Here's why:

* Low viscosity: Lava with low silica content is very fluid and flows easily. This allows it to spread out over large areas, creating the characteristic broad, shield-like shape of these volcanoes.

* High temperature: Lava with low silica content is also hotter than lava with high silica content. This contributes to its fluidity.

* Basaltic composition: Shield volcanoes are typically composed of basalt, a type of volcanic rock with low silica content.

Water content: While water can affect lava viscosity, its influence is less significant in the formation of shield volcanoes. The lava flows are usually very hot, which would vaporize any significant amount of water.

In contrast to shield volcanoes, volcanoes with a high silica content tend to have:

* High viscosity: This makes the lava flow slowly and build up steep cones.

* Explosive eruptions: The trapped gases in the viscous lava can build up pressure, leading to explosive eruptions.

Examples of shield volcanoes:

* Mauna Loa in Hawaii

* Kilauea in Hawaii

* Olympus Mons on Mars

Therefore, the lava that forms shield volcanoes has low silica content and low water content, making it very fluid and contributing to the characteristic shape of these volcanoes.