"The same substance emitted from a volcano is rhyolite, and characterizes the explosive Pelee-Krakatoa-St. Helens type of eruption. Actually, dacite is quite predominant at Mount St. Helens. The explosive nature of St. Helens is because the magma is like that of your mother's home-made fudge, as compared to the molasses-type magma emitted by the Hawaiian volcanoes. St. Helens eruptions are quite explosive because "the fudge" caps building pressures up to a point when all hell breaks loose."