Wednesday, May 8, 2013

Geophysics Problem of the Day: Undergraduate Plate Tectonics Midterm

I like exams that are:
1. Not too easy
2. Not too hard
3. Interesting for students; they can learn something while doing the exam.

 Here is an adapted version of  the midterm that I gave in undergraduate plate tectonics course. The students found question 1 confusing, but enjoyed questions 2 & 3.  From my point of view it was an effective exam: a good range of scores from low to high.

Please let me know what you think of the midterm. Any suggestions for how to make the first question  less confusing for the students would be helpful. The goal is to test their understanding of the interplay of temperature, rheology, and melting to create new crust beneath a mid-ocean ridge, and to contrast that with the thermal-mechanical behavior underneath continents.

I. Below is a copy of a cartoon of the geotherm underneath the average ocean crust.

     Answer the following questions referring to the cartoon, copied from the Annual Review Earth Sci. Article by Fischer, Ford, Abt, and Rychert, (Ann Rev. Earth Sci. 2010)

a. Lithosphere vs Asthenosphere
i. Show and label the lithosphere and the asthenosphere.
ii. Show and label where heat flow is mostly by conduction and where heat flow is mostly by convection.iii. Approximate the average depth corresponding to the horizontal line on the plots. (Don’t forget units!)

b.Geotherm and Melting
i.  Draw an additional geotherm corresponding to what it might look like underneath a continent.
ii.  Draw an additional geotherm, corresponding to what it might look like directly underneath a mid-ocean ridge.
iii.  Superimpose a curve representing the melting temperature (solidus) as a function of depth.
iv. Show and label where partial melt may occur—below continents, below average ocean crust, and/or below the mid-ocean ridge.

c.Viscosity and Rock strength
i. Using the top axis, plot rock strength as a function of depth below average ocean crust.
ii. Using the top axis, plot rock strength as a function of depth below average continental crust.

--> Answer the questions about the below map from a recent paper by Ekstrom, Nettles & Dziewonski (Phys. Earth. Planet. Inter, 2010). 
-->Caption:  Centroid locations and moment tensors for the 38 largest (MW  7.5) earthquakes of 2004–2010. Plate boundaries (Bird, 2003) are shown by gray lines. The full moment tensor is shown by shading; the nodal lines of the best-double-couple focal mechanism are also shown. The dot indicates the projection of the compression (P) axis onto the focal sphere. The size of the focal mechanisms is a linear function of magnitude. (note: the numbers above the focal mechanisms is from the paper and not important for this question.)

1. Faulting.
a.  Name the three major types of faults.

b.  Find, circle, and label good examples of moment tensor solutions corresponding to each of the three major fault types listed in a. above.

2. Boundaries.
On the map, find and clearly label an example of each the following types of plate boundaries:
i.   Convergent Boundary
ii.  Divergent Boundary
iii. Transform Boundary

3. Plate names and relative velocities. 
a. On the map, choose any two plates that share a boundary and label them A and B
b. Using A as the reference frame, draw two arrows corresponding to the plate velocity of B with respect to A.

c. Extra Credit:  Give the plate names of the two plates you have chosen.

 III.  Use the map shown below to answer the following questions.
The following questions refer to the boundary between points labeled A and B on the projected map.


 1.  The boundary between A and B is a
a.      mid-ocean ridge
b.     trench (or subduction zone)
c.     transform fault
d.     fracture zone

2.  The plate velocity at point A with respect to point B is best described as:
a.     Moving towards the right compared with point B
b.     Moving towards the left compared with point B
c.     The same velocity as point B

3.  The depth of the ocean floor at point A is
a.     Deeper than at point B
b.     Shallower than at point B
c.     About the same depth as point B

4.  The age of the ocean floor at point A is
a.      Older than at point B
b.     Younger than at point B
c.     About the same age as point B

 Extra credit 3.  Describe the quantity that is mapped in this image. (i.e. what do the colors refer to?)

 Extra credit 4.  What are the units of the numbers on the legend? (e.g. meter, kilogram, second, etc.)


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