how does mid-ocean ridge form?

Answer 1

It forms from two oceanic plates spreading apart. Oceanic plates are relatively thin and dense formed primarily of basalt. When the oceanic plates spread, there would be a void between them that must be filled. The reduced pressure presumably creates magma which spills upward to fill the void. The magma cools into rock and the spreading continues. Oceanic crust is very new geologically and there is very little if any older than the dinosaurs. This is becuase it keeps getting recycled on the other other end by subduction. The mid ocean ridge forms at the spreading center, the plates are pushed under when they encounter another plate. Since the magma is fresh and relatively less dense, it tends to form a high spot. This is where the "ridge" comes from. As it spreads, cools and becomes more dense it resides deeper in the ocean (generally). This forms a long ridge where the crack or spreading center is located.

Answer 2

Oceanic ridges form in areas where the hot (upwelling) mantle rises and then moves laterally due to convection. So convection is the driving force for plates to move, for plates to come apart. Areas where the cooler mantle sinks are subduction zones.
In mid-ocean ridges you will form oceanic crust as explained above. As the crust extend it filled in with magma extracted from the mantle. The mantle is largely a solid with small amounts of melt, that allows it to flow, very slowly.
In one word, mid ocean ridges form by convection.

Answer 3

Although I agree with both of the previous answerers, there is another theory: Slab Pull. This theory states (generally) that as oceanic crust cools it becomes more dense, and dives into the mantle. The other side of the plate then brings up moltem basalt from the mantle to replace it. I do not ascribe to this theory, but it should be mentioned as an alternative to Seafloor Spreading.