Volcanic eruptions can be explosive or gentle, depending on the type of magma and gas trapped inside.

Some magma is runny and flows easily, causing gentle eruptions where lava pours out slowly. Other magma is thick and sticky, trapping gases more tightly. When pressure builds in this thick magma, it can explode violently, sending ash and rocks high into the air. For example, shield volcanoes like those in Hawaii have gentle lava flows, while stratovolcanoes like Mount Fuji erupt explosively. The amount of gas and magma type controls how the volcano behaves.