Wednesday, October 02, 2013

Geologic Dance

If you have kids of a certain age, you may remember watching Cinderelmo with them ad nauseum. Actually, it was a rather silly and sweet video with a really, really catchy song, Everybody's Doing This Dance.


Step to the right, shake, shake shake...Turn, turn around...

I could watch my toddler shake and spin for hours.

One day, when we were walking home from daycare, she spontaneously started to sing and dance to this tune. A couple approached their home in a car during this performance in front of their house.  I worried that she was impeding their progress.  They stopped, watched the performance with a big smile, and then continued into their driveway after Iris finished the song and resumed walking.

About that time, I met a geophysicist who mentioned that some of the channel islands are spinning.  I immediately imagined them stepping to the right, stepping to the left, spinning one way, then the other--just like Cinderelmo.

Simple shear of southern California during Neogene time suggested by paleomagnetic declinations, published in Journal of Geophysical Research, Solid Earth, 1985.  (I added the paragraph breaks for easier reading.)
We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°.

Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise.

This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region.

Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system.

Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific-American plate interactions both rifted the continental crust to create this pattern and rotated the western-most structures within a dextral simple shear zone which had a half width of about 400 km.
In plain English, it means that they looked at the magnetic alignment of rock layers around the southern California region and learned that sections are rotating.

 Recall that the earth is a giant magnet, with a slowly varying magnetic field.  The iron in rocks aligns with the magnetic field when it is first pushed up to the surface and still a hot liquid.  When it cools, the alignment is fixed.  So, you can read off the magnetic alignment of the earth's crust and obtain a record of past earth magnetic field orientation.

It's like a tape recorder.  The deeper you go, the farther back in time you peer.  Of course, this works if the ground stays put.  In rift zones--say at the boundary between the North American and Pacific plates--the ground moves and the picture gets more complicated.

If you can identify the same geologic layers in different locations, and they are magnetically oriented differently, you can infer past ground movement.  Different layers may have different rotational (magnetic alignment) offsets.  In fact, our local mountain ranges have rotated clockwise 50° and then counter-clockwise 15° for a net rotation of 35°.
The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°.
Pretty nifty, but it gets even more interesting.  I took a screenshot of the Historical Earthquakes & Significant Faults in Southern CA interactive map at the Southern California Earthquake Data Center. (SCEDC is at Caltech and open today. Their USGS coworkers are locked out due to the government shut-down.)

Notice the fault line that runs through the northern Channel Islands of Anacapa, Santa Crus, Santa Rosa and San Miguel (due south of the city of Santa Barbara)?  There is a corresponding fault line to their north running right through Santa Barbara.  The North American and Pacific plates are kind of locked here (and in other E-W or transverse ranges of Coastal California).

If you slide two fluids across one another, little eddies will form. (Or rub your hand across a table cloth and see little folds come up.) This is the geologic equivalent. Slide two plates in opposite directions with a bit of loose crust between them.  The crusty bits will rotate like geologic ball bearings!  The fault map explains why some of the Channel Islands rotate, and others do not.  

Of course, they are all sliding roughly NE along with the Pacific plate.  The islands are like slowly moving ships and some of them are wagging their tails as they go by.  You would have to watch from the beach for a very, very long time to discern the movement.  Use your geologic imagination cap.

BTW, today, our nation is being tested.  Remember that, in some parts of our country, teachers cannot teach geology that contradicts the biblical notion that the earth is ~4000 years old.

Remember also the students and teachers massacred by Boko Haram. (The group's name translates as "Western education is forbidden".)  Those bullies have targeted schools repeatedly and the Nigerian government has been unable, or unwilling, to stop them.  The only thing their government has done is to shut down the schools, allowing Boko Haram to get what they wanted.

How do we stop global bullying?

6 comments:

  1. That is absolutely fascinating! I hadn't even thought that an island could rotate, though of course our planet is essentially like a massive living thing itself and we are like tiny specks of dust bouncing about on the extreme periphery.

    ReplyDelete
  2. i teach 4th grade and you have given me a much deeper understanding of the Earth's plates. Thank you!

    ReplyDelete
    Replies
    1. Oh, Good. This is an excerpt of lesson plan I wrote about geomagnetism. I'll post the other part about magnetic lava in Hawaii.

      Delete
  3. Holy smokes - awesome explanation. Thank you.

    ReplyDelete
  4. I keep coming back to this, as I bring it up in my classroom. I do the demonstration with the stacked bath towels but it doesn't show how things are rotating. Very interesting.

    ReplyDelete
  5. I keep coming back to this. I do a bath towel stack demonstration, but it doesn't show the rotation. Very interesting.

    ReplyDelete