INQUIRING THE OCEAN'S AGE

Oamaru Mail, Volume XXXVIII, Issue 10519, 29 July 1910, Page 6

INQUIRING THE OCEAN'S AGE

"The Ocean old—Centuries old," sang Longfellow. He was certainly very conservative in his estimate. To a geologist it would seem as absurd to state the ocean's age in centuries as to say that Methuselah was "minutes old." A recent method of estimating it is based on a consideration of the ocean's saltness. According to geologists the salt in the ocean is the accumulation of what has been dissolved out of the earth by the rains of long ages, and washed down into the sea. The water evaporates and is again and again condensed to fall as rain; but the salt stays in the ocean basin. According to this view the sea has been and is still growing salter and Salter. If we. can find out how salt it is, and how much Salter it is getting annually, we shall be able to tell how long it has taken for its saltness to accumulate; in other words, its age. This is the celebrated method of estimating the age of the ocean devised by Mr J. Joly. In technical language, it consists substantially in dividing the sodium content of the sea-water by the yearly contribution from the land,., which is ascertained by analysing river waters and ganging the streams. Says George F. Becker, in a criticism of this method, published in Science: — "It is assumed on uniformitarian principles that what variation there has been in the annual salt tribute is undiseoverable. In a long-forgotten memoir Edmund Halley made a very similar suggestion and anticipated Lyell in propounding a strictly uniformitarian doctrine of the accumulation of salt. "Oceanic sodium is at -least chiefly derived from lime-soda feldspars, which as essential constituents are practically confined to Archean and later igneous rocks. The original surface of the earth must have consisted of such rocks to the exclusion of all others, while at the present day the greater part of the land area is covered with sedimeiitaries. Now the rate of decomposition of rocks is chiefly dependent on exposure. Even in areas of ancient feldspathic massives decomposition docs not seem to penetrate to great depths. ... A layer of decomposition products 100 feet thick seems to arrest decay. . . . In short, buried massives decompose at a rate whir!) is scarcely sensible.

"ft, is quite conceivable that in the fur distant future till the massive rocks misfit he thoroughly decomposed down to sea level or a trifle below. Tlie continents would then he exclusively dctrital. ("iider such conditions there couk! he no further important additions to the .sodium content of the ocean. . .

"Thus, in the distant past, there must have been a time when a far greater mass of massive rork was decomposed each year than now decays in the same period: and a limit to this process can also be foreseen. The total area of exposed massives has surely diminished and will continue to diminish Climate and temperature may perhaps have been in the past much what tliev are to-day; the rate of chemical denudation per unit area may not hnv changed considerably, hut the most rigid uniformitariau would not maintain that the total area of exposed mas .sive rocks has been constant. The inference .seems unavoidable that sodium accumulation . . . progressed more va pklly (though possibly not with greater intensity) in the distant past and will come substantially to an end when a certain very finite layer of surface material has been exhausted."

i Tlic ocean's ago, on Joly's theory of j salt-accumulation, he calculates to be | 9-1,544,000 years. Mr Becker attempts I a rough estimate based on a rate of f aeciimulatioii that is decreasing anI nually, and his conclusion is that the j ocean's age lies between 38,000,000 and | 48,000,000 years. He says, in summing [ up:

"It appears that Mr Joly's linear relation between oceanic sodium and its increment must lead to an excessive estimate of the earth's age, at least when tho increment is duly determined. Thus that method assigns a limit, a knowledge of which is very valuable as a cheek- on other computations. On the other hand, the ages computed from his data . . . seem to me suspiciously low. Various trains of reasoning lead me, at least, to believe that 50,000,000 years is not a maximum but a minimum age; if,so, and if the exponential hypothesis is applicable, then Mr Joly's datum for the annual sodium increment is too large." I