Mountain ranges and tectonic plates -- how can I make my world map more plausible?

Question

This is the first world map I’ve ever made. I tried a smaller one a while ago, but now I’m attempting a map of a full planet. As always, this world is Earth-like, etc. I also have yet to finish it, and have left markers of what direction tectonic plates are moving in and the kinds of plate boundaries present. The circles along the boundaries are where I plan to place islands. The “mountain ranges” pictured are also just where I plan to put them, they won’t necessarily be tall mountains coast-to-coast, it’s just a way for me to note where to put them. Feedback is very much appreciated! By the way, the hole in the top middle is meant to be a volcanic hotspot.

Answer 1

Plates are generally very large, with a few smaller chunks here and there. When the pressure of the mantel pushes up between two plates, it forms a divergent boundary. The magma cools, creating new crust and shifting the rest of the plate to make room. On the other end of the plate, this shifting often pushes it into another plate. When two plates are pushed together a convergent boundary is formed, where the more dense plate is forced under the less dense plate. This forms a deep ocean trench and pushes the lighter plate up, often resulting in expansive fold mountain ranges, like the Rockies and Andes. Occasionally, two light continental plates are pushed into each other, resulting in extremely large mountains, such as the Himalayas, but this is rare. The third type is the transform boundary, which occurs when the edges of two plates slide against each other in opposite directions, creating a fault line.

Your world looks to be made up of lots of little plates. This isn't inherently bad, as the structure of the world might just be different from our own. However, having so many plate boundaries means way more volcanoes and earthquakes. Depending on the setting, this could work in your favor, but if that's not what you're going for, consider reducing the number of plates and thereby the number of boundaries.

The bigger issue I see is the prevalence of continental convergent boundaries. Nearly every continent is made of up two plates pushing into each other forming mountains down the middle. These kinds of boundaries are rare because both plates must have similar density; if one is more dense than the other it will be forced underneath creating an oceanic trench and coastal mountains. Having lots of continental convergent boundaries isn't necessarily a problem though. Again, it's a different world; just because they are so rare on Earth doesn't mean your planet can't have more. (Just bear in mind that these kinds of boundaries result in massive mountain ranges, which can severely isolate parts of the world. Again, if that suits your aesthetic, great, but keep it in mind.)

The real issue is the size and shape of these convergent continents. To appear as they do on your map, two equally sized plates would need to converge such that their edges line up perfectly, which is just not going to happen. Either there is a large continent that a small one converges with, creating an outcropping of sorts (exhibit A), or two equally sized continents converge, but at a skewed, offset angle and position (exhibit B).

Exhibit A

Exhibit B

Edit: After taking a closer look at some of the details of your map, I found a few more specific issues.

Issue 1: Plates cannot just drift on their own. They need to be pushed, either by a divergent boundary on the other side or another plate. Here are a couple places where plates seem to move on their own without being pushed. Unlike convergent boundaries, which can collide at any angle, plates with divergent boundaries must move away from each other in opposite directions, so, unfortunately, you can't just mark these boundaries as divergent. They just aren't possible.

Issue 2: Plates are solid and must move in a single uniform direction. Here are some instances of single plates that appear to be moving in multiple directions. Unfortunately you cannot just split these into multiple plates, as they would then fall under Issue 1, above.

You may be able to average out these conflicts, as shown below, maintaining the convergent boundaries, since they can be at any angle. Just be careful about what side effects this will introduce with all the other plates around it and make sure to account for those. (For example, in the second image, changing the plate to move to the upper left will maintain the existing convergent boundaries, but will cause the right boundary to no longer be transform. But you can't just make it divergent as it would then fall under Issue 1.)

Issue 3: Plates moving toward each other at any angle are convergent. In this case, the plates appear to be moving in roughly the same direction, but not quite. This slight angle is enough to make the boundary convergent, not transform. This may be a typographical error, given the other arrows on the left plate (not shown). If that is the case, and the plates are moving parallel to each other, there may not be a boundary at all. Transform boundaries are possible between plates moving in the same direction, as long as the plates are moving at different speeds. This is uncommon though, as their speed tends to average out as they drag each other along.

Issue 4: All boundaries must be accounted for to accurately model geographical transformations. In this example, the upper plate (A) is marked as moving down against the lower left plate (B) forming a convergent boundary, but there is no boundary marker between the upper plate (A) and the lower right plate (B). This is important as the relative motion of these plates would create a convergent boundary that would result in a coastal mountain range on the northern edge of that continent.

Technically this also falls under Issue 1 as there is no force that would cause plate C to move to the right. So what would actually happen is the plate C would be pushed down by plate A, creating a convergent boundary between plate C and plate D. I suppose the resistance caused by plate D might impose some push back force against plate C which would carry through to plate A resulting in a secondary convergent boundary (like a three car pileup). I don't know of any real world instances of this, but it might be more likely with smaller plate sizes and could be interesting to explore. Note that, should such a thing exist, the total convergent force would not increase, it would just be spread out over a larger area (i.e. half the force (give or take) of plate A would get absorbed, creating mountains, while the other half is transferred through plate C (i.e. moving it) and absorbed by plate D, creating more mountains).

