Around 700 million years ago, the Earth cooled so much that scientists believe massive ice sheets encased the entire planet like a giant snowball. This global deep freeze, known as Snowball Earth, endured for tens of millions of years.
Yet, miraculously, early life not only held on, but thrived. When the ice melted and the ground thawed, complex multicellular life emerged, eventually leading to life-forms we recognize today.
The Snowball Earth hypothesis has been largely based on evidence from sedimentary rocks exposed in areas that once were along coastlines and shallow seas, as well as climate modeling. Physical evidence that ice sheets covered the interior of continents in warm equatorial regions had eluded scientists – until now.
In new research published in the Proceedings of the National Academy of Sciences, our team of geologists describes the missing link, found in an unusual pebbly sandstone encapsulated within the granite that forms Colorado’s Pikes Peak.
Earth iced over during the Cryogenian Period, but life on the planet survived.
NASA illustration
Solving a Snowball Earth mystery on a mountain
Pikes Peak, originally named Tavá Kaa-vi by the Ute people, lends its ancestral name, Tava, to these notable rocks. They are composed of solidified sand injectites, which formed in a similar manner to a medical injection when sand-rich fluid was forced into underlying rock.
A possible explanation for what created these enigmatic sandstones is the immense pressure of an overlying Snowball Earth ice sheet forcing sediment mixed with meltwater into weakened rock below.
Dark red to purple bands of Tava sandstone dissect pink and white granite. The Tava is also cross-cut by silvery-gray veins of iron oxide.
Liam Courtney-Davies
An obstacle for testing this idea, however, has been the lack of an age for the rocks to reveal when the right geological circumstances existed for sand injection.
We found a way to solve that mystery, using veins of iron found alongside the Tava injectites, near Pikes Peak and elsewhere in Colorado.
A 5-meter-tall, almost vertical Tava dike is evident in this section of Pikes Peak granite.
Liam Courtney-Davies
Iron minerals contain very low amounts of naturally occurring radioactive elements, including uranium, which slowly decays to the element lead at a known rate. Recent advancements in laser-based radiometric dating allowed us to measure the ratio of uranium to lead isotopes in the iron oxide mineral hematite to reveal how long ago the individual crystals formed.
The iron veins appear to have formed both before and after the sand was injected into the Colorado bedrock: We found veins of hematite and quartz that both cut through Tava dikes and were crosscut by Tava dikes. That allowed us to figure out an age bracket for the sand injectites, which must have formed between 690 million…
