This morning, while reading the newspaper, my eye caught this on the front page:
Photo from the Rochester Democrat and Chronicle.
This naturally piqued my attention.
So I found the original paper.
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.
Penny Higgins - Paleontologist • Photographer 