# Exam_2005 - Universitetet i Oslo

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UNIVERSITETET I OSLO
Det matematisk-naturvitenskapelige fakultet
Exam in: GEL2140 Geophysics and global tectonics
Day of exam: Thursday June 2nd 2005
Exam hours: 2.30 pm – 5.30 pm
This examination paper consists of 5 pages.
Appendices: None
Permitted materials: Calculator
Make sure that your copy of this examination paper
NB! Exercise 1 and 2 count 35% each, exercise 3 counts 20% and exercise 4 counts 10%
Exercise 1.
a) Which corrections (data reductions) are commonly applied to gravity measurements?
What does the formula δg = h(0,3086-0,04192ρ)mgal express? What is h and ρ ?
b) Define Bouguer anomaly.
Figure 1 shows a rather plane mountain plateau which is 1000m high and 150km wide.
Assume that the gravity measured in B is gm = 981714,3mgal and that the theoretical
gravity in A, as given by the International Gravity Formula, is gλ = 981918,0mgal.
Calculate the Bouguer anomaly in A. Why is it negative?
1000m
crust
30km
mantle
70km
asthenosphere
•B
150km
•A
ρ1=2,60Mg/m3
ρ2=3,10Mg/m3
H
ρ3=3,30Mg/m3
Figure 1.
c) What is isostatic equilibrium? Describe Airy’s and Pratt’s isostatic compensation
mechanisms.
Compute the thickness H of the root when the area is in isostatic equilibrium as shown in
figure 1.
d) Assume that the mountain plateau gets covered by an ice sheet with uniform thickness
and density ρice = 0,9Mg/m3. As a result, the lithosphere under the ice sheet is sinking a
little deeper in the asthenosphere, but without getting thicker. When the area has reached
a new isostatic equilibrium, the surface of the ice sheet is at an altitude 400m higher than
the surface of the plateau before it was covered by ice.
Use this information, together with the values in figure 1 and ρice, to compute the
thickness of the ice sheet.
Exercise 2.
a) What is remanent magnetisation?
b) Define magnetic inclination and magnetic declination. What is apparent pole?
c) The following expression is often used in connection with palaeomagnetic investigations:
tan I = 2 tan λ
What do I and λ represent?
The application of the expression in palaeomagnetism is usually based on an important
assumption. Which one?
d) Paleomagnetic directions have been measured on lava rocks collected at a site located at
60&ordm;N. The average inclination is 63&ordm; and the declination is 10&ordm;.
How many degrees have the rocks moved in north – south direction since they were
magnetised? Have they been rotated?
e) Figure 2 (next page), which is taken from the textbook of Mussett and Khan, shows
apparent polar wander curves for Europe and Siberia. What do the curves tell about the
plate tectonic history of the two land masses?
S: Silurian
D: Devonian
C: Carboniferous
P: Permian
T: Triassic
Figure 2.
Exercise 3.
a) What do we understand by lithosphere and asthenosphere?
b) Describe shortly the various plate boundaries with respect to movements and distribution
of earthquakes.
c) What characterize the composition and thickness of continental and oceanic crust?
d) When two plates collide, what happens to normal oceanic and continental crust? Give
examples.
Exercise 4.
NB! The questions in parentheses concerns topics covered by another textbook than the one
being used now.
Multiple choice questions: Just write the question number and a, b or c for the right answer
in what you hand in.
1) What is the boundary between the crust and mantle called?
a) The Benioff boundary
b) The Meguma zone
c) The Mohorovicic discontinuity
2) What is an ophilite?
a) Subducted oceanic crust
b) Obducted oceanic crust
c) Material from the earth’s core
3) The oceanic crust consists of mainly:
a) Granitic rocks
b) Gabroic rocks
c) Metamorphosed sediments
(4) Can you observe strike slip faults in rift basins?)
a) Yes
b) No
(5) The depth versus age curve for the seafloor can be expressed by the following equation:)
a) D= 350 + 2500√ T
b) D= 2500 + √ T
c) D= 2500 + 350√ T
6) We find the slab graveyard:
a) In the trench
b) In the accretionary prism
c) At the core/mantle boundary
7) Surrounding the Indian Ocean we find:
a) Only passive margins
b) Only active margins
c) Both active and passive margins
8) A passive margin has been created by:
a) Rifting followed by sea floor spreading
b) Rifting followed by compression
c) Subduction followed by extension
(9) A successful rift:)
a) Contains evaporates
b) Splits a continent into two pieces
c) Prevents subduction
(10) A listric normal fault:)
a) Is the most typical fault in old oceanic crust
b) Can be observed in rifts
c) Can only be observed in sandbox experiments
a) Is subsiding very slowly
b) Is spreading with rates less than 4 cm/year
c) Is moving with rates less than 4 cm/year with respect to a hot spot
(12) Can a mountain chain collapse?)
a) Yes
b) No
13) What is the age of the oldest oceanic crust in the ocean basins?
a) 130-150 my
b) 100-120 my
c) 160-180 my
(14) The oldest oceanic crust is found in the:)
a) Western Pacific
b) Arctic ocean
c) Eastern Pacific
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