What causes mid-ocean ridges to form?

http://en.wikipedia.org/wiki/Mid-ocean_ridge

Mid-ocean ridges are formed as a result of divergent plate boundaries in the Earth's crust. These boundaries occur where tectonic plates are pulling apart from each other. Here's how it happens:

1. Convection currents: Heat from the Earth's core creates convection currents in the asthenosphere, the semi-fluid layer beneath the Earth's crust. These currents cause the plates to move.

2. Upwelling magma: As the plates separate, magma from the mantle rises up through the cracks or rifts formed between the plates. This upwelling magma is less dense than the surrounding rocks and tends to accumulate in the rift zone.

3. Magma solidification: The upwelling magma, known as basaltic lava, eventually comes into contact with cold ocean water. This rapid cooling causes the magma to solidify and form new rock. Over time, multiple eruptions and solidifications build up layers of new rock, creating a ridge-like structure on the ocean floor.

4. Plate separation: As the magma solidifies, it pushes the plates apart on either side of the ridge. This continuous process of magma upwelling and solidification at the divergent plate boundary leads to the formation and growth of mid-ocean ridges.

It is important to note that mid-ocean ridges are not all at the same depth, and their appearance varies depending on factors such as the rate of plate separation and the type of plate boundary.