Here's why:

* Fluid Lava: Shield volcanoes are known for their fluid, fast-flowing lava. This is because the magma has a low silica content, which makes it less viscous (thinner).

* Basaltic Composition: The magma that erupts from shield volcanoes is typically basaltic in composition. Basalt is a dark-colored, fine-grained volcanic rock with a low silica content (usually around 45-55%).

* Gentle Slopes: The low viscosity of the lava allows it to spread out over large distances, creating the characteristic gentle slopes of shield volcanoes.

In contrast, volcanoes with high silica content (like rhyolite or dacite) have much thicker, more viscous lava that tends to build up steep-sided cones.