Foxes Lair
  • Home
  • About
    • About Foxes Lair
    • History
    • Landscape and Soils
  • Things To Do
    • Scavenger hunt
    • Picnic Spots
    • Walking Tracks
    • Visit the Arboretum
    • Geocaching and Orienteering
    • Ride Your Bicycle
  • Things To See
    • Birds
    • Wildflowers
    • Trees in the Narrogin district
    • Narrogin spiders scorpions ticks
    • Vertebrates
    • Fungi and lichens
  • Other Places to Visit
    • FAMILY bush attractions
    • WILDLIFE bush attractions
    • WILDFLOWER bush attractions
    • all reserves
  • Foxes Lair seasonal guide
  • 1Foxypress
    • Foxypress
    • Vanishing Farms
  • Contact

Our Ancient Granites

6/11/2020

0 Comments

 
PictureGranite formation during subduction
Greetings fellow Foxies,
Some years ago, a visiting Scot remarked that our granite rocks looked old and tired compared with the ones at his homeland. I had to agree. Unlike Scotland, our Yilgarn craton bedrock has been stable for millions of years. No geologically recent volcanism or mountain building events. And best of all no haggis!
 Granite forms when geological plates collide and one slips under the other (subduction, see this blog).
WA granites formed over 2 billion years ago
Over hundreds of millions of years, the top of our granite bedrock has been very slowly breaking down to soil that has been washed or blown away causing mountains to flatten.
About 300 million years ago the land was also ground down by glaciers. Overall, long term erosion has lowered the ground level by up to 5 km.
​It is over this time that our granites have cracked and weathered (become tired).
The bedrock has been fractured many times as the craton has been shunted around during supercontinent cycles. Intact solid rock outcrops like Yilliminning rock less frequent. Most granite outcrops have horizontal and vertical cracks (called joints) that weather to large boulder heaps and cracked rock outcrops.

Picture
Aerial view of Boyagin Rock
Picture
Highlighted cracks on Lizard Rock
PictureCracks in a 2 metre square area of exposed pallid clay on a ridge
The vertical joints are obvious on an aerial photo, as ridges and valleys that often trend east/west, northeast/ southwest, north/south or northwest/southeast, and make right angle turns. 
Locally, the most obvious area is southwest of Narrogin, where much of the old laterite cover has been removed and soil has formed on basement rock.
Newman Block is a great example. Lines of vegetation reflect changes in the granite ‘grain’ as well as faults and dykes. The image on the left of pallid zone clay on a ridge displays an amazing number of joints that are in the underlying bedrock.
Eager to learn why the joints follow particular directions I started reading on and on and on…… It is a long and fascinating story. 

​Our local bedrock has a northwest/southeast trend or ‘grain’ from the formation of the Yilgarn Craton as ‘islands’ of continental rock collided about 2.8 billion years ago. The red lines between the domaines/ and terranes show subduction zones where one slid under the other as they collided. These zones are called orogens.
The orogens on the north, northeast and southwest sides formed when the Yilgarn Craton collided with other land masses as the Australian continent was being built up. The Pinjarra orogen on the west side has had at least three collisions with other land masses including the Zimbabwe craton and India.
Picture
Assembly of the Yilgarn Craton from southwest to northeast
Picture
Red lines are orogens where Yilgarn Craton has collided with other land masses
During this time, the craton has joined and separated from at least three supercontinents as it has drifted over the globe.
The images below show the position the WA in three supercontinents before they split apart. I have taken the liberty of rotating the supercontinents so that WA is in its present north/south orientation. 
The radiating multi-coloured lines on the Nuna supercontinent are joints caused by pressure from mantle hotspots that have filled with intrusive rock to form dykes (see this blog). These also affect the direction of joints in continental rocks.
Picture
Nuna about 1.4 billion years ago
Picture
Rodinia 750 million years ago
Picture
Gondwana 520 million years ago
When India and Antarctica separated from WA, reduction of weight from the combined continents on the underlying mantle created an upward pressure that caused the Darling Range to slowly rise on the west coast accompanied by parallel faults inland. On the south coast a line of uplift parallel to the coast (Jarrahwood Axis) caused east/west trending faults and formerly south flowing rivers to reverse flow and eventually join the Swan River. On the south side of this uplift the coast slumped forming a slope down to the ocean.
The Australian plate is inexorably drifting north and is subducting underneath the Eurasian Plate. In a couple of hundred million years Darwin could be a suburb underneath Bali, or possibly a ski resort!
This subduction is extremely slowly causing the Northern Australian coastline to sink, and the south coastline to rise (unfortunately not as fast a sea level rise from climate warming).
Stress from tilting of the Australian Plate is a cause of earthquakes like the Meckering quake.
 
It is no wonder that our granites look old and tired. Still they create interesting shapes and landscapes.

0 Comments



Leave a Reply.

    Author

    Doug Sawkins is a friend of Foxes Lair 

    Categories

    All
    Animals Other
    Birds
    Disorders Plant Animal
    Fungi Lichens
    History
    Insects Bugs Other Arthropods
    Landscapes Soils
    Other Reserves And Places
    Reptiles
    Spiders Other Arachnids
    Tree
    Walks Other Facilities
    Wasp
    Wildflowers Orchids
    Wildflowers Other Summer Autumn
    Wildflowers Other Winter Spring
    Wildflowers Parasitic

    Archives

    December 2022
    October 2022
    July 2022
    June 2022
    May 2022
    April 2022
    March 2022
    January 2022
    November 2021
    October 2021
    September 2021
    August 2021
    July 2021
    June 2021
    May 2021
    April 2021
    March 2021
    February 2021
    January 2021
    December 2020
    November 2020
    October 2020
    September 2020
    August 2020
    July 2020
    June 2020
    May 2020
    April 2020
    March 2020
    February 2020
    January 2020
    November 2019
    October 2019
    July 2019
    April 2019
    March 2019
    February 2019
    January 2019
    December 2018
    November 2018
    September 2018
    July 2018
    May 2018
    April 2018
    March 2018
    February 2018
    January 2018
    December 2017
    November 2017
    October 2017
    September 2017
    July 2017
    May 2017
    April 2017
    February 2017
    January 2017
    December 2016
    November 2016
    October 2016
    September 2016
    August 2016
    July 2016
    June 2016
    May 2016
    April 2016
    March 2016
    February 2016
    January 2016
    December 2015
    November 2015
    October 2015
    September 2015
    August 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    February 2015
    January 2015
    September 2014
    August 2014
    April 2014
    March 2014
    February 2014
    May 2012
    March 2012
    October 2011
    September 2011
    August 2011
    April 2011

© 2015 All Rights Reserved. Doug Sawkins, Australia.