Researchers offer explanation for missing rock layers
Models of mantle currents offer an explanation for the global Great Unconformity.
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A research team from the University of Wollongong has offered an explanation for one of the largest gaps in the geological record. According to the study, prolonged uplift and subsidence of ancient continents caused a vast quantity of rock to disappear or become covered.
In many parts of the world, relatively young sedimentary rocks lie directly on much older crystalline rock, such as granite. A period ranging from hundreds of millions of years to more than a billion years is sometimes missing between the two layers. Geologists call this the Great Unconformity.
A striking example is found in the Grand Canyon, in an area that was once part of the ancient continent Laurentia. Comparable geological gaps have been described in Europe, Australia, Africa and Asia. It is therefore not a single local break, but a pattern found across several continents.
Santosh Dhungana and Nicolas Flament investigated the pattern using models of convection in the Earth’s mantle. In these models, hot layers of rock flow very slowly deep beneath the Earth’s surface. That movement can gradually push large parts of a continent upwards and later cause them to subside again.
According to the researchers, the ancient continental cores Laurentia, Baltica and North China rose during the Neoproterozoic, a period from about one billion to 540 million years ago. These areas then began to subside. Erosion during the uplift removed rock; when the land later sank, seas and sediment could cover the eroded surface.
The model results matched the locations where the Great Unconformity is best documented. This supports the hypothesis that processes deep within the Earth’s mantle played an important role. The researchers thus present a mechanistic explanation for a phenomenon for which several explanations had previously been proposed.
The study was published in Nature on 5 November. The model offers a possible explanation that can be tested further against geological data. The research does not claim that every missing layer disappeared in exactly the same way.
With their model, the researchers link processes deep within the Earth to what geologists observe at the surface. According to this explanation, missing rock layers can indirectly provide information about continental movements and the development of the Earth over very long timescales.
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The geological ages, regions studied, modelling method and publication date can be found in the university’s explanatory material. The text clearly distinguishes between observations and the model-based explanation.
- confirmed The Great Unconformity can encompass a gap of hundreds of millions to more than a billion years. — The University of Wollongong gives this range. source
- confirmed The researchers modelled convection in the Earth’s mantle. — The method is described in the university’s explanatory material. source
- confirmed The study appeared in Nature on 5 November. — The university source gives the publication and date; the Nature page contains the study. source
Editor's note
The study is peer-reviewed and was published in Nature. The explanation comes from mantle-convection models and should be understood as a substantiated hypothesis about a geological pattern, not as direct observation of all the missing rock.Sources
More on this in Dutch media
- AD — „geologie aardmantel”
- de Volkskrant — „geologie aardmantel”
- RTL Nieuws — „geologie aardmantel”