Column Heights
The column will stop rising once it reaches 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. 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.
Eruption column heights are a useful way of measuring eruption intensity since for a given atmospheric temperature, the column height is proportional to the fourth root of the mass eruption rate. Consequently, given similar conditions, to double the column height requires an eruption ejecting 16 times as much material per second. The column height of eruptions which have not been observed can be estimated by mapping the maximum distance that pyroclasts of different sizes are carried from the vent—the higher the column the further ejected material of a particular mass (and therefore size) can be carried.
The approximate maximum height of an eruption column is given by the equation.
- H = k(MΔT)1/4
Where:
- k is a constant that depends on various properties, such as atmospheric conditions.
- M is the mass eruption rate.
- ΔT is the difference in temperature between the erupting magma and the surrounding atmosphere.
Read more about this topic: Eruption Column
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