Narrabeen beach in Sydney is an interesting place. Some people like it for the nearby lagoon, some like it for the ocean baths. But if you keep walking, you will come to the towering cliffs, filled with Permian age shales. These rocks just happened to play a major role in proving the theory of continental drift. A theory that literally changed the world.
Shales are fine grained rocks and are formed in calm waters. These sediments were perfect for preserving fossils of what once lived in a young Pangaea.
If you are lucky, big stones will have fallen from the cliffs, smashing open the rocks and revealing the secrets left inside for 250 million years. Though you have to look up in case you are not so lucky and one of those rocks falls on you.
Glossopteris
These fossils that you might find are in all likelihood a glossopteris fossil. They were the dominant tree found in the area and are related to the gymnosperms we have today. Glossopteris is characterised by its tongue like leaves and small pinecone like seed pods. You are likely to find their shape imprinted into the rock so it looks like a mould or the leaves.
I have been lucky enough to find a fascinating specimen that is a startling black and orange colour. Black carbon imprints from the leaves and bright orange sections where iron rich water has seeped into the rock and rusted.
Glossopteris was once found all over Gondwana. But after fusing with the other continents to become Pangaea, glossopteris and the rest of life on Earth came face to face with the Permian Triassic Mass Extinction, and event also known as the “Great Dying” (read more about mass extinctions and The Great Dying here). This event wiped out 96% of all life on Earth. Though glossopteris did not survive, it still had an important role to play.
Continental Drift and Pangaea
Alfred Wegner was ridiculed for suggesting that continents moved. He discovered that especially in the case of Africa and South America, they looked like they could fit together nicely. As if they once formed one, super, continent.
Wegner also noticed how certain rock types seemed to line up neatly. If you piece the continents together to form an ancient continent called “Pangaea”.
Now back to glossopteris. Glossopteris thrived in midlatitude Gondwana, which included Australia, Antarctica, Africa, South America and Indian continental plates. Fossils of the same organisms, especially plants, (specifically glossopteris) seemed to line up where he predicted them to based on this theory. This formed a major part of Wegner’s argument for continental drift. Mostly because there was very little evidence for any other possible mechanisms for this.
Glossopteris was particularly instrumental to Wegener’s argument. It’s fossils are seen in places across vast oceans, and its seeds were too big to be able to be carried by the wind. The only possible way for it to exist in such distant places, is if they were once connected.
But that was preposterous and no one believed him. This was partly because his predicted rate of movement was about two orders of magnitude too fast. No one could explain how the continents moved and it didn’t help that the geologists just didn’t like him. This was because he was primarily a meteorologist.
Earth’s Changing Magnetic Field
After that, scientists discovered that the Earth’s magnetic field flips every now and again. This phenomena is recorded in the magnetic, iron rich mid ocean ridges. Where the ocean floor slowly spreads apart, giving us a lovely gradation of new to old rock, with their corresponding magnetic fields.
Contrary to popular opinion, Wegner did live to see his work accepted in general. Though there were still sceptics as to how it worked. North America was the last to place to accept this theory. They clung to the idea that there was either a sunken land bridge or the oceans flooded and dried periodically allowing plants and animals to migrate across from Europe for most of Wegner’s life and only came around in the 1960s, 10 years after everyone else.
That’s when the scientific community finally decided that Wegner was right. The continents spread apart over time, allowing these magnetic bands to form, for continents to share geology and animals and plants like glossopteris to migrate across continents.
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