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Causes Of Mild Geological Climates

The Uniformity Of Climate

Hypotheses Of Climatic Change


Some Problems Of Glacial Periods

The Climatic Stress Of The Fourteenth Century

The Variability Of Climate

The Changing Composition Of Oceans And Atmosphere

The Climate Of History

Glaciation According To The Solar-cyclonic Hypothesis[38]

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Terrestrial Causes Of Climatic Changes

The Effect Of Other Bodies On The Sun

The Sun's Journey Through Space

Post-glacial Crustal Movements And Climatic Changes

The Earth's Crust And The Sun

The Origin Of Loess

The Solar Cyclonic Hypothesis

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Climate Of History

The Changing Composition Of Oceans And Atmosphere

Terrestrial Causes Of Climatic Changes

The major portion of this book has been concerned with the explanation
of the more abrupt and extreme changes of climate. This chapter and the
next consider two other sorts of climatic changes, the slight secular
progression during the hundreds of millions of years of recorded earth
history, and especially the long slow geologic oscillations of millions
or tens of millions of years. It is generally agreed among geologists
that the progressive change has tended toward greater extremes of
climate; that is, greater seasonal contrasts, and greater contrasts from
place to place and from zone to zone.[70] The slow cyclic changes have
been those that favored widespread glaciation at one extreme near the
ends of geologic periods and eras, and mild temperatures even in
subpolar regions at the other extreme during the medial portions of the

As has been pointed out in an earlier chapter, it has often been assumed
that all climatic changes are due to terrestrial causes. We have seen,
however, that there is strong evidence that solar variations play a
large part in modifying the earth's climate. We have also seen that no
known terrestrial agency appears to be able to produce the abrupt
changes noted in recent years, the longer cycles of historical times, or
geological changes of the shorter type, such as glaciation.
Nevertheless, terrestrial changes doubtless have assisted in producing
both the progressive change and the slow cyclic changes recorded in the
rocks, and it is the purpose of this chapter and the two that follow to
consider what terrestrial changes have taken place and the probable
effect of such changes.

The terrestrial changes that have a climatic significance are numerous.
Some, such as variations in the amount of volcanic dust in the higher
air, have been considered in an earlier chapter. Others are too
imperfectly known to warrant discussion, and in addition there are
presumably others which are entirely unknown. Doubtless some of these
little known or unknown changes have been of importance in modifying
climate. For example, the climatic influence of vegetation, animals, and
man may be appreciable. Here, however, we shall confine ourselves to
purely physical causes, which will be treated in the following order:
First, those concerned with the solid parts of the earth, namely: (I)
amount of land; (II) distribution of land; (III) height of land; (IV)
lava flows; and (V) internal heat. Second, those which arise from the
salinity of oceans, and third, those depending on the composition and
amount of atmosphere.

The terrestrial change which appears indirectly to have caused the
greatest change in climate is the contraction of the earth. The problem
of contraction is highly complex and is as yet only imperfectly
understood. Since only its results and not its processes influence
climate, the following section as far as page 196 is not necessary to
the general reader. It is inserted in order to explain why we assume
that there have been oscillations between certain types of distribution
of the lands.

The extent of the earth's contraction may be judged from the shrinkage
indicated by the shortening of the rock formations in folded mountains
such as the Alps, Juras, Appalachians, and Caucasus. Geologists are
continually discovering new evidence of thrust faults of great magnitude
where masses of rock are thrust bodily over other rocks, sometimes for
many miles. Therefore, the estimates of the amount of shrinkage based on
the measurements of folds and faults need constant revision upward.
Nevertheless, they have already reached a considerable figure. For
example, in 1919, Professor A. Heim estimated the shortening of the
meridian passing through the modern Alps and the ancient Hercynian and
Caledonian mountains as fully a thousand miles in Europe, and over five
hundred miles for the rest of this meridian.[71] This is a radial
shortening of about 250 miles. Possibly the shrinkage has been even
greater than this. Chamberlin[72] has compared the density of the earth,
moon, Mars, and Venus with one another, and found it probable that the
radial shrinkage of the earth may be as much as 570 miles. This result
is not so different from Heim's as appears at first sight, for Heim made
no allowance for unrecognized thrust faults and for the contraction
incident to metamorphism. Moreover, Heim did not include shrinkage
during the first half of geological time before the above-mentioned
mountain systems were upheaved.

According to a well-established law of physics, contraction of a
rotating body results in more rapid rotation and greater centrifugal
force. These conditions must increase the earth's equatorial bulge and
thereby cause changes in the distribution of land and water. Opposed to
the rearrangement of the land due to increased rotation caused by
contraction, there has presumably been another rearrangement due to
tidal retardation of the earth's rotation and a consequent lessening of
the equatorial bulge. G. H. Darwin long ago deduced a relatively large
retardation due to lunar tides. A few years ago W. D. MacMillan, on
other assumptions, deduced only a negligible retardation. Still more
recently Taylor[73] has studied the tides of the Irish Sea, and his work
has led Jeffreys[74] and Brown[75] to conclude that there has been
considerable retardation, perhaps enough, according to Brown, to equal
the acceleration due to the earth's contraction. From a prolonged and
exhaustive study of the motions of the moon Brown concludes that tidal
friction or some other cause is now lengthening the day at the rate of
one second per thousand years, or an hour in almost four million years
if the present rate continues. He makes it clear that the retardation
due to tides would not correspond in point of time with the acceleration
due to contraction. The retardation would occur slowly, and would take
place chiefly during the long quiet periods of geologic history, while
the acceleration would occur rapidly at times of diastrophic
deformation. As a consequence, the equatorial bulge would alternately be
reduced at a slow rate, and then somewhat suddenly augmented.

The less rigid any part of the earth is, the more quickly it responds to
the forces which lead to bulging or which tend to lessen the bulge.
Since water is more fluid than land, the contraction of the earth and
the tidal retardation presumably tend alternately to increase and
decrease the amount of water near the equator more than the amount of
land. Thus, throughout geological history we should look for cyclic
changes in the relative area of the lands within the tropics and similar
changes of opposite phase in higher latitudes. The extent of the change
would depend upon (a) the amount of alteration in the speed of rotation,
and (b) the extent of low land in low latitudes and of shallow sea in
high latitudes. According to Slichter's tables, if the earth should
rotate in twenty-three hours instead of twenty-four, the great Amazon
lowland would be submerged by the inflow of oceanic water, while wide
areas in Hudson Bay, the North Sea, and other northern regions, would
become land because the ocean water would flow away from them.[76]

Following the prompt equatorward movement of water which would occur as
the speed of rotation increased, there must also be a gradual movement
or creepage of the solid rocks toward the equator, that is, a bulging of
the ocean floor and of the lands in low latitudes, with a consequent
emergence of the lands there and a relative rise of sea level in higher
latitudes. Tidal retardation would have a similar effect. Suess[77] has
described widespread elevated strand lines in the tropics which he
interprets as indicating a relatively sudden change in sea level, though
he does not suggest a cause of the change. However, in speaking of
recent geological times, Suess reports that a movement more recent than
the old strands "was an accumulation of water toward the equator, a
diminution toward the poles, and (it appears) as though this last
movement were only one of the many oscillations which succeed each other
with the same tendency, i.e., with a positive excess at the equator, a
negative excess at the poles." (Vol. II, p. 551.) This creepage of the
rocks equatorward seemingly might favor the growth of mountains in
tropical and subtropical regions, because it is highly improbable that
the increase in the bulge would go on in all longitudes with perfect
uniformity. Where it went on most rapidly mountains would arise. That
such irregularity of movement has actually occurred is suggested not
only by the fact that many Cenozoic and older mountain ranges extend
east and west, but by the further fact that these include some of our
greatest ranges, many of which are in fairly low latitudes. The
Himalayas, the Javanese ranges, and the half-submerged Caribbean chains
are examples. Such mountains suggest a thrust in a north and south
direction which is just what would happen if the solid mass of the earth
were creeping first equatorward and then poleward.

A fact which is in accord with the idea of a periodic increase in the
oceans in low latitudes because of renewed bulging at the equator is the
exposure in moderately high latitudes of the greatest extent of ancient
rocks. This seems to mean that in low latitudes the frequent deepening
of the oceans has caused the old rocks to be largely covered by
sediments, while the old lands in higher latitudes have been left more
fully exposed to erosion.

Another suggestion of such periodic equatorward movements of the ocean
water is found in the reported contrast between the relative stability
with which the northern part of North America has remained slightly
above sea level except at times of widespread submergence, while the
southern parts have suffered repeated submergence alternating with great
emergence.[78] Furthermore, although the northern part of North America
has been generally exposed to erosion since the Proterozoic, it has
supplied much less sediment than have the more southern land areas.[79]
This apparently means that much of Canada has stood relatively low,
while repeated and profound uplift alternating with depression has
occurred in subtropical latitudes, apparently in adjustment to changes
in the earth's speed of rotation. The uplifts generally followed the
times of submergence due to equatorward movement of the water, though
the buckling of the crust which accompanies shrinkage doubtless caused
some of the submergence. The evidence that northern North America stood
relatively low throughout much of geological time depends not only on
the fact that little sediment came to the south from the north, but also
on the fact that at times of especially widespread epicontinental seas,
the submergence was initiated at the north.[80] This is especially true
for Ordovician, Silurian, Devonian, and Jurassic times in North America.
General submergence of this kind is supposed to be due chiefly to the
overflowing of the ocean when its level is slowly raised by the
deposition of sediment derived from the erosion of what once were
continental highlands but later are peneplains. The fact that such
submergence began in high latitudes, however, seems to need a further
explanation. The bulging of the rock sphere at the equator and the
consequent displacement of some of the water in low latitudes would
furnish such an explanation, as would also a decrease in the speed of
rotation induced by tidal retardation, if that retardation were great
enough and rapid enough to be geologically effective.

The climatic effects of the earth's contraction, which we shall shortly
discuss, are greatly complicated by the fact that contraction has taken
place irregularly. Such irregularity has occurred in spite of the fact
that the processes which cause contraction have probably gone on quite
steadily throughout geological history. These processes include the
chemical reorganization of the minerals of the crust, a process which is
illustrated by the metamorphism of sedimentary rocks into crystalline
forms. The escape of gases through volcanic action or otherwise has been
another important process.

Although the processes which cause contraction probably go on steadily,
their effect, as Chamberlin[81] and others have pointed out, is probably
delayed by inertia. Thus the settling of the crust or its movement on a
large scale is delayed. Perhaps the delay continues until the stresses
become so great that of themselves they overcome the inertia, or
possibly some outside agency, whose nature we shall consider later,
reenforces the stresses and gives the slight impulse which is enough to
release them and allow the earth's crust to settle into a new state of
equilibrium. When contraction proceeds actively, the ocean segments,
being largest and heaviest, are likely to settle most, resulting in a
deepening of the oceans and an emergence of the lands. Following each
considerable contraction there would be an increase in the speed of
rotation. The repeated contractions with consequent growth of the
equatorial bulge would alternate with long quiet periods during which
tidal retardation would again decrease the speed of rotation and hence
lessen the bulge. The result would be repeated changes of distribution
of land and water, with consequent changes in climate.

I. We shall now consider the climatic effect of the repeated changes in
the relative amounts of land and water which appear to have resulted
from the earth's contraction and from changes in its speed of rotation.
During many geologic epochs a larger portion of the earth was covered
with water than at present. For example, during at least twelve out of
about twenty epochs, North America has suffered extensive
inundations,[82] and in general the extensive submergence of Europe, the
other area well known geologically, has coincided with that of North
America. At other times, the ocean has been less extensive than now, as
for example during the recent glacial period, and probably during
several of the glacial periods of earlier date. Each of the numerous
changes in the relative extent of the lands must have resulted in a
modification of climate.[83] This modification would occur chiefly
because water becomes warm far more slowly than land, and cools off far
more slowly.

An increase in the lands would cause changes in several climatic
conditions. (a) The range of temperature between day and night and
between summer and winter would increase, for lands become warmer by day
and in summer than do oceans, and cooler at night and in winter. The
higher summer temperature when the lands are widespread is due chiefly
to the fact that the land, if not snow-covered, absorbs more of the
sun's radiant energy than does the ocean, for its reflecting power is
low. The lower winter temperature when lands are widespread occurs not
only because they cool off rapidly but because the reduced oceans cannot
give them so much heat. Moreover, the larger the land, the more
generally do the winds blow outward from it in winter and thus prevent
the ocean heat from being carried inland. So long as the ocean is not
frozen in high latitudes, it is generally the chief source of heat in
winter, for the nights are several months long near the poles, and even
when the sun does shine its angle is so low that reflection from the
snow is very great. Furthermore, although on the average there is more
reflection from water than from land, the opposite is true in high
latitudes in winter when the land is snow-covered while the ocean is
relatively dark and is roughened by the waves. Another factor in causing
large lands to have extremely low temperature in winter is the fact that
in proportion to their size they are less protected by fog and cloud
than are smaller areas. The belt of cloud and fog which is usually
formed when the wind blows from the ocean to the relatively cold land is
restricted to the coastal zone. Thus the larger the land, the smaller
the fraction in which loss of heat by radiation is reduced by clouds and
fogs. Hence an increase in the land area is accompanied by an increase
in the contrasts in temperature between land and water.

(b) The contrasts in temperature thus produced must cause similar
contrasts in atmospheric pressure, and hence stronger barometric
gradients. (c) The strong gradients would mean strong winds, flowing
from land to sea or from sea to land. (d) Local convection would also be
strengthened in harmony with the expansion of the lands, for the more
rapid heating of land than of water favors active convection.

(e) As the extent of the ocean diminished, there would normally be a
decrease in the amount of water vapor for three reasons: (1) Evaporation
from the ocean is the great source of water vapor. Other conditions
being equal, the smaller the ocean becomes, the less the evaporation.
(2) The amount of water vapor in the air diminishes as convection
increases, since upward convection is a chief method by which
condensation and precipitation are produced, and water vapor removed
from the atmosphere. (3) Nocturnal cooling sufficient to produce dew and
frost is very much more common upon land than upon the ocean. The
formation of dew and frost diminishes the amount of water vapor at least
temporarily. (f) Any diminution in water vapor produced in these ways,
or otherwise, is significant because water vapor is the most essential
part of the atmosphere so far as regulation of temperature is concerned.
It tends to keep the days from becoming hot or the nights cold.
Therefore any decrease in water vapor would increase the diurnal and
seasonal range of temperature, making the climate more extreme and
severe. Thus a periodic increase in the area of the continents would
clearly make for periodic increased climatic contrasts, with great
extremes, a type of climatic change which has recurred again and again.
Indeed, each great glaciation accompanied or followed extensive
emergence of the lands.[84]

Whether or not there has been a progressive increase from era to era
in the area of the lands is uncertain. Good authorities disagree widely.
There is no doubt, however, that at present the lands are more extensive
than at most times in the past, though smaller, perhaps, than at certain
periods. The wide expanse of lands helps explain the prominence of
seasons at present as compared with the past.

II. The contraction of the earth, as we have seen, has produced great
changes in the distribution as well as in the extent of land and water.
Large parts of the present continents have been covered repeatedly by
the sea, and extensive areas now covered with water have been land. In
recent geological times, that is, during the Pliocene and Pleistocene,
much of the present continental shelf, the zone less than 600 feet below
sea level, was land. If the whole shelf had been exposed, the lands
would have been greater than at present by an area larger than North
America. When the lands were most elevated, or a little earlier, North
America was probably connected with Asia and almost with Europe. Asia in
turn was apparently connected with the larger East Indian islands. In
much earlier times land occupied regions where now the ocean is fairly
deep. Groups of islands, such as the East Indies and Malaysia and
perhaps the West Indies, were united into widespreading land masses.
Figs. 7 and 9, illustrating the paleography of the Permian and the
Cretaceous periods, respectively, indicate a land distribution radically
different from that of today.

