Download 2015-16 - School of Earth Sciences

UNIVERSITY OF BRISTOL
Summer 2016 Examination Period
School of Earth Sciences
Year 3 Examination
EXAM PAPER CODE EASC30053
GLOBAL TECTONICS AND GEODYNAMICS
Time allowed:
3 hours
This paper contains 4 sections, A-D.
All questions in each section must be answered.
Each section carries equal weight, and the sub-division of marks
for each question is indicated.
OTHER INSTRUCTIONS
Answer each question in section A in a new blue answer book,
and each of the other sections in a separate blue answer book.
Clearly indicate the question answered, and remember
to put your candidate number on each blue booklet.
ADDITIONAL INFORMATION
NON-GRAPHING AND NON-PROGRAMMABLE CALCULATORS MAY BE
USED IN THIS EXAMINATION
TURN OVER ONLY WHEN TOLD TO START WRITING
Section A:
Answer all parts of this question. This section counts for 40% of the total
mark for this examination and each part is weighted equally.
Use diagrams and equations to illustrate your answer wherever possible.
1. Why and how does mantle convection occur in the Earth? Explain the
physical processes and parameters that govern mantle convection, using
brief descriptions, equations with defined terms, and annotated diagrams.
2. In approximate chronological order, describe the main strands of evidence
upon which the theory of plate tectonics is founded.
3. Contrast the compositions of erupted magmas at continental and oceanic rift
settings and account for the differences highlighted.
Section B:
Answer all parts of this question. The question counts for 20% of the total
mark for this examination.
a) This part consists of brief, independent questions.
i.
Which parameters of a material do the P-wave and S-wave velocities
depend on?
[5%]
ii.
If the new Planet 9 recently discovered in our solar system were
(hypothetically) observed to have a normalised moment of inertia of
𝐼
= 0.5, where M is its mass and R its radius, what would you
𝑀𝑅 2
conclude about its internal density structure?
[5%]
iii.
Define the Clapeyron slope and name three important phase changes
in the mantle.
[5%]
iv.
Explain the difference between the geomagnetic poles and the
magnetic poles.
[5%]
v.
According to Griffith, what is required to propagate a crack?
[5%]
b) The adiabatic bulk modulus (or incompressibility) of a material at constant
𝑑𝑃
temperature is given by 𝐾 = −𝜌 𝑑𝜌 where ρ is density and P is pressure. For a
material with a bulk modulus of 150 GPa, how much pressure would you
have to apply to the material to increase its density by 50%?
[25%]
c) For highly compressible materials (such as gases), the isothermal equation of
state can be written as 𝜌 = 𝑐𝑃𝛾 where ρ is density, c and γ are constants, and
P is pressure. In this question we will consider the resulting pressure profile
with depth within a spherical gaseous Jupiter-like planet.
i.
Assuming hydrostatic conditions, i.e. that pressure in the planet is
solely due to the weight of overlying fluids, state the equation for the
infinitesimal change in pressure dP due to an infinitesimal change in
depth dz in terms of gravitational acceleration and density.
[10%]
ii.
Making the hydrostatic assumption, use the equation of state to derive
an expression for the pressure at a depth z inside the planet. You may
assume: the gravity g is constant; the pressure is zero at the surface
1
(z=0) and 𝛾 = 2. Demonstrate that this expression is equal to 𝑃(𝑧) =
1
2
(2 𝑔𝑐𝑧) .
[20%]
iii.
On Jupiter, pressures reach 100 GPa at a depth of 12,000 km. Using the
expression in ii., estimate the pressure [in GPa] at the centre of the
planet at the depth of 72,000 km.
[10%]
iv.
Describe three important assumptions you have made in your
calculations and how they might be violated.
[10%]
Section C:
Answer all parts of this question. The question counts for 20% of the total
mark for this examination.
The Euler Pole between the Indian and Eurasian Plates lies at 24oN, 18oE with an
angular rotation rate of 0.5 degrees/Ma. Figure 1 shows the focal mechanisms
for earthquakes occurring in the India-Asia collision zone, separated according
to type.
Fig 1.
a) Calculate the rate of motion (in mm/yr) between the two plates at 30oN,
75oE.
[30%]
Note, to convert between latitude and longitude and Cartesian vectors, use the
following relationship:
b) For each panel (A, B, C), identify the type of mechanism represented. [10%]
c) For each of the labeled events (A1, B1, C1, C2), estimate the approximate
strike and dip of each of the nodal planes. If possible, indicate which is the
fault plane and which is the auxiliary plane giving reasons.
[30%]
d) Briefly describe the pattern of faulting and how that relates to the kinematics
of the India-Asia collision zone.
[30%]
Section D:
Answer all parts of this question. The question counts for 20% of the total
mark for this examination.
Fig. 2
a) Briefly describe the main mineralogical changes that occur during melting of
the mantle beneath mid-ocean ridges.
[10%]
Fig 2 shows an experimentally derived phase diagram to investigate mantle
melting at 1GPa.
b) To what approximate depth does 1GPa correspond?
[10%]
c) i) What is the temperature of onset of peridotitic mantle melting predicted by
this diagram?
[10%]
ii) Account for any discrepancy between the temperature you report in part
i) and temperatures typically inferred for the convecting mantle at 1GPa?
[10%]
It is often argued that the mantle contains some recycled basaltic crust. Recycled
oceanic crust can be represented by a composition R.
d) What is the sub-solidus mineralogy of this recycled crust at 1GPa?
[10%]
e) Recycled oceanic crust is argued to have melted substantially by the onset of
peridotite melting. What degree of melting has composition R undergone by
the onset of peridotite melting?
[30%]
f) Briefly discuss how the melting behaviour of recycled oceanic crust can help
account for relative radiogenic Nd isotopic compositions of most Icelandic
basalts relative to those of ocean islands, such as the Canaries, that sit on old
lithosphere
[20%]