Tuesday, 24 March 2015

On erosion

On our second geology walk a couple of questions arose about erosion.
  1. Why were there granite boulders on one side of the road and not the other? and
  2. Do streams always follow fault lines?
The first, illustrated here
was rapidly solved by the fact that Paddy's River (running in the gully between the observer and the tors) had removed about 40m of soil and other material (I think known collectively as regolith)  to get to the granite bedrock.

I felt heuristically that the answer to the second question was "No".  Streams will follow faults, if a fault is available where the stream wishes to go, but do not require a fault.  Trying to get my head around the idea in more detail led to consulting books (purchased at low cost through the ACT Lifeline Book Fair).  Most of what follows comes from "Dynamic Earth" by Skinner, Porter and Park" which is very consistent with the much older "The Earth" by Tarbuck and Lutgens, but I have tried to summarise so any errors are my fault.

The first step is to consider what is a Fault?  This is a fracture in rock where there is visible movement in the rocks on either side of the fracture.  There are many types of fault but I - and thus you, dear reader - ain't going there today.  Where there is no visible movement the fracture is called a joint.    These arise due to the brittle nature of rock in the upper crust.  They are crucial in the process of erosion, with which I will concern myself for the next couple of paragraphs.

In the cited text, index entries for erosion go to a discussion of wave action on the coast.  That isn't very important in the Canberra area.  Frances commented - very correctly - that a few million years ago Canberra would have been on the coast, so wave action could have had a role in forming the landscape.

The erosive action of wind on bare earth can lead to significant loss of soil: up to 1m in a few years in extreme cases (eg the dust bowls of the mid-Western USA in the 20th century)  Again this is not that important in the Canberra area: if anything we are a beneficiary when the dust from the over-grazed Western Plains gets blown here.  If you'll excuse a hobby horse going for a trot, when the understorey is burnt (eg by a habitat reduction blaze) then it increases the chance of wind erosion.  The outcome of wind action can be depressions in the eroded areas (forming lakes when it rains) and sand dunes where the particles get dumped.

Most erosion comes to what my text refers to as "weathering".This can be physical or chemical.

Physical weathering occurs when water enters the joints and
  • deposits minerals which form crystals.  The growth of these crystals can rupture the rock along the line of the joint. or
  • freezes (where the ice expands and wedges the rock apart).
Other physical weathering may occur where fire causes heat differentials that crack the rock or by the roots of plants growing into the cracked joints.  Another form of weathering is the action of lichen on the rocks.
Chemical weathering occurs when chemicals have dissolved in water and the resultant solution changes the minerals in the rock an may leach away the substance of the rock.

All of the above form unevenness in the surface of the rock which allows water to start flowing across the surface.  As it does so it will abrade the rock increasing the unevenness and adding particles to the water, in turn adding to its effectiveness as an agent of erosion.

There is probably much scope to refine what is above which I will try to do as time permits.

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