Quarzodiorite balls from Viveiros da Granja (Pedras Cebola)

Geossítio 14 Quarzodiorite balls from Viveiros da Granja (Pedras Cebola)

The chaos of blocks in the Viveiros da Granja…

In the area of the former Viveiros Florestais da Granja, huge rounded blocks stand out, the result of the natural wear and erosion of the rock that outcrops here: the quartzodiorite of Arouca, also known as the Arouca Massif. The most resistant blocks remain in the landscape, whose rounded shape is due to the spheroidal or “onion-skin” disjunction caused by the outer layers of the rock standing out, as if they were the layers of an onion. This geosite is part of the 13.3 km long, circular PR4 short-route geotourism trail, which takes you to discover the northern slope of Serra da Freita. The mythical trail races “Maratona Filhos da Freita”, “Dolorosa de Moldes” and “Ultra Trail da Serra da Freita” pass through Viveiros da Granja.

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 is in a WNW-ESSE direction. It is considered to be a late to post-D3 granite, which for the granitoids of the Central Iberian Zone (ZCI) corresponds to an age of between 321-290 Ma, according to work published in 2019. This unique geological occurrence of rounded blocks that persist in the landscape is the result of the weathering and erosion of the rock mass, the result of the already advanced orthogonal fracturing of the quartzodiorite massif and the spheroidal disjunction of the rock. The blocks most resistant to the physical and chemical alterations to which the massif has been subjected are those that persist in the landscape, in the form of chaotic blocks after the surrounding terrain, previously sandy and removed, has been lowered.