Eruption column

An eruption column consists of hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many kilometres into the air above the peak of the volcano. In the most explosive eruptions, the eruption column may rise over 40km, penetrating the stratosphere. Stratospheric injection of aerosols by volcanoes is a major cause of short-term climate change.

A common occurrence in explosive eruptions is for column collapse to occur. In this case, the eruption column is too dense to be lifted high into the air by the force of the explosion, and instead falls down the flanks of the volcano in the form of a pyroclastic flow.

Formation of eruption columns

Eruption columns form in explosive volcanic activity, when the high concentration of volatile materials in the rising magma caused it to be disrupted into fine volcanic ash and coarser tephra. The ash and tephra are ejected at speeds of several hundred metres per second, and can rise rapidly to heights of several kilometres, lifted by enormous convection currents.

Eruption columns may be transient, if formed by a discrete explosion, or sustained, if produced by a continuous eruption or closely spaced discrete explosions.

Column heights

Several factors control the height that an eruption column can reach. Intrinsic factors include the diameter of the erupting vent, the gas content of the magma, and the velocity at which it is ejected. Extrinsic factors can be important, with winds sometimes limiting the height of the column, and the local thermal temperature gradient also playing a role. The atmospheric temperature in the troposphere normally decreases by about 10 K/km, but small changes in this gradient can have a large effect on the final column height. Theoretically, the maximum achievable column height is thought to be about 55km. In practice, column heights ranging from about 2-45 km are seen.

Eruption columns over 10-15 km high break through the tropopause and inject ash and aerosols into the stratosphere. Ash and aerosols in the troposphere are quickly removed by rain and other precipitation, but material injected into the stratosphere is much more slowly dispersed, in the absence of weather systems. Substantial amounts of stratospheric injection can have global effects: after Mount Pinatubo erupted in 1991, global temperatures dropped by about 0.5°C. The largest eruptions are thought to cause drops of up to several degrees, and are potentially the cause of some of the known mass extinctions.

Hazards

Column collapse

Eruption columns may be so laden with dense material that they are too heavy to be supported by convection currents. When this happens, the column or more commonly part of the column may collapse under gravity, generating a pyroclastic flow which can travel down the flanks of a volcano at speeds of over 100 km/hour. Column collapse is one of the most common and dangerous volcanic hazards.

Aircraft

Several eruptions have seriously endangered aircraft which have encountered the eruption column. In 1982, two aircraft flew into the upper reaches of an eruption column generated by Galunggung volcano, and the ash severely damaged both the exterior of the planes and their engines. Both craft were forced to descend rapidly due to loss of engine power and make emergency landings in Jakarta.

Similar damage to aircraft occurred due to an eruption column over Redoubt volcano in Alaska in 1989. Following the eruption of Mount Pinatubo in 1991, aircraft were diverted to avoid the eruption column, but nonetheless, ash dispersing over a wide area caused damage to 16 aircraft, some as far as 1000 km from the volcano.