This is a map of the major oceanic spreading centers. This is sometimes considered to be one ~70,000 km-long volcano. Here, the plates are pulled apart by convection in the upper mantle, and lava intrudes to the surface to fill in the space. Or, the lava intrudes to the surface and pushes the plates apart. Or, more likely, it is a combination of these two processes. Either way, this is how the oceanic plates are created.
The lava produced at the spreading centers is basalt, and is usually abbreviated MORB (for Mid-Ocean Ridge Basalt). MORB is by far the most common rock type on the Earth's surface, as the entire ocean floor consists of it. We know that spreading occurs along mid-ocean ridges by two main lines of evidence: 1) the MORB right at the ridge crest is very young, and it gets older on either side of the ridge as you move away; and 2) sediments are very thin (or non-existent) right near the ridge crest, and they thicken on either side of the ridge as you move away. Mid-ocean ridges are also the locations of many earthquakes, however, they are shallow and generally of small magnitude.
We have never witnessed an eruption along a mid-ocean ridge, although a few times earthquake swarms have been detected along them (mainly by secret US Navy listening devices). When scientists have investigated soon after, fresh-looking basalt, plumes of hot chemical-laden water, and recently-killed marine organisms have been observed, indicating that an eruption almost certainly had occurred.
Click here for more information on submarine volcanoes!