Why are some volcanoes so extremely dangerous?

Suddenly and entirely without warning, the Merapi volcano on the island of Java in Indonesia can erupt violently. In the last 100 years alone, over 1,500 people have died as a result of it. In the “ROTT'N'ROCK” research project, volcano researcher Valentin Troll seeks answers to why some volcanoes are so unpredictable. Rock samples taken from Merapi show how the hot and acidic gases emanating from the volcano cause the rock to disintegrate. After some time, the volcano becomes so unstable that it can no longer withstand the pressure of the magma inside and explodes in a violent eruption.