Friday Non-Lifeform Blogging: The Antarctic Shield.
?Frank Todd
Antarctica, an immeasurable blue-white expanse populated by penguins and icebergs. We hear regularly about the ozone hole growing or icesheets melting, serving to add yet more weight to our growing load of existential dread – but other than that it’s just there, you know?
We forget Antarctica is a real continent made of solid rock, not ice – some of the oldest rock on the planet – and whether we all die horribly in a self-inflicted ecological cataclysm or not, it’ll still be there.
Click on the image to see a graphical representation of geologic time, courtesy USGS
Just attempting to comprehend the sheer breadth and depth of the processes involved in its creation impresses our the nature of our own impermanence on mere mammals like us . I find it strangely comforting that the earliest Homo sapiens found is estimated at a mere 154,000 to 160,000 years old – if 2,500 millennia are a mere eyeblink in the face of the universe then we are not even a twitch.
Image courtesy University of Melbourne
East Antarctica is a large Precambrian shield, and is similar to shield areas in Brazil, Africa, India, and Australia which are also known as cratons.
Shield rocks are very stable and those found Antarctica are over 3 billion years old, mostly metamorphic gneiss, schist and granites, overlaid by ocean sediments like shale, sandstone and limestone. The rocks were recrystallized during an orogeny, a mountain building episode caused by plate collisions, in the early Paleozoic Era (about 500 million years ago).
It’s sounds odd, but I have found while reading about the whole process of craton formation to envisage the continental plates as the surface of a pot of thick soup … as the surface cools, then heats from below, there are always larger surface accretions that persist and around which the other smaller solidified pieces rotate. I expect that if several billion years tectonic history could be animated that’s pretty much how it would look. Cratons/croutons/soup – it works for me.
The University of Melbourne is studying the tectonics of Antarctica and this image shows how that research has identified the oldest cratonic blocks known. These are the chunks that the smaller more temporarily solid plates split off and rotate around.
One of the things I’d particularly like to know is what will happen tectonically when the weight of the shrinking icesheet and glaciers is reduced ? Will there be some sort of rebound as the pressure is released? The energy has to go somewhere. And what’s the likely time-scale?
If anyone can answer these questions, it’s the University of Melbourne, whose excellent site has been a mine of information and which I highly recommend to any layperson.
As the ice melts, there will be more and more opportunities to study the geology and paleontology of the shield.
“Geologists have found evidence that there was once a forested supercontinent in the Southern Hemisphere, which they call Gondwanaland. Before the Earth’s constantly shifting plate movement began to break it up 150 million years ago, Antarctica was a core piece of this assembly; its adjoining land has since become Africa, Madagascar, India, Australia and South America. The Antarctic Plate drifted south at little more than a centimeter each year, but geologic time eventually yields cataclysmic results: The journey moved it into ever colder, high-latitude climates, at a rate of about 4?C for each million years; eventually life conditions had changed dramatically, and Antarctica arrived at a near polar position. This astounding history of rock and life on Earth has left a stratigraphic and fossil record, locked in and beneath the ice, the sea, and in the bedrock below both.”
I do find it somewhat ironic that we’re finding out so much about deep time events just as it becomes clear our own time on the planet may be short. There may be forests in Antarctica again, but it’s unlikely we’ll see it . I can only hope the cephalopods inherit the earth and I get my choice of re-incarnation.
