Geoscience Reference
In-Depth Information
The first few lines of the abstract of Morgan's 1968 paper, “Rises, Trenches,
Great Faults, and Crustal Blocks,” based on his unanticipated and poorly under-
stood talk at the 1967 AGU meeting, capture the essence of plate tectonics:
The earth's surface is considered to be made of a number of rigid crustal blocks. It is assumed
that each block is bounded by rises (where new surface is formed), trenches or young fold
mountains (where surface is being destroyed), and great faults, and that there is no stretching,
folding, or distortion of any kind within a given block. On a spherical surface, the motion of
one block (over the mantle) relative to another block may then be described by a rotation of
one block relative to the other block.
4
Morgan then proceeded to locate the pole of rotation that best accounts for the
fractures along the Mid-Atlantic Ridge, finding it at 62°N ± 5°, 36°W ± 2°, near
the southern tip of Greenland. The relatively small error bands show how well the
different fracture arcs point to the same geographic pole position. Morgan used the
geometry of the ridge features to estimate spreading rates and compared them to
ratescalculated fromthemagnetic anomalies. FortheMid-Atlantic Ridge,theEast
Pacific Rise, and the Pacific-Antarctic Ridge he got a good match. Using those
three ridges and the estimated motion for each plate, Morgan was able to predict
rate that he calculated.
Great Faults, and Crustal Blocks,”
Journal of Geophysical Research
73, no. 6 (1968).
Hess Is Right
Lamonter Xavier Le Pichon took the next step. He had heard Morgan's 1967 talk
and later recalled:
It seems extraordinary that, in the hall packed with the best geophysicists and geologists in the
United States, nobody got excited by or even interested in the implications of Morgan's ideas.
