Basalts are the rock type. Peridotite is the mantle rock. When peridotite moves upwards it undergoes isentropic decompression melting and the liquid is basaltic in composition. A metaphor is the formation of a cumulus cloud (fair weather). The air has some dissolved water in it...i.e. H₂O in vapor form. Now the sun heats the surface of earth and that HEATS the air near the surface and so that air with its "dissolved water" rises. As the air rises it cools and when the air cools enough, the dew point eventually equals the T and BANG tiny drops of water form...i.e. a cloud forms...

So, you can think of the phase change when peridotite (all solid) goes to a mixture of liquid (basalt) and residual solid in a similar way.

Basalt is the main rock type associated with spreading center volcanism. Basalt is also the kind of rock that is erupted on Hawaii; it is generally a dark gray or black fine-grained rock. When basalt is erupted underwater (for example at a spreading center), the lava cools and solidifies very quickly forming a distinctive pillow shape; hence, these are known as pillow basalt.

As the plates pull apart, mantle rocks rise passively toward the surface to fill that space. If you are familiar with phase diagrams, you may know that the melting point of a material depends on both temperature and pressure. Rocks deep in the earth are very hot but they are solid because they are under very high pressure. As the rocks move toward the surface, the pressure decreases and the rocks start to melt; this is known as decompression melting.