Final answer:
A pyroclastic flow is the volcanic hazard that can travel long distances and set buildings on fire before contact, much more dangerous than the effusive lava flows seen in places like Hawaii.
Step-by-step explanation:
The volcanic hazard described that can flow tens of kilometers and catch buildings on fire before making contact is known as a pyroclastic flow. This high-density mix of hot lava blocks, pumice, ash, and volcanic gas can reach very high speeds, and as it moves it can engulf and ignite buildings due to the extreme heat. Unlike lava flows which typically move slower allowing for some measures of evacuation, pyroclastic flows can occur with little warning and are one of the most deadly hazards associated with explosive volcanism. People living near volcanoes like those in Hawaii are more familiar with less hazardous, effusive eruptions that produce relatively gentle flows of lava that slowly run down the sides of volcanoes.