Research led by a University of Wollongong (UOW) PhD candidate may have solved one of Earth's most perplexing geological mysteries – what caused the Great Unconformity?
Published in Nature, the study argues the gentle rise and fall of whole continents over hundreds of millions of years, caused by hot currents deep below the Earth's crust, led to the confounding disappearance of huge swathes of Earth's geological record.
PhD candidate and lead author Santosh Dhungana said the Great Unconformity had long puzzled scientists but theories about the geological gap had not been thoroughly tested.
"Imagine a book where a huge section in the middle has been ripped out. The Great Unconformity is that gap in Earth's story, with the narrative jumping from the early chapters straight to a much later part of the story. Our research suggests these pages weren't lost by chance," he said.
Layers of rock usually stack up in chronological order, with oldest at the bottom and youngest at the top. But in locations throughout the world, sedimentary rocks of the Cambrian period, from around 540 million years old, sit directly on top of much older crystalline rocks such as granite, which can be more than a billion years old. The gap between the two periods is the Great Unconformity.
One of the most striking examples of the Great Unconformity is found in the Grand Canyon in the United States, once part of the ancient continent known as Laurentia (modern North America). Evidence of the geological gap has also been found in Europe, Australia, Africa and Asia. Depending on the location, the missing record stretches from 200 million years to well over a billion.
By modelling patterns of mantle convection over hundreds of millions of years, Mr Dhungana and co-author and supervisor Associate Professor Nicolas Flament were able to piece together the puzzle of the Earth's history and geological formation.
Deep beneath the surface, Earth's mantle is made of hot rock that is solid and moves very slowly. These currents, known as mantle convection, can gently lift huge areas of land , or let them sink back down.
Mr Dhungana and Associate Professor Flament found that across the ancient continental cores of Laurentia, Baltica (northern Europe) and North China, currents in Earth's mantle triggered the continents to rise during the Neoproterozoic Era, which spans one billion to about 540 million years ago. This era ended with the Cambrian period, which brought an explosion of animal life on Earth.
The land's long rise was followed by a long period of sinking during the Phanerozoic eon, the last 540 million years. As these ancient continental cores gradually subsided into the ocean, seas and sediment buried the worn rock surface, forming the Great Unconformity that survives to this day.
The mantle convection modelling matched the geological footprint. The research pointed to the rise and fall of the very regions where the Great Unconformity is best preserved and documented, in North America, northern Europe, and North China.
"Sometimes what is missing can tell us more about the story of Earth than what we can see. A single, long period of uplift followed by a long period of sinking would have been the ideal recipe for removing the rock and preserving the worn surface," Associate Professor Flament said.
"Many explanations for the Great Unconformity had been proposed but few had been tested against how the Earth actually behaves over time. It is a thrill to be able to propose a solution to this geological puzzle."
