Here's why:

* Source: The magma originates from the Earth's mantle, specifically from the asthenosphere, which is a partially molten layer.

* Composition: The mantle is primarily composed of peridotite, a rock rich in olivine and pyroxene. When this peridotite melts, it produces magma with a high content of silica (SiO2), iron (Fe), magnesium (Mg), and calcium (Ca), and relatively low amounts of potassium (K) and sodium (Na).

* Basalt: This specific composition results in magma with a low viscosity (it flows easily) and a relatively low melting point. When this magma erupts at the mid-ocean ridge, it cools and solidifies to form basalt, a dark-colored, fine-grained igneous rock.

So, the process of plate tectonics at mid-ocean ridges creates the perfect conditions for the formation of basaltic magma.