Semeru Volcanoe

MOUNT SEMERU, among the most active of Indonesia’s 120 live volcanoes, erupted recently, spewing hot clouds of volcanic ash and debris.

Volcanoes erupt because of how heat moves beneath Earth’s surface. Deep within the planet, it is so hot that rocks slowly melt to become a thick, flowing substance called magma. Since it is lighter than the solid rock around it, magma rises, collecting in chambers near the surface.

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 exited onto the surface is called lava.

The explosivity of an eruption depends on the composition of the magma. Thin, runny magma allows gases to escape easily; upon eruption, such magma simply flows out of the volcano (like in Hawai’i). Thick, sticky magma does not allow gases to escape, and is thus much more explosive. Such eruptions — called explosive eruptions — can blast towers of ash miles into the air, as seen at Semeru.

These explosions can hurl house-sized boulders (like with Semeru).