Deborah

As magma cools down slowly, underground at depth, crystals of minerals grow. That changes the viscosity of the magma and its chemical composition. The longer it sits underground, the more ends up like lumpy sticky rock porridge, which can trap its gases. When that magma reaches the surface, the eruption is more explosive and destructive because of those trapped gases, then if it had been simple mafic, runny magma. I was also taught that any eruption that interacted with water or wet rock was 4 times as explosive than if no water was involved, because of the expansion of water into steam.