Here's why:

* Oceanic Rift Systems: These are areas where tectonic plates are pulling apart, allowing magma to rise from the Earth's mantle.

* Basalt Composition: Basalt is a mafic igneous rock, meaning it is rich in magnesium and iron. This composition is typical of magma that originates from the mantle.

* Melting Processes: The melting of the mantle beneath oceanic rift systems produces basalt magma, which then rises to the surface and erupts.

Therefore, basalt is the most common type of magma erupted along oceanic rift systems.