среда, 10 января 2018 г.

Stones that give birth Castanheira is a small village in…

Stones that give birth

Castanheira is a small village in Portugal that hosts a very rare geological phenomenon called pedras parideiras, which translates into English as stones that give birth, or rock delivering stones, if you prefer. What happens here is that small rocks are expelled from a mother-stone: a granitic block that measures roughly 1000×600 meters.

The small nodules (or baby-rocks as I prefer to call them) can vary between 1 to 12 cm in diameter. They are discoids and biconvex in shape (some of them remind me of a turtle shell) and they are constituted by the same minerologic elements of the granitic mother-stone. The baby-rock’s outer layer is composed of biotite and the inner layer has a quartz and potassium feldspar nucleus. The nodules are expelled by the mother-stone due to physical weathering (temperature change and freezing/thawing) which causes the nodules to detach and spread out. When they do so, they leave a dark relief on the surface of the mother-stone. That relief is full of biotite, the main component of the nodule’s outer layer.

This rare phenomenon of granitization has been occurring for 320 million years and each nodule requires hundreds of years to “be born”.

Nowadays, we no longer see as many nodules as in the past. This geosite in the village of Castanheira is part of the Arouca Geopark and was recognized by UNESCO by its geological importance, but despite the warnings not to take any rocks, visitors still take samples with them. Unfortunately, people are not aware of the significance of this phenomenon and how rare it is, since this is the only place in the world where it occurs with these characteristics. A fertility legend does not help either to keep the nodules in place: the legend says that if a woman wants to become pregnant, she can increase her chances by placing one of these baby-rocks under her pillow.

A similar phenomenon occurs near S. Petersburg in Russia. Also very strange is the case of the cliff that “lays eggs” in south-east China. The stones look like eggs and are expelled from a cliff every 30 years or so, according to local residents. Here the rocks are much bigger and heavier than the “babies” from the portuguese version: the spheres can vary between 30 to 60 cm in diameter and could weight an outstanding 300 kg. Scientists have a few theories for the origin of these “stone-eggs”, but the reason for this occurrence is still not well known and more studies will be required to come up with an official explanation. Similar to the portuguese stones, an ancient legend is also part of this geological singularity: locals believe these rocks bring good fortune and help produce baby boys and that is why they bring them home and worship them.







Photo credits:

http://bit.ly/2mb4cz3 – photo by Cssantos

http://bit.ly/2m4lvRj – photo by Ricardo Oliveira

http://bit.ly/2m6G4N3 – photo by Cláudio Franco/claudiofranco.net

http://bit.ly/2FdtyEi – photo by Cssantos

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