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.
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