A mudflow is the downhill movement of soft, wet earth and debris made fluid by rain or melted snow. Mudflows occur when water mixes with soil and rock. They are most common in mountainous regions when a long dry season is followed by heavy rains.
Mudflows from volcanic eruptions, also known as lahars, are the most dangerous. This type of mudflow is composed of a slurry of hot gases, rocky debris and water. Lahars are as thick as liquid concrete and can move up to 80 miles per hour. They can be caused by lava, melting snow and glaciers during an eruption.