The oceanic ridges, place of spreading of the tectonic plates and of formation of new oceanic crust, are home to an intense tectonic, volcanic and hydrothermal activity. Several components are required for the functioning of a hydrothermal system:
- a fluid, sea-water in this case ;
- a driving force. The energy is provided by the magma chamber that contains molten lava at 1200°C, and located several kilometers in depth ;
- a "plumbing" system, materialized by faults and fissures produced by the spreading of the plates.
The sea-water penetrates the ocean floor, permeable because of the fissures and the pores of the rocks.
Because of the presence of the magma chamber, the temperature rises by several hundred degrees per kilometer. The tectonics (i.e. the overall deformations due to the spreading of the plates that is taking place at the ridges) is the main process that creates the permeability: the rocks are cracked, fissured, and even locally crushed. Some fissures can reach several meters in width and hundreds of meters in length.
The sea-water is heated at depth, strongly reacts with the rocks it travels through, and becomes enriched in dissolved metals.
Warmer, and consequently lighter, the water rises and comes out of the ocean floor at temperatures reaching sometimes 400°C. The thermal shock with the deep-sea water induces the precipitation of the metals and sulfides it carried, which build up to form chimneys.