Eruption column

An eruption column is a cloud of hot volcanic ash suspended in volcanic gas emitted during an explosive volcanic eruption. The ash forms a column that may rise many kilometres into the air above the vent of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km, 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 air convection, and instead falls down the flanks of the volcano to form a pyroclastic flow or surge. On some occasions, if ash isn't dense enough to fall, it may create pyrocumulonimbus.

Eruption columns form in explosive volcanic activity, when the high concentration of volatile materials in the rising magma causes 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.

The solid or liquid material in an eruption column is lifted by processes which vary as the material ascends:

The column will stop rising once it attains an altitude where it is no longer less dense than the surrounding air. 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 6-7 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 55 km (34 mi). In practice, column heights ranging from about 2–45 km (1.2–28.0 mi) are seen.

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