BREAKING! How Part of Canada got Stuck to Australia – #ScienceNews

This morning, while reading the newspaper, my eye caught this on the front page:

  Scientists chart land drift from 1.7 billion years ago  Doyle Rice  USA TODAY  Eons ago, the land Down Under wasn’t so far away after all.  Rocks recently discovered in Australia bear striking similarities to those found in North America, a study found. The sandstone sedimentary rocks the scientists uncovered are not "native" to present-day Australia but are common in eastern Canada.  The rocks were found in Georgetown, Queensland, Australia, which is roughly 250 miles west of Cairns in the northeastern part of the continent.  Scientists said one region of modernday Australia was once! attached to North America but broke away 1.7 billion years ago.  After drifting around for about 100 million years, the chunk crashed into what’s now Australia,  forming the "supercontinent" Nuna.  Researchers determined that when Nuna broke apart about 300 million years afterward, that chunk of land did not drift away. It instead became a new piece of real estate permanently stuck to Australia.  "This was a critical part of global continental reorganization when almost all continents on Earth assembled to form the supercontinent called Nuna," said study lead author Adam Nordsvan of Curtin University in Perth, A! ustralia. "This new finding is a key step in understanding ho! w Earth’s first supercontinent Nuna may have formed."  Nuna, sometimes referred to as Columbia, was one of several supercontinents that existed before the most well-known and recent one, Pangea.  The study was published in Geology.
Title reads: North American rocks migrated Down Under.
Photo from the Rochester Democrat and Chronicle.

This naturally piqued my attention.

So I found the original paper.

Nordsvan, Collins, Li, Spencer, Pourteau, Withnall, Betts, and Volante, 2018, Laurentian crust in northeast Australia: Implications for the assembly of the supercontinent Nuna: Geology, doi:

Let me distill this in my own way…

In Earth’s past there have been many so-called supercontinents, where all of the planet’s major landmasses were stuck together to make a giant continent. The one most people have heard of is Pangaea, which was around during the time of the dinosaurs and started breaking up as the dinosaur era came to an end. It is the breakup of Pangaea that resulted in the jigsaw puzzle like fit of South America and Africa. They were actually together at one time!

Each major landmass on Earth has one or more parts that are particularly coherent. These are called the craton and represent rocks that have not been affected by large mountain-building events for at least a billion years. These cratons have been the major units moving around on the Earth’s surface for billions of years and grow just a little bit every time the land masses come together to make another supercontinent. North America has a single craton. Other continents, like Australia, are made up of more than one craton that may or may not stay together when supercontinents break up.

Geologists can use specialized techniques to figure out where each craton was at a given time through a combination of radiometric data and paleomagnetic data from sediments that have been deposited on top of cratons. Radiometric techniques provide an age of the rocks and paleomagnetic data can tell researchers where the North Pole was, with respect to the craton, at the time those sediments were deposited.

Prior to the development of Pangaea, there was a supercontinent known as Rodinia. We don’t know as much about it as we do Pangaea, because the formation and subsequent breaking up of Pangaea destroyed a lot of the evidence. But we do know about Rodinia because of paleomagnetic data and radiocarbon ages. Here is a link to an article I wrote a while ago on Rodinia.

Before Rodinia, around 1.5 billion years ago, there was another supercontinent called Nuna (also sometimes called Columbia). Evidence for this one is even more difficult to collect because of two subsequent cycles of building and breakdown of supercontinents. Paleomagnetic evidence and dating methods help reconstruct the ancient continent.

This new paper is a discussion of part of the North Australian Craton called the Georgetown Inlier. The inlier is a bit of the North American Craton (also called the Laurentian Craton, or LC) that got attached to the North Australian Craton (NAC) as the NAC first collided then broke away from the LC, during the time of the Nuna supercontinent.

To determine that the Georgetown Inlier was originally part of the LC, the authors looked at both measurements of paleoflow (the direction that water flowed when the rock was being deposited) and counts of individual zircon crystals of various ages. These were compared with rocks of similar age from both the LC and the NAC. Based on these comparisons, the authors showed that the Georgetown Inlier was much more like rocks from the LC than any from the NAC.

From there, the authors proposed a model by which the movement of rocks from the LC to the NAC could be explained in association with the assembly and subsequent disassembly of the supercontinent Nuna.

The authors packed a lot into this little paper. I hope my little explanation here unpacks it a little for you.


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