ACTIVITY 5: Metamorphism
Purpose:
To consider evidence for the effect of pressure on rocks through
observations of the outcomes of a simulated distortion of fossils.
Notes:
The apparatus and materials required are: modelling material; several
cockleshells, or similar shells; 50g of plaster of Paris powder;
disposable plastic cups; stirring rod; water.
Do not let pupils pour plaster of Paris
down the sink.
Development of knowledge and understanding:
Metamorphic rocks are formed by the action of heat and increased
pressure on the original rocks. This activity concentrates on
the effects of pressure acting on an original sedimentary rock.
If the pressure is intense enough and acts for a long time, the
rock recrystallises. Some of the new minerals are platy in shape
and become aligned at right angles to the main pressure. This
produces new planes along which the rock will split, in preference
to the original bedding planes. The new alignment planes, called
cleavage planes, are made use of when slate is split for roofing
materials. More intense metamorphism will destroy the fossils
and form coarser-grained metamorphic rocks.
Demonstration 5: Detecting the distortion
(This activity is based on ESTA’s “Hidden Changes
in the Earth” in “The Science of the Earth 11-14” series.)
Learning objective:
To produce distortions in castings of seashells and analyse a distorted
seashell casting to predict the relative size and direction of
distorting pressures, and to study diagrams to the same effect.
Many metamorphic rocks, such as slate, are formed deep below ground,
under great pressure. They sometimes contain fossils which have
been badly squashed. The results of the squashing gives clues about
he directions of the pressures which squeezed the rocks.
Work in pairs to make a squashed 'fossil', but do not show other
groups what you are doing.
- Soften the modelling clay.
- Make a mould by pressing the outside of a shell carefully into
the clay. Remember to make a rim to contain the plaster.
- Carefully remove the shell, to leave the imprint in the clay.
- Squeeze the mould so as to change the shape of the shell imprint,
by first choosing whether to squeeze it from top to bottom; from
side to side, or whether to apply pressures as shown in the third
picture, i.e. shear pressures. Whichever you choose do not distort
the shape too much. Remember how you squeezed the mould, it will
be important later.
- Mix up some plaster in an old plastic cup. Place less than
1cm of water in the cup and stir in enough plaster to make a
runny cream.
- Pour the plaster into the distorted mould and leave it to set
for a few minutes.
- Leave any remaining plaster to set in the cup. Wash the stirring
rod.
- When your plaster fossils have set, scratch your initials on
the base and then carefully take your fossil cast out of the
modelling material.
- Pass your fossil on to a nearly group. See if they can work
out the directions of the pressures which you used to distort
the fossil.
- Do the same for theirs. Did you get it right?
- How could the same distortion have been produced by forces
acting in different directions?
- Study each of the drawings below. They show several trilobites
found in slates. The top left is an undistorted trilobite; the
other pictures have been distorted by pressures in the Earth.
- These fossils have been distorted by moderate pressures which
have changed the rock from a mudstone to a slate. What would
happen to the fossils if the pressures had been much greater?
- By about how much has each trilobite been squeezed? By about
how much have the surrounding rocks been squeezed? By about how
much has the region in which the rocks are found been squeezed?
How might this scale of deformation have been caused?
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