Final answer:
Lahars are formed when volcanic ash mixes with water, resulting in debris flows that can cause significant environmental and infrastructural damage.
Step-by-step explanation:
Lahars form when ash mixes with water, typically during a volcanic eruption when volcanic debris flows are triggered. Examples of such events include the 1980 eruption of Mount St. Helens, where massive floods of mud and rock flowed down river valleys. These debris flows can have a devastating impact on the surrounding environment, causing significant damage to structures and posing a threat to life.