From the U.S. Geological Survey Volcano Hazards Program FAQs:

"Definitely. Many stratovolcanoes have a plumbing system of hot acid water that progressively breaks down hard rock to soft, clay-rich material. The volcano is gradually weakened, and large parts may suddenly fail. Resulting water-rich landslides are especially dangerous because they can occur without any volcanic or seismic warning.

The risk of mudflows [lahars] formed this way is especially high along rivers downstream from Mount Rainier, because of the large population on floodplains, the huge weakened edifice of the volcano, and a long history of large flows that occurred when the volcano was otherwise dormant."

Learn More

• Types of Volcano Hazards (Website), U.S. Geological Survey
  An overview of the hazards posed by volcanoes

• Volcanoes and Lahars (Website), Washington Department of Natural Resources
  Resources to learn more about volcanoes, their hazards, who is at risk, and how to prepare for emergencies. Particular focus on volcanoes in Washington.

• Volcano Hazard Assessments (U.S.) (Website), U.S. Geological Survey
  Published hazard assessments for many individual volcanoes in the United States

• Investigating Volcanic Landslide Hazards (Case Study), American Geosciences Institute
  Case study demonstrating how modern surveying methods are used to assess volcanic landslide and debris flow hazards

• Identifying and Reducing Landslide Risk (Webinar) American Geosciences Institute
  2019 webinar providing an overview of landslide risk assessment, mapping, and impacts in the United States, and featuring case studies of work done in and with communities to reduce landslide risk.