When a volcano is at least partially created by its lava flows, the viscosity of that lava can determine its steepness. What viscosity must a shield volcanoes lava be, with its broad, non-steep sides?

Low Viscosity magma

Explanation:

Low viscosity magma is usually thinner in comparison to the high viscosity magma and comprised of relatively lesser amount of dissolved gases. They move easily without any obstruction at a faster rate. When any volcanic eruption occurs where there is low viscosity magma, it allows the magma to move in thin layers and rapidly and quietly.

Thus, a shield volcano is comprised of a broad, non steeper sides with low viscous magma.