Main article: Geography
Geographic features on a planet or a moon strongly influence the hydrology and the climate pattern after terraforming. The effect of each Geographic feature must be studied to see how it affects wind and water circulation.
Active or dead Edit
When referred to a celestial body (planet or moon), the terms alive and dead refer to Geologic activity. This means that the planet has active volcanos or cryovolcanos, active plaque tectonics, mountains, canyons, faults and other features that are new and where tectonic processes are still ongoing.
However, the classification is much more complex then this:
- Extremely active - celestial bodies where massive volcanism is visible and/or where the crust is very new, without signs of visible craters. Examples include Io, Europa and Enceladus.
- Visibly active - a celestial body where geological activity is visible (active volcanoes, surface is clearly new, resurfacted). Examples include Venus, Earth, Ganymede and Pluto.
- Appearently inactive - a celestial body where we don't see significant geological activity, but where we have indications that the core is still hot or limited geological processes might occur. Examples: Mercury, Mars, Ceres, Dione, Titan and possibly Miranda.
- Recently dead - a celestial body that was active some time ago, It is no longer active, but the scars of its former activity can be seen. Best examples are Luna, Rhea, Tethys, Iapetus, Ariel, Umbriel and Charon.
- Dead - a celestial body that lacks any sign of tectonic activity and is completely or almost completely covered with craters. Examples include Callisto and all the small moons and asteroids.
As one can see, almost all larger rocky or icy celestial bodies in the Solar System have or had tectonic activity. Sometimes, this affects their entire surface, while in other cases it is limited.
- Mercury is Geologically active and is shrinking . We don't see volcanoes and massive canyons on the surface, but we can see small fault scarps— cliff-like landforms that resemble stair steps. However, most of the landscapes is dominated by craters.
- Venus also seems to be geologically active  as it has volcanoes, but lacks of plate tectonics. It looks like Venus undergone a global volcanic activity, which is supposed to have created a runaway greenhouse effect. As for now, we don't have enough data about what is going on there.
- Earth is geologically active and shows significant signs of activity.
- Luna had some volcanism and tectonic activity in its past  but now it is an almost dead body. Even if some scientists argue that it might still have a small molten core, that core has not the power to affect the surface.
- Mars was active in its past and it had  plate tectonics. However, its core is not warm enough to sustain these processes. Mars has inactive volcanoes and massive canyons.
- Ceres is an interesting place  and shows signs of geological activity in a different way. It looks like Ceres never had a molten core. Temperature in its core was enough to melt water, but not enough to melt rocks, so its core is partially differentiated. However, this water has the power to reach the surface and created the mountain in Ceres's bright spot.
- Io is the most active moon in the Solar System and its surface is affected by strong forces . Even if we see no direct signs of plate tectonics, we see very high mountains on an apparently endless plain and volcanoes. The lack of craters show that Io's surface is resurfacting fast.
- Europa is an active satellite and it shows to have plate tectonics  but made of ice instead of rock. Unlike Earth, the plates of Europa are much smaller.
- Ganymede is geologically active  and it has both new and old terrain. It might have plate tectonics. The icy crust is slowly resurfacting.
- Callisto appears to be a dead world, but some  suggest it might have cryovolcanic activity. However, since its core appears to be undifferentiated, heat to the cryovolcans might come from meteor impacts.
- Enceladus is an amazingly active world and its cryovolcanoes throw water plumes into orbit around Saturn. Its surface shows young and old features. However, when Cassini reached Enceladus's North (inactive) pole , it found many small cracks and canyons. So, this moon is globally active and we can speculate it to have quakes.
- Dione has many craters, but also signs of tectonic activity. It might have a subsurface ocean.
- Tethys has signs of ancient geological activity  and might still be able to fire a water plume.
- Rhea seems has deep canyons and some  speculated it to be still active.
- Titan appears to be geologically active and might have plate tectonics  as well.
- Iapetus has an equatorial mountain range  that nobody could give a good theory of how it got formed.
- Oberon seems to be a recently dead moon, with canyons formed when its subsurface ocean froze .
- Titania also seems to be a recently dead moon, with canyons formed when the subsurface ocean frozen.
- Ariel appears a recently dead body, with many canyons. It could be possible that this moon had recently been outgassing carbon dioxide.
