the new ussr illustrated

welcome to the Urbane Society for Skeptical Romantics, where pretentiousness is as common as muck

some local geology: granite and glaciers

with one comment

victor_harbor_17_march_2007_20070317_002

a bunch of granite dumped on Granite Island

While at Victor Harbour, we did the usual walk around Granite Island, marveling at these massive lichen-covered granite boulders and reading the signs about their origins, and their hardness and consequent permanence, compared to, say, limestone.
Granite is a composite of 3 minerals – quartz (bluish), feldspar (pink and white) and mica (black biotite). On the island it’s found in great heaps of rocks, called xenoliths, subject to weathering known as tafoni – though the examples there aren’t spectacular, compared to others, such as Kangaroo Island’s Remarkable Rocks. This granite has upwelled from – well, somewhere – back in the Cambrian, about 520 million years ago. Granite is igneous rock, generally formed from molten lava under the surface which slowly pushes its way through cracks and spaces to just below the surface, over millions of years, where it’s finally revealed through soil erosion – at least that’s the story I’m getting through my reading. What I see, though, is a mixture – boulders in heaps, at the tops of hills, that look like they’ve rained down from the sky; great cliff faces that look more like upwellings; and, in gullies, a combo of large and small boulders that look the end-product of an avalanche.
Well, a lot can happen in 500 million years, but I’ll try to make sense of it, not only for Granite Island but the region around it. Here’s an intro from the geological society of Australia:

Grey metamorphic rocks are exposed in natural outcrops, road cuttings and along the sea coast over much of southern Fleurieu Peninsula and Kangaroo Island.
They are called the Kanmantoo Group by geologists and were deposited into a rapidly subsiding ocean basin as fine grained sand and silt eroded from large land masses to the west and south in the Cambrian Period about 520 million years ago.
After the basin filled, this sequence of sediments was buried deeply below the earth’s surface and altered (metamorphosed) by heat and pressure into their present form. They were also intruded by masses of molten granite (called the Encounter Bay Granite) and were then thrust up into a mountain range in a major earthmoving event called the Delamerian Orogeny which ended about 475 million years ago.

So it would seem that the Delamerian Orogeny was responsible for the granite formations on Granite Island and thereabouts. They were igneous intrusions resulting from the uplift and folding of the lithosphere (the earth’s crust and mantle) due to the clash of tectonic plates (the meaning of orogeny). This particular orogeny, occurring at the end of the Cambrian period and into the Ordovician, created the Flinders and Mount Lofty Ranges (the Adelaide geosyncline). In those days, the area was part of the supercontinent called Gondwana – in fact the Delamerian was one of several orogenies that contributed to its formation.
Fast forward a few hundred million years, to the Paleozoic era, and Gondwana was located around the South Pole, though parts of it extended almost to the equator. In those days the highlands of the Adelaide geosyncline, which had eroded down over the years, were often covered in ice caps, though the planet overall was warmer than today. Geologists find evidence of glacial activity from that period, from Port Elliot round to Hallet Cove:

Boulders of Encounter Bay Granite and Kanmantoo Group rocks, plucked off the surface and moved many kilometres by the ice from their original location, are a common feature of this glacial terrain. They are called erratics.

There’s so much more to explore in this line, obviously, and it’s a perfect example of a little scratching at the surface of a subject revealing, for me, a whole world of ‘known unknowns’, to quote the immortal Donald Rumsfeld. Science is amazing in its accumulations from researchers across the globe. So now, when I see strange boulders in out of the way places and unrelated, apparently, to the rocks around them, I’ll think of glaciation and erratics as a possible explanation.

2012.06.04_Auswick Norber erratics

Advertisements

Written by stewart henderson

February 8, 2014 at 9:41 am

Posted in geology

Tagged with ,

One Response

Subscribe to comments with RSS.

  1. Great post!

    Omega User

    February 9, 2014 at 1:35 pm


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: