Editing Eruption column (section)

==Structure==

The solid and liquid materials in an eruption column are lifted by processes that vary as the material ascends:<ref>{{cite web | url = http://www.sci.sdsu.edu/volcano/ | title = How volcanoes work – The eruption model (QuickTime movie) | work = San Diego State University | access-date = 2007-06-30 | archive-url = https://web.archive.org/web/20070701000926/http://www.sci.sdsu.edu/volcano/ | archive-date = 2007-07-01 | url-status = dead }}</ref>

* At the base of the column, material is violently forced upward out of the crater by the pressure of rapidly expanding gases, mainly steam. The gases expand because the pressure of rock above it rapidly reduces as it approaches the surface. This region is called the ''gas thrust region'' and typically reaches to only one or two kilometers above the vent.
* The ''convective thrust region'' covers most of the height of the column. The gas thrust region is very turbulent and surrounding air becomes mixed into it and heated. The air expands, reducing its density and rising. The rising air carries all the solid and liquid material from the eruption entrained in it upwards.
* As the column rises into less dense surrounding air, it will eventually reach an altitude where the hot, rising air is of the same density as the surrounding cold air. In this neutral buoyancy region, the erupted material will then no longer rise through convection, but solely through any upward momentum which it has. This is called the ''umbrella region'', and is usually marked by the column spreading out sideways. The eruptive material and the surrounding cold air has the same density at the base of the umbrella region, and the top is marked by the maximum height which momentum carries the material upward. Because the speeds are very low or negligible in this region it is often distorted by stratospheric winds.