A Brief Blogstory of Melbourne - Part 1
Exploring the past is one of my passions, and I thought it might be nice to apply this to the place where I live. So, i present to you…
A Brief Blogstory of Melbourne
Like the majority of big cities in the world, Melbourne sits along a river. We’re next to a bay that opens into Bass Strait, and we get a temperate seasonal climate. We’re well north of Antarctica and well south of the tropics, but this geographical position is a relatively recent one for us - and it won’t last.
The Earth is 4,500 million years old, but around Melbourne, the oldest rocks you’ll find are only 1/5th of that age. To get an idea of what it was like before then, you have to look around other parts of the world. This first bit of the blogstory of Melbourne will be a quick look at the “Cryptozoic”, the time from the beginning of the world to the first animals.
At the Melbourne Museum, if you go through the Life Sciences section to the very back, you’ll see a small slab of polished rock with bright red layers mounted on the wall. These are layers of iron oxide. They are the major source of iron in the world today, and they all formed between 4,500 and 2000 million years ago, and never since.
Iron forms this rich, red stuff when it rusts up. And it rusts up when there is oxygen in the air. These layers of iron oxide show that it rusted over, periodically, in a cycle leading to band after band of iron.
Consider that this is an age when the most complex life was single-celled. Bacteria, and algae. There was very little oxygen around, so the oceans would have been rich with iron. But then, every now and then, you might see a bloom of algae. Algae would release oxygen into the water (as they do today), and huge amounts of iron floating dissolved in the seas would rust out, forming these distinct bands on the sea floor.
After 2000 million years ago, we stop seeing these banded iron formations. And 1900 million years ago, “red beds” become common (and still form today); red beds are different, because they form on land. They show us that the oxygen is now in the atmosphere, not just the water. Algae have polluted both the seas and the atmosphere with this volatile, explosive compound. And most of the iron that was once dissolved in the ocean is completely rusted out.
This little story illustrates the different kind of world Earth was for most of its history. You have algae in the ocean, towers of bacterial mats forming large colonies, oceans rich in iron, and atmospheres poor in oxygen.
Back then, the continents where in a different arrangement. Continents are driven around the globe over vast stretches of time. They’re pushed by spreading ocean floors on the one hand - places where new ocean floor is welling up from below, and they’re pulled by sinking ocean floors on the other, where old dense rock sinks into the Earth. (This concept is vital to understanding much of what will come).
About 900 to 700 million years ago, Melbourne would have been just north of the equator - but don’t expect that this means it would’ve have been warm and sunny. Rocks from other places around Australia tell us that most of the continent was covered in glaciers. If you want to know what the world would look like back then, imagine Antarctica, but stretched out to cover half the Earth. (Incidentally, Australia was connected to future-Antarctica on one side, and future-North America along what is now the East Coast. A new ocean opened up between Australia and North America though, pushing them away from each other in the same way that Africa and South America have been pushed away from each other by the opening of the Atlantic.)
600 million years ago, after the air had been filling with free oxygen (and the seas with dissolved oxygen) for a long time, and after the glaciers had finally retreated, the very earliest fossils of complicated many-celled creatures - “animals” - can be found. They lived in a sea that covered low-lying areas of land, and they were all soft and squishy. There are no traces of bones or carapaces, just imprints from delicate jelly-like creatures. They’re called the “Ediacarans” (after a place called Ediacara Hills, near where they were first found).
And there have been a range of theories about what, exactly, they were. Were they all the ancestors of modern kinds of animals, as people first assumed? Or where they a range of different branches of the family tree that died without leaving descendants?
Some people think that they weren’t anything like modern animals. They lived in a world where nobody had teeth or claws to dig into them, so they could have been constructed in a totally different way to anything alive now. They could have been enormous single-cells with thick walls, like one species of modern algae. One suggestion was that they were related to lichens, but living in shapes that couldn’t be supported in modern environments.
Whatever these things were, they seemed to be all gone in the next hundred million years. And the next instalment will begin there, with a look at the site of Melbourne during the Cambrian Period…
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More information:
If you’re interested in this vast period of time before complex life, it’s sometimes hard to find references. Here are a few good ones:
*See part of a banded iron formation down at the Museum Victoria’s Melbourne Museum (i believe it’s a slab from Western Australia).
*Right beside this slab, the Museum has some samples of the much more recent Ediacaran fossils for you to look at.
*There’s a book called “Wildlife of Gondwana” (1993), by Pat Vickers-Rich (found at Monash University) and Tom Rich (at the Museum). It starts with a description of the Banded Iron and a discussion of the very earliest life on Earth. On page 36 you’ll find full-colour photographs of the mysterious Ediacaran fossils.
*There are many scientific papers on the Ediacarans or “Vendobionta”; for an analysis fo Ediacarans as an entirely alien kind of animal (also discussed in a book: “The Garden of Ediacara: Discovering the First Complex Life” by Mark McMenamin, Columbia University Press, 1998):
-”Vendobionta and Psammocorallia: lost constructions of Precambrian evolution” by Adolf Seilacher. Journal of the Geological Society, London, Vol. 149, 1992, pp. 607-613
-”The phylum Vendobionta: a sister group to the Eumetazoa?” by Leo Buss and Adolf Seilacher. Palaeobiology, Vol. 20, 1994, pp. 1-4.
If anyone has any other tips about good places to find information on the longest period of Earth’s history - the expanse of time between 4,500 million and 600 million years ago, please comment…
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OK, that’s all very interesting — but when did Jessica Alba appear? ;)
Oh, that would have to be when the Earths surface was still molten… because… it was so hot.
*waits for the groans…*
it took me a while to psych myself up to read such a lengthy post, and it was interesting. But I was dissapointed not to find the forecasts of doom and destruction that the “and it won’t last” statement hinted toward.