UNIVERSITY OF EAST ANGLIA School of Environmental Sciences Main Series Undergraduate Examination 2012-2013 Candidate s no.: GEODYNAMICS: EARTH S ENGINE ENV-2A43 Time allowed: 2 hours. Answer ALL questions in Section A, and TWO questions from Section B. Sections A and B have equal weighting. Write all answers to Section A in the spaces provided on the paper. Answer two questions from Section B one in each answer book provided. Write your candidate s number on this paper AND on the answer books. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. ENV-2A43 Module Contact: Dr Jenni Turner, ENV Copyright of the University of East Anglia Version 1
2 SECTION A 1. a) Why do the same volumes of oceanic crust and continental crust have different densities? b) Give two examples of how new crust may be added to a continent. c) Give one example of how the idealised Penrose model for the layered structure and composition of oceanic crust may be altered near an ocean ridge. d) Why does the elevation of the sea floor decrease with increasing distance away from an ocean ridge? e) What is the relationship between the age of oceanic crust and its depth below sea level? 2. a) Sketch and label a transtensional basin within a strike-slip fault system, show the direction of stresses that control basin formation. [3 marks]
3 b) Briefly explain how a rift may form in the absence of tectonic plate tensional stresses. [2 marks] 3. a) By using the plot in Figure 2 determine the direction and the rate of motion at the plate boundary B-C in Figure 1. [4 marks] b) Show on Figure 1 the direction and rate of plate motion from your solution in Figure 2. Figure 1: Velocity map of relative Figure 2: Plot for scaled solution tectonic plate motion of the problem in Figure 1 PLEASE TURN OVER
4 4. Using shell diagrams below, and assuming a shallow focus earthquake, draw the path through the Earth of seismic rays, and label each segment of each ray: a) PcP on Figure 3a [2 marks] b) P4KP on Figure 3b [2 marks] c) PKPPKP and PKP 650 PKP on Figure 3c [4 marks] Figure 3: Shell diagrams
5 d) What physical change in state occurs inwards at the core-mantle boundary? Explain briefly why P wave velocity decreases but density increases inwards across this boundary. [4 marks] e) What physical changes occur at the ~650 km boundary and what mineral is involved? [3 marks] 5. a) The inversion of seismic data can be used to map magma storage regions. What basic principles does this rely on and why might interpretation be non-unique? [4 marks] PLEASE TURN OVER
6 b) Explain why magmatic underplating often occurs around the Moho using the principle of buoyancy. [3 marks] c) During the final stage of magma ascent the exsolution of volatiles becomes important in driving eruptions. Give three brief reasons for this. [3 marks] 6. a) Define the aspect ratio as applied to lava flows or dykes. [2 marks] b) What are the two principle controls on aspect ratio in effusive eruptions. [2 marks] c) What is the rheology of magma?
7 d) Name two eruptive scenarios where magma rheology governs behaviour. [2 marks] e) Briefly explain your answer for one of these scenarios. [3 marks] PLEASE TURN OVER
8 SECTION B [Answer TWO Questions] 7. The opening and closing of an ocean basin is simply summarised by the Wilson cycle, but no two ocean basins are the same. Explain, using your knowledge of the subsurface processes, why there are variations in ocean ridge cross-section profiles, ocean crust composition, and subduction zone characteristics. Include in your answer the type of evidence which allows us to identify these variations. [25 marks] 8. Discuss to what extent plate tectonic and magmatic processes control the occurrence of basaltic volcanism at subduction-related settings. [25 marks] 9. Relate the characteristics of strombolian, vulcanian and plinian activity to the physical properties of the magma and processes that occur, giving examples. Analyse whether each type of volcanism has a particular association with magma type and volcano-tectonic setting. [25 marks] END OF PAPER