The shields are build form the entire lava or magma and are entirely fluid nature and hence it's named for its low profile as compared to the explosive nature of the stratovolcanoes.
The geologic structure of the volcano can be distinguished from the other three forms by their unique magmatic composition. They have lavas of less fluid or viscous nature.
And the active shield experiences a near-continuous eruption for a longer period that results in a gradual build-up that can reach to very large dimensions.
The shield volcanoes usually have a large shield that slopes to the 2 to 3 degrees that gradually steepens with the gain in height.
