9.2

Collapses can come in many forms

Collapse events can be grouped into three types: those involving a magmatic eruption; those involving non-magmatic explosions, and those that are ‘cold’ and have no volcanic activity associated with them. The first type is called a Benzimianny-type, which refers to the magmatic eruption and collapse of the Benzimianny volcano in Russia in 1955. The second is called a Bandaitype, which refers to the collapse of the Bandai volcano in Japan whose north flank collapsed in 1888 due to steam blasts. The third type is sometimes referred to as an Unzen-type after a small sector collapse occurred in 1792 which did not involve any volcanic activity.

Benzimianny, Bandai, and Unzen type collapses can lead to a variety of events:

  • Landslides: general term for mass movement. Implies a gradual movement rather than the more sudden movement of an avalanche.
  • Debris avalanches: moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope, such as what killed 57 people at Mount St. Helens, USA. As the moving debris rushes down a volcano and into river valleys, it incorporates water, snow, trees, bridges, buildings, and anything else in the way.

A debris avalanche animation

  • Caldera: A large crater formed by a volcanic explosion or by collapse of a volcanic cone. A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it and will collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers.

Caldera demonstration model

Collapse of the Puʻu ʻŌʻō crater floor on March 15th, 2011:

Figure showing the evolution of the Crate Lake caldera in Oregon. (Modified from diagrams on back of 1988 USGS map “Crater Lake National Park and Vicinity, Oregon.” United States Geological Survey.)

  • Sector and flank collapse: flank collapse does not involve the magmatic conduit, while sector collapses takes most of the top of the volcano off.

Figure showing the difference between a shallow flank collapse and a more catastrophic sector collapse (Courtesy of Tibaldi, A.)

  • Lahars: If the moving debris is water-saturated, it is called a lahar, or mudflow. These can be extremely deadly, such as the lahar from Nevado del Ruiz volcano in Colombia which killed more than 20,000 people. The video below shows the power a lahar can generate as a lahar moves down the Curah Lengkong river in January, 2002, at the Semeru volcano in Indonesia. The channel is 25 m across. (Franck Lavigne, 2002)

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