Wednesday, May 20, 2015

J Tuzo Wilson's Hawaiian Dream

Again via Twitter I came across this collection of essays on eminent geologists titled Rock Stars, produced by The Geological Society of America History of Geology Division.

I haven't gone through all of them yet, but there is a lovely piece on J. Tuzo Wilson, the geophysicist most famous for his work on the then incipient field of moving continents and oceanic crust aka plate tectonics.

We know Wilson from geology coursework on the "Wilson Cycle", the opening and closing of ocean basins over hundreds of millions of years. But Derek York who has written the essay has more insights. He remembers that Wilson had a knack of solving problems by visualizing rather than working equations. One of his famous insights into the origin of the Hawaiian island chain came to him this way-

Tuzo’s mind had a fascinating way of solving problems. Unlike most physicists, who find their solutions via mathematics, Tuzo solved problems almost entirely with visual images and then presented the solutions in extremely clear prose. He had a remarkable ability to look into the heart of extreme complexity and see simplicity itself. The nearest mind that I can think of to compare with Tuzo’s was that of Michael Faraday who, instead of integrating differential equations to calculate the electric field, imagined a charged particle to be an octopus with tentacle-like lines of force reaching out into the space around it.

To solve the problem of the origin of the Hawaiian Islands, for example, Tuzo imagined someone lying on his back on the bottom of a shallow stream, blowing bubbles to the surface through a straw. The bursting bubbles were the Hawaiian Islands, and they lay in a line because they were swept along the surface by the moving stream. Thirty years later, leading geophysical theorists use supercomputers to solve horrendous equations that Tuzo”solved” in the visualizing region of his brain.

In today's jargon, the moving  stream is a tectonic plate in motion and the bubbles are rising molten material originating ultimately from a fixed heat source in the mantle. 

Another famous Wilson piece of reasoning was the sense of movement on transform faults linking mid ocean ridges. He predicted that for earthquakes occurring underwater in the middle of the ocean "rocks everybody believed had moved right to left during the earthquake had moved left to right, and vice versa".

What did he mean by that? Well, the sense of motion along transform faults which connect strands of mid-oceanic ridges is opposite to what one would expect from a classical strike slip fault. See the image below.

Fig. A is a bed offset by a sinistral strike slip fault with a left lateral motion.  This means that the piece of crust north of the fault  has moved towards left.

Fig.B shows strands of a mid oceanic ridge. The red line linking the ridges is the transform fault. Notice however that the sense of motion along the transform fault is opposite that of a classical strike slip fault  i.e. the piece of  crust north of the transform fault is moving towards the right as it would  since the ridge is producing new oceanic crust and pushing older crust away.

There are other interesting essays, I enjoyed the one on Lawrence Sloss And The Sequence Stratigraphy Revolution.

Dive in..

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