What causes a hydrothermal field to form?
Hydrothermal fields are driven by hot or melted rock (magma) under the surface of the earth. These heat up the sea water that has percolated through cracks etc, causing it to rise again to the surface often streaming out from the sea floor under pressure like the geysers found on land. This is why we find hydrothermal fields near volcanically active areas where the magma lies close enough to the surface to heat up the water that circulates through the rocks of the sea floor.
This sea water is cold (around 2°C) when is slips into cracks in the rocks. When it approaches the magma it can be heated up to 400°C! The heated water rises rapidly, dissolving metals and minerals as it goes. It streams out into the water above the sea-floor where it cools rapidly and the metals and minerals precipitate out forming the chimneys.