Those Millions Of Years

Auckland Star, Volume LXX, Issue 219, 16 September 1939, Page 2

Those Millions Of Years

By--A. W. B. Powell

WHEN a geologist states that a rock or a fossil is 500,000,000 years old the layman smiles and takes it with a grain of salt. He reminds one of the story of a Maori forest ranger who stated that a certain tree was 1 500 years eight months old because an expert told him some eight months previously that the tree was 1500 years old.

It may not lie generally known, but modem geological a-ne estimates are based in principle upon a very definite factor (lie rate of decay of radio-active minerals. The rocks of the earth's surface, the granites and gneisses in particular. composing the continents, the basalts and even sedimentary rocks to ;i lesser degree, are one ami all radioacti\c. J hat is. tliev contain minute amounts of those elements which are continually and antomaticallv changing into forms of lower ;itomie weight and developing heat in the act of transformation.

Pliese radioactive substances are all traceable to two parent elements—the very heavy elements uranium and illinium, gradually these elements break clown until they Assume the properties of ordinary lead. In the course of disintegration the atomic weight of uranium descends from 238 to 20(1 and in the ease of thorium from 232 to •'-'OS -Vow as ordinary lead has an atomic weight of 207 it will he seen that even if these radioactive elements have »iven off all their energy thev resolve into a. lead that is distinguishable bv bavin-' a di He rent atomic weight.

Tn l!tn."i Professor Bolt wood broke new grounds in suggesting the use of radioactive minerals as an age index. Since then a vast amount of research has been devoted to the subject with which is associated the renowned names of I'utherford. Shoddy, Strutt. .Tolv, Boltwood and Holmes.'it was Xew Zealand's inte Lord Hutlicrford, who had much to do with determining the actual rate of decay of the radio-active elements.

Here lies the crux of the matter of accurate geological time estimates, for the rate of decay of the two eleu'ients now being known, the age of a radioactive mineral can likewise 1m» estimated by mathematical determination based upon the stage of disintegration it lias reached. That is tlie loss of atomic weight of tlie specimen multiplied by Ihe known rate of decav.

Sequence and relative age of rucks a.iul tiicir contained fossils has long l>een known, but it required the key as supI'lied by the radio-active data to translate geological time into terms of periods >f years.

J lie two chief former methods of :cologv used in determining the of the earth were the rate of land denudation, or the rate of deposition of m'diments, and the rate of sodium hloride derivation from the land and its accumulation in the oceans. Unfortunately for the sedimentation method the rate of deposition of sediments varies vastly according to situation. In the 'cenn depths far from land this rate must be a mere fraction of an inch in a thousand years or so, whereas in a situation oiT the mouth of a large river as nuch as a foot in a hundred years may iccur.

The known thickness of the standard sequence _ of sedimentary rocks is given as 70 miles, so even on the uncertain basis of rate of sedimentation a vast span of time is involved.

Joly. one of the expounders of the method of time estimation from the factor of sodium content of the ocean bids us to imagine the oceans in their original freshness, free, or nearlv so. from salt and then infers that th<* salinity has increased gradually at a fairly constant rate as, through the passage of time, this salt has been washed down from the land. Both the sedimentation and the sodium content met hods fail owing to the lack of a stable rate of accumulation. The radioactive method has the great advantage of being based upon a constant rate of decay of two elements which does not alter in any combination of temperature and pressure. The now generally accepted geological age estimates give the oldest earth'"formation* an antiquity of 2000 million years. .V) 0 million years for the Cambrian, the first appearance of well preserved fossils, and a mere one million years since the earliest known human remains. By analogy with the standard sequence of geological time we get a rough idea of the age of our New Zealand sediments. For instance, parts of Preservation Inlet, South West Otago, are Ordovician. -ISO million years okl', the Amuri limestone is Cretaceous, 9.1 million, and large areas around Wanganui and Hawke's Bay range between the ages of seven and one million years.

Around Auckland the sandstone and mndstone cliffs are now considered to be lower Miocene, roughly about 19 million years old. while the harder rugged grevwaeke rock, such as most of the cliffs of Waiheke Island are formed of, belongs to the Triassic to Jurassic period between 190-155 million years back.

Recent researches show that ,4eepA\ater ocean deposits are frequently highly radio-active, particularly 'in respect to the deposit known as red-clay and manganese nodules. excellent examples of manganese nodules, two to three inches across, were taken by the cable Recorder a few.'years a<*o at 2750 fathoms, 200 miles east of Sydney. Some idea of their" strong radio-activity can be gauged from the fact that whereas Xe w Zealand limestones, granites and volcanic rocks registered between 0 and 20 by a certain test the Sydney nodule reached the high figure of 330. The origin of these deep water manganese nodules and their exceptional radium content has caused much speculation. Older authorities considered that the nodules had their origin by the slow accretion of particles of cosmic dust that settled to the great depths wheTe they were not overwhelmed by land derived sediments. Modern theory suggests that salts of uranium and radium practically universally present in minute quantities in sea water are more heavily concentrated in great depths owing to increased oxidation occasioned by the great pressure. The diagram illustrates what may be termed the geological clock. The earth's vast age is reduced to a period of 24 hours and the various geological formations are expressed proportionately in hours or minutes. Thus, it will be seen that 13 hours of the 24 passed fiefore we reach the Cambrian which is estimated at .VHUIOO.OOO years from the present. -Also that man's record otnnipieS about the last 10 minutes and the historic period, shown by a narrow black sector, covers but a few brief minutes.

Geological time can never be as precise as that of our personal experience, as the scale or our short span of existence is too minute by comparison. The researches by the application of the principles of radio-activity, however, enable us at least to scoff less at the geologist and his millions.