Semeru erupts: What causes volcanic eruptions? Why are some more explosive?

As these chambers fill up, pressure exerted on the surrounding rock increases. Eventually, some of the magma pushes through the cracks in the rock, and vents onto the surface. This is a volcanic eruption; the magma that has erupted is called lava.

The explosivity of an eruption depends on the composition of the magma. Thin, runny magma allows gasses to escape easily; upon eruption, such magma simply flows out of the volcano (like in Hawai’i). Thick, sticky magma does not allow gasses to escape, and is thus much more explosive. Such eruptions see magma blast into the air, solidifying into deadly tephra, which can range in size from fine dust to massive, house-sized boulders (like with Semeru).