Geoscience Reference
In-Depth Information
Large Rigid Plates
Tuzo Wilson had opened his 1965 article on transform faults with this prescient
paragraph:
Movements of the Earth's crust are concentrated in mobile belts, which may take the form of
mountains, mid-ocean ridges or major faults with large horizontal movements. These features
and the seismic activity along them often appear to end abruptly, which is puzzling. The prob-
lem has been difficult to investigate because most terminations lie in ocean basins. [These]
mobile belts divide the surface into
several large rigid plates
.
1
On the second page, he wrote: “The plates between the mobile belts are not readily
turned seafloor spreading and continental drift into plate tectonics, though no one
would use the phrase itself until Dan McKenzie and Jason Morgan in a 1969 paper
in
Nature
.
3
Wilson's drawings showed transform faults and rigid blocks in two dimensions,
butofcourselargeobjectsontheEarth'ssurfacemustconformtoits3-D,spherical
shape. Bullard and his colleagues had recognized this and used spherical geometry
to fit the Atlantic continents back together. The breakthrough to plate tectonics
came when Morgan extended Wilson's two-dimensional concepts to the three-di-
mensional surface of a sphere.
JustasWilsonhadnoticedthatearthquakesalongtheMid-Atlantic Ridge'sfrac-
ture zones “end abruptly” and do not extend beyond the ridge crest. Morgan no-
ticed that the fractures are not quite straight lines but more like the curving arcs
of concentric circles. Each fracture arc focuses to the same point on the globe.
That point can be thought of as a pole around which one rigid block (or plate) has
rotated relative to the other. To obtain their fit, Bullard and colleagues had in ef-
fect taken two now-separate blocks—the continents on either side of the Atlantic
Ocean—found the optimum pole of rotation, and swung one continent around that
pole until it snuggled against the other side. Morgan took the opposite sequence,
showing how rotation about a common pole could separate two continents and
open an ocean basin. This was an essential step because it allowed Morgan to use
the concept of a rotational pole to treat the surface of the Earth in purely geomet-
rical terms.
