MAGNETISM and THE SAN FRANCISCO BAY AREA


Exercises

(A) Playing with magnets

Place a bar magnet under a piece of paper. Sprinkle iron filings or used staples around the magnet. What happens to the filings or staples? How do they align themselves with respect to the magnet? They align themselves according to the direction of the magnetic field. At the poles of the magnet, the iron filings are straight up and down; in the middle of the magnet, the filings are aligned parallel to the length of the magnet. Compare the pattern with that shown below.


The Earth's magnetic field

Now do a similar exercise in 3 dimensions. You will need clay or play dough, a cylindrical neodymium magnet formed by 2 standard-sized disks (1 cm by 1/2 cm; available from Dowling Miner Magnetics in Sonoma, California), and used staples. Roll the magnet inside the clay or play doh. Make the diameter of the ball 2 to 4 times the size of the magnet. Now explore the magnetic field of the ball by placing used staples on the ball. What happens to the staples? How do they align themselves on the surface of the ball? At the magnetic poles of the ball, the staples stand up vertically. The staples lie flat against the ball at the magnetic equator. Between the poles and the equator, the staples stand up in different angles with respect to a plane tangent to the ball. This angle is called the magnetic inclination. What is the inclination of the staple at approximately the latitude of the Bay area?

(B) Northeast of Point Reyes are some east-west-trending anomalies. Do these anomalies parallel any mapped faults or coincide with rocks rich in iron and magnesium? What is the cause of these anomalies? (clue: look at the specifications of the aeromagnetic survey flown in this area)

(C) Why are the magnetic contours more convoluted (wiggly) over most of the Bay area but start to smooth out over the Central Valley?

(D) At the average rate of plate motion (5 cm/yr), how long will it take Los Angeles (on the Pacific plate) to be opposite San Francisco (on the North American plate) if Los Angeles and San Francisco are about 650 km apart? The last big earthquake in the San Francisco Bay area was in 1906, where at Point Reyes, the Pacific plate moved as many as 6 meters relative to the North American plate in a matter of seconds (for comparison, the Loma Prieta earthquake caused about 1 meter of offset). How long would it take to move that same amount (6 m) using the average rate of plate motion?

(E) To visualize what kind of deformation occurs within a bend of the San Andreas Fault, make a copy of the figure below. Cut the paper into sections A and B. For each section cut the paper along the trace of the San Andreas Fault and move the pieces of paper in the direction shown by the arrows. Describe what happens in the area of the bend of the San Andreas for both cases.


Plan view of fault

Answers to Exercises

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