According to National Geographic, “Two important traits that characterize volcanoes are explosiveness and viscosity” (“Forces of Nature-Volcanoes”, 1996). Shield volcanoes are not explosive because they have basaltic magma with low viscosity. They also have low slopes connected to a central vent that form a broad domical shape.
Shield volcanoes are very wide. The lower part of a shield volcano has a slope of 2 to 3 degrees; this is flatter than the middle, which has a slope of about 10 degrees. This broad domical shape is a result of the thousand flows of lava that expands from the central vent. Many shield volcanoes in North California and Oregon have diameters of about 3 to 4 miles and heights of 1,500 to 2,000 feet.
Shield volcanoes are not explosive. The explosiveness of a volcano is determined by the viscosity of the magma inside a volcano. “Viscosity is the resistance to flow,” as defined by Professor Stephen A. Nelson in Tulane University (“Volcanoes, Magma, and Volcanic Eruptions”, 2010). Shield volcanoes have basaltic magma, with the lowest viscosity compared to other volcanic magma: andesitic magma and rhyolite magma. In terms of viscosity, the lower the viscosity, the more liquid-like the magma would be. No matter how low the viscosity of magma may be, it is still very much higher than the viscosity of water.
Shield volcanoes were formed by solidified basaltic lava with fluid basaltic lava in the center. During an eruption, this fluid basaltic lava is expelled from the central vent and fissures of the volcano and can travel at speeds up to 12 kilometers an hour. This speed results in the construction of gentle slopes on the side and wide summit areas. Because ninety percent of the volcano is lava with less dissolved gas, there are almost no pyroclastic materials (such as volcanic ashes and cinders) that can cause the volcano to be explosive. Shield volcanoes are only explosive when water gets into the vent.
During an eruption, lava flows out in all directions from the center vent. Some shield volcanoes spill out lava from long fissures instead of the central vent on the summit, this is very common in depressions. As flow after flow of lava floods the encompassing countryside, they form broad plateaus not far away from the volcano. These broad plateaus are called volcanic plateaus or lava plateaus. Lava plateaus of this kind can be seen in Iceland, Southeastern Washington, Eastern Oregon, and Southern Idaho.
To conclude, shield volcanoes have a broad domical shape from its fluid basaltic lava. Basaltic lava prevents shield volcanoes from having pyroclastic materials, thus they are not explosive.