So far as appears from the scattered facts of geological history, the
changes in the distribution of land seem to have been marked by the
following characteristics: (1) Accompanying the differentiation of
continental and oceanic segments of the earth's crust, the oceans have
become somewhat deeper, and their basins perhaps larger, while the
continents, on the average, have been more elevated and less subject to
submergence. Hence there have been less radical departures from the
present distribution during the relatively recent Cenozoic era than in
the ancient Paleozoic because the submergence of continental areas has
become less general and less frequent. For example, the last extensive
epeiric or interior sea in North America was in the Cretaceous, at least
ten million years ago, and according to Barrell perhaps fifty million,
while in Europe, according to de Lapparent,[85] a smaller share of the
present continent has been submerged since the Cretaceous than before.
Indeed, as in North America, the submergence has decreased on the
average since the Paleozoic era. (2) The changes in distribution of land
which have taken place during earth history have been cyclic.
Repeatedly, at the close of each of the score or so of geologic periods,
the continents emerged more or less, while at the close of the groups of
periods known as eras, the lands were especially large and emergent.
After each emergence, a gradual encroachment of the sea took place, and
toward the close of several of the earlier periods, the sea appears to
have covered a large fraction of the present land areas. (3) On the
whole, the amount of land in the middle and high latitudes of the
northern hemisphere appears to have increased during geologic time. Such
an increase does not require a growth of the continents, however, in the
broader sense of the term, but merely that a smaller fraction of the
continent and its shelf should be submerged. (4) In tropical latitudes,
on the other hand, the extent of the lands seems to have decreased,
apparently by the growth of the ocean basins. South America and Africa
are thought by many students to have been connected, and Africa was
united with India via Madagascar, as is suggested in Fig. 9. The most
radical cyclic as well as the most radical progressive changes in land
distribution also seem to have taken place in tropical regions.[86]

(After Schuchert.)]

Although there is much evidence of periodic increase of the sea in
equatorial latitudes and of land in high latitudes, it has remained for
the zooelogist Metcalf to present a very pretty bit of evidence that at
certain times submergence along the equator coincided with emergence in
high latitudes, and vice versa. Certain fresh water frogs which carry
the same internal parasite are confined to two widely separated areas in
tropical and south temperate America and in Australia. The extreme
improbability that both the frogs and the parasites could have
originated independently in two unconnected areas and could have
developed by convergent evolution so that they are almost identical in
the two continents makes it almost certain that there must have been a
land connection between South America and Australia, presumably by way
of Antarctica. The facts as to the parasites seem also to prove that
while the land connection existed there was a sea across South America
in equatorial latitudes. The parasite infests not only the frogs but the
American toads known as Bufo. Now Bufo originated north of the equator
in America and differs from the frogs which originated in southern South
America in not being found in Australia. This raises the question of how
the frogs could go to Australia via Antarctica carrying the parasite
with them, while the toads could not go. Metcalf's answer is that the
toads were cut off from the southern part of South America by an
equatorial sea until after the Antarctic connection between the Old
World and the New was severed.

As Patagonia let go of Antarctica by subsidence of the intervening
land area, there was a probable concomitant rise of land through
what is now middle South America and the northern and southern
portions of this continent came together.[87]

These various changes in the earth's crust have given rise to certain
specific types of distribution of the lands, which will now be
considered. We shall inquire what climatic conditions would arise from
changes in (a) the continuity of the lands from north to south, (b) the
amount of land in tropical latitudes, and (c) the amount of land in
middle and high latitudes.

(a) At present the westward drift of warm waters, set in motion by the
trade winds, is interrupted by land masses and turned poleward,
producing the important Gulf Stream Drift and Japan Current in the
northern hemisphere, and corresponding, though less important, currents
in the southern hemisphere. During the past, quite different sets of
ocean currents doubtless have existed in response to a different
distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and
several other periods, the present American barrier to the westward
moving tropical current was broken in Central America. Even if the
supposed continent of "Gondwana Land" extended from Africa to South
America in equatorial latitudes, strong currents must still have flowed
westward along its northern shore under the impulse of the peculiarly
strong trade winds which the equatorial land would create. Nevertheless
at such times relatively little warm tropical water presumably entered
the North Atlantic, for it escaped into the Pacific. At several other
times, such as the late Ordovician and mid-Devonian, when the isthmian
barrier existed, it probably turned an important current northward into
what is now the Mississippi Basin instead of into the Atlantic. There it
traversed an epeiric, or mid-continental sea open to both north and
south. Hence its effectiveness in warming Arctic regions must have been
quite different from that of the present Gulf Stream.

