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Test Exam for the Earth System Class of ESPACE
23.1.2006
Extra question 20 points
The figures below show the path of S and P waves through our planet
.
 Describe the major differences between the 2 kind of waves
P waves are faster compressional (or longitudinal waves, thus the obscillation is on
the same direction of the energy propagation). They move trough solid, liquid and
gaseous media.
S waves are slower transverse waves (or shear waves, the obscillation is in a direction
perpendicular to the direction of energy transfer. They move only through solid media
(where it is possible to have shear stresses!)
 What do represent the blue, red, yellow and white parts in the figure?
The white is the solid inner core, the yellow the liquid outer core. Red and blue is the
mantle, the red is the area sampled by the waves and the blue the shadow zone.
 Which image correspond to S and which image correspond to P waves path?
a) P waves b) S waves (they take a longer time to reach at the same point and do not
travel through the outer core.
 What can you infer from the previous figures about the core of our planet?
The S waves are not traveling through the outer core thus it is liquid.
Test Exam for the Earth System Class of ESPACE
23.1.2006
Question #1 60 Points
The previous figure shows the magnetic anomalies on the Atlantic oceans south of
Iceland (bottom) and a cartoon version of the magnetic strip of the oceans at the top:
 How would explain the pattern of magnetic anomalies?
Since through geological observations we know that the magnetic field is not always
pointing north but has reversal, the magnetic strips are one of the proofs that the oceans
are created at the ridge. The ocean floor acts as a tape recorder. Once the magma that
come out at the ridge cool below the Curie temperature the rocks “remember” the polarity
of the magnetic field at the time of the creation of the that given ocean floor.
 Why do you think the real data at the bottom show a much more complex pattern?
The real data have obviously experimental error in the measure and this would already
make the field more complex, furthermore it is recorded at the surface of the ocean
(~3000m away) as perturbation to the average surface field. The major reason is the
secondary magnetization due to chemical reaction in the geothermal area of the ridges.
 Why the magnetic pattern is symmetric with respect to an axis? What is that axis?
The axis of symmetry is the ridge where the new ocean floor is created. It is symmetric
because we have symmetric spreading
 The cartoon picture shows that the bathymetry (depth of the oceans) increases
moving away from the symmetry axis how would you explain this behavior?
The ocean floor get to be older and cooler moving away from the ridge (cooler
lithosphere is denser) thus to be in Isostatic equilibrium with the rest of the mantle it must
be deeper.
 What the magnetic field tell us about the Earth core?
In order to have magnetic field we need to have fluid and conductive material inside the
planet. So since we have magnetic field we have another proof that the core must be fluid
(and low viscosity!)
Test Exam for the Earth System Class of ESPACE
23.1.2006
Question #2 40 Points
The 2 figures at the top show a map of the topography and bathymetry (top) and of the
distribution of earthquakes (green) and volcanoes (red) on a global scale. How can you
use them to define the major plates? Why? Draw on the earthquake map at least 2 plates
and write their names. How fast a typical plate moves (respect to what?) ?
Earthquakes and mountain are due to the deformations of the plates. Since one of the
assumptions of plate tectonics is that the plates are rigid and all the deformation is
localized at the border we can use the earthquakes or the major mountain chain as proxy
to find the plate boundaries (note that this can have problems with old mountain ranges as
Urals and Appalachian or in area with small relative motion between the couple of plates
as NA and EU in Kemtchaka). The volcanoes are in general associated to 3 type of
magma production: hot spot, subduction of material to a depth where it is hot enough to
melt, spreading areas. A part for the first kind not related to plate motion but related to
the motion of the mantle, the other processes are related to plate boundaries thus also the
volcanoes are a good proxy for the position of plate boundaries.
Test Exam for the Earth System Class of ESPACE
23.1.2006
A typical relative velocity for the movement of 2 plates is of the order of tens of mm/yr
one respect to the other. Another good reference frame is the hot spot reference that
assume that the hot spots are fixed respect to the average position of the mantle.