The shape of shield volcanoes is a function of

proportion of pyroclastic debris included in the edifice

lava viscosity

local variations in the tilt of the land surface

the explosive nature of the eruptions that produce the shield

lava viscosity

Explanation:A new shield volcano forms when a crack on the surface of the Earth begins erupting thin layers of basalt lava. Over a long period of time the thin layers of lava erupted from the volcano's vent creates a shield shaped volcano. Shield volcanoes have gently sloping sides.