(b) We will next consider the influences of changes in the amount of
equatorial and tropical land. As such lands are much hotter than the
corresponding seas, the intensity and width of the equatorial belt of
low pressure must be great when they are extensive. Hence the trade
winds must have been stronger than now whenever tropical lands were more
extensive than at present. This is because the trades are produced by
the convection due to excessive heat along the heat equator. There the
air expands upward and flows poleward at high altitudes. The trade wind
consists of air moving toward the heat equator to take the place of the
air which there rises. When the lands in low latitudes were wide the
trade winds must also have dominated a wide belt. The greater width of
the trade-wind belt today over Africa than over the Atlantic illustrates
the matter. The belt must have been still wider when Gondwana Land was
large, as it is believed to have been during the Paleozoic era and the
early Mesozoic.

An increase in the width of the equatorial belt of low pressure under
the influence of broad tropical lands would be accompanied not only by
stronger and more widespread trade winds, but by a corresponding
strengthening of the subtropical belts of high pressure. The chief
reason would be the greater expansion of the air in the equatorial low
pressure belt and the consequent more abundant outflow of air at high
altitudes in the form of anti-trades or winds returning poleward above
the trades. Such winds would pile up the air in the region of the
high-pressure belt. Moreover, since the meridians converge as one
proceeds away from the equator, the air of the poleward-moving
anti-trades tends to be crowded as it reaches higher latitudes, thus
increasing the pressure. Unless there were a corresponding increase in
tropical cyclones, one of the most prominent results of the strengthened
trades and the intensified subtropical high-pressure belt at times of
broad lands in low latitudes would be great deserts. It will be recalled
that the trade-wind lowlands and the extra-tropical belt of highs are
the great desert belts at present. The trade-wind lowlands are desert
because air moving into warmer latitudes takes up water except where it
is cooled by rising on mountain-sides. The belt of highs is arid because
there, too, air is being warmed, but in this case by descending from

Again, if the atmospheric pressure in the subtropical belt should be
intensified, the winds flowing poleward from this belt would necessarily
become stronger. These would begin as southwesterlies in the northern
hemisphere and northwesterlies in the southern. In the preceding chapter
we have seen that such winds, especially when cyclonic storms are few
and mild, are a powerful agent in transferring subtropical heat
poleward. If the strength of the westerlies were increased because of
broad lands in low latitudes, their efficacy in transferring heat would
be correspondingly augmented. It is thus evident that any change in the
extent of tropical lands during the geologic past must have had
important climatic consequences in changing the velocity of the
atmospheric circulation and in altering the transfer of heat from low
latitudes to high. When the equatorial and tropical lands were broad the
winds and currents must have been strong, much heat must have been
carried away from low latitudes, and the contrast between low and high
latitudes must have been relatively slight. As we have already remarked,
leading paleogeographers believe that changes in the extent of the lands
have been especially marked in low latitudes, and that on the average
there has been a decrease in the extent of land within the tropics.
Gondwana Land is the greatest illustration of this. In the same way, on
the numerous paleogeographic maps of North America, most
paleogeographers have shown fairly extensive lands south of the latitude
of the United States during most of the geologic epochs.[88]

(c) There is evidence that during geologic history the area of the lands
in middle and high latitudes, as well as in low latitudes, has changed
radically. An increase in such lands would cause the winters to grow
colder. This would be partly because of the loss of heat by radiation
into the cold dry air over the continents in winter, and partly because
of increased reflection from snow and frost, which gather much more
widely upon the land than upon the ocean. Furthermore, in winter when
the continents are relatively cold, there is a strong tendency for winds
to blow out from the continent toward the ocean. The larger the land the
stronger this tendency. In Asia it gives rise to strong winter monsoons.
The effect of such winds is illustrated by the way in which the
westerlies prevent the Gulf Stream from warming the eastern United
States in winter. The Gulf Stream warms northwestern Europe much more
than the United States because, in Europe, the prevailing winds are

Another effect of an increase in the area of the lands in middle and
high latitudes would be to interpose barriers to oceanic circulation and
thus lower the temperature of polar regions. This would not mean
glaciation in high latitudes, however, even when the lands were
widespread as in the Mesozoic and early Tertiary. Students of glaciology
are more and more thoroughly convinced that glaciation depends on the
availability of moisture even more than upon low temperature.

