N O T E S CONTINENT AND OCEAN BASIN EVOLUTION BY SEA FLOOR SPREADING ("Commotion in the Ocean"), by Robert S. Dietz, Ph.D.

(A Digest by the Editor)*

The author visualizes the continents a s island-like rafts of low den- sity rock (sial) isostatically floating in an ultrabasic mantle (sima), having some analogies with icebergs floating in water(Fig. l), but closer ones to rafts of slag floating on a pool of iron in a blast furnace. The temperature environment differs radically, but for correct model simili- tude, the earth must be thought of a s a viscous body of considerable fluidity.

Fig. 1 . Comparison between the island-like continents and icebergs

*Editor's Note: When Dr. Robert S. Dietz presented his "Distinguished Lecture" paper "Continent and Ocean Basin Evolution by Sea Floor Spreading" at a luncheon meeting of the Houston Geological Society during the GSA Convention and the International Saline Conference, a se r ies of unfortunate episodes, cul- minating in a projector failure, combined to hamper the presentation.

Because his paper represents an interesting and thought-provoking philo- sophic approach to fundamental geologic problems including the origin of conti- nents and oceans and subsequent orogeny; and because we feel that probably only a small percentage of his audience last Fall got an accurate conceptionof his theory and its implications, we a r e presenting here a partial digest of his paper, together with some of the rough sketches which illustrate the informal lecture version.

It i s our hope that these excerpts will provide an accurate idea a s to what the theory is and will stimulate your interest in its implications with respect to continental drift, continental and ocean basin orogeny, and with respect to the validity of the assumptions on which the "Mohole Project" is based.

The author's concept is schematically diagrammed in Fig. 2 and is outlined in Table I.

Fig. 2

Here is a schematic of sea floor spreading. Convection cells are the fundamental driving force. The mid-ocean divergence is marked by a rise and a rift. The sea floor is "crustless" - - no riding plate separates the cell spreading from the ocean floor. This is the principal novel aspect of this concept. The continents are built up over the convergences.

TABLE I

1. Large scale thermal convection cells, fueled by decay of radioactive minerals, operate in the mantle, providing the earth's primary diastrophic force.

2. These cells do not invade the continental blocks since they are

low density sialic rafts. They are protected from convection cell invasion by their bouyancy -- like slag in a blast furnace. Hence, the continents are a true "crust".

3. In contrast, the sea floor is invaded by these convection

currents so that it is "crustless". Instead, it possesses only a "rind".

4. "Outcropping" beneath the oceans, the horizontal limbs of

these cells act as conveyor belts. They carry the sea floor rind and deep sea sediments back to (marginally and beneath) the continents, where conversion into sial takes place.

5. By frictional coupling with the convection cells, continents are

drifted to downwelling sites where they attain stability, much as an iceberg is carried to a downwelling zone in the sea.

Fig. 3. North Atlantic Ocean Fig. 4. South Atlantic Ocean Figures 3 and 4 are physiographic diagrams of the North and South

Atlantic Oceans, respectively, as interpreted and drawn by Heezen and Tharp. They show the abrupt continental slopes rising out of the abyss, and the well-known jigsaw puzzle fit between South America and Africa, long cited as evidence of continental drift.

More importantly, they show the Mid-Atlantic Ridge which ex-tends

the entire length of the Atlantic abyss. (This "Mid-Atlantic Ridge" and the pronounced rift extending along its axis are spectacularly illust-rated in Life Maezine's Pictorial Atlas of the World.)

Fig. 5.

Similar mid - ocean ridges seem to course through all of the ocean basins. (Fig. 5) They are second only to the continents and ocean basins hemselves in grandeur. Clearly they are of fundamental geologic importance.

The median positions of these ridges in the ocean basins are truly remark able. They can hardly be coincidental. They must in some way be rela-ted to the continents. Menard supposed that the continents controlled their position. But the hypothesis of sea floor spreading assumes the converse; that the mid-ocean rises occur in zones of upwelling between two convection cells in the mantle; that the divergent movements of the "rind" of a "crustless" mantle away from that zone of upwelling provides the tension respon-

sible for the rif t along the axis of the ridge and also the rafting of the sial of the continental masses to a reas of downwelling between convection cells in which the surficial movement i s convergent.

This hypothesis of s e a floor spreading assumes that new s t r ips of s e a floor form from mantle substance along both sides of the median r i ses , a s the older "rind moves outward and toward the continents. " (Fig. 6)

Under this concept, even the oldest sea floor rocks must be comparatively young. Consequently the "Mohole" attempt t o penetrate the earth 's original crust in the ocean basins would be doomed to disappointment since the oldest rocks should be found in the neculei of the continents.

Continental Drift

The highly mobile sea floor en- visaged in the hypothesis of sea floor spreading suggests the probability of c o n t i n e n t a l drift. But unlike the Wegener hypothesis ( in which t h e continents a r e the active elements ploughing their way through a weak s i m a ) Dietz' theory assumes that passive continents a r e carr ied along by mantle convection and will come to r e s t when dynamically balanced over a convection cell convergence zone.

T h e contrasting hypotheses of continental drift a r e diagrammed in Fig. 7.

Note that if the convection pat- tern within the mantle changes, then a v a s t rearrangement of geologic elements and patterns must occur.

