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Home / Science / The world's largest geode, formed when the Mediterranean disappeared, reveals a new study

The world's largest geode, formed when the Mediterranean disappeared, reveals a new study



In a deserted mine in southern Spain, there is a room of pure crystal.

To get there, you have to descend deep into tunnels, climb a ladder into an unobtrusive hole in the rocks and squeeze yourself through a jagged tube of gypsum crystals barely wide enough for a person. If you make it this far, you'll be in the largest geode in the world: the Pulpí Geode, a 11-cubic-foot cavity littered with ice-clear crystals and sharp as spears on any surface.

While you may never have been in a geode before, you've probably held one or at least seen it before.

"Many people have small geodes in their homes," Juan Manuel García-Ruiz, a geologist at the Spanish National Research Council and co-author of a new paper on the history of the Pulpí Geode, told Live Science. "It is usually defined as an egg-shaped cavity within a crystal-lined rock."

These crystals can form after water seeps through tiny pores in the surface of a rock, transporting even minuscule minerals into the hollow interior. Depending on the size of the rock cavity, crystals can continue to grow for thousands or millions of years, forming caches of amethyst quartz and many other shiny minerals.

The crystal columns at Pulpí are made of gypsum ̵

1; the product of water, calcium sulfate and a lot of time – but since the Geode's unexpected discovery in 2000, not much else has become known about them in one study Published in the magazine Geology on October 15, García-Ruiz and his colleagues tried to shed new light on the mysterious cave by limiting how and when the geode formed.

The Crystal map

García-Ruiz is no stranger to giant crystals. In 2007, he published a study about Mexico's fantastic Crystal Cave a basketball court-sized cave of plasterboard buried as telephone poles buried 300 meters below ground. City of Naica. Uncovering the story of this "Sistine Chapel of Crystals," as García-Ruiz called it, was facilitated by the fact that the crystals still grew in the wet guts of the mine.

In Pulpí, however, the mine was completely dry, and the Geode's crystals had not grown for tens of thousands of years. In addition, Geode's gypsum spines are incredibly pure – so transparent that "you can see your hand through them," García-Ruiz said. This means that they do not contain enough uranium isotopes to perform radiometric dating. This is a standard method of analyzing how different versions of elements radioactively decay to date very old rocks .

A researcher is overshadowed by telephone pole-sized plaster columns in Mexico's Crystal Cave.

[Photocredit:AlexanderVanDriessche/CreativeCommons30)

"We had no idea what had happened," said García-Ruiz. "So we had to do a cartography of the entire mine to understand the very complicated geology."

The researchers analyzed and dated rock samples around the mine for seven years to find out how the area had changed since its formation hundreds of millions of years ago. The driving question of the team: Where does the calcium sulfate in the Pulpí Geode come from?

Eventually, the researchers reduced the geode formation to a window of about 2 million years (not bad for the 4.5 billion year old calendar) of geological time). The crystals must be at least 60,000 years old, the team thought, because this was the latest age of a piece of carbonate crust growing on one of Geode's largest crystals. Since the crust is on the outside of a crystal, the crystal underneath must be even older, said García-Ruiz.

In the meantime, the composition of other minerals in the mine suggests that calcium sulfate was introduced into the area only after one hour. Event called Messinian Salinity Crisis – the almost complete emptying of the Mediterranean before about 5.5 million years ago was suspected.

Due to the size of the gypsum crystals, it is likely that they began forming less than 2 million years ago through a very slow-growing process called Ostwald ripening in which large crystals form by dissolution of smaller, said García-Ruiz. Take a look into your freezer to get an everyday example of this process. When the ice has passed its heyday, small ice crystals begin to break away from the rest of the enjoyment. Over time, these small crystals lose their shape and recombine into larger crystals, giving old ice a markedly gritty texture.

The Pulpí Geode may not be as tasty as ice cream, but just knowing that there are magical places like this brings with it its own sweet satisfaction. Partly thanks to the research team's mapping efforts, tourists can now visit the Pulpí Geode, and García-Ruiz certainly would not blame you. García-Ruiz squeezed past the jagged gypsum gate into the cave of the Geode for the first time a few years ago and remembered a feeling: "euphoria".

Originally published on Live Science more science? You can get 5 editions of our partner magazine "How It Works" for 5 USD for the latest amazing scientific news.

(Credit: Future plc)


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