Imagine opening a history book and finding several chapters torn out. This was what geologists looking into the rock records of Earth’s past discovered on several continents in the 1870s.
They dubbed it the Great Unconformity. Where layers of rock often stack up in a steady sequence from oldest to youngest, in many places around the world there is a gap of hundreds of millions of years.
For 150 years, nobody has known what erased those missing chapters. In new research published in Nature, we finally provide an explanation: deep processes inside Earth lifted some continents high above the sea, where their surfaces were scoured down by the grinding movement of enormous sheets of ice.
The Great Unconformity
Layers of rock often stack up in chronological order, with the oldest at the bottom and the youngest at the top.
Yet in some places, such as the Grand Canyon in North America, you can touch rock layers from two different ages, hundreds of millions of years apart. A flat layer of sedimentary rock, about 540 million years old or younger, rests directly atop a crystalline “basement” up to two billion years old.
Somewhere in between, a considerable chunk (up to a quarter) of Earth’s history has simply vanished.
Important things happened in that time. The sedimentary layers on the younger side of the Great Unconformity appear at the same time as a huge diversification in the forms of life on Earth. This is the so-called Cambrian explosion, which began around 540 million years ago.
Scientists have speculated that entire continents were eroded during the Great Unconformity, which may have altered the chemical balance of seawater and allowed animal life to flourish.
The puzzle of missing rock
Geologists have found evidence of the Great Unconformity throughout North America and parts of northern China, as well as beneath younger rocks in northern Europe. Yet its origin has remained a puzzle.
Some scientists have argued it was caused by erosion by massive ice sheets during the Snowball Earth period between 720 million years and 635 million years ago. Others have pointed to the reshuffling of continental blocks, mountain-building processes, heat-induced movement inside the Earth, volcanic activity, or global sea level change.
Testing these hypotheses has been a challenge because the primary evidence consists of missing rock. As a result, we need to work from what we already know to fill in the gaps.
How hot rock can move continents
We had a theory that the creation of the Great Unconformity might have been influenced by heat-driven motion in Earth’s mantle. The mantle is a 2,900-kilometer-thick layer of hot solid rock that slowly moves over tens to hundreds of millions of years. Hot mantle rises from deep beneath and cold mantle sinks, pushing and pulling the surface above.
This huge, slow movement can raise or lower the surface by hundreds of meters. Continents sit on top of the mantle and get pushed up and down by its motion.
We used the Australian supercomputer Gadi to model how the movement of the mantle had affected continents over the past billion years.
Our results consistently showed that mantle convection caused ancient continental blocks Laurentia (which now makes up much of North America), Baltica (parts of northern Europe), and North China to be lifted up from around a billion years ago to 540 million years ago. Over the past 540 million years, they have been pulled downward.
This up-and-down movement is the ideal scenario for the formation and preservation of the Great Unconformity. When these continental blocks were lifted up, we see the crystalline underlayer was at the surface – and the layers of sediment above were eroded away. Then when the blocks sank, new layers of sediment began to form, which are preserved in rock to this day.
Lifted continents scoured clean by ice
Importantly, the uplift predicted by our models coincided with the Snowball Earth period. During this period, enormous ice sheets scraped over the surface of continents for tens of millions of years.
Glaciers and ice sheets are more efficient than rivers at eroding continents, even on gentle slopes. The ice sheets would have slowly slid downhill across the uplifted continents, eroding away everything in their path.
In contrast to Laurentia, Baltica, and North China, we found that Australia and South America experienced only two cycles of uplift and subsidence from 900 million years ago. This scenario is not conducive to the formation and preservation of the Great Unconformity – and indeed we see little evidence of it on these continents.
Our research suggests the missing pages in Earth’s geological archive may be due to slow mantle convection that gently lifted entire continents, then made them subside over hundreds of millions of years.
In that sense, the Great Unconformity is the fingerprint of deep Earth processes. Forces deep beneath our feet have left planet-scale marks in the geological record, and shaped the environment in which complex life first evolved.
Santosh Dhungana, PhD Candidate, Environmental Futures, School of Science, University of Wollongong, Nicolas Flament, Associate Professor and ARC Future Fellow, Environmental Futures, School of Science, University of Wollongong
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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