ACTIVITY 8: Deformation
Purpose:
To show how faulted rocks at outcrop can provide evidence of the
size and direction of the forces which produced the fault.
Notes:
The apparatus and materials required are: transparent plastic box
(e.g. chocolate box or component drawer); spatula or dessertspoon;
tray; a piece of board to fit snugly into the box; 500g of dry
fine sand; 25g of flour; a photograph of faulted rocks (optional).
Only thin layers of flour are needed, sprinkled along the front
of the box alone, in order to save flour and to allow the materials
to be reused several times.
Development of knowledge and understanding:
Rocks frequently become fractured during their history, but school
laboratory investigations on real rocks are difficult to carry
out with any finesse. The activity described uses layers of fine
sand and flour which behave like layers of rock.
Pupils should appreciate that faulted rocks at the Earth's surface
contain clues about the ancient pressures which deformed them.
The near-horizontal faults produced by compressional pressures
are called thrust faults. More steeply inclined faults are produced
by tensional stresses and these can be formed in the same transparent
box as in Demonstration 8, by putting the vertical board in the
centre of the box, filling one side with sand and flour as before
and moving the board gently away.
Large scale pressures acting within the lithosphere are caused
by plate tectonic movements. Where plates are converging, the compressional
stresses produce near horizontal thrust faulting. Where plates
diverge, the tensional stresses produce steeper faults, call normal
faults.
Demonstration 8: Make your own folds and faults
(This activity is based on ESTA’s “Earth’s Surface
Features” in “The Science of the Earth 11-14” series.)
Learning objective:
To simulate the movements which produce faulting in rock structures.
When pressures are applied to solid materials they may bend or
break. When sands or sandstones bend, folds are produced; when
they break, faults are formed. Find out what types of folds and
faults are produced by compression by following these instructions.
- Place the board vertically inside one end of the box.
- Build up several thin layers of flour and sand. Do not fill
it more than half-full. Spread the flour along the front of the
box only.
- Very carefully, push the vertical board across the box, so
that it begins to compress the layers. When you notice the layers
beginning to bend, stop pushing the board. Hold the board upright
and draw a scaled diagram of the result.
- Continue pushing the layers with the board until the sand is
about to overflow the box. Hold the board upright and draw a
scaled diagram of the result.
( to repeat animation press F5)
- Then add arrows to your diagram to show the directions of
the forces which were acting whilst you compressed the layers
with the board.
- Are the layers still horizontal, or are they folded?
- Did one set of layers slide over the rest? If you have been
careful, you will have produced a fault in which layers of rock
are pushed up and over other layers. These types of faults are
often nearly horizontal.
- How could you use the same apparatus to find out what happens
when sands and sandstones are stretched (put under tension)? Find
out here...
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