At plate boundaries where sea-floor spreading occurs, magma wells up to fill the space created by the plates that are moving apart. Check out this United States Geologic Survey (USGS) educational animation to see visually how it works:

Also if you went down in a submersible vehicle to an area of sea-floor spreading you would most likely see lots of hydro-thermal vents, where water heated by the magma jets out into the ocean. If you search around on-line you can find some great videos of hydro-thermal vents and all the unique animals that live near the warm water at great depths in the ocean.

Great question!

Spreading centers (or oceanic ridges) are where new oceanic crust is generated. The crust that underlies all of Earths ocean basins is formed at these types of plate boundaries. As you get further away from the ridge in the middle of an ocean (for example the Mid Atlantic Ridge or the East Pacific Rise), the crust gets older, because new crust is formed in the middle and spreads outwards.

The hot and partially melted mantle (most of the mantle is actually solid, but soft and ductile, like clay) is closer to the surface at ocean spreading centers than in any other tectonic environment on Earth. Deep, hot mantle rises up. As the hot mantle rock gets closer to the surface, the pressure decreases, and this causes it to melt more easily. Not all of the mantle rock melts it is usually less than 30% liquid. The magma (melted rock) has composition of basalt a rock with a lot of magnesium, iron, silicon, and aluminum (more magnesium and iron, less silicon than granite). Some of the magma cools and solidifies at the surfacein contact with seawater forming pillow basalts, but most of it cools below the surface to form gabbro (same composition as basalt, but cools below the surface) and peridotite (more magnesium and iron than basalt).

Geologists used to think that spreading centers were driving plate tectonics. We thought that oceanic crust was being pushed out at these boundaries. We now think that a more important driving mechanism is subduction where oceanic crust is sinking back into the mantle. The sinking crust pulls the rest of the plate along, and this causes the spreading center to spread faster.

I made a very generalized sketch of whats going on at spreading center.

Creation of new crust - as the crust splits apart, magma from the mantle comes up to fill the cracks, solidifies, and becomes part of the new plates that are separating. Generally, the magma cools beneath the surface, but there is always volcanic activity at spreading plate boundaries, usually of the effusive (not explosive) type. A good place to look at this on land is the East African rift that runs from Djibuti to Mozambique, but the undersea spreading centers are far more developed.

That is a great question! As you probably know, the outer, rocky layer of the Earth (which is referred to as the lithosphere) is broken up into a series of a dozen or so large tectonic plates (as well as a bunch of smaller ones). These plates are able to move around because the layer below the lithosphere, called the asthenosphere, is veryweak--it is solid, but it is very hot, so it can flow kind of like taffy and can carry the lithospheric plates along with it. The movement of the plates, which is called plate tectonics, explains why we have mountains and earthquakes and volcanoes where we do and generally why the Earth looks different in different places and isn't just the same everywhere.

At mid-ocean ridges, two plates move apart from each other--for example, in the middle of the Atlantic Ocean there is the Mid-Atlantic Ridge, where North and South America move away from Europe and Africa.But as the plates move away from each other they don't just leave a big hole between them--instead the underlying asthenosphere rises up to fill the gap. As I've said, the asthenosphere is normally solid,but this is because it is normally at very high pressure due to the weight of the lithosphere on top of it. As this asthenosphere rises to the surface, the pressure on it goes way down, but it is still very hot, and as a result the it starts to melt. This molten rock or magma erupts on the seafloor to make a kind of volcanic rock called basalt,and it is this basalt that makes up the new seafloor (oceanic lithosphere).

As the two plates continue to move away from each other, more and more seafloor is made at the mid-ocean ridge to fill in the gap between them--all of the rocks that make up the Atlantic seafloor are newly made and didn't exist when that ocean started to form about 140million years ago. But the Earth isn't getting bigger over time, so the making of all this seafloor at mid-ocean ridges must be balanced by the destruction of seafloor elsewhere--this happens in subduction zones at deep-sea trenches, where old, cold seafloor is recycled back into the asthenosphere.

You can find some great videos explaining seafloor spreading at these links: