Chapter 16
Section 16 of Climatic Changes, The Nature and Causes by Ellsworth Huntington. This LibriVox recording is in the public domain. Recorded by Leon Harvey. Chapter 16. 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 associated with great movements of the Earth's crust, whereby mountains are uplifted and continents are peeved. In attempting to explain this association, the general tendency has been to look largely at the past instead of at 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 the 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 localisation of glaciers, the distribution of Lewis, 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 of 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 with any seriousness. Nevertheless, it is worthwhile to raise the question whether climatic extremes may cooperate with other agencies in setting a time when the US 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 close enough so that their gravitational stresses cause crustal deformation, all life would presumably be destroyed. As to the Sun, there has hitherto been no conclusive evidence that 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 associated with extreme climates. indeed there appears to have been little contrast from zone to zone for figs breadfruit trees tree ferns and other plants of low latitudes grew in greenland nevertheless both at the end of the mesozoic and 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.
Thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately. According to the solid cyclonic hypothesis, such a conclusion 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 circumstances
and would leave little record in the rocks. or there might be glaciation high latitudes 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 altitude of continents and mountains have not changed much in 20,000 or 30,000 years.
Yet the present moderately mild abog, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the assumption 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 S. 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 illustration of exclusion, I may mention that between 1800 and 1808, which are years taken at random, I find in Mount's catalogue 407 entries. Only 37 of these, which were accompanied by structural damage, have been retained. Other catalogues, such as those of Perry and Fuchs, have been treated similarly.
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 their 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.
Table 7 is displayed on the page. The regularity of column D is so great as it makes it almost certain that we are here dealing with a real relationship. Column F, which shows the average number of earthquakes in the months succeeding any given condition of the sun, is still more regular except for the last entry.
The chance that six numbers taken at random will average themselves in any given order is 1 in 720. In other words, there is one chance in 720 that the regularity of column D is accidental. But a 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 at 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 aftershocks 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 favour 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 1 in 14,400 and greater than 1 in 500,000.
It may be 1 in 20,000 or 100,000. In any event it is so slight, 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 this strict method of correlation coefficients.
This, however, is impossible for the entire century, for the record is by no means homogeneous. The earlier decades are represented by only about one-fourth as many earthquakes as the latter 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 19th century, but renders this method of correlation and coefficients inapplicable. During the period from 1850 onward, the record was much more nearly homogeneous, though not completely so. Even in these latter 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 50 years being as follows. January 2.8 February 2.4 March 2.5
April 2.4 May 2.4 June 2.3 July 2.4 August 2.4
September 2.5 October 2.6 November 2.7 December 2.8. The correlation coefficients between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period,
1850 to 1899, are as follows. Sunspots and earthquakes of the same month, positive 0.042 or 1.5 times the probable error. Sunspots of a given month and earthquakes of that month and the next, positive 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. Positive 0.112 or 4.1 times the probable error. These coefficients are all small, but the numbers of individual cases, 600 months, is so large that the probable error is greatly reduced, being only standard deviation 0.027 or standard deviation 0.028. Moreover, the nature of our data is such that even if there is a strong connection between solid changes and earth movements, we should not expect a large correlation
coefficient. In the first place, as already mentioned, the earthquake data are not strictly homogeneous. Second, an average of about two and a half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the Earth's crust. Third, the sun spots are only a partial and imperfect measure of the activity of the sun's atmosphere. Fourth, the relation between solar activity and earthquakes is almost certainly indirect. In view of all these
conditions, the regularity of table 7 and the fact that the most important correlation coefficient rises are more than four times the probable error, makes it almost certain that the solar and terrestrial phenomena are really connected. We are now confronted by the perplexing question of how this connection could take place. Thus far, only three possibilities present themselves, and each is open to objections. The chief agencies concerned in these three possibilities are heat,
electricity, and atmospheric pressure. Heat may be dismissed very briefly. We have seen that the Earth's surface becomes relatively cool when the sun is active. Theoretically, even the slightest change in the temperature of the Earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements off the crust. Practically, the heat of the surface ceases to be of appreciable importance at a depth of perhaps
20 feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun. The second possibility is based on the relationship between solar and terrestrial electricity. When the sun is active, the Earth's atmospheric electrical potential is subject to slight variations. It is well known that when two opposing points of an ionized solution are
oppositely charged electrically, a current passes through the liquid and sets up electrolysis, whereby there is a segregation of materials and a consequent change in the volume of the parts near the respective electrical poles. The same process takes place, although less freely in a hot mass such as forms the interior of the earth. The question arises whether internal electrical currents may not pass between the two oppositely charged poles of the earth,
or even between the great continental masses and the regions of heavier rock which underlie the oceans. Could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? Could the results vary in harmony with the sun? Bowie has shown that numerous measurements of the strength and direction of the Earth's gravitational pull are explicable only on the assumption that the upheaval of a continent or a mountain range is due in part
not merely to pressure or even to flow into the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. The query arises whether this change of volume may be related to electrical currents at some depth below the Earth's surface. The objections to this hypothesis are numerous.
First, there is little evidence of electrolytic differentiation in the rocks. Second, the other part of the Earth's crust is a very poor conductor, so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. Third, electrolysis due to any such mild causes as we have here postulated must be an extremely slow process. Too slow, presumably, to have any appreciable result within a month or two.
Other objections join with these three in making it seem improbable that the Sun's electrical activity has any direct effect upon movements of the Earth's crust. The third or meteorological hypothesis which makes barometric pressure the main intermediary between solar activity and earthquakes seems at first sight almost improbable as the thermal and electrical hypothesis. Nevertheless, it has a certain degree of observational support of a kind which is highly lacking in the other two cases. Among the extensive writings on the periodicity of earthquakes, the one main fact stands out with great distinctness. Earthquakes vary in number according to the season.
This fact has already been shown incidentally in the table of earthquake frequency by months. If allowance is made for the fact that February is a short month, there is a regular decrease in the frequency of severe earthquakes from December and January to June. Since most of Milne's earthquakes occur in the Northern Hemisphere, this means that severe earthquakes occur in winter about 20% oftener than in summer. Table 8 is displayed on the page. The most thorough investigation of this subject seems to have been that of Davison.
His results have been worked over and amplified by Knott, who has tested them by Shuster's exact mathematical methods. His results are given in Table 8. Here the Northern Hemisphere is placed first, then it comes East Indies and the Malay Archipelago lying close to the equator, and finally the Southern Hemisphere.
In the Northern Hemisphere, practically all the maxima come in the winter, for the month of December appears in 15 cases out of 25 in column D, while January, February, or November appear in six others. It is also noticeable that in 16 cases out of 25, the ratio of the actual to the expected amplitude in column G is 4 or more,
so that a real relationship is indicated, while the ratio falls below 3 only in Japan and Zante. The equatorial data, unlike those in the northern hemisphere, are indefinite, for in the East Indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude.
Even in the Malay archipelago, which shows a maximum in May, The ratio of actual to expected amplitude is only 2.6. Turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northern hemisphere. July or August appears in 5 out of 6 cases.
Healy ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. Nevertheless, it is practically 4 in Chile and exceeds 5 in Peru and Bolivia, and in the data for the entire southern hemisphere. The whole relationship between earthquakes and the seasons in the northern and southern hemispheres is summed up in figure 12, taken from Nott.
The northern hemisphere shows a regular diminution in earthquake frequency from December until June and an increase in the rest of the year. In the southern hemisphere, the course of events is the same so far as summer and winter are concerned, for August with its maximum comes in winter, while February with its minimum comes in summer. In the southern hemisphere, the winter month of greatest seismic activity has over 100% more earthquakes than the summer months of least activity.
In the northern hemisphere, this difference is about 80%, but this smaller figure occurs partially because the northern data include certain interesting and significant regions like Japan and China, where the usual conditions are reversed. If equatorial regions were included in figure 12, they would give an almost straight line. The connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. A meteorological hypothesis seems to be the only logical explanation. Wherever sufficient data are available, earthquakes appear to be most numerous when climatic conditions cause the Earth's surface to be most heavily loaded or to change its load most rapidly.
The main factor in the loading is apparently atmospheric pressure. This acts in two ways. First, when the continents become cold in winter, the pressure increases. On an average, the air at sea level presses upon the Earth's surface at a rate of 14.7 pounds per square inch, or over a tonne per square foot, and only a little short of 30 million tonnes per square mile.
An average difference of one inch between the atmospheric pressure of summer and winter over 10 million square miles of the continent of Asia. For example, means the continent's load in winter is about 10 million million tonnes heavier than in summer. Second, the changes in atmospheric pressure due to the passage of storms are relatively sharp and sudden.
Hence they are probably more effective than the variations in the load from season to season. It is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. It is also suggested by the fact that violent storms are frequently followed by violent
earthquakes. Earthquake weather, as Dr. Schlesinger suggests, is a common phrase in the typhoon regions of Japan, China and East Indies. During tropical hurricanes, a change of pressure amounting to half an inch in two hours is common. On September 22, 1885, at Falls Point Lighthouse on the Bay of Bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much
over two hours, and finally rose fully two inches inside two hours. A drop of two inches in batomagic pressure means that a load of about two million tons is removed from each square mile of land. the corresponding rise of pressure means the addition of a similar load. Such a storm, and to a less degree every other storm, strikes a blow upon the Earth's surface, first by removing millions of tonnes of pressure and then by putting them on again.
Such storms, as we have seen, are much more frequent and severe when sunspots are numerous than at other times. Moreover, as Vito long ago showed, one of the most noteworthy evidences of the connection between sunspots and the weather is a sudden increase of pressure in certain widely separated high-pressure areas. In most parts of the world, winter is not only the season of highest pressure and of most frequent changes of Vito's type, but also of severest storms.
Hence a meteorological hypothesis would lead to the expectation that earthquakes would occur more frequently in winter than in summer. On the Chinese coast, however, and also on the oceanic side of Japan, as well as in some more tropical regions, The chief storms come in summer in the form of typhoons.
These are the places where earthquakes also are most abundant in summer. Thus, wherever we turn, storms and related barometric changes seem to be most frequent and severe at the very times when earthquakes are also most frequent. Figure 12 is displayed on the page, Seasonal Distribution of Earthquakes, after Davison and Nott. Other meteorological factors, such as rain, snow, winds and currents,
probably have the same effect on earthquakes through their ability to load the Earth's crust. The coming of vegetation may also help. These agencies, however, appear to be of small importance compared with the storms. In high latitudes and in regions of abundant storminess, most of these factors generally combine with parametric pressure
to produce frequent changes in the load of the Earth's crust, especially in winter. In low latitudes, on the other hand, there are few severe storms, and relatively little contrast in pressure and vegetation from season to season. There is no snow, and the amount of groundwater changes little. With this goes the twofold fact that there is no marked seasonal distribution of earthquakes
and that except in certain local volcanic areas, earthquakes appear to be rare. In proportion to the areas concerned, for example, there is little evidence of earthquakes in equatorial Africa and South America. The question of the reality of the connection between meteorological conditions and crustal movements is so important that every possible test should be applied.
As a suggestion of Professor Schlesinger, we have looked up a very ingenious line of inquiry. During the last decades of the 19th century, a long series of extremely accurate observations of latitude discloses a fact which had previously been suspected,
but not demonstrated. Namely, that the Earth wobbles a little about its axis. The axis itself always points in the same direction, and since the Earth slides irregularly around it, the latitude of all parts of the Earth keeps changing. Chandler has shown that the wobbling
thus induced consists of two parts. The first is a movement in a circle with a radius of about 15 feet, which is described at approximately 430 days. This so-called Illurian movement is a normal gyroscopic motion, like the slow gyration of a spinning top. This depends on purely astronomical causes, and no terrestrial causes can stop it or eliminate it. The period appears to be constant, but there are certain puzzling irregularities. The usual amplitude of this movement, as Schlesinger
puts it, is about 0.27 degrees, but twice in recent years has jumped to 0.40 degrees. Such a change could be accounted for by supposing that the Earth has received a severe blow or a series of milder blows standing in the same direction. These blows, which were originally suggested by helmet a most interesting interview of our suggestion as to the blows struck by storms the second movement of the pole has a period of a year and is roughly an eclipse whose longest radius is fourteen feet and the shortest four feet or to put it technically there is an annual term with a maximum amplitude of about zero minutes point two zero this however varies irregularly the result is that the pole seems to wander over the earth's surface in the spiral fashion illustrated in figure thirteen
It was early suggested that this peculiar wandering of the pole in an annual period must be due to meteorological causes. Jeffries has investigated the matter exhaustively. He assumes certain reasonable values for the weight of air added or subtracted from different parts of the Earth's surface, according to the seasons. He also considers the effect of precipitation, vegetation and polar ice, and variations of temperature and atmospheric pressure in their relation to movements of the ocean.
Then he proceeds to compare all these with the actual wandering of the Pole from 1907 to 1913. While it is as yet too early to say that any special movement of the Pole was due to the specific meteorological conditions of any particular year, Geoffrey's work makes it clear that meteorological causes, especially atmospheric pressure, are sufficient to cause the observed or irregular wanderings. Sleight wanderings may arise from various other sources, such as movements of the rocks, when geological faults occur, or the rush of a crater rising from movements of the air This fact, coupled with the mathematical certainty that meteorological phenomena must produce some wandering of the pole, has caused most astronomers to accept Geoffrey's conclusion
If we followed their examples, we are led to conclude that changes in atmospheric pressure, and in the other meteorological conditions, strike blows which sometimes shift the Earth several feet from its normal position in respect to the axis. Figure 13 is displayed on the page, wandering of the pole from 1890 to 1898, after Moulton. If the foregoing reasoning is correct, the great and especially the sudden departures from the smooth gyroscopic circle described by the pole in the Eulerian motion would be expected to occur at about the same time as unusual earthquake activity. This brings us to an interesting inquiry made out by Milne, and amplified by Knott. Taking Albrecht's representation of the irregular spiral-like motion of the pole, as given in figure 13, they show that there is a preponderance of severe earthquakes at times when the direction of motion of the Earth, in reference to its axis, departs from the smooth Eulerian curve.
A summary of their results is given in Table 9. The table indicates that during the period from 1892 to 1905, there were nine different times when the curve of figure 13 changed its direction, or was deflected by less than 10 degrees during a tenth of a year. In other words, during those periods, it did not curve as much as it ought according to the Illyrian movement.
At such times, there were 179 world-shaking earthquakes, or an average of about 19.9 per tenth of a year. According to the other lines of Table 9, in 32 cases the deflection during the 10th of the year was between 10 degrees and 25 degrees, while in 56 cases it was from 25 degrees to 40 degrees. During these periods the curve remained close to the Illurian path and the world-shaking earthquakes averaged only 8.2 to 12.9. Then when the deflection was high, that is when meteorological conditions threw the earth far out of its Illurian course, the earthquakes were again numerous. The number rising to 23.4 when the deflection amounted to more than 55 degrees.
Table 9 is displayed on the page, deflection of path of pole compared with earthquakes. In order to test this conclusion in another way, we have followed a suggestion of Professor Schlesinger. Under his advice, the Illurian motion has been eliminated, and a new series of earthquake records has been compared with the remaining motions of the poles, which presumably arise largely from meteorological causes.
