SO461 Geological Oceanography

Fall 2007 Test 1

 Diagrams 4 @ 20 points 80 Short answer 4 @ 5 points 20 Total 100

Authorized reference:

1. This test will be open book.  Students can use their own copy of the textbook, with any notes they have permanently written in it.  No papers can be attached to the text, and class notes and handouts will not be allowed. Books will not be shared during the exam.
2. Calculators will not be allowed.  For the calculations, you may set it up and leave the answer in terms of a sum, product, or quotient, or you can make reason simplifications to get a rough idea of the answer.

Short Answer: The questions should be answered with several sentences (no more than a short paragraph). You must answer four of the five questions.

Diagrams: These should clearly indicate understanding of the procedures and concepts involved, and should be well thought out with appropriate terms and details to support your answer.

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

You are not to discuss this exam with any students not in your section until after 1430 today.

## Figure 1.  The three magnetic anomaly profiles on the previous were acquired in the same region, and all have the same horizontal scales (in km) and vertical scales (in nannoteslas).  The time diagram on the bottom shows the durations of normal and reversed periods during the last 50 Ma.

What are these magnetic profiles actually measuring?

Label at least three anomalies on each profile with your estimate of the age in millions of years, and show where they occur on the time line..

On each profile, mark the location or the ridge or indicate the direction in which you would look to find the ridge that created this seafloor.

How does the spreading rate vary for each profile?  For one profile, show how you would calculate it, and then indicate whether the other two would have faster or slower spreading rates.

What is an Euler pole, and how do these profiles relate to one?

## Figure 1.  Rates are 10 mm/yr, 15 mm/yr, and 25 mm/yr

Figure 2.  The diagram on the previous page shows a sequence of rocks that you found while walking through the scenic mountains of Oman on your graduation leave.  The thickness scale is in meters.  Discuss what you can infer about the geologic history from these rocks.  For full credit you should have something to say about each rock layer, and a brief summary/conclusion.

• Depth can change over time (as seafloor cools, for instance)

• Latitude can change over time, as a plate moves

• So where this was has changed over time, both in depth and latitude (and longitude as well, but that is much harder to measure)

Figure 3.  The diagram above shows the focal mechanisms for selected earthquakes in a region of the Indian Ocean that is about 700 km across.

Pick two representative earthquakes that will do the best job of explaining what is happening, and show the likely orientation of the fault plane and the type of fault involved.  Be specific and include numbers; list and defend any assumptions you make.

• The focal mechanism provides two possible fault orientations, each with a dip and strike.  You should be able to estimate those.

• At a ridge, typically both mechanisms will be present on different faults on each side of the axial rift valley.

• For the ridge transforms, the fault plane will probably be the one with the strike perpendicular to the strikes of the normal faults (which will be parallel to the ridge axis).

On the diagram below, sketch a possible plate boundary or boundaries and indicate the plate motions suggested by the earthquake data.

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 Fig. 2.14, p.17 of the text.