Between 717 and 661 million years agoduring the late Proterozoic, the Earth went through the so-called “Sturtian glaciation”. It was the most extreme glacial period that has ever occurred, with ice covering our planet entirely from the North Pole to the South Pole and an average global temperature reaching –50 °C. This theory, developed by the Australian geologist Douglas Mawson in the first half of the twentieth century, it is known as the theory of “Snowball Earth” or “Snowball Earth”. Even if over time numerous testimonies of this very long glaciation have been identified on the planet, decisive proof was missing that in the Proterozoic the ice caps covered the continents even in the equatorial areas. Now a team of geologists from the University of Colorado has managed to provide it by dating details sandstones from the Rocky Mountains in Colorado: they date back to the Sturtian glaciation and were formed due to the weight of the ice sheets on a region that at that time was located at the equator.
The rocks that testify to the Sturtian glaciation
The “snowball Earth” theory has so far been largely based on the discovery of rock materials deposited by glaciers in areas that were in coastal or shallow sea environments in the Proterozoic. Now, however, researchers have found one try even within a continent which in that interval of time was gathered together with the others near the equator to form the supercontinent Rodinia. In particular, at Pikes Peak, the highest peak in the Front Range mountain range Rocky Mountains in Coloradorocks with unusual characteristics had already been identified for some time. It’s about sandstones present in the form of infiltrations within granites, which are believed to have formed from a fluid mixed with sand “injected” into fractures. It has been hypothesized that these sandstones were created by the enormous pressure exerted by the overlying ice cap, which towards the end of the glaciation would have pushed the water coming from the melting of the ice mixed with sediments into the fractures.
The dating of sandstone rocks in Colorado
To confirm the hypothesis these infiltrations, defined as “Tava injectites”, however, they had to be dated. Researchers have only now managed to do this thanks to veins of minerals containing iron (such as hematite), which cut transversally through the sandstones. Iron minerals, in fact, contain very low quantities of radioactive elements present in nature, includinguraniumwhich transforms over time radioactive decay in lead. Since we know the time it takes for uranium to transform, by measuring the relationship between the isotopes of uranium and those of lead present in iron minerals, it was possible to date the latter. As a result, it was possible to trace the sandstones back to an included time interval between 690 and 660 million years agothat is, the Sturtian glaciation.
Now the goal is to identify rocks of this type also in other locations in North America, in order to obtain an overall vision of how the Earth appeared during the glaciation. In recent years, many studies have been carried out relating to the “snowball Earth” theory: attempts have been made, for example, to understand how it was triggered, why it was so intense, why it lasted 56 million years and how some organisms were able to survive. Given the complexity and uniqueness of this event, it will still take time to have a complete reconstruction.
