Primary lahars are flows which are formed as a direct result of a volcanic eruption. They tend to be bulky (107–109 m3) and record high speeds (>20 m/s). Their maximum flows are commonly between 103–105 m3/s. These features provide the ability to flow long distances, even hundreds of kilometers downstream.

How often do lahars occur?

2. The size and frequency of lahars— During the past few millennia lahars that have reached the Puget Sound lowland have occurred, on average, at least every 500 to 1,000 years. Smaller flows not extending as far as the lowland occur more frequently.

What causes primary lahar?

Primary lahars often result from interaction of water with pyroclastic flow deposits. As a result the lahars may be very hot (Figure 56.2(B)); temperatures close to boiling have been recorded. This can increase the hazard as the high temperatures lead to scalding of people and animals caught in the lahar.