What defines a shield volcano?

A shield volcano is characterized by its broad, dome-shaped profile and primarily composed of basaltic lava flows. The lava that erupts from shield volcanoes is typically low in viscosity, allowing it to flow easily and travel long distances. This results in the gentle sloping sides that are hallmark features of shield volcanoes. The basaltic composition contributes to the fluid nature of the lava, leading to the formation of large, shield-like structures that can cover extensive areas.

In contrast, steep slopes usually indicate a different type of volcano, like stratovolcanoes, which are built up from layers of volcanic rock and pyroclastic materials. Symmetrical cones are also typical of stratovolcanoes rather than shield volcanoes. Layering with igneous intrusions usually describes dikes and sills within the crust and does not pertain to the surface features of shield volcanoes specifically. Hence, the broad, dome-shaped structure formed by basaltic lava is what distinctly defines shield volcanoes.