Based on the new image with the red circle highlighting the unusual formation, and your description of the ash expanding, then contracting into a column, changing color, and expanding again, you are observing a fascinating and complex interaction within the volcanic eruption column, likely involving:
* Vortex Rings or Eddies: The most striking feature highlighted is the swirling, somewhat tubular or ring-like structure. This strongly suggests the formation of vortex rings or large-scale eddies within the turbulent ash plume.
* How they form: As the massive column of ash and gas rises rapidly, it creates significant turbulence. The friction between the rising column and the surrounding cooler air, as well as variations in the column's ascent speed, can cause the edges of the plume to curl inwards and form these swirling structures. They are essentially localized rotating masses of ash and gas.
* Why they are visible: The "ring" becomes visible because the ash particles within the vortex are concentrated and often move in a more organized, helical path, making the boundary of the vortex distinct from the surrounding less organized ash.
* Density and Temperature Gradients:
* "Expanding then contracting into a column": This could be a visual effect of the vortex forming. As the ash enters the core of a forming vortex, it might appear to "contract" into a denser, more defined "column" or ring-like shape, before the vortex itself expands outwards.
* "Changing color":
* Ash Concentration: Denser concentrations of ash within the vortex can appear darker due to more light absorption.
* Particle Size/Composition: Different parts of the plume might contain varying concentrations of coarser or finer ash particles, or even different mineral compositions, which can affect their light-scattering properties and thus their perceived color.
* Water Vapor/Condensation: As the hot, gas-rich plume rises, the water vapor within it can cool and condense. If certain parts of the vortex are particularly rich in condensing water vapor (forming tiny cloud droplets), they might appear lighter or whiter compared to areas dominated by dry ash.
* Interaction with Atmospheric Layers:
* Even within the main umbrella cloud, different sections can be affected by localized temperature inversions or wind shear at various altitudes, contributing to the formation and persistence of these vortex structures.
In summary, the "strange place" you've highlighted is very likely a large vortex ring or a series of strong eddies forming within the turbulent volcanic eruption column. This is a common, though visually dramatic, phenomenon in powerful explosive eruptions, where the immense energy creates complex flow patterns in the atmosphere. The changes in density, concentration, and possibly condensation within these vortices contribute to the observed "expansion, contraction, color change, and re-expansion" effect.
