Submarine volcanic activity is confined to (1) rift zones of mid-ocean ridges (MOR) and transform faults, (2) hot spots (HS) and belts of hot spots («hot lines »), (3) systems of island arcs (IA) and (4) marginal seas (MS). Volcanic activity may occur on the oceanic flanks of deep sea trenches and marginal swells between trenches and the ocean floor. Rift zones of MOR exhibit the deep-sea fissure eruptions of low-potassium oceanic tholeiites with some variations of mineralogical and chemical compositions. They produce pillow and ropy flows, volcanic cones generally being not formed. As a result of these eruptions, more than 1 X 10¹⁰ t of lava per year pour out on the ocean floor. […].