Here's why:

* Basaltic lava is very fluid and has a low viscosity (think of it like honey). This means it can flow easily and spread out over large distances.

* Low silica content: Basaltic lava is relatively low in silica, which makes it very hot and fluid.

* Gentle eruptions: Eruptions of basaltic lava are usually non-explosive and produce large volumes of lava that flow smoothly.

These characteristics allow the lava to create the distinctive, broad, shield-like shape of shield volcanoes.