- Umbriel is not well studied  but appears to be a dead body.
- Miranda is dominated by scars from its geologic activity  and might have tectonic activity even today. This moon needs to be further explored.
- Triton could have tectonic activity . Its surface seems to be influenced by underground processes and it retrograde orbit produces tidal heating.
- Pluto is for sure active , even without a source of tidal heating and without a significant core to conserve heat.
- Charon had its own icy plate tectonics  but its subsurface ocean is now frozen.
All these observations show that many celestial bodies are or at least were active. We can speculate that also in other solar systems planets and moons are or were active.
Our current understanding of volcanism and tectonic movements is limited. The only planet that has been studied up to a satisfactory level is Earth. However, at least in theory, we can come with a few models. Please note that there are many theoretical models and the ones listed below are very simplified.
Crust structure Edit
Single layer crust. This model works if the crust of a planet or moon is almost entirely made by a single type of material. In case of an Outer Planet, which contains a high amount of water ice and has a subsurface ocean, the crust is made by water ice. In this model, as the planet cools, a crust gets formed. Currents in the mantle or subsurface ocean pushes the surface in various directions. The crust is formed in certain points and submerges in others. As this happens, mountain ranges and canyons are formed.
Double layer crust. This model supposes that there are two different constituents of the crust. There will be a lighter and a heavier material. At the beginning, they could be uniformly distributed on the surface. However, as tectonic movements occur, the upper layer is pushed and compressed into larger masses. Only the lower layer is submerged and resurfacted. As this process continues, the upper layer accumulates into more compact and more elevated structures, like continents, while the lower layer, found in thinner layers, forms the bottom for future oceans.
Compact crust. If the crust is wide and compact, internal pressure in the mantle can accumulate up to dangerous levels. Then, when a crack gets formed, you can see extreme events like a global volcanism. However, some celestial bodies have a variable thickness of the crust. Enceladus has a much narrower crust at its tiger stripes then at its North pole.
Altitude leveling Edit
The crust works like a boat. If you add more weight it will submerge. If you remove weight, it will rise. With other words, you cannot have too high mountains or too deep canyons. Volcanic and tectonic activities are responsible for creating mountains and canyons, but not beyond a certain limit.
If the crust is thick, it can support much higher mountains and deeper canyons, like we can see on Mars compared to Earth. This is an important challenge for terraforming, because high terrain blocks air currents and influences climate. Also, deeper oceans require more water to be filled.
First of all, if a celestial body has an ice crust and a subsurface ocean, it will probably lose all once terraforming begins. The use of Ground Insulation can be problematic if the planet is active, because insulation layers can be broken. Ground insulation can still be useful if the celestial body is inactive or apparently inactive. If the subsurface ocean is dominated by strong currents, the use of Artificial Continents can be risky.
For very active planets and moons, like Io and Enceladus, terraforming might be questionable. With so many volcanoes erupting, it is not known if a biosphere on Io can survive. Strong active volcanism also means there are many quakes. Also, surface elevation changes from time to time, meaning that on a terraformed planet ocean shores will change and rivers can turn backwards.
For a moderately active planet and moon, terraforming is feasible. Tectonic activity has the power to create higher places like continents and lower areas that become bottom of future oceans. Since too high mountains and too deep canyons cannot survive, there will not be too high obstacles for air currents and not too deep places to store all planet's water. On the surface, there will be plateaus, faults, canyons and rifts, mountains (of volcanic and tectonic origin) and depressions. These are the basic structures water needs to erode and create an Earth-like Geography.
For an inactive planet or moon, things are more complicated. As seen on Mars, at the last phase of its geological activity, huge mountains and deep canyons formed. And as seen on Mercury, there are huge altitude variations between place to place. This will block air currents into nearly remote areas. Deep depressions will accumulate a large part of the water, which means that we need far more water to create oceans.
Existence of former tectonic activity is very important in creating the landscape after terraforming. Initial cracks are used by the first rivers flowing on the planet and can later become valleys. Deep canyons can be flooded completely. Mountains will be important too. Sometimes, when water is added to a planet during terraforming, a catastrophic event known as Noah Flood is needed to create valleys and an Earth-like Geography. This process is more easy to be done if the planet has a tectonic-based Geography then a volcanic-based one.