For this purpose, use has been made of the very full records of earthquakes published under the oyspices of the International Seismological Commission for the years 1903 to 1908, the only years for which they are available. These include every known shock of every description, which was either recorded by seismographs or by direct observation in any part of the world.
Each shock is given the same weight, no matter what its violence or how closely it follows another. The angle of deflection has been measured as Milne measured it, but since the Illurian motion is eliminated, our zero is approximately the normal condition which would prevail if there were no meteorological complications. Dividing the deflections into six equal groups according to the size of the angle, we get the results shown in Table 10. Table 10 is displayed on the page, Earthquakes in 1903-1908 compared with the departures, the projected curve of the Earth's axis from the Illurian position. Here were some 20,000 earthquakes that are employed.
The result agrees closely with that of Milne for a different series of videos and for a much smaller number of earthquakes. So long as the path of the pole departs less than about 45 degrees from the smooth gyroscopic Illurian path, the number of earthquakes is almost constant, about 8 and a quarter per day.
When the angle becomes large however, the number increases by nearly 50%. as the work of Milne, Nott and Jefferies is confirmed by a new investigation. Apparently earthquakes and crustal movements are somehow related to sudden changes in the load imposed on the Earth's crust by meteorological conditions. This conclusion is quite as surprising to the authors as to the reader, perhaps more so. At the beginning of this investigation,
we had no faith whatever in any important relation between climate and earthquakes. At its end, we are inclined to believe that the relation is close and important. It must not be supposed, however, that meteorological conditions are the cause of earthquakes and of movements of the Earth's crust. Even though the load that the climatic agencies can impose upon the Earth's crust runs into millions of tonnes per square mile, it is a trifle compared with what the crust is able to support.
There is, however, a great difference between the cause and the occasion of a phenomenon. Suppose that a thick sheet of glass is placed under an increasing strain. If the strain is applied slowly enough, even so rigid in material as glass will ultimately bend rather than break. But suppose that while the tension is high, the glass is tapped. A gentle tap may be followed by a tiny crack.
A series of little taps may be the signal for small cracks to spread in every direction. A few slightly harder taps may cause a whole sheet to break suddenly into many pieces. Yet even the hardest tap may be the merest trouble compared with the strong force which is keeping the glass in a state of strain, and which will ultimately bend it if given time. The earth as a whole appears to stand between steel and glass in rigidity.
It is a matter of common observation that rocks stand high in this respect, and the consequent difficulty with which they can be bent without breaking. Because of the earth's contraction, the crust endures a constant strain, which must gradually become enormous. This strain is increased by the fact that sediment is transferred from the lands to the borders of the sea, and there forms areas of thick accumulation. From this has arisen the doctrine of isostasia, or of the equalization of crustal pressure.
An important illustration of this is the oceanward and equatorial creep which has been described in Chapter 11. There we saw that when the lands have once been raised to high levels, or when a shortening of the Earth's axis by contraction has increased, the oceanic bulge at the equator, when the reverse has happened because of tidal retardation, the outer part of the Earth appears to creep slowly back towards a position of perfect isostatic adjustment. If the sun had no influence upon the earth, either direct or indirect, isostasy and other terrestrial processes might flex the earth's crust so gradually that changes in the form and height of the lands would always take place slowly, even from the geological point
of view. Thus, erosion would usually be able to remove the rocks as rapidly as they were doomed above the general level. If this happens, mountains would be rare or unknown, and its climatic contrast would be fireless marked than is actually the case on our earth where crustal movements have repeatedly been
rapid enough to produce mountains. Nature's methods rarely allow so gradual an adjustment to the forces of isostasy. While the crust is under a strain, not only because of contraction, but because of changes in its load through the transference of sediments and the slow increase or decrease in the bulge at the equator, the atmosphere more or less persistently carries on the tapping process.
