1946-1974
Calm Before
the Storm
Commotion In the Ocean
Lecturer Gave His View of the Bottom Line
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The
Theory
The
Debate
The
Lecture
The
Evolution
Seafloor
spreading, mantle convection and the "new" continental drift were generating
quite a disturbance in the geosciences community in 1962 when Robert
S. Dietz went on the road as an AAPG Distinguished Lecturer.
Old theories that had lain dormant for decades have been revived. Their respective advocates proposed that the distribution of continents and ocean basins on the earth was due either to the planet contracting, expanding or oscillating in size.
Some scientists, reluctant to modify traditional interpretations of geosynclinal theory, made emotional arguments against the new ideas.
Dietz, a marine geologist whose research contributed substantially to the development of global plate tectonics theory, also was an effective communicator who helped the profession bridge the gap between the axioms of the old "land geology" and the implications of modern marine studies.
His method was direct, simple and spiced with humor.
"Continent and Ocean Basin Evolution by Seafloor Spreading," was the official title of Dietz' AAPG lecture. It became commonly known, however, by its subtitle, "Commotion in the Ocean," an appropriate description in light of the hullabaloo its premise had raised in the profession.
Dietz, then with the U.S. Navy Electronics Laboratory in San Diego, summarized the greatest geologic breakthrough of the century with a few succinct lines:
"... Seafloor spreading is envisioned as the fundamental process creating continents and ocean basins. Accordingly, the sea floor moves out in opposite directions from the mid-ocean rises. The gap is filled by new strips of sea floor created from the ultrabasic mantle. By this giant conveyor belt action, protocontinental rock is eventually piled up as rafts of sial; continental islands in the world-encircling sima. Thermal convection cells in the mantle provide the fundamental driving force and the mid-ocean rises mark their divergence while the continents tend to lie over the convergences.
"The principle novelty of this concept is that no fixed layer separates the sea floor from the convection process; rather the ocean bottom is the exposed and outcropping limbs of this convection. Accordingly, it is useful to consider the supra-mantle substance beneath the ocean ... as only a 'rind.'
"In contrast, the buoyant sialic continents that ride above this convection are not invaded by it so that they alone are the true crust.
"Although perhaps alarming at first thought, seafloor spreading is an orderly, evolutionary and actualistic process consonant with geologic history. Continents grow in area and thickness with time and the volume of the ocean basins increases as well to accommodate juvenile water.
"The continents are domains of compression and the ocean basins domains of tension; but the earth as a whole neither contracts nor expands."