These deposits are most often found at the base of the volcanoes and in some cases they extend many tens of miles from the cone itself. If these deposits are mapped in a region, there is significant concern for safety. They were mobilized by rocks falling from the volcano during a major earthquake. They were ejected from the caldera and could bury a town. A road-cut in Morgantown is only about 6 feet in height, yet it has cost hundreds of thousands of dollars over the years to continually repair damage done by the continued movement of the slope onto the road.
Local folks call it a landslide.
You can now suggest what the material is that is moving and why it began to move. This fall, after weeks of rain, and extensive flooding, at least 321 people were confirmed dead in south eastern Mexico. An entire school disappeared along with the teacher and all but two of her students. The town was built in a caldera. The town was lost as a result of soil piping.