Geossítio 13 Espinho Quartzodiorite (Cebola Stones)
Rocks that “peel” like onions…
This geosite can be seen on the slope of the road, near the village of Chão de Espinho. Here you can understand how the rounded blocks that persist in the landscape around this site are formed: initially, a network of fractures (diaclases) is generated that divides the massif into blocks similar to parallelepipeds. Over time, the wear and erosion of the blocks (especially at the vertices and edges) causes the rock to flake off in more or less concentric layers, as if they were onions.
Here we see the rock that outcrops throughout the central area of Arouca, namely in the historic center, at Calvário or at Viveiros da Granja, the “Granitoide de Arouca”. It was once widely used in the construction industry, particularly in the building of the Monastery and other old buildings in the historic center of Arouca.
The mythical “Maratona Filhos da Freita” and “Dolorosa de Moldes” trail races pass through Pedras Cebola.
Este geossítio faz parte do itinerário A: Freita, The Enchanted Mountain.
Saiba mais sobre a Rota dos Geossítios .
Informação Geológica
The Arouca Massif, the rock found at the Pedras Cebola geosite, corresponds to a biotite granitoid rock with plagioclase, varying in composition from granodiorite to quartzodiorite, with an ellipsoid shape whose major axis runs in a WNW-ESSE direction. It is considered to be a late- to post-D3 granite, which would correspond to an age of between 321-290 Ma for the granitoids of the Central Iberian Zone (ZCI), according to studies published in 2019.
Here we observe a still early stage in the evolution of the orthogonal fracturing of the Quartzodiorite Massif. The diaclastic network of this body divides it into larger and smaller blocks, with the appearance of coarse parallelepipeds. In turn, weathering is more pronounced on these diaclastic planes, due to the more intense circulation of seepage water there. Since the degree of weathering at the vertices is higher than at the edges, the blocks tend to become increasingly rounded and move away from each other. In addition, the spheroidal or “onion-skin” disjunction can be clearly seen here, and it is also noticeable that the outermost surface of the block is very sandy, in contrast to the core of the less altered quartzodiorite block.
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