Please note that the following is not a comprehensive coverage and should be used in conjunction with
the Mark Scheme and Examiners’ Report.
Q.1.
Figure 1a is a simplified section of a destructive plate margin
showing the foci of earthquakes that occurred in recent months.
Choice of 4 on outside of bend as plate subducts at the trench.
DATA RESPONSE. Information on the Figure.
(a) Describe the distribution of the earthquakes in Figure 1a.
!
……………………………………………………………
!
…………………………………………………………...
!
……………………………………………………………
!
……………………………………………………………
(3)
(b) (i) The plate bends as it begins to subduct. This may result in tensional
forces on the outside of the bend.
Label one earthquake focus (T
which may be due to tensional forces.
) in the subducting plate
(1)
Main points are :
1. Foci are clustered around the trench.
2. ONLY within oceanic lithosphere.
3. ONLY near trench are they close to the edge of the plate.
4. Spread out at depth.
5. Appear to form two lines.
6. ALL WITHIN the plate NOT at edges.
ANY three OR one or two with extra detail.
Need to make 3 valid observations.
(ii) At depth the earthquakes are more likely to be due to forces
within the subducting plate.
! ! Two arrows (σmax
σmax) in the subducting plate
! ! show the likely directions of the maximum principal stresses. !
!
!
!
Suggest why the forces labelled are acting in this way.
(2)
! ! ………………………………………………………………………
! ! …………………………………………………………………. . . .
Two forces have to be identified for two marks. One for the
descending plate e.g. ridge push and one from below e.g
resistance of the asthenosphere / upper mantle.
Figure 1b shows the variation of temperature with depth close to the
destructive plate boundary.
Figure 1b
(c) Refer to Figure 1b.
(i) Complete the table below by stating the depths and temperatures at
the earthquake foci Y and Z.
(2)
Focus
Depth / km
Temp / °C
Y
20-25
200
Z
175-200
1155-1250
Depth :
Temperature :
20 to 25 allowed
EXACTLY 200°C - on the isotherm
Note that only the 200°C is not given
any leeway because it is on the isotherm.
Depth :
175 to 200km allowed
Temperature : 1155° to 1250°C approximately half way
between the 1000° and 1400°C isotherms
(ii) State and account for the range of temperatures found at a
depth of 100km.
! Temperature range : from…<600…….ºC to …>1400……..ºC.
Account : Cold slab subducting into hot asthenosphere.
Temperature range : the horizontal blue line at 100km depth cuts all
the isotherms from 600° to 1400°C. This means that the MINIMUM
temperature is LESS than 600°C, but not as low as 400°C. Similarly,
the MAXIMUM temperature is ABOVE 1400°C but not as high as
1600°C.
!
(iii) Seismic waves travel faster in the subduction plate than they do
in the asthenosphere. Suggest reasons to explain this.
(3)
The asthenosphere is hotter than the subducting slab. (1)
!……………………………………………………………………………
!The slab is rigid while the asthenosphere is plastic (1)
……………………………………………………………………………
Seismic waves travel faster in rigid than plastic mediums (1)
14 marks
Seismic wave velocity increases as the moduli increase i.e. as the
resistance to change in volume and change in shape increase. The
descending plate is colder, more rigid and so the moduli are
greater than the asthenosphere (which is plastic NOT semi liquid
at a locality such as this). Increasing density SLOWS the waves.
Section B
Answer one question from this section.
3.
Describe the formation of ocean floor magnetic anomalies and evaluate their importance in supporting the theory of plate tectonics.
4.
(a)
Describe the variation in surface heat flow across a spreading ocean basin and active continental margin.
(b)
Discuss the importance of surface heat flow in supporting the theory of plate tectonics.
5.
(25)
(25)
Describe how the orientation of principal stresses within the lithosphere affect the nature of any fault structure produced.
Outline the conditions that might lead to a rock folding rather than faulting.
Let’s try Question 5.
Two parts.
First part just descriptive. Need to relate stresses to the faults produced.
Second part is asking a question : when does a rock fold and when does it fault ?
Bit more involved. Do I cover everything I can think of or do I know a lot about
one factor in particular ?
(25)
Normal
Diagrams by far the easiest way to obtain at least credit for the knowledge
aspect. It would NOT be necessary to include all three. One would be sufficient
to make the general point and then the other two could be described.
Understanding will have to be shown by addition of explanatory text.
For example :
Reverse
As shown in the diagrams, the type of fault produced depend
upon the orientation of the principal stresses. In the case of
normal faults, the maximum stress is vertical and the
minimum horizontal. This results in a tensional environment
and so lengthening of the rock. In the case of reverse fault the
effect is compressional and the crust shortens.
Tear faults form when the maximum and minimum are both
horizontal. This results in shear and the rocks slide past one
another in a horizontal direction. There is no change in the
length of that portion of the crust.
Tear
It will be seen that the orientation of the fault plane is always
parallel to the intermediate principal stress. It is the relationship
between the orientations of the maximum and minimum
stresses that determines the type of fault that will form.
It would be expected that examples of each of the fault types would be included.
These could be major structures e.g. Great Glen, Moine Thrust and the great Glen
Fault; or, more local examples that may have been studied e.g Pennine faults.
Again, the best way to answer the second part of the question is to use a
labelled diagram or, diagram plus explanation, or a combination. In this case the
diagram has only been partly labelled, making it quite difficult to cover the
main points easily using text.
However, here’s an attempt :
The diagram shows how a rock reacts (strains) to applied pressure (stress).
The upper curve is straight to start off with and the rock behaves elastically. That
means that if the stress is removed then the rock returns to its original size. The
curve starts to bend at the elastic limit. At stresses beyond the elastic limit the
rock starts to behave plastically and will not return to its original size if the stress
is reduced. Eventually the rock will flow so much it will fracture (where the curve
ends). If the rock happens to fail in the elastic region, a fault is formed. If the rock
gets into the plastic region it will fold until it is folded so much it will fail.
You will probably agree that it is quite difficult to put the above into words, and
you could ask - if it can be put into those words, what’s the need for the
diagram ? Good point ! The choice is up to you. One is not BETTER than the
other, or will get more marks, BUT, examiners have found from experience that
the high-scoring candidates are the ones that usually make the most use of
labelled diagrams. The diagram as it stands would obtain little credit as it is
insufficiently labelled. Without the above description it is (almost) useless.
So, all I’m really interested in is how many marks did I get ?
ANSWER : 19 out of 25. 17 without the examples. The diagrams as they stand above, with no explanation, would get 14 marks.
So where did the other 6 marks go - and is this is YOUR essay so is it the best you can do ?
For 20-22 marks there would need to be a little more DEPTH and / or BREADTH.
It is the second part of the question (and questions are nearly always written such that this is the case) that really allows a candidate to show higher skills. This is
the reason for including the other two curves above. What would cause a rock to behave like this ? Increasing temperature ? Pore pressure ? Time ?
Also, what’s the significance between the rock faulting in the elastic part or at the end of the curve ? Might this relate in any way to earthquakes ? Metamorphism?
This last bit is not prescriptive and the examiner does not have a mark scheme listing which of these things MUST or MUST NOT be in the essay.
The examiner is looking for evidence of understanding and the bringing together of different aspects of the subject. At examiners’ meetings we call this “a hint of
flair”. This could something you disagree with your teacher about, or you don’t really understand ! Examiners are looking for evidence of original thought, and not
necessarily something they agree with. If you can make the examiner stop and think then you have almost certainly got these higher skill marks.
What do I have to do for FULL MARKS ? You may have got them already. Have you included : diagramsʐexplantionsʐ examplesʐother factorsʐperhaps some
evaluationʐor special insight into one of the factorsʐ.Yes ? Then you’ve just got yourself 25 out of 25.