SO461 Geological Oceanography

Fall 2007, Test 2

 

Short answer

4 @ 10 points

40

 

Longer answer

3 @ 20 points

60

 

Total

 

100

 

 

 

Short Answer: The questions should be answered with several sentences (no more than a short paragraph). You must answer four questions, and have a choice from five.

Longer Answer: These should clearly indicate understanding of the procedures and concepts involved, and use correct and appropriate terminology.

 

The exam is open book, which includes any notes permanently written in the book. No other references or calculators are allowed. Do not leave anything blank, and note that I am looking for an understanding of the important concepts and appropriate use to terminology. An answer than does this will get more credit than finding and copying a sentence in the book that includes the word.

 

 

Budget your time. If you try to look up everything in the text book, you will run out of time.

 


 

 

Short Answer. Answer 4 of the 5 questions in this section, for 10 points each.

 

1. The diagram below shows an earthquake at occurred on a fault. The fault appears to offset two linear features, A and A.

 

        What do you think is the orientation (dip and strike) of the fault? Why can you say this?  There are two possible strikes, about N0 and N90E, with dips of about 85W and 85N respectively.  The East-West strike is more likely because it is parallel to the fault trace.

        What kind of fault do you think this is? Be specific, and discuss the geometry shown.  Transform fault.

        Could there be a fault with a similar geometry but different focal mechanisms?

 

 

 

 

 



2. The two diagrams below cover areas of the seafloor about 1250 km across. Discuss the features you see in the diagrams, and what accounts for the differences between the two areas.

 

 

 Mid-Atlantic Ridge                                         East Pacific Rise

  


3. Use a sketch diagram to show how we classify the metamorphic rocks, and the major groupings in our classification. Are there any possible cases where it would be understandable to take gneiss for granite?

 


4. The diagram below shows a map of a plate boundary, and the velocities between the two plates along the boundary at four points in the hot spot reference frame. You can make any reasonable assumptions you want, and approximate values.

        Given the geometry, what kind of a plate boundary do you think this is? Why?

        How do we define plate motions, and what can you say about the plate motion between the two plates from the velocity pattern?

 

 
Plate motion at each of the 4 locations.  

While the 72 mm/yr is the maximum in this area, we do not know that it is 90 degrees from the Euler pole.

We do not know where the Euler pole is, because the volcanic arc does not lie on a great or small circle related to the Euler pole. 


5. The diagram below shows a seismic reflection line from the offshore Santa Monica Basin in southern California.

 

        If this is related to transtension or transpression, what does that mean?

        Why does the scale on the right side say water depth (m), even when it records the returns form the ocean crust below the seafloor?

        What do you think is the actual depth at the bottom of the record? What assumptions would you have to make for this assessment?

 

 

 



Longer Answer. Answer all 3 questions in this section, for 20 points each.

 

 

 

 

The map and east-west cross section above show the distribution of earthquakes somewhere on earth, and nine focal mechanisms.

        Could the two diagrams represent the same area? Why or why not?

        What kind of plate boundary (or boundaries) are on the map?

        What type(s) of fault occur in this region, and how do they relate to the plate boundary (or boundaries)?

        Would igneous rocks be created in this region? Where and why?

        Would metamorphic rocks be created in this region? Where and why?

 

Earthquakes color coded by focal mechanisms.
Earthquakes color coded by focal mechanisms.

Note the thrusts are all very shallow, and occur both along the subduction zone and in the orogenic belt to the east.



 

The two diagrams below show a map with three plates, and a velocity diagram showing the plate motions in the hot spot reference.

 

 

Boundary

Type

Explain reason

A-B

 Ridge

 

B-C

 Transform

  

A-C

 Trench

  

 

 

Which boundary will show the fastest motion, and why can you say this?



 

3. Discuss how recent technology allows us to compute instantaneous rates of plate rotation. How do these poles differ from total reconstruction poles and stage poles, and which poles we would want to use to create a movie showing the breakup of the Pangea supercontinent?