Here's why:

* Basalt is a type of mafic magma, meaning it's low in silica and high in magnesium and iron.

* This composition makes basalt magma very fluid, allowing it to flow easily and spread out over large areas.

* Shield volcanoes are characterized by their broad, gently sloping shape, formed by the accumulation of these highly fluid lava flows.

Let me know if you have any other questions about volcanoes!