Page Six 



EVOLUTION 



April. 1928 



How Old Is the World? 



B\ Allan Strong Broms 



THE existence of ocean tides tells us one important 

 thing about the earth's interior, — it is rigidly solid. 

 Were it fluid (even a thin layer under the cool outer 

 crust) there would be tides in that fluid and the thin 

 crust would heave and fall upon it in time with the 

 waters of the ocean. The crustal tides would thus efface 

 the ocean tides. As this does not occur, we know that 

 at present the earth's interior is rigid as a solid. 



But though the interior rocks yield but slightly as 

 tides, they are not unyielding under great pressures. 

 Under the enormous weights of the overlying rocks, 

 they flow slowly and thickly, yielding most where the 

 pressure is heaviest. On this viscous interior the sur- 

 face rocks float. Where they are heavy, they sink deep 

 and produce the beds of the oceans. Where they are 

 light, they float high, producing our land areas. 



Heavy Substratum 



We are very sure that the heavy rocks of the ocean 

 beds and tliose deeply underlying the continents are 

 largely basaltic. Outflows of volcanic lavas indicate 

 this. The lighter continental rocks, which float like a 

 scum on the heavier substratum, are largely granitic. 



It happens that the basalts melt at a lower tempera- 

 ture than the granitic rocks. This means that as the 



CONTINENT 



/GRANitic :;.sCu_M •:.■■ V. 



' ' ' FLbATING'.-V- •'■ 

 ■.filOH". 



OCEAN 



TIDES EFFACED 



SEDIMENTS BEING 

 DEPOSITED IN 

 SHALLOW SEAS 



DENUDED LAND 



y////////////7//////////m .,,,.,,,,^.,,.^. 



//• /CIRCULATION^ AIDS ESCAPE '////,,/,//// F\.\}\D' ///// /, 

 IMULATED / HEAT. // '/////''//'' ,AA,' -i-lll^ ,/ /' 



OF ACCUMULATED /HEAT. //',////''//'/, , 



'//W^'^^W/W^M^W/y^p^^^ 



interior heat of the earth accumulates (due, as we dis- 

 covered in the last article, to the heat-emitting radio- 

 active elements thorium and uranium distributed through 

 the rocks), the basalts will eventually melt into a fluid 

 layer under the still solid, unmelted crustal rocks. 



As the basalts melt, they expand, with two important 

 results. On the one hand, the expansion slightly in- 

 creases the earth's diameter and circumference. To ex- 

 pand, the crust cracks. Into the cracks then flow the 

 molten lavas from below. They soon cool and harden, 

 and thus effectually prevent any later reduction of the 

 crustal surface to fit a shrinkage of the interior. 



On the other hand, the expansion of melting means a 

 reduction in density of the basaltic substratum. This 

 reduces its power to support the overlying continents, 

 which thereupon sink deeper. As the continents sink, 

 their lowlands are invaded by shallow arms of the sea. 

 Within these transgressional seas are then deposited lay- 

 ers of sediments washed in by streams from the neigh- 

 boring lands. Layer upon layer is laid down, adding to 

 the weight and forcing the continental masses locally 

 deeper into the underlying subtstratum. 



A Period of Rapid Cooling 



But when the substratum rocks become liquid, they 

 flow. Twice a day they rise and fall in great tides, — 

 and of course, the crust and oceans with them. More 

 important, the deeper and hotter lavas rise, displaced by 

 the cooler (and heavier) upper lavas which sink. A 

 vertical circulation thus sets in, its eff'ect being to carry 

 heat rapidly towards the surface. These rising, hot lavas 

 also attack and melt the bottom of the solid crust, par- 

 ticularly the basaltic ocean beds, reducing their thick- 

 ness and therefore their power to keep the interior heat 

 from escaping into outer space. The heat from radio- 

 activity, which, according to Professor Joly, has been 

 accumulating for perhaps thirty or forty million years, 

 now rapidly escapes through the ocean beds in some 

 six to twelve million years. 



But the interior heat cannot thus escape through the 

 thick, unmelted continents. It must go right on accu- 

 mulating without relief, to what end we can hardly say. 

 But fortunately, the tidal pull of the moon must cause 

 a slow westward drift of the protuberant land masses. 

 In due time, every part of the overheated substratum is 

 uncovered and given its chance to cool off. 



Mountain Building 

 As the substratum cools and hardens, it shrinks. The 

 substratum tides disappear and the ocean tides reappear. 

 The earth crust, enlarged to fit the expanded substratum, 

 now wrinkles into great folds as it settles down upon 

 the shrinking interior, — downward folds making valleys 

 and ocean-deeps, upward folds making mountain ranges, 

 both kinds being great, unstable, pressing arches that 

 break and fault and crush. Ranges bearing north and 

 south, such as those of the Americas, doubtless relieved 

 the east and west pressures, while others bearing east 



