Creating a realistic world map - Erosion

Question

This Query is part of the Worldbuilding Resources Article.

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Once geological processes have created a landscape a number of factors come into play in order to reshape it. This includes weather and climate effects such as wind, rain and ice reshaping the terrain.

• What are the processes that reshape land-masses and alter the terrain after it has initially formed by geological processes?

• How can those processes be easily drawn upon to create realistic looking maps?

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Note:

This is part of a series of questions that tries to break down the process of creating a world from initial creation of the landmass through to erosion, weather patterns, biomes and every other related topics. Please restrict answers to this specific topic rather than branching on into other areas as other subjects will be covered by other questions.

These questions all assume an earth-like spherical world in orbit in the habitable band.

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See the other questions in this series here : http://meta.worldbuilding.stackexchange.com/questions/2594/creating-a-realistic-world-series

Answer 1

If you've decided to go with Plate tectonics, the biggest thing to remember is the world is ever changing...you are simply producing a snap shot of a particular point in that change. Geological processes, although still completely active, can be neglected as they operate on a scale far outside of our own time frames.

Ice vs water, hot vs cold. The sun has cycles...many of them with time frames much beyond our lifetimes, so they are much harder to observe. That said, there was likely a time where the mass of land that is currently Greenland was warm and tropical. Reversely, there is the 'snowball earth' case where the globe quite literally froze down to snowball with a thin strip of open water at the equator. This rotation between snow and ice will be your biggest global erosion.

First, you will have 'major' erosion effects...apply these to your world first:

1. Glaciers are amazingly heavy...this weight quite likely melts water near the bottom of the glacier that's in contact with the land, so you get the equivalent of a giant scrub brush up against the land with a few million tons of pressure pushing it along. This produces a layer of soft sedimentary rock that can actually be amazingly thick. This process also works as a 'reset' mechanism...whatever land formations and features existed prior to be covered by a glacier likely don't exist any more. Remember glaciers 'flow' and the ice that's at the very end of a glaciers reach once started out at the top of the glacier.

Additionally...glaciers have the interesting ability to carry rocks (large boulders) on it's surface. When they finally do melt, these boulders can be left in the strangest of spots, a giant flat plain with an amazingly large boulder sitting on it (easiest answer for an unexplained rocks = glacier put it there).

2. water flow. Once again, always in change. There are several theories that have, without our intervention, the Mississippi river changing it's route to the ocean about once every 1500 years. The rivers you place may not have been there very long and may be changing which way they flow in the future.

   2a. Pick your continental divide. Water on one side of this divide will flow one direction towards the ocean (ocean being the 'low point', lakes can always be side destinations) while water on the other side will flow to the other ocean. Gives a good starting point for rivers. Most large scale continental rivers will find their beginnings high in the mountains as glaciers feed tributaries that eventually become the massive flow of a river. Remember water always flows downhill, but it won't always be the most direct route since other obstacles may exist. Steeper declines tend to produce straighter rivers (force involved lets them cut through that much more) vs relatively level land that produces meandering rivers wandering slowly around hills (and could easily change direction through their lifetimes).

   2b. Plains are formed by ice, grinding the rocks to dirt and sand, and leaving it hard and flat packed. Overtime rivers cut into this flat land and create the river ways...with enough time, these river ways cut deep into the relatively soft earth left behind in a glaciers path eventually leaving behind features such as the grand canyon. Clay also plays a role here...high amounts of clay in the soil tend not to erode as quickly (not water soluble), which greatly inhibits a rivers ability to cut out features such as canyons.

   2c. large scale events. If we had a glacier existing to the size of north america before it begins to melt...it would start off it's melting process as creating small lakes on the surface of the glacier. As the ice continues to melt, these lakes merge with each other and eventually become a considerable water mass held up by quickly melting ice. When this breaks, the entire contents of a lake that's potentially larger than several midwest states combined spills out in one giant event. If it releases into the ocean, this can cause an immediate rise in ocean levels. This is theory...but it's thought people (homo sapiens in the time of coexistence with Neanderthals) once lived in whats now the black sea. One of these glacial lakes dumping into the Hudson bay caused a global sea level rise that initiated the flooding of that former human territory and ended up creating the black sea. Alternatively, the entire mass of water can be dumped onto land and allowed to flow south...there is an event in North America where all species over a certain size went extinct and a good portion of smaller ones as well (it's the reason why the North American elephant no longer exists). Running theory is a glacial lake was dumped out across North America and created much of the great plains that we know (a mass water dump like this creates hills and plains formations)

3. Vegetation. Life is actually the counter of erosion...life fights to retain stability in an ever changing world after all. Trees and plant matter tirelessly work to keep their soil..well, theirs. Root systems give dirt the stability it needs not to be washed away in a simple rainfall. One of the scary examples of this (I'd have to go digging for pics), is the island nation of Haiti. It went through some heavy sanctions that ultimately cut them off from Oil. The response was for the population to use the trees and in a very short time, the island of Haiti underwent 98 - 99% deforestation. If you can find it somewhere, get a pic of Haiti from google earth prior to 2004, and then find a more recent one. Since the deforestation, the river delta's have rapidly grown as, without the forestation to protect it's soil, the island has literally melted.

General rule - the more the plant life (forests especially), the less the of an effect the erosion forces of water will have.

Most other effects (such as wind and tide) tend to have more localized effects, where they definitely leave their mark, however they do not do it on a mapping scale.

