Geologists say the underground blast at Yellowstone on July 23 was caused by a buildup of steam and hot water, which propelled rocks hundreds of feet into the air.

The analysis confirms the USGS’s initial conclusion that the blast was caused by an accumulation of hot water and steam in underground conduits that had gotten clogged by a buildup of silica. It’s a similar process to what occurs in geysers like Old Faithful, but unlike geysers, there is no well-worn path for the steam and water to travel to the surface. Instead, the pressure built up until it was stronger than the rock above it. Then, kaboom!