Volcanic rock balls at Pico do Areeiro

In the uppermost region of Madeira island, near Pico do Areeiro, there is a rock formation that has rock balls embedded in it. This interesting balls can be seen in its original place, embedded in a layer of rock, or they can be seen nearby standing on the floor after they have weathered out and fell down.

In a previous post I have also reported rock balls (that could be accretionary lapilli) that were found in an ash layer. But these balls are different from those: the host rock is different, their size is bigger, its color is different and the texture is also different.

The diameter of the balls range between 2 and 6 cm. They are rounded, some of them are quite spherical. Its size and color reminds meatballs, its texture is also similar to the texture of meatballs but the surface also presents wrinkles and cracks/grooves, much like they had been baked at high temperatures.

The balls are scattered in the rock layer, distributed along the layer’s height:




The layer is completely shattered, presenting cracks that are compatible with the process of frost weathering (expansion of ice when water freezes that puts considerable stress on cracks and crevices resulting in the fracturing of the rock):


Is it a case of spheroidal weathering?

The texture of the balls could also suggest spheroidal weathering but I think this is not the case. Why? Because if we observe the balls embedded in the original rock, there is no onion skin, no peeling layers around them. The fractures around the balls are not concentric, they are radially disposed around the balls. The internal structure of the balls is not layered either:


Are they cannonballs bombs?

What are cannonballs bombs?

Cannonballs bombs are volcanic bombs that have a distinctive rounded shape that is produced by post-impact mechanical rounding processes when the bombs roll at high speed down the slopes of a scoria cone.

Volcanic bombs typically have smoother and harder surfaces. However, if they actually were volcanic bombs they had to somehow end up together in that layer. Yes… the volcanic bombs could end up in the bottom of a volcano slope, where the flowing lava would incorporate and bake them as it flowed over… and this would explain their baked appearance…

But maybe they are not cannonballs either. These balls are not rock solid as volcanic bombs use to be. And the mineral composition of the balls does not seem to be very different from the surrounding rock.

Concretions balls?

Another possible explanation is that the balls are concretions. If there is enough porosity inside the rock, chemical elements can migrate and new minerals can form. A concretion can form inside such rock when a mineral began to form and grow around a nucleus – in a porous rock the new mineral can fill spaces between particles and act as a cement (this would explain the spherical form of the balls without the need of having balls rolling down a slope). Then the frost weathering would act on that layer of rock producing cracks, the cracks would develop around the concretions. The surrounding rock then would be weathered out exposing the balls. Once exposed the process of frost weathering would also act on the balls fracturing its surface giving them its cracked appearance.

I am not sure how the volcanic balls were formed, to understand their formation it would be helpful to know better what is the composition of the balls and the surrounding rock.

For now what I know is that some of them also look quite nice standing on a shelve where they will not be subjected to frost weathering anymore.

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