teacher’s notes
student’s notes
Anything that cuts through another layer must be younger than the one it cuts through.
This is called a cross-cutting relationship. This is shown by the pushing of the glass
rod through the layers of sand.
The tearing of the paper represents a fault process that has affected all the layers,
it must have happened after all of the layers were deposited.
Answers to questions
Q
1.
The
lowest
layer
of
yellow
sand
Q
2.
The
lowest
layer
of
yellow
sand
Q
3.
The
top
layer
of
red
sand
Q 4. The glass rod going through all of the sand layers
Q 5. The lowest layer of yellow sand was first, then layer of red sand above it, and
so on until all of the layers were cut through by the glass rod
Q 6. The lowest yellow sheet of paper
Q 7. The lowest yellow sheet of paper
Q 8. The top red sheet of paper
Q 9. All the sheets were torn (‘a fault’)
Q 10. The lowest yellow sheet was first and so on (depending on colours used)
until final red sheet at top, then all of the sheets were torn.
Activity 2 – demonstration of cross-cutting
Carry out the Volcano in the lab. activity but use several layers of different coloured sand.
The layers should be quite thin, giving a total depth of sand of about 1 cm. Alternatively,
use Figures 4 and 5, which show typical results, as a basis of discussion and questions.
These figures are schematic and the thickness of the sand layers has been increased
for clarity.
‘igneous rock’. The igneous rock on the surface is clearly younger than the layers below.
However, the most important point is that the igneous rock that forms in the tubes
cutting through the ‘sedimentary rock layers’ is also younger than the layers it is
cutting through.
Answers to questions
Q 11. Before heating, the oldest ‘rock’ is the layer of wax.
Q 12. Before heating, the youngest ‘rock’ is the layer of water.
Q 13. The other layers must have been there first in order for the wax to be able
to cut through them.
Extension topics
The extension work consists of a series of questions relating to superposition
and cross-cutting in different situations.
Answers to questions (extension topics)
Q 14. Later holes cut across earlier holes so that later patches cut across earlier
patches. This illustrates cross-cutting relationships. Any lines painted on top of the
patches must have come last. This illustrates superposition. So
the electricity cable is the oldest
the gas is the next oldest
the water main is the youngest / most recently laid
double yellow lines are younger than both electricity and gas but it is not
possible to say whether they are younger than the water main
Q 15. (a) Mudstone
(
b
)
Superposition
Q 16. (a) Limestone
(
b
)
Superposition
Q 17. (a) Sandstone
(
b
)
Superposition
Q 18. (a) Dolerite
(
b
)
Cross-cutting
Activity 1b – paper layers demonstration of superposition
Apparatus
sheets of coloured paper
Place layers of coloured paper, one sheet on top of another, yellow at the bottom,
red on top, any other colours in between, see Figure 2. Now tear across all the
sheets of paper, Figure 3.
Figure 2 Layers of coloured paper
Figure 3 Layers of coloured paper after tearing
Points to bring out
Students need to appreciate that the layers of sand and paper are meant to
model layers of rock. Although we have put them on top of each other quickly, the
geological processes would occur over a much longer time scale.
The layer that is lowest in the sequence is the oldest. Layers above get
progressively younger. This is called superposition.
Figure 4 The volcano in the lab. before heating
Figure 5 The volcano in the lab. – a typical result after heating
Points to bring out
Emphasise that the red wax represents solid ‘rock’ (probably solid igneous rock).
Above this are three layers of sand that represent layers of sedimentary and
metamorphic rocks. Particularly emphasise that the water above this also
represents a sedimentary rock layer.
The wax is heated at one spot to represent a ‘hot spot’ in the mantle. This melts the
wax (‘rock’), which rises through the covering layers of ‘rock’. As the wax sets it
forms