 Its first phase of activity was involved with water, possibly dammed by glacial ice which filled the Clearwater River valley.The volcano heated glacial water then flooded down the volcano's vent, creating violent steam explosions and broken lava fragments.Once the steam explosions had subsided, the broken lava fragments settled back into the glacial water, creating the unvolcano-like form of White Horse Bluff which is mostly made of fragmental volcanic glass called hyaloclastite.