The 2010 eruption is the largest explosive event at Merapi volcano since 1872. The high energy of the initial 26 October explosions cannot be explained by simple gravitational collapse and the paroxysmal explosions were preceded by the growth of a lava dome not large enough to ensure significant pressurization of the system. We sampled pumice from these explosive phases and determined the pre-explosive depths of the pumices by combining textural analyses with glass water content measurements. Our results indicate that the magma expelled was tapped from depths of several kilometers. Such depths are much greater than those involved in the pre-2010 effusive activity. We propose that the water-rich magma liberated enough gas to sustain the explosivity. Our results imply that the explosive potential of volcanoes having dome-forming, effusive activity is linked to the depth from which fresh magma can be evacuated in a single explosion, regardless of the evacuated volume.