Growing lava domes are unstable and shed hot debris, sometimes in large volumes, that may create pyroclastic flows. These flows may sweep out as far as 12 km (7 mi) from their source. Depending upon the season and snow conditions on the mountain, such events can swiftly erode and melt snow and ice to generate flows of water and sediment called lahars that surge tens of kilometers (tens of miles) down valleys.
Pyroclastic flows can also produce ash clouds that rise thousands of meters (tens of thousands of feet) into the atmosphere and drift downwind for hundreds of kilometers (miles) to produce tephra fall. Dome-collapse pyroclastic flows, tephra fall, and lahars are common on Mount Hood and have occurred during all eruptive periods of the past 30,000 years. They are very likely to occur in future eruptions.