Saturday, 7 February 2015

Geology of the Canberra Region

The overall aim of this is to reinforce what I learnt in a course on this topic.  Hopefully, it will also contain some material of interest to others.

For several years a number of my friends have done things with or for the local branch of the University of the Third Age.  Frances enrolled last year to learn Latin, do a course on Pommie Cathedrals and to learn to play the ukele.

I have always passed on this as I thought you had to be a SNAG (Sensitive New Age Guy) to be involved and I am more of a TOAD (Terrible Old Age Dork).  However a course about the landscape around Canberra seemed to have some merit so I have bitten the bullet.

I attended the first of 3 lectures on Thursday and it was quite interesting.  I am probably going to recast the narrative of the course somewhat later, and there are some bits which on reflection my memory does join together so I am not going to critique the course in detail: I will comment on some particularly interesting elements below and when we have finished may try to set out something along the following lines:

  1. What are rocks made of?  A bit of basic mineralogy so that one can have a look and say what sort of rock things are.
  2. How are rocks made? Igneous vs sedimentary vs metamorphic.
  3. How do rocks get shaped into the forms visible in the landscape? Volcanism, folding, tectonic shifts, erosion, glaciation.
  4. Which of the above are evident in the broad structures in the landscapes of Australia and the Southern Tablelands?
  5. An outline of the major landscape elements of the Canberra Region
  6. Specific highlights of the Canberra Region
In what follows I have borrowed a couple of images from the course notes (which had in turn been borrowed from elsewhere).
 A key feature of the lecture, probably relevant to point 3 above, was the two natural cycles, for rock ...
 ... and water.

A good amount of time was devoted to plate tectonics which showed not only:

  • how the 'super continents' (of which Gondwana is the most recent - and thus most famous - example) split and join; but also
  • how the whole lot drifts around on the face of the planet.

WRT the latter point it seems that the Earth is basically like a gyroscope with the axis more of less in a constant position relative to the orbit round the Sun.  Thus the equator (perpendicular to that axis) is also constant and the climate of the continents will vary as they shuffle about.  The movement also offers many opportunities for molten rock to escape as the plates but into, or slip across each other.  This also leads to mountain building as the plates slide over one another  - the last major bout of this was 200m years ago, which also - not just a coincidence I suspect - the age of the ocean floors.

As an example of the forces involved in earthquakes and such manifestations of tectonics, a 'quake in Tibet.China gave a rupture 426km long with a vertical displacement of 7m.

An interesting snippet was that Australia physically moved 7cm closer to Indonesia last year (the lecture didn't assess how much it moved in that direction in a political dimension).  The processes through which the (pre)historic movements of rocks are tracked is very interesting and comes in two parts;

  • The age of rocks can be assessed by the abundance of various elements as the radioactive ones (eg Uranium235 decay through a number of intermediate elements to become the stable Lead206.  Knowing the half-life of U235 lets those who can, calculate how long it is since the rock formed (presumably from magma).
  • Where they were are times in the past can be calculated by examining their magnetic properties.  This relies on knowing which way the polarity of Earth was arranged at various times and the fact that minerals line up according to this polarity when their temperature drops below the Curie temperature (about 5800C.
The names of at least some geological ages represent the areas where the typical rocks were found.  For example:
  • Ordovician is named for the Ordovars, a tribe who have been accused of being Welsh. Silurian  comes from the Silures (another Welsh team) while Camrian relates to the Latin name for the entire Land of Song!
  • Perm in central Russia led to the Permian; and Jura is a region in France.
Along the same lines Gondwana is named after the Gondi folk from somewhere in Central India.

A final (for now) snippet is that the summit of Mt Everest, a tad over 8,000m AMSL is Ordovician rock, which formed at the bottom of a sea 4,000m deep.  So the uplift is some 12km!

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