Part 16 (2/2)
[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of Geologic History, by Willis and Salisbury, 1910, pp.
273-274.]
[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in Turkestan, Carnegie Inst. of Wash., No. 26, 1905.
In North America the stages have been the subject of intensive studies on the part of Taylor, Leverett, Goldthwait, and many others.]
CHAPTER XVI
THE EARTH'S CRUST AND THE SUN
Although the problems of this book may lead far afield, they ultimately bring us back to the earth and to the present. Several times in the preceding pages there has been mention of the fact that periods of extreme climatic fluctuations are closely a.s.sociated with great movements of the earth's crust whereby mountains are uplifted and continents upheaved. In attempting to explain this a.s.sociation the general tendency has been to look largely at the past instead of the present. Hence it has been almost impossible to choose among three possibilities, all beset with difficulties. First, the movements of the crust may have caused the climatic fluctuations; second, climatic changes may cause crustal movements; and third, variations in solar activity or in some other outside agency may give rise to both types of terrestrial phenomena.
The idea that movements of the earth's crust are the main cause of geological changes of climate is becoming increasingly untenable as the complexity and rapidity of climatic changes become more clear, especially during post-glacial times. It implies that the earth's surface moves up and down with a speed and facility which appear to be out of the question. If volcanic activity be invoked the problem becomes no clearer. Even if volcanic dust should fill the air frequently and completely, neither its presence nor absence would produce such peculiar features as the localization of glaciers, the distribution of loess, and the mild climate of most parts of geological time. Nevertheless, because of the great difficulties presented by the other two possibilities many geologists still hold that directly or indirectly the greater climatic changes have been mainly due to movements of the earth's crust and to the reaction of the crustal movements on the atmosphere.
The possibility that climatic changes are in themselves a cause of movements of the earth's crust seems so improbable that no one appears to have investigated it with any seriousness. Nevertheless, it is worth while to raise the question whether climatic extremes may cooperate with other agencies in setting the time when the earth's crust shall be deformed.
As to the third possibility, it is perfectly logical to ascribe both climatic changes and crustal deformation to some outside agency, solar or otherwise, but hitherto there has been so little evidence on this point that such an ascription has merely begged the question. If heavenly bodies should approach the earth closely enough so that their gravitational stresses caused crustal deformation, all life would presumably be destroyed. As to the sun, there has. .h.i.therto been no conclusive evidence that it is related to crustal movements, although various writers have made suggestions along this line. In this chapter we shall carry these suggestions further and shall see that they are at least worthy of study.
As a preliminary to this study it may be well to note that the coincidence between movements of the earth's crust and climatic changes is not so absolute as is sometimes supposed. For example, the profound crustal changes at the end of the Mesozoic were not accompanied by widespread glaciation so far as is yet known, although the temperature appears to have been lowered. Nor was the violent volcanic and diastrophic activity in the Miocene a.s.sociated with extreme climates.
Indeed, there appears to have been little contrast from zone to zone, for figs, bread fruit trees, tree ferns, and other plants of low lat.i.tudes grew in Greenland. Nevertheless, both at the end of the Mesozoic and in the Miocene the climate may possibly have been severe for a time, although the record is lost. On the other hand, Kirk's recent discovery of glacial till in Alaska between beds carrying an undoubted Middle Silurian fauna indicates glaciation at a time when there was little movement of the crust so far as yet appears.[125] Thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately.
According to the solar-cyclonic hypothesis such a condition is to be expected. If the sun were especially active when the terrestrial conditions prohibited glaciation, changes of climate would still occur, but they would be milder than under other circ.u.mstances, and would leave little record in the rocks. Or there might be glaciation in high lat.i.tudes, such as that of southern Alaska in the Middle Silurian, and none elsewhere. On the other hand, when the sun was so inactive that no great storminess occurred, the upheaval of continents and the building of mountains might go on without the formation of ice sheets, as apparently happened at the end of the Mesozoic. The lack of absolute coincidence between glaciation and periods of widespread emergence of the lands is evident even today, for there is no reason to suppose that the lands are notably lower or less extensive now than they were during the Pleistocene glaciation. In fact, there is much evidence that many areas have risen since that time. Yet glaciation is now far less extensive than in the Pleistocene. Any attempt to explain this difference on the basis of terrestrial changes is extremely difficult, for the shape and alt.i.tude of continents and mountains have not changed much in twenty or thirty thousand years. Yet the present moderately mild epoch, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the a.s.sumption that the sun's atmosphere may sometimes vary in harmony with crustal activity, but does not necessarily do so at all times.