Want the 'non-plate tectonic' version of this? Earth does a great job in redistributing energy, which ultimately comes with more movement and more erosion. Mars/Venus would be less dependent on this and volcanic activity would become a partial driver.

Answer 2

1- Glaciations: This phenomenon appears in regions close to the poles. The last glaciations covered North America but also Northern Europe and Asia. On an earth-like planet, places far from the poles are not affected unless the temperatures drop to a very low point.

Our planet had several ice ages in the past. This creates a movement of the ice sheets. They move south when the temperature gets colder. When they melt, they start moving again. All the movements cause erosion and can move a large amount of land with it. The fjords in Norway are a good example of this process.

When they melt, sometimes the water can reach the ocean, but not always. The natural geography of the land, but also the weight and movement of the ice can lead to the creation of lakes when the ice is melting. That is why Canada and Finland have so many lakes.

2- Water flow: It is important to know that water is always flowing downhill and always use the shortest route available to reach the sea, or the closest large water body. The shortest route depends on the surrounding landscape, an obstacle can prevent water from flowing in a particular direction, such as a higher elevation. Rivers rarely splits and if they do, it's generally on small rivers and it's only temporary. The erosion process will be uneven and eventually, there will be only one river.

More about river bifurcation can be found here: How (un)likely is a split of one major river into two others?

The exception for this is the delta. Deltas are form with the accumulation of sediments carried by the river (dirt, sand, organic sediments). The formation of the delta can be temporary or permanent depending on the origins of the sediments.

Lastly, the flow of water erodes the landscape. Younger rivers tend to be straight with smoother banks. Older rivers are more sinuous and can have steep banks due to erosion. Some river, old enough can create deep valleys.

On lakes: lakes tend to have only one output river (if any). Since water always take the shortest route, you are unlikely to have to have two places at the same dept.

3- The freeze/unfreeze cycle can also create interesting formation such as the pingo. The water into the ground changes the shape of the land at each cycle : http://en.wikipedia.org/wiki/Pingo

But they are usually too small to be represented in most maps.

*These are the factors I know but there could be others.

Answer 3

The literature can use different names for the same erosion. It also tends to have a seperate name for processes that can be easily grouped. As a result this is a distilled summary of major erosion processes.

What are the processes that reshape land-masses and alter the terrain after it has initially formed by geological processes?

1. Aeolian Erosion
   • Saltation causes erosion at lower heights to be greater when the wind blows softly, creating pedestal rocks.
   • Freely blown particles erode at all levels.
2. Thermal Erosion
   • Thermal expansion of heated salt crystals and cooled water causes pressure inside the cracks and pores they collect in, resulting in scree and tafoni.
   • Thermal gradients from sunlight, fire, or any other source of heat. Pressure is built up from inhomogenous materials and/or sharp temperature change in a material. When the pressure gets high enough you get wear on the material. One result is surface exfoliation.
3. Hydraulic Erosion
   • Precipitation erodes the surface layer and takes the path of steepest descent with a basin. Basin may have sub-basins in a hierarchy.
   • Tidal erosion causes coastlines to degrade into silt and sand, and carves cliffs in taller and sturdier materials.
   • Healthier soil holds more water and contributes to higher levels of rain in the surrounding areas
   • Rivers have a water source (typically percipitation), and they erode differently depending on gradient and volume of the water. Larger volume results in slightly larger objects being moved but mostly increases rate of erosion. Larger gradients increase size of objects moved. Rivers form V-shaped valleys over time.
   • Flooding redistributes erosion and tends to flatten areas.
4. Glaciation
   • Forming in cold conditions, they scoop out U-shaped valleys. Carries loose rocks like a conveyor belt. And grinds down the rocks it sits ontop of.
5. Biologic Erosion
   • Bacteria, plants, and animals modify the soil structure throughout their lifecycle. This changes erosion characteristics by increasing chemically reactions and water retention.
   • Different plants affect their region differently, mangroves reduce tidal erosion. Grass reduces wind and water erosion, and so on. Plants with shallow roots stabilize against surface erosion and plants with deeper roots anchor steeper slopes against landslides. They can also mechanically weather the soil by growing into cracks and pores to break rock apart.
   • Mechanical erosion from creatures moving can form trails that will even wear down materials harder than themselves such as stone _{(now look at your keyboard)}.
6. Chemical Erosion
   • Hydrolysis wears down rocks such as limestone where it creates limestone caverns. The reaction is driven faster in the presence of carbonic acid from acid rain. The effected rocks are carbonates and silicates which tend to get dissolved, and aluminosilicates which produce a secondary, usually weaker, material.
   • Hydration of rocks increase pressures in the material resulting in increased pressure erosion.
   • Oxidation can physically weaken the surface layer of a material, making it brittle and allowing erosion to happen more easily. Pressure Erosion
   • Pressure release in the material (usually from removal of surface layers) causes inclusions to expand and fracture the surrounding material.

How can those processes be easily drawn upon to create realistic looking maps?

From a bare tectonic map you have rock-type information. You also have basic heating information and major wind currents that can be modeled based off the bare tectonic map. Precipitation can be modeled to get base moisture levels. Basins are already present at this point in the geologic structures, you can use a gaussian blur to extract a scale-space representation of the basins and sub-basins that you can use for segmenting if you don't already have that information. You can use the preceding information to develop the information for soil types and biologic population. These will effect soil quantity and fertility, which will decrease erosion in general, increased precipitation and moisture levels, as well as increase erosion on selected soil types in the presence of certain species (trees will break down craggy rock quicker). Higher populations will reduce the anti-erosion effect of plants and increase mechanical wear.