Scientists managed to clarify the geological history of the unusual cave.
Geode Pulpi is located near the Spanish town of the same name, at a depth of about 50 meters. This is one of the most unusual caves on Earth. The underground ovoid cavity, about 11 cubic meters in volume, whose walls are dotted with jagged crystals, resembles Superman's Fortress of Solitude.
Geodes are small cavities in sedimentary or volcanic rocks, partially or completely filled with mineral matter. Usually these formations are small - a few centimeters in diameter. Cavities one meter in size are already considered very large, and the Pulpi geode can be called gigantic. Scientists at the University of Granada have reconstructed the geological history of this geode, mapped the structures around it, and tried to understand how it formed. An article about this research was published in the journal Geology.
The crystals that fill the unique cave are selenite, a type of gypsum characterized by a parallel-fibrous aggregate structure. Over the 20 years that have passed since the discovery of the Pulpi geode, the geochemical origin of these crystals has remained a mystery to scientists. Finding out the mechanism of their formation proved to be a difficult task, because the hydrothermal system of the geode has long been inactive.
Researchers led by geologist Juan-Manuel García Ruiz put forward a hypothesis that could explain the formation of an unusual cavity and selenite outgrowths in it. Most likely, the crystals grew by a "self-sustaining mechanism" due to the continuous supply of salts, which was ensured by the dissolution of anhydrite (anhydrous calcium sulfate).
This process took place at a temperature of about 20 ° C and was reinforced by the so-called Ostwald ripening. This is the condensation of the supersaturated phase of a substance, in which the growth of large grains of crystals occurs due to smaller ones, with the dissolution of drops without their preliminary adhesion.
Dating the formation of the cavity turned out to be another challenge. The selenite in the geode is very clean and practically free of chips, which makes it difficult to study. Scientists suggest that the crystals formed after the Messinian peak of salinity - the almost complete drying of the Mediterranean Sea, which happened about 5.6 million years ago. "They are most likely younger than two million years old, but older than 60,000 years, because that is the age of the carbonate crust covering one of the large selenites," says Garcia Ruiz. This is a fairly large range, and in subsequent studies, scientists will try to narrow it down as much as possible.
This study provides an opportunity to better understand how the geochemical processes leading to the formation of such cavities take place. The work of Spanish scientists shows that structures like the Pulpi geode are not anomalies and can be explained within the framework of existing geological concepts.
Geode Pulpi is not the only interesting underground find in recent years. Earlier, scientists from Mexico discovered a ritual cave of the Maya Indians, and a scientist from Indianapolis University found a new species of spider in the caves of India.