of the shield. You can tell by the results that this is quite a ways short of being ready for primetime.

Figure 13. Greyscale map of elevations. Should be 16-bit...

Figure 13. Greyscale map of elevations. Should be 16-bit…

Figure 12. Perspective view of the final result, prominently showing the attempt at a land slip.

Figure 12. Perspective view of the final result, prominently showing the attempt at a land slip.

 

I have also posted the resulting heightmap in hopefully 16-bit png, and a simple opengl perspective view.

If you have any comments, complaints, questions, kvetches, kibitzes, or better ideas, please feel free to leave a comment.

If you use this technique, please drop a line to let me know how it worked for you, any rough spots you ran across or any alterations you made to get over those rough spots. I’d love to see what people with greater talent than myself might be able to do with this technique.

Thank you for your attention and whatnot,
The Astrographer.

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