In conclusion it may be noted that each of the several climatic
influences of increased land area in the high latitudes would tend to
increase the contrasts between land and sea, between winter and summer,
and between low latitudes and high. In other words, so far as the effect
upon high latitudes themselves is concerned, an expansion of the lands
there would tend in the same direction as a diminution in low latitudes.
In so far as the general trend of geological evolution has been toward
more land in high latitudes and less in low, it would help to produce a
progressive increase in climatic diversity such as is faintly indicated
in the rock strata. On the other hand, the oscillations in the
distribution of the lands, of which geology affords so much evidence,
must certainly have played an important part in producing the periodic
changes of climate which the earth has undergone.

III. Throughout geological history there is abundant evidence that the
process of contraction has led to marked differences not only in the
distribution and area of the lands, but in their height. On the whole
the lands have presumably increased in height since the Proterozoic,
somewhat in proportion to the increased differentiation of continents
and oceans.[89] If there has been such an increase, the contrast between
the climate of ocean and land must have been accentuated, for highlands
have a greater diurnal and seasonal range of temperature than do
lowlands. The ocean has very little range of either sort. The large
range at high altitudes is due chiefly to the small quantity of water
vapor, for this declines steadily with increased altitude. A diminution
in the density of the other constituents of the air also decreases the
blanketing effect of the atmosphere. In conformity with the great
seasonal range in temperature at times when the lands stand high, the
direction of the wind would be altered. When the lands are notably
warmer than the oceans, the winds commonly flow from land to sea, and
when the continents are much colder than the oceans, the direction is
reversed. The monsoons of Asia are examples. Strong seasonal winds
disturb the normal planetary circulation of the trade winds in low
latitudes and of the westerlies in middle latitudes. They also interfere
with the ocean currents set in motion by the planetary winds. The net
result is to hinder the transfer of heat from low latitudes to high, and
thus to increase the contrasts between the zones. Local as well as zonal
contrasts are also intensified. The higher the land, the greater,
relatively speaking, are the cloudiness and precipitation on seaward
slopes, and the drier the interior. Indeed, most highlands are arid.
Henry's[90] recent study of the vertical distribution of rainfall on
mountain-sides indicates that a decrease sets in at about 3500 feet in
the tropics and only a little higher in mid-latitudes.

In addition to the main effects upon atmospheric circulation and
precipitation, each of the many upheavals of the lands must have been
accompanied by many minor conditions which tended toward diversity. For
example, the streams were rejuvenated, and instead of meandering perhaps
over vast flood plains they intrenched their channels and in many cases
dug deep gorges. The water table was lowered, soil was removed from
considerable areas, the bare rock was exposed, and the type of dominant
vegetation altered in many places. An almost barren ridge may represent
all that remains of what was once a vast forested flood plain. Thus,
increased elevation of the land produces contrasted conditions of slope,
vegetation, availability of ground water, exposure to wind and so forth,
and these unite in diversifying climate. Where mountains are formed,
strong contrasts are sure to occur. The windward slopes may be very
rainy, while neighboring leeward slopes are parched by a dry foehn wind.
At the same time the tops may be snow-covered. Increased local contrasts
in climatic conditions are known to influence the intensity of cyclonic
storms,[91] and these affect the climatic conditions of all middle and
high latitudes, if not of the entire earth. The paths followed by
cyclonic storms are also altered by increased contrast between land and
water. When the continents are notably colder than the neighboring
oceans, high atmospheric pressure develops on the lands and interferes
with the passage of lows, which are therefore either deflected around
the continent or forced to move slowly.

The distribution of lofty mountains has an even more striking climatic
effect than the general uplift of a region. In Proterozoic times there
was a great range in the Lake Superior region; in the late Devonian the
Acadian mountains of New England and the Maritime Provinces of Canada
possibly attained a height equal to the present Rockies. Subsequently,
in the late Paleozoic a significant range stood where the Ouachitas now
are. Accompanying the uplift of each of