Debris Avalanche

Debris avalanches usually occur on large, steep volcanoes like Mt Egmont, and are one of the most hazardous but least common of volcanic dangers.  They are mainly caused by instability of the volcano's slope.

When a slope of a volcano is not stable it can easily collapse (possibly triggered by volcanic earthquakes) causing debris to be transported away from the slope.  The bigger the avalanche the bigger its speed and thus its danger.

Landslides

A landslide is a downslope movement of a mass of bedrock under the influence of gravity.  It implies a gradual movement rather than the more sudden movement of an avalanche.

Hazards they Produce

Debris avalanches and landslides can produce numerous dangers.  The mixture of debris from a landslide or avalanche with water may produce harmful lahars. They also can dam rivers or cause flooding.  Perhaps one of the most important hazards that can be produced by avalanches or landslides is a tsunami.

 

Tsunamis

 

Waves from the April 1, 1946 tsunami move up the Wailuku River in Hilo, Hawaii.  Although the tsunami was generated from an earthquake it shows its potential impact: 159 people got killed and around $26 million in damages was caused.

Tsunamis are large sea waves that have long wave periods.  When these waves reach coastal areas, they can go far inland which can cause a lot of damage.

Information supplied by: http://volcano.und.nodak.edu