The rocks on the earth’s surface are like a book that tells the entire history of our planet.
Each of the soil layers that form mountains, canyons, cliffs and other geographical features they are like the pages of that book that contain information about when and how the landscape that we see today was formed.
The rocks closest to the Earth’s crust are the first pages of the book, the ones that tell the beginning of the story.
From there, to As more layers are superimposed, we learn from more recent times of our planet.
But, what if that book will there be missing pages?, how could we understand a story if we can’t read all the chapters?
That’s just what happens with the geological record of the Earth.
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Between the oldest and most recent rock layers, there is a huge gap, a lot of missing pages .
This gap for which we have no information is equivalent to a period that in some parts of the world corresponds, even, up to 1,000 million years, according to the United States Geophysical Union (AGU).
This immense void is known as the Great Unconformity, and it is one of the greatest enigmas in geology.
“A billion years is almost a quarter of the Earth’s history, that’s a a lot of lost information, a lot of time that is lost”, geologist Barra Pea tells BBC Mundo k, PhD candidate in Geological Sciences at the University of Colorado Boulder and a specialist in the Great Unconformity.
In an article by 2020, a group of researchers from that university described the Great Unconformity as a “geological amnesia”.
What is the Great Unconformity due to, what clues exist about this lost time and why? is it important to solve the mystery?
Layers upon layers
In geology, the passage of time is recorded in the layers of rock and sediment that are deposited one on top of the other.
The lower layers are the oldest and those that accumulate on top are increasingly most recent.
The type of rocks and the location of each layer give the s researchers information on the way and the time in which that region of the soil was formed.
The study of these layers it is called stratigraphy and it serves to know what the Earth was like at the time that layer was formed.
One of the most where these layers can be clearly identified is the Grand Canyon in Arizona, United States.
Each horizontal line shows where, when and how the rocks that make up that layer were deposited.
And it was precisely in the Great Canyon where the Great Unconformity was first identified.
It was the geologist John Wesley Powell, in 1869, who first noticed that there was a piece of time that was not written or in that book of stones.
A gap in the ground
“Unconformities” in soil layers occur when rocks or sediments erode and time elapses before before a new layer is produced, as explained by the United States Geological Survey (USGS).
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After that period, at some point a new rock layer will form on the eroded surface, but it will be impossible to fill that gap during which no sediment was accumulated.
This explains why in several parts of the world it is possible to observe very old rock layers on which layers of very young rocks settle, without that between them there is a layer of middle-aged rocks.
By finding out the age of the oldest and youngest stones, the researchers deduce the period of time for which they have no record.
And where are those lost rocks?
According to Peak, in some places the absence of the layers it is because the rocks were destroyed by water or erosion, which turned into sand that ended up being deposited, for example, in ancient oceans.
But in other places it is not clear whether the layer even existed at some point.
“It may be that in those places there has never been sand or clay deposited that later became rock, that in that landscape really nothing much was happening , as if it had simply been a sitting there for a long time”, says the geologist.
What is the reason for the Great Unconformity?
The short answer is that nobody knows.
Peak, however, says that there are at least four main hypotheses that try to explain that lost era.
The first has to do with the formation of an ancient supercontinent called Rodinia, which was formed between 1 ago,000 million and 800 millions of years .
That was before the famous supercontinent Pangea was formed.
When Rodinia was assembling, Due to tectonic movements, a large amount of rocks were exposed to atmospheric conditions, which could favor the qu and that new rocky material that was being formed would be destroyed.
“When the rocks are new they are very susceptible to those processes that destroy them “, says Peak.
The second refers to a very similar process: the formation of another supercontinent called Pannotia, some 580 million years.
A third possible explanation also has to do with Rodinia, but not with the formation of this supercontinent but with its fragmentation, that occurred some time ago 750 millions of years.
In this process of separation of the land masses, there is also It has exposed a lot of material that geologists call “fresh rock”, which is easily eroded.
The climate hypothesis
And Peak mentions a fourth possible reason, which no longer has to not to do with the supercontinents, but with the climate changes throughout the history of the planet.
The geologist explains that there is evidence that During a period within what corresponds to the nonconformity there was a stage of cooling of the Earth, some 675 millions of years .
In that period it is likely that almost the entire planet was covered by glaciers.
Then, the hypothesis is that this ice has removed what were then the outermost layers.
“If the entire globe is covered by ice, there is not much material being deposited to form new rocks,” says Peak.
No answer
For each of these hypotheses there are studies that reinforce or try to refute them.
For Peak, however, the truth is that for now there is not a single answer that explains the Great Unconformity.
“Perhaps There is no single, simple answer that explains the phenomenon at a global level“, says the expert.
“It may not only be due to the formation of the continents or the climate, but also to the regional characteristics that control the way in which the rocks are eroded”.
“On a global level, we cannot say that there is a single cause”.
The keys that the Great Unconformity keeps
To determine the age of the rocks and therefore the layers that are missing, geologists look at the radioactive decay of rocks, that is, they analyze the chemical elements in which rocks decompose over time.
In the case of Peak, for example, it also analyzes the process of cooling of the rocks, to find out which period they belong to.
Putting together the pieces that explain the Great Unconformity is one way to learn what early Earth was like.
That, according to Peak, is important because during that period there were great changes, not only geological but also biological, for which it can provide information about the evolution of life on the planet.
An article from the University of California Santa Barbara from 2020 about the Great Unconformity mentions that this phenomenon is linked to another great enigma of science: the sudden emergence of complex life in the Ediacaran periods (between 635 million years and 170 million years) and the Cambrian (between 541 million years and 485 millions of years).
“The Cambrian explosion was Darwin’s dilemma“, says Francis Macdonald, a geologist at UC Santa Barbara.
“This is a question you have 060 years. If we manage to solve it, we will be rockstars“.
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