The North Pacific: an Example of Tectonics on a Sphere
Earth's shell is a few rigid plates — and each pair moves as a single rotation about a pole.
The continents move — but not as continents. Earth's whole surface is cracked into a few huge rigid plates, and McKenzie and Parker found that each pair turns about a single point, like a door on its hinge.
The idea, unpacked
Look at a map of the world's earthquakes and a pattern jumps out: they fall along thin lines, leaving enormous quiet areas in between. McKenzie and Parker took the quiet areas seriously. Those, they said, are rigid plates — slabs of Earth's outer shell that do not bend or stretch — and all the action happens only where the plates meet.
Then comes the elegant part. On a globe, sliding a rigid plate around is the same as spinning it about an axis through the centre of the Earth. Wherever that axis comes out at the surface is the plate pair's “pole of rotation.” Near that pole the two plates barely move past each other; a quarter of the way around the globe, they move fastest. Two numbers — where the pole is and how fast the spin — describe everything.
Reading the spin from earthquakes
By the mid-1960s the pieces were on the table but not yet assembled: Wegener's drifting continents, Hess's spreading sea floor, the magnetic stripes of Vine and Matthews, and Tuzo Wilson's new “transform faults.” What was missing was a way to make it exact. Dan McKenzie, a young Cambridge geophysicist, and Robert Parker, who wrote the computer programme that drew the maps, found it in an old theorem of Euler and a clever choice of projection.
Their test was beautiful. If two plates really turn about a pole, then on a special map centred on that pole the direction each earthquake slips should line up into neat parallel rows. They plotted the slip directions of earthquakes all around the North Pacific — the Aleutians, the San Andreas — and the rows lined up. The paper ran in Nature in December 1967. Jason Morgan, in America, had reached the very same idea independently a few months earlier; the two share the credit for founding the theory.
Why it mattered
This is the moment a set of bold hypotheses became a science you could calculate with. Continental drift had been argued for fifty years and mostly dismissed; seafloor spreading explained the engine but not the geometry. McKenzie and Parker supplied the geometry — exact, testable, and confirmed by earthquakes over a quarter of the planet. Within a year the method was extended to the whole Earth, and “plate tectonics” became the framework that now organises all of geology: where earthquakes strike, why mountains rise, how oceans open and close.
Two records on one spindle
Think of a vinyl record turning on its spindle. A point near the spindle hardly moves; a point at the rim races around. The spindle is the pole of rotation, and the speed grows the farther out you go — exactly as sin of the angle from the pole. Now imagine two such discs meeting along a seam: how they grind, pull apart, or slide past each other along that seam depends only on where the shared spindle sits. That seam is a plate boundary, and the spindle is the Euler pole.
Before and after
In the Library, this paper is the hinge of the plate-tectonic story. Wegener (1912) saw that the continents move; Hess (1962) found the moving conveyor of the sea floor; Vine and Matthews (1963) read its speed in magnetic stripes. McKenzie and Parker (1967), with Morgan, turned all of it into one exact law of rigid plates rotating on a sphere — and Le Pichon (1968) then drew the whole planet's plates. Everything after, down to the GPS in your phone measuring centimetre-a-year drift, speaks this language.
Individual aseismic areas move as rigid plates on the surface of a sphere. Application of the Mercator projection to slip vectors shows that the paving stone theory of world tectonics is correct and applies to about a quarter of the Earth's surface.