Shaping the Tectonic World View
by Carol Hasenberg
Outgoing GSOC President Sheila Alfsen introduced Dr. Tanya Atwater by discussing the origins of GSOC and its name “The Oregon Country”. The first decades of exploration and research into the geology of the Oregon Country answered the questions of What and Where, she said. It was clear that Oregon contained a strip of volcanism that was present all around the edges of the Pacific Ocean. The big question left to answer was the Why. And that is what the work of Dr. Atwater and many other scientists did over the last 50 years. Plate tectonics has been one of the greatest scientific discoveries of all times.
Atwater began her address by acknowledging that yes, she was able to be involved in the development of this discovery shortly after the birth of the idea. She told the GSOC audience that although she had been asked to relate an oral history of her work in the talk, her brain did not really function that way. Her memory does not work so much in chronological details but in the strength of the ideas. So instead she presented her work in a series of vignettes illustrating the powerful ideas that she helped to develop. A crucial aide in the talk and one that the reader may follow at home is her EMVC website which contains a number of animations illustrating particular details of plate tectonic processes. The site address is emvc.geol.ucsb.edu. So the reader is encouraged to open it up and follow along!
Dr. Atwater studied geology in college, but she thought that the program involved more memorization than her brain could handle, so instead she opted to do geophysics as her profession. It was so much easier for her to work with the mathematics of geophysics. And that is how she found herself working in Chile reading seismographs where a conference of international scientists studying the south Pacific sea floor convened. At that meeting a paleomagnetic trace through the spreading center of the south Pacific Ocean was presented by Jim Heirtzler. This trace showed areas of alternating magnetic orientation that formed symmetric patterns as one headed away in either direction from the high point of the mid ocean ridge. The existence of this phenomenon, so like that found in the Atlantic Ocean, had only one logical explanation-the process of sea floor spreading. And sea floor spreading confirmed and provided a driving mechanism for the theory of Continental Drift proposed by Alfred Wegener in 1912.
So Atwater's career focus was inspired at this meeting. She has spent her career researching and modeling tectonic processes and has made particular studies of the western North American region. She has produced educational animations of the California coastline changes in shape from the San Andreas transform fault movements. She has also made animations of the breakup of supercontinent Pangea. These are available on the EMVC website.
Atwater recommended a book called "Plate Tectonics: Insider's History of the Modern Theory of the Earth" by Naomi Oreskes. In this book Oreskes has collected the writing of key scientists involved in developing plate tectonics theory, one chapter of which was written by Atwater. One of the things that Atwater mentioned in discussing the book was that the development of plate tectonic theory was a multi-disciplinary effort, and each scientist was convinced most strongly by the evidence from her or his field. For examples, the geophysicists were convinced by the paleomagnetic data, and the seismologists were convinced by the locations and frequency of earthquakes around the plate boundaries.
The next idea discussed by Atwater is that the tectonic plates are in general quite rigid and their shapes are only altered by growth along the boundaries, subduction, and crinkling under extreme pressure such as that found in boundaries like the Himalayas. How that is known is that after traveling thousands of miles apart, continental (shelf) margins can be pieced together quite accurately.
In tandem with this notion, Atwater then discussed the role of transform faults in the growth of plate boundaries. These are the faults that run perpendicular to the “stair steps” in the mid oceanic spreading centers. For a while their role and creation were poorly understood. But researchers have come to understand that these are simply scars that are generated at a stair step while the generated material is traveling in two different directions along the step. The steps themselves are the result of irregularities in the plate boundaries that have divided. Atwater had made a really good model of this action with a box and fabric, and this model is discussed on the EMVC website.
Another idea discussed by Atwater that is global in scope for plate tectonics is the fact that upwelling of magma does not drive the creation of new plate material on the seafloor; rather, it is the pull of the weight of the subducting slabs at the cold plate margins which drives the process. In this sense the "boiling pot of jam" analogy fails to correctly model the behavior of the earth's crust. Atwater illustrated this by having the audience study the African continent and the spreading margins which surround it. It is clear that over time this tectonic plate has grown larger, and the spreading centers have shifted with respect to one another. So the spreading centers do not remain in fixed locations on the earth's surface-they migrate as the process continues.
Atwater also discussed the more detailed, location specific work she has done. She has studied how the lateral motion of the Pacific Plate along the North American plate at the San Andreas Fault has altered the coastline of California in the last several million years. She showed the audience at least two examples of this. In the first example a block of the coastline broke off from the main mass and rotated clockwise to become the Santa Barbara Peninsula that we see today, and the space it used to occupy now holds the Los Angeles Bay. A key marker layer of pebble conglomerate from a volcano in northern Mexico, called the Poway conglomerate, ended up in three different locations and indicated the movement to the researchers. Another example of movement along the San Andreas Fault is the Pinnacles and Neenah formations, which are the same in origin but have been separated by 315 miles of movement along the fault. Atwater has developed animated illustrations of the changing coastline over time on her website and also has done some manual models which can demonstrate the changing coastline to school children in Southern California.
Atwater’s talk concluded with questions from the GSOC audience. One listener asked what was the mechanism that started the original breakup of Pangea. The answer was that hot spots in the mantle start to break up a continent. When asked what had inspired her to believe in her own work at that age, Tanya said that a confidence building upbringing and the sheer arrogance of youth helped her to have the courage to present her papers to geophysical conferences which numbered “1000 men to one woman”. She had people tell her that they were interested to see "the girl" present. As there was no dress code for professional women at that time, she would show up “wearing love beads and barefoot" to conferences.
The talk concluded with an announcement of two sessions that were scheduled between Atwater and students from PSU. Atwater has an important role in passing her experiences and confidence to a new generation of geoscientists.