1. Plate Tectonics: The Earth's crust is divided into tectonic plates that are constantly moving. At mid-ocean ridges, these plates pull apart, creating a gap.

2. Mantle Upwelling: As the plates move apart, the space is filled by hot, solid mantle rock rising from deep within the Earth. This upward movement is called mantle upwelling.

3. Decompression: As the mantle rock rises, the pressure on it decreases significantly. This decrease in pressure, known as decompression, lowers the melting point of the rock.

4. Partial Melting: Even though the mantle rock is still mostly solid, the reduction in pressure causes a small portion of it to melt. This molten rock is called magma.

5. Magma Intrusion & Eruption: The magma is less dense than the surrounding solid rock, so it rises further. It can intrude into the surrounding crust or erupt at the surface, creating new oceanic crust.

Key Points:

* Pressure and Melting Point: The melting point of a rock is directly related to the pressure surrounding it. Higher pressure means a higher melting point.

* Decompression Melting: When pressure decreases, the melting point of the rock also decreases, causing it to melt partially.

In summary: The combination of plate movement, mantle upwelling, and decompression leads to partial melting at mid-ocean ridges, generating the magma that creates new oceanic crust.