There are actually very few volcanoes that do not explode, but the ones that do explode with a high sound and lots of hot magma, and some are very low and don't have a lot of magma. An explosive eruption always begins with some form of blockage in the crater of a volcano that prevents the release of gases trapped in highly viscous andesitic or rhyolitic magma. The high viscosity of these forms of magma prevents the release of trapped gases.