The most rare geological phenomenon in the World
Rock that gives birth, know as "Pedra Parideira".
How does the phenomenon of Calving Stones occur?
The growth of calving stones is due to thermal oscillations and the action of erosion on biotitic nodules. They eventually break free from the parent rock and accumulate in the soil, leaving a cavity in the granite lined with a biotitic layer.
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| Portugal, calving stones and your nodules. |
This geological process takes a long time, so it can take about 300 million years for the nodules to be released. These have dimensions that vary between 1 and 12 centimeters in diameter and have a core with quartz and feldspar minerals.
In popular parlance, this phenomenon came to be called Calving Stones (pedra parideira), for it refers to a stone that “calving”, from giving birth, bearing small children.
I believe that because it is a rock, the most correct term to use is "calving stone" and not "birthing stone", although both terms are acceptable.
Where can you see Calving Stones?
The Calving Stones can be seen in only two places in the world:
in the city of Saint Petersburg, Russia and in Arouca, Portugal.
So having a Calving Stone nodule in your collection will be even rarer.
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| An inhabitant of the region with several nodules of the Calving Stone. |
In Portugal, the Calving Stones are located in the village of Castanheira, in the heart of the Serra da Freita plateau, divided between the municipality of Arouca and Vale de Cambra. Amidst this idyllic landscape, divided by grazing activities, lies this famous natural and geological heritage, which extends over an area of about 1 km².
The first account described
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| First Geographic Dictionary of villages in Portugal |
In 1751, this phenomenon is described for the first time in the "Geographic Dictionary" (TOMO II: page 505), by Fr. Luiz Cardoso, who describes it based on the reports of the inhabitants:
"Cliffs that the natives call the Stones that stop, deducing the name that these stones throw other small pebbles in certain months of the year, leaving the pits after throwing them."
In Portugal they are known as Pedra Parideira and are a rare geological phenomenon.
The nodules assume discoid and biconvex shapes and are composed of the same mineralogical elements as granite, the outer layer is composed of biotite and the inner layer has a quartz and potassium feldspar core. These nodules, when descaling from the bedrock cores by thermoclast/cryoclast, leave an outer layer in low relief in the bedrock cores and spread around it.
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| Calving stone nodule. |
The Parideiras Stones symbolize fertility in the ancestral tradition of the region, this tradition is still present in the local populations. Sleeping with a birthing stone under your pillow is believed to increase fertility.
They are a rare phenomenon on Planet Earth, which is why visitors to these places are asked not to collect stones for personal use.
Due to erosion, some nodules are released from the "mother stone" and accumulate in the soil, leaving a cavity in the granite. That's why the inhabitants of the village of Castanheira called this rock "Pedra Parideira", for being "the stone that looks like stone".
It was here that the Arouca Geopark, a geosite of extreme international relevance, was installed but little known, even within the country. The objective of this private law association is to conserve, promote and enhance its cultural, natural and geological heritage.
How does the release of the nodules from the mother rock happen?
The explanation for this phenomenon according to José Lobo and Bruno Novo, from Visionarium, thermoclasty is a type of weathering agent, caused by the variability of temperature on the surface of rocky materials, causing a variation in volume.
Envelopes swell as a reaction to elevated temperatures and contract as a reaction to cooling. As rocks are generally polymineralic aggregates, and due to the fact that each mineral presents different values of expansion coefficient, different expansion and contraction speeds arise. The outermost parts of the rocks, subject to strong diurnal thermal amplitudes, fracture.
Disaggregation by gelation is one of the most effective in terms of fracturing, although it is a seasonal mechanism that occurs predominantly in high mountain areas. This agent actively contributes to the “birth” of the biotite nodule. The water contained in the fractures, when the temperature is lower than 0ºC, starts to freeze in the most superficial part. As the outside temperature drops, ice wedges grow inside the fractures. When water freezes, it increases in volume (about 10%), consequently exerting great pressure inside these fractures, causing them to widen and extend. Therefore, it promotes the disaggregation of the rocks, and the consequent “birth” of the biotitic enclave.
The Parideiras Stones gradually emerge on the surface of the rock, come off and accumulate on the ground. For this reason, the peasants of the region call the rock “the stone that stops stone”, that is, the rock that produces another rock.
The assertion of the existence of Birthing stone in other latitudes is not proven.
See more images on Wikimedia Commons
or in Mindat.org
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