How did the oceans form?

Due to the complexity of the processes, it is very difficult to reconstruct the sequences of events of the planetary evolution, and in particular to the changes in the hydrospheric environment.

The geological record presents us with fragmentary information. The dimensions and shape of the oceans changed. There were times when part of the water remained frozen in the coldest zones (glacial epochs), the level and extent of the oceans decreasing, and others in which all the ice melted, rising the sea to its maximum levels.

The shapes of the continents, and therefore of the oceanic basins, also varied. Some continents were divided, the fragments, thus formed, migrated slowly and, in certain cases, merged with other fragments giving rise to new continental masses of different contours. Concomitantly, the oceans changed shape. Some oceanic depressions, such as the Atlantic Ocean, were established in relatively recent geological times (about 100 million years ago). Others are much older, such as the Pacific Ocean, whose genesis is uncertain.

During the billions of years that passed, the oceanic waters received enormous volumes of salts, until they stabilized in a relative way in the current composition. Some of these salts were immobilized and buried at the bottom of the sea for a long time. Some reappeared in the new mountains formed in the orogenetic margins of the continents.

Also, from the beginning, groundwater was exposed to the planet's internal heat sources. The latter, whose origin may be related to mineralogic phase changes or radioactive origin, were a main factor in the terrestrial dynamics. Much of the geological processes of the crust occurred in the presence of water 
The liquid or gaseous water is introduced through the fissures dragging varied solutes that finally will crystallize in the form of minerals. A large part of the minerals in rocks originate in this way (for example, feldspars and quartz). These processes of mineralization are called hydrothermal (when they occur in the presence of liquid water) or neumatolíticos (when they occur due to the action of steam). They may also be influence by other fluids, such as methante. 
Many rocks originate in these environments: most of the metamorphic rocks, the migmatites, almost all the reef rocks and others. Similarly, the mineralogical record includes numerous hydrated minerals originating in underground aqueous environments, such as micas, amphiboles, clays and gypsums.

Volcanic phenomena are also due to the presence of water. One of the main causes of the eruptions is the vaporization of hot water as the pressure that kept it in liquid state decreased. The steam "bubbles" released from the boiling water are the "piston" that pushes lava and volcanic clasts along fractures and chimneys, and ends up spilling them out. At the same time, the mixture of liquid and gaseous water has a lubricating effect that facilitates the flow of the lavas. If not, these, whose viscosity is very high, could not drain through the narrow effusion channels. The large columns of "smoke" that come out of the volcanic craters are mostly formed by water vapor emitted during the effusive processes. Similarly, the geysers and fumaroles, so frequent in volcanic zones, mainly include hot aqueous ejections.

Water is also the main factor in the genesis of sedimentary rocks. With very few exceptions, sediments are formed due to the entrainment of particles and materials by liquid water currents (rivers, marine and lacustrine currents, etc.) or solid (glaciers).

When the sediments are buried, they undergo compaction and dehydration processes. Part of the water, subjected to conditions of high pressures and temperatures, migrates out of the sediments, reducing the water content of the same.

D.Antón., Drought in a water world