Accretionary lapilli?

Recently I came across these little stone balls that made me thought of accretionary lapilli.


What are accretionary lapilli?
Accretionary lapilli are rounded tephra balls. Formed during volcanic eruptions when ash is thrown into the air, they grow when ash particles join together and more ash particles accumulate around a central nucleus. Particles are brought together by means of surface tension (water from atmospheric moisture) and/or electrostatic attraction. This way ash aggregates into balls which usually have an internal structure formed by distinct concentric layers. The particles of ash can be firmly cemented before reaching the ground (the cement is thought to be the result of a chemical reaction between sulfuric acid and the volcanic rock) which allows them to survive the impact. If not well cemented, the accretionary lapilli can be flattened when hit the ground or desintegrate on impact. If they fall on loose ash they can roll on it and grow like a snowball.

The samples were found on a tephra layer on Camara de Lobos. In the following picture you can see the dark layer of ash where the balls were found:

The balls are not so dark as the surrounding material. In fact if we look closer there seems to be a white cement gluing the dark particles of basalt of which the balls are made. They are hard, I can’t crush them between my fingers.

I broke some to see if they presented the internal structure formed by concentric layers:

As you can see no concentric layer is perceivable. Maybe the particles are not enough small to form the typical concentric layers. Maybe they were not formed in the air. Maybe they formed by “snowball” rolling on the top of the dark ash layer (as you can see in the image of the ash layer, the balls are found mainly closer to the top of that layer). Or maybe they were formed by some other process. Maybe by some mineralization process that occurred after the ash layer have been settled.

It does not seem to be the product of water erosion either. Pebbles can be rounded by a water stream or waves in a beach. So, pieces of material from a different tephra layer could have been rounded by water and end up at the top of that dark ash layer. However, usually water does not round pebbles to a ball form, and some of the balls I found are connected in pairs ruling out the process of water erosion. This connection in pairs suggest that the balls were not mechanically rounded by water but rather were in the process of growing when one ball touched the surface of another ball.

I am not sure if they actually are accretionary lapilly or concretions of some sort, but some interesting process must have been at work to form such curious rounded stones, some of them are almost perfectly spherical.

PS – In another location there are bigger volcanic rock balls embedded in a rock layer.

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