How do volcanoes produce lahars that can race down valleys like wet concrete?

Lahars form when loose volcanic ash and rock mix with water from heavy rain, melting snow and ice, crater lakes, or broken dams. Because the mixture is dense and fluid, it can surge down valleys at highway speeds or faster, carrying boulders and debris. The 1985 Nevado del Ruiz disaster in Colombia is one of the deadliest lahar events in history, killing about 23,000 people when a lahar overwhelmed the town of Armero. Lahars can happen during an eruption or long after one, when weather remobilizes old deposits. That is why volcano hazard maps often focus on river channels leading away from a volcano.