For the past one hundred and fifty years, geologists have been trying to find an answer to the riddle of "disappeared time" from the history of the Earth. In some places of the planet, the geological record falls through for a billion years, and this phenomenon has received in the scientific community, somewhere, even the ominous name "Great Disagreement." However, in early January, a group of researchers published an article explaining this mysterious phenomenon. The work was published in the journal "Proceedings of the National Academy of Sciences", and it provides fairly convincing evidence that huge glaciers cut down more than ten kilometers of the Earth's surface during the historical period known as "Snowball Earth".
Great disagreement
“Big disagreement” is not given this name by accident - it is really big, and it really is disagreement. This is a completely official geological term that describes the "vacuum" observed in some parts of the planet in the history of rocks. As we all know or surmise, the further into the earth, the older deposits are found on our way.
However, back in 1869, the geologist John Powell, traveling through the Grand Canyon, discovered that the local rocks lacked a layer of geological record lasting a billion years, immediately preceding the period when life flourished on the planet in all its variety of animal and plant species.
Panorama of the Grand Canyon from the south.
Soon enough, Powell's colleagues found out that this phenomenon was not limited to the southwest of North America. In total, there is not enough land on the surface, just think, about ten billion cubic kilometers of rocks. They just disappeared. This is a quarter of the history of the planet, and for many generations of scientists it was a matter of honor to find out what happened in those distant times.
By and large, there are only two possible scenarios - either for some reason during a giant period of time sedimentary rocks were not formed, or there was a titanic scale of erosion that destroyed the geological record. The researchers, quite logically, looked for evidence in rocks above and below the disappeared layer. In the same Grand Canyon, just above it is the Tapeats Sandstone, which formed about 525 million years ago in the Cambrian period, and below the Vishnu Schist shale, which is more than 1.6 billion years old.
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It is worth noting here that the layers of rocks located above have a much larger volume per unit of time than those below. The lower layers are compressed, since for a huge amount of time they were under great pressure, but stronger than implied by the mass of the layer that is on top. And this is consistent with the erosion hypothesis, since the additional pressure can be explained by the weight of the disappeared layer.
Zircon
But how do you know where all this missing breed has gone? She did not evaporate, in the end! The research mentioned at the beginning of the article approached this issue from an unexpected side - it studied microscopic zircon crystals. This mineral is one of the first to form when molten liquid rock (magma) begins to cool. And it is very durable, so it can withstand geological processes that destroy other minerals.
Zircon.
More important, however, is that zircon at the moment of its formation becomes, as it were, a cast of the ongoing geochemical processes. Scientists can determine the age of these crystals by comparing isotopes of uranium, a very slowly decaying radioactive element. Specifically, in the framework of this study, scientists analyzed the content of oxygen isotopes and a silvery metal called hafnium in zircon crystals. Their ratio is significantly different in the continental and oceanic crust, so this analysis can show where the magma from which the zircon was formed came from - from under the land or from under the ocean.
Snowball Earth
The results obtained by scientists can be shocking - a huge layer of earth, up to 14 kilometers in total, was dumped into the oceans, and then pushed back under the land and processed into magma in the process of subduction. This is consistent with the hypothesis that 650 million years ago, the entire planet, or most of it, was covered in ice. This theory, as we remember, is called "Snowball Ground". It was once considered ridiculous, but it is getting more and more confirmation.
Scientists who have analyzed zircon have also studied ancient impact craters found on Earth. They note that there are many more of them on the planet that appeared after the global glaciation, as giant ice sheets, apparently, simply scraped off smaller traces of asteroid impacts. Naturally, more ancient craters have survived, but they are huge, up to several kilometers deep, and the cataclysms we are talking about today could not destroy them to the ground. Thus, the complex of evidence given by the researchers can really say that giant glaciers 650 million years ago "sanded" the land surface.
Weaknesses of the theory
Of course, this study also has weak points. One of them, obviously, is that the land would have warmed up millions of years before the end of the "Great Discord", so it is not clear why the deposits of this period were not preserved. The authors believe that the ice left nothing to erode, and it took time for the new "earth" to form. However, more research is needed to confirm this hypothesis. Some of their colleagues believe that erosion occurred before the formation of the "Earth-snowball" and this provoked a global cooling on the planet.
But perhaps the most intriguing thing about this study is that it can explain the life spurt that took place after the planet was “thawed”. There is a hypothesis according to which the changes on the Earth caused by such a large-scale erosion could cause the emergence of a huge number of new species of living beings - a phenomenon known as the "Cambrian explosion".
The researchers note in the article that the shallow waters left by glaciers filled with minerals and water would be extremely favorable for a wide variety of marine life and all kinds of evolutionary transformations. They treat this assumption with a healthy dose of skepticism, since their specific research does not prove it in any way. But, if confirmed, it will be an inspiring story. The coldest and most severe winter in the history of the Earth, giving rise to an incredible variety of life forms, which subsequently developed, including into humans. Isn't it poetical?