Eruption plumes normally form during volcanic eruptions and are always formed during subglacial eruptions if the eruption can melt its way through the ice. The plume carries large amounts of gases and tephra into the atmosphere, but a large part of the tephra falls close to the volcano. The ash makes the plume darker, but the plume is lighter in color where it is largely made up of gases, mainly water vapor.

The plume rises from the eruption site due to the gas thrust from the eruption and is then a mixture of tephra and gas. At this stage the plume is denser than the surrounding atmosphere, but as it rises, the plume takes in high quantities of ambient air and the density gets reduced. If the mixing continues, the plume can become positively buoyant and then continues to rise convectively. This transition happens at an altitude of a few hundred meters to a few kilometers, depending on the eruption strength (power of the gas thrust), and normally makes up the largest part of the plume. The plume eventually loses its buoyancy and starts to spread out, with wind controlling the direction of the plume. At this stage, only the finest ash particles are still in the plume, with larger fragments already having fallen out and settling near the eruption site.

Although the eruption of Eyjafjallajökull was a medium-size eruption with the volcanic plume rarely rising above 7 km, it was unusual in its duration and amount of ash dispersed to the south and southeast of Iceland. During the first two days of the eruption, the wind forced the plume downwards. That, coupled with a relatively weak gas thrust yet high amount of tephra in the plume, caused the plume to be rather low above the eruption site and a “close to ground” transport of ash occurred. The photo was taken on the 16th of April and shows the low-lying plume and the “close to ground” transport.