C W I I E N T U DRIFT CIRRIED BY YU(N

COWECIIW ~ I F T -- m LMTIW OF STAEIILI~~ OVER CMNERCEW(I

Fig. 7

Continents over Divergences or Convergences

If convection currents in the mantle are accepted as the fundamental cause of continents, there are but two choices -- either to assume that the continents formed over the divergences (e. g., Vening-Meinesz, 1944) or that they formed over the convergences. The two possibilities are diagrammed in Fig. 8.

Fig. 8

Since the protosial most logically must be generated over diver-

gences, the development of continents over the divergences has the "ad-vantage" of not requiring crustal drift. But the continents would then be domains of tension, while continents in fact appear to be compressional platforms -- built of plicated mountain roots like a closed accordion. Ten-sion would also tend to fragment continents and one would not expect the great continental slopes to rise up sharply out of the sima as they do.

The virtually compelling evidence that continents are compres-

sional structures leads the author to believe that, they tend to be poised over the convergences and that the conveyor belt action postulated in the sea floor spreading hypothesis provides a reasonable method for trans-ference of protosial from the divergences to the convergences.

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The author argues for:

(a) a steady state earth as opposed to either a shrinking or an expanding one,

(b) an evolutionary, as contrasted with a catastrophic, origin for the ocean basins,

(c) an evolutionary growth of continents, (d) a secondary, as opposed to primary, origin of the crust and

oceans, with both the hydrosphere and the crust gradually being generated from the mantle at a ratio of about 5 parts of sial to one part of water (13: 1 by weight).

Permanency

The various types and degrees of permanency which follow from the sea floor spreading concept are set forth below:

TABLE II

OCEAN (Hydrosphere)

Oceans are permanent features and growing in depth with time as new water is squeezed out of the mantle. Growth rate about 1 m/million years.

CONTINENTS

Continents are permanent features of the earth and are growing in area and thickness. They do not founder (no Atlantis). They undergo continental drift of a moderate and controlled type if convection cells change position. Tend to lie over convergences.

OCEAN BASINS

Permanent features of the earth but due to continental drift are not permanent in position. Grow deeper with time and tend to grow smaller in area, but increasing amount of water on earth offsets this tendency.

SEA FLOOR

Sea floor is not permanent. New strips are added along divergences (at mid-ocean ridges) and horizontal sea floor spreading (carrierbelt action) carries sea floor and its overlying rind toward and under the continents where it is destroyed.

Apparent Youth of the Sea Floor

One of the surprises of marine geology has been the apparent youthfulness of the sea floor a s outlined below.

T A B L E I 1 1

YOUTH O F T H E S E A F L O O R

Oldest fossils so far dredged from an ocean (both Atlantic and Pacific) a r e no older than Cretaceous. (E. L. Hamilton. D. Erickson, and others. )

Rocks from Mid- Atlantic Ridge gave a Cenozoic age by radio-active dating.

Guyots (Tablemounts) of the Pacific have proven not to be Precambrian as Hess (1946) supposed but Mesozoic or younger. Mid -Pacific mountains, seemingly as old as any, a r e Cretaceous.

Thickness of sediments in ocean basins suggest age no older than Paleozoic - - 1/10 of geologic time.

Geomorphic history of sea floor features fades into nothingness before mid-Mesozoic.

The meagre evidence presently available suggests that the sea floor is very young. The sea floor spreading hypothesis accounts for this phenomenon on the assumption that conveyor belt movement of the "rind" of the mantle is continu- ally wiping the slate clean and that, in consequence, the search for the original crust of the earth at the Mohorovic Discontinity beneath the ocean floor is hopeless - - even to find Precambrian rocks on the ocean floor seems most unlikely.

The final portions of the paper present interesting contrasts between the tectogene view on the origin of geosynclines and a time-sequence diagram of their development as deduced from the sea floor spreading hypothesis (Figs. 9, 10 & 11); and equally interesting diagrams of the Deep Structure of Continents; of Marginal Mountain Ranges and Sea Floor Underthrusting the development of island arcs; the probable composition of successive layers beneath the sea (Figs. 12 & 13), with a note that the Moho beneath the continent and that beneath the sea floor represent two different transition zones.

The author expresses the opinion that the oceanic layers a r e well preserved in eugeos ynclinal mountains such a s the Coast Ranges of California.

But to present all of those illustrations and the accompanying discussions would leave you little to look forward to, whereas the purpose of this partial condensation is to stimulate your interest in this fascinating attempt to resolve the basic problems of continent and ocean basin origin, so we shall add only his own brief conclusions.

Fig. 9

I N. Y. V T. N. HAM. ME.

Fig. 10

Fig. 11

Fig. 13

Conclusions

I have offered a concept of Sea Floor Spreading which I think may have been fundamental in the development of the earth's surficial structures. The concept is cyclic, simple, evolutionary, and actualistic. These aspects have at least philosophic merit. I believe, too, that some measure of consonance is achieved with the grand geologic panorama in the depth of time, although such an evalua- tion is highly subjective.

This essay is rife with speculation, admittedly so, and with little apology. Sorting out the puzzle of the earth requires more than mere modification of conventional ideas. Of course, this enigma has been with us for a long time, so it i s not likely to be solved today o r tomorrow. At best, one can hope to be on the right track and to be "conformable" with the record "writ on stone." At worst, these disturbing "megathoughts" should provide a stimulus for a new investigation.