Turning now to the main problem of how climatic changes may be connected with movements of the earth's crust, let us follow our usual method and examine what is happening today. Let us first inquire whether earthquakes, which are one of the chief evidences that crustal movements are actually taking place in our own times, show any connection with sunspots. In order to test this, we have compared _Milne's Catalogue of Destructive Earthquakes_ from 1800 to 1899, with Wolf's sunspot numbers for the same period month by month. The earthquake catalogue, as its compiler describes it, ”is an attempt to give a list of earthquakes which have announced changes of geological importance in the earth's crust; movements which have probably resulted in the creation or the extension of a line of fault, the vibrations accompanying which could, with proper instruments, have been recorded over a continent or the whole surface of our world. Small earthquakes have been excluded, while the number of large earthquakes both for ancient and modern times has been extended. As an ill.u.s.tration of exclusion, I may mention that between 1800 and 1808, which are years taken at random, I find in Mallet's catalogue 407 entries. Only thirty-seven of these, which were accompanied by structural damage, have been retained. Other catalogues such as those of Perry and Fuchs have been treated similarly.”[126]
If the earthquakes in such a carefully selected list bear a distinct relation to sunspots, it is at least possible and perhaps probable that a similar relation may exist between solar activity and geological changes in the earth's crust. The result of the comparison of earthquakes and sunspots is shown in Table 7. The first column gives the sunspot numbers; the second, the number of months that had the respective spot numbers during the century from 1800 to 1899. Column C shows the total number of earthquakes during the months having any particular degree of spottedness; while D, which is the significant column, gives the average number of destructive earthquakes per month under each of the six conditions of solar spottedness. The regularity of column D is so great as to make it almost certain that we are here dealing with a real relations.h.i.+p. Column F, which shows the average number of earthquakes in the month succeeding any given condition of the sun, is still more regular except for the last entry.
TABLE 7
DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTS
A: _Sunspot numbers_ B: _Number of months per Wolf's Table_ C: _Number of earthquakes_ D: _Average number of earthquakes per month_ E: _Number of earthquakes in succeeding month_ F: _Average number of earthquakes in succeeding month_
A B C D E F
0-15 344 522 1.52 512 1.49 15-30 194 306 1.58 310 1.60 30-50 237 433 1.83 439 1.85 50-70 195 402 2.06 390 2.00 70-100 135 286 2.12 310 2.30 over 100 95 218 2.30 175 1.84
The chance that six numbers taken at random will arrange themselves in any given order is one in 720. In other words, there is one chance in 720 that the regularity of column D is accidental. But column F is as regular as column D except for the last entry. If columns D and E were independent there would be one chance in about 500,000 that the six numbers in both columns would fall in the same order, and one chance in 14,400 that five numbers in each would fall in the same order. But the two columns are somewhat related, for although the after-shocks of a great earthquake are never included in Milne's table, a world-shaking earthquake in one region during a given month probably creates conditions that favor similar earthquakes elsewhere during the next month. Hence the probability that we are dealing with a purely accidental arrangement in Table 7 is less than one in 14,400 and greater than one in 500,000. It may be one in 20,000 or 100,000. In any event it is so slight that there is high probability that directly or indirectly sunspots and earthquakes are somehow connected.
In ascertaining the relation between sunspots and earthquakes it would be well if we could employ the strict method of correlation coefficients. This, however, is impossible for the entire century, for the record is by no means h.o.m.ogeneous. The earlier decades are represented by only about one-fourth as many earthquakes as the later ones, a condition which is presumably due to lack of information. This makes no difference with the method employed in Table 7, since years with many and few sunspots are distributed almost equally throughout the entire nineteenth century, but it renders the method of correlation coefficients inapplicable. During the period from 1850 onward the record is much more nearly h.o.m.ogeneous, though not completely so. Even in these later decades, however, allowance must be made for the fact that there are more earthquakes in winter than in summer, the average number per month for the fifty years being as follows:
Jan. 2.8 May 2.4 Sept. 2.5 Feb. 2.4 June 2.3 Oct. 2.6 Mar. 2.5 July 2.4 Nov. 2.7 Apr. 2.4 Aug. 2.4 Dec. 2.8
The correlation coefficient between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period, 1850-1899, are as follows:
Sunspots and earthquakes of same month: +0.042, or 1.5 times the probable error.
Sunspots of a given month and earthquakes of that month and the next: +0.084, or 3.1 times the probable error.
Sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of January, February, and March compared with earthquakes of February, March, and April; sunspots of February, March, and April with earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the probable error.
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