Here, the plates are pulled apart by convection in the upper mantle, and basalt lava intrudes to the surface to fill in the space. Or, the basalt intrudes to the surface and pushes the plates apart. Or, better yet, it is a combination of these two processes. Either way, this is how the oceanic plates are created. Viscosity of the magma controls the type of volcano.
Viscosity is controlled by the composition and temperature of the magma.
Hotter lavas are less viscous. Olivine crystals in vesicular basalt. In building stone at Hawaiian Volcano Observatory. The larger olivine crystals are several millimeters in diameter. Gases are not easily released from stiff, viscous magmas or lava.