Edit 2 - General Tips:

If I were creating a world map based on underlying plate tectonics, here is what I would do:

1. Start with defining where your plates are. Make them varied in size and shape.
2. Decide which plates are oceanic and which are continental (note that the edge of a continental plate is not the same as the edge of the continent. The edge of the plate might be hundreds or even thousands of miles offshore. For example: unlike the Pacific, there is no Atlantic oceanic plate. The North American, South American, Eurasian and African plates all connect in the middle of the Atlantic ocean at a giant divergent boundary called the Mid-Atlantic Ridge)
3. Decide which boundaries are divergent. These will determine the motion of the other plates. Note that a single plate cannot have divergent boundaries on two sides, as it would fall under Issue 2 above.
4. Use the motion of the divergent plates to determine the motion of the other plates. This will tell you which are convergent and which are transform. Use these boundaries to determine placement of mountains.
5. As desired, move and shift these boundaries around until you get the shape and layout you want, making sure to never shift something in such a way that it would screw up the plate mechanics. You should have a good amount of leeway, as long as you keep in mind the things I've mentioned (the only ones that might be tricky to move are the divergent ones, since it could change a transform into a convergent, or vice versa).
6. Finally, add in the shorelines of the actual continents. Convergent boundaries between ocean and continent will have a coast line that generally follows the edge of the plate (which is actually a few hundred miles offshore). Divergent boundaries will almost never lie along a coast as the coasts are effectively getting pushed away from the boundary over millions of years, but these coastlines tend to loosely follow the general shape of the boundary they are getting pushed away from. Seismic activity and other geologic events can significantly change the shape of the coastline though, so the actual placement of the coastline is pretty arbitrary relative to the edge of the plate. For the most part, you can pretty much shape your continents and land masses however you want at this point, regardless of the underlying plates, as long as you keep the few things mentioned above in mind.

Answer 2

Good question and great answers already.

I've been doing "realistic" maps for a while, and what I found best was to create a nice, real-world feel, was the following:

1. Plate Tectonics

I start with a blank sheet and draw seemingly random shapes from which to derive the plate tectonics: first you need to end up with 12-18 different shapes, nothing too complex. From there, I would figure out the relationships, which were already mentioned and explained amazingly well (i.e., two plates moving against each other will yield you mountain ranges, two plates drifting apart will give you a deep ocean trench).

2. Outlines

Again, trying to focus on rough brushes first, to figure out what is "land" and what is "ocean", keeping in mind the plate tectonic relationships you created. This is the most fun part if you ask me, as you can play with the shapes and the plate tectonics will hint you as where you'll find archipelagos and mountain ranges:

• Two plates crashing in the middle of your continent will look like the Himalayas or Andes, and smaller in-land plates may yield you India-like regions. Plates which are being crushed from both sides may yield you regions with lots of volcanoes.
• Coastlines in the middle of a plate will yield you, for example, an extensive underwater platform as found in the east side of South America. While coastlines at the tectonics' margin could yield you a Chilean-like landscape (high mountains and a sharp drop into deep waters).

3. Mountains and altitudes

To keep it simple, I try to outline the "mountain" sections/regions, and will add a few odd ones where it could make sense or wrap up my map. Within the "mountain" regions I add "peaks" or just "higher mountains". Throughout the rest of the map I place "hills" and "lowlands".

Hills could be anywhere - they could be extremely old mountain ranges which were eroded by eons of wind, water or tremors, or simply generated by weathering.

Lowlands could be swampy or regularly-flooded regions. I usually end up turning most of these lowlands into lakes, or at least, covering part of their areas with permanent water bodies.

4. Climates

This is tricker, as I do not know how much into detail you'd like to go. You could delve into the cells and atmospheric circulation of winds, or you can just use an approximation. For example, one could say the world has what, 5-6 major forested areas (i.e., Canada, Amazonia, Central Africa, Central Asia and Southeast Asia) and 5-6 major desert areas (i.e., west of the USA, central South America, north of Africa, the Middle East, Mongolia) so I usually try to create that many regions if I want my world to match Earth. If I want a wetter world, I add more forested areas, less deserts, or vice versa.

5. Rivers

I usually roll some dice to determine how many major rivers I will have on each mountain range or, sometimes, hilly or flat area. All rivers should flow downhill, so that they end either at the ocean, at a lake or other rivers.

Just from looking at the world map, you can see that most mountain ranges have several major rivers and that will help you determine which areas will be marshy (i.e., a major river going through forested areas) or floodplain-y (i.e., a major river crossing a desert). I liked this post for some pointers.

6. Cities and civilisation

A very basic rule is for you to place cities only where people would find water and, preferably, also freshwater. Cities usually will need a food source, a means of generating economic value and to defend themselves. So you should try to have all three, and that will require you to start detailing your map further: is there a shallow ore vein here? A nice grassland where crops and/or cattle can grow? Is the river sufficiently big to allow for trade ships to reach this other city? Again, some pointers here that I liked.

If my story/game demands it, I will rate the settlements I build with simple a point system. The more of these basic aspects a city has (i.e., food, water, defense, economic resources, major rivers) the higher it will score and, possibly, the stronger it will become. The bigger the city, arguably, could yield you a bigger country/nation/kingdom/empire. You could mix it up and have some major cities be city-states too. This will depend on your story and interests, obviously.

Just as with the world's history, having a region with amazing resources could yield you an advanced, populous and expansionistic empire. Having little to no resources may yield you backwards, sparesely-populated or even nomadic groups.

I hope these ideas will help you create a realistic map! Feel free to comment back your thoughts. Good luck!