Artificial continents could be the best way to create some land on an Oceanic Planet. Current technology might offer a few solutions.
An artificial continent must have a lower density then oceanic water. Now, as we well know, oceanic water contains a specified amount of salts and is, therefore, heavier then distillated water. So, a material with density of 1, should float on an ocean.
First, the continent must be made of materials that can resist to forces created by the underground ocean. Water is more mobile then lava is for plaque tectonics.
Second, it must be non-flammable.
Third, the continent must be non-biodegradable. It must remain unaffected by chemical reactions with gasses in the atmosphere or with substances within the water.
An Earth - like planet or an Inner Planet can be subject to major temperature differences between equator and poles, creating strong winds and strong water currents. On the other hand, an Outer Planet has very weak winds and almost no water currents, so that continents will be safer.
There are a few solutions, provided by nature, but current technology and by future technology.
Solution by nature Edit
On Earth, in many swamps and deltas, we can see floating islands. Plants grow on the surface, above water, with their roots connected one to the other. These kind of islands can reach sizes of hundreds of meters. It can be possible, by using genetically modified plants, to create even larger islands, up to a few km in radius or even larger. This solution is good to create habitable islands, but not enough to create a continent.
Floating cities Edit
This technology is currently under development in places like Holland and was proposed even for the US . Basically, a city will be made of independent barges, connected one with the other. This is a good solution for limited areas, but still not enough for building a continent. Over a long period of time, the barges will need maintenance. If not, they will be corroded and will sink.
Floating organic compounds Edit
There are many solid organic compounds, the most known of them are alkanes. Many of them are solid and soft and are floating above water. They can be produced in laboratory from carbon dioxide and water or more efficiently from methane (which is the most simple alkane). So, a planet or satellite whose atmosphere contains methane (Titan is the best example) is a good candidate. In future, genetically modified plants or bacteria might be able to create large amounts of alkanes.
This technology might be good enough to create a floating continent. There are, still, two major problems. First, alkanes are soft (in fact, wax is usually made of them) and are flammable. There are, however, organic compounds that are less or almost not flammable. Covering the continents with them, or with a layer of ground, might be enough to prevent a disaster.
Floating rocks Edit
There are a few chemical elements, like Lithium, that are light enough to float on water. However, lithium is rare and highly reactive. Even so, there are lithium compounds that can float. Floating rocks might be made of light materials (like lithium) or they can contain bubbles of gasses, locked inside their composition.
On Earth, floating rocks are rare, but future chemists might be able to create them.
Ice continents Edit
The most easy way to create a floating continent is by allowing temperature to remain just below freezing. This way, we can create close to the poles large ice caps and close to the equator an open ocean. However, an icy continent is not what settlers might wish to have.
It might be possible to create only a floating layer, reaching only up to two meters thick. Currents will make cracks in the layer, then it will brake it in tiny parts. Then, currents will form straits of open water, while compacting the layer in other parts. In this scenario, we will have millions of tiny plaque tectonics, in an endless movement, accretion and separation.
Another model is with a layer of 100 meters thick. In this second scenario, cracks will also occur, but will be more rare and will result in larger plaques. Again, we will see parts of open ocean and places where plaques will come together, forming tiny mountains.
A more stable model is with a layer of over 1 km. In this case, movement will be far slower. Cracks and mountains will form too, but much slower. Settlers will have enough time to develop their towns.
In order to increase stability of each plaque, we can use two technologies. The first one requires to enforce plaques with wide cables of stronger materials (they can be of organic origin, they can be made of steel or they can be made of carbon fiber). The second technology requires to anchor the continents to the ground, far below the ocean. Again, this can be done with huge cables.
Make real continents Edit
It is possible, in some places, to build some real continents. If the ocean is shallow enough, we can excavate matter from the bottom and move it to create high mountains, reaching above surface. This technology might prove expensive, but at least we can create some real islands and in some cases real continents.
For an ocean planet where the bottom is a plain and water is at average 1 km deep, it can prove useful. However, if the ocean is over 20 km deep, we might only be able to create some islands. The islands are still good enough to build a base on their top or to anchor a larger, floating continent.
Planetary insulation Edit
Main article: Ground Insulation.
The idea is to cover the entire planet with a protection layer that will prevent ice below a certain point to heat and melt. This layer can be made of organic matter or of solid rock. This might work at best if the celestial body is not completely differentiated (it is a mixture of rocks and ices). Some theoretical models suggest that Callisto is made this way. If this is the case, we can melt only a limited amount of ice (to create oceans and atmosphere). The rocky material can create continents and islands. Also, rocks and a layer of artificial organic materials can cover the ocean floor, to prevent heat reach deeper ices.
A layer of rock of 1 km thick can be enough to isolate a layer of ice for many millennia. However, a layer of only a few meters thick will not work for a long period of time.
It is known that water is heavier then ice. So, in time, water will melt the ice, infiltrating into any crack (that can be created by gasses that will melt much faster). Over time, chunks of ice can separate from the bottom and reach the surface, where it will melt faster. So, a celestial body containing much ice will end-up as an Oceanic Planet sooner or later.
The insulation layer will not work forever. Heat from surface will slowly pass through it. On the other hand, tectonic activity can create cracks into a narrow layer. Water can slowly penetrate many rocks, bringing heat with it. Over a long time period, if the planet has its core heated, this heat will reach from interior, since the ice can no longer cool down through the surface.
In many theoretical models, creating an insulation layer can be a cheaper and more efficient solution then the creation of artificial floating continents. However, after a time that will last from a century to maybe millions of years, the planet will become an oceanic world. This is an example of a Short Lived Earth.
Creating an artificial continent will require the use of present or near-future technologies on a planetary scale. It might be the best solution for an oceanic planet. However, the newly formed continents will never be as stable as plaque tectonics because they will float on water, which is far more fluid then an ocean of magma.