The violence of that process varies greatly, and the variations depend largely on the severity of the climatic contrasts. If the main outlines of the cyclonic hypothesis are reliable, one of the first effects of a disturbance of the sun's atmosphere is increased storminess upon the Earth.
This is accompanied by increased intensity in almost every meteorological process. The most important effect, however, so far as the US crust is concerned, would apparently be the rapid and intense changes of atmospheric pressure which would arise from the swift passage of one severe storm after another. Each storm would be a little tap on the tensely strained crust.
Any single tap might be of little importance, even though it involved a change of a billion tons in the pressure on an area no larger than the state of Rhode Island. Yet a rapid and irregular succession of such taps might possibly cause the crust to crack,
and finally to collapse in response to stresses arising from the shrinkage of the earth. Another and perhaps more important effect of variations in storminess, and especially in the location of the stormy areas, would be an acceleration of erosion in some places and retardation elsewhere. A great increase in rainfall may almost denude the slopes of soil, while a diminution to the point where much of the vegetation dies off has a similar effect.
If such changes should take place rapidly, great thicknesses of sediment might be concentrated in certain areas in a short time, thus disturbing the isostatic adjustment of the Earth's crust. This might set up a state of strain which would ultimately have to be relieved, thus perhaps initiating profound crustal movements. Changes in the load of the Earth's crust due to erosion and the deposition of sediment,
no matter how rapid they may be from the geological standpoint, are slowly compared with those due to changes in barometric pressure. A drop of an inch in barometric pressure is equivalent to the removal of about 5 inches of solid rock. Even under the most favourable circumstances, the removal of an average depth of 5 inches of rock or its equivalent in soil over millions of square miles
would probably take several hundred years, while the removal of a similar load of air might occur in half a day or even a few hours. Thus, the erosion deposition due to climatic variations presumably play their part in crustal deformation chiefly by producing crustal stresses, while the storms, as it were, strike sharp sudden blows.
Suppose now that a prolonged period of worldwide mild climate, such as is described in Chapter 10, should permit an enormous accumulation of stresses due to contraction and tidal retardation. Suppose that then a sudden change of climate should produce a rapid shifting of the deep soil that it accumulated on the lands, with a corresponding localization and increase in strains. Suppose also that frequent and severe storms play their part, whether great or small, by producing an intensive tapping of the crust. In such a case, the ultimate collapse would be correspondingly great, as would be evident in the succeeding geological epoch. The seafloor might sink lower, the continents might be elevated, and mountain ranges might be shoved up along lines of special weakness.
This is the story of the geological period as known to historical geology. The force that causes such movements would be the pull of gravity upon the crust surrounding the Earth's shrinking interior. Nevertheless, climatic changes might occasionally set the date when the gravitational pull would finally overcome inertia, and thus usher in the crustal movements that close old geological periods and inaugurate new ones. This, however, could occur only if the crust were under sufficient strain.
As Lawson says in his discussion of the elastic rebound theory, the sudden shifts of the crust, which seem to be the underlying cause of earthquakes, can occur only after the accumulation of strain to a limit and, this accumulation involves a slow creep of the region affected. In the long periods between great earthquakes,
the energy necessary for such shocks is being stored up in the rocks as elastic compression. If a period of intense storminess should occur when the Earth as a whole was in such a state of strain, the sudden release of the strains might lead to terrestrial changes which would alter the climate still further, making it more extreme, and perhaps preventing the storminess due to the solar disturbances to bring about glaciation. At the same time, if volcanic activity should increase, it would add its quota to the tendency toward glaciation.
Nevertheless, it might easily happen that a very considerable amount of crustal movement would take place without causing a continental ice sheet or even a marked alpine ice sheet. Or again, if the strain in the Earth's crust had already been largely released through other agencies before the stormy period began, the climate might become severe enough to cause glaciation in high latitudes without leading to any very marked movements of the Earth's crust. as apparently happened in the mid-Silerian period. End of section 16
