Some volcanoes erupt often and some are inactive for long periods of time. The amount of time between volcanic eruptions depends on the volcano itself, according to the Department of Geosciences at Oregon State University.
Three things that can trigger a volcanic eruption are magma buoyancy, pressure from gases in the magma and magma refilling the magma chamber. When rock melts, it produces magma that is lighter than the rock around it. This causes buoyancy that makes the magma rise to the surface and the volcano to erupt. Magma contains water, sulfur dioxide and carbon dioxide that cause bubbles in the magma, making the magma partially molten. This causes the volcano to erupt explosively. When new magma enters a chamber already filled to capacity with old magma, it pushes the magma to the top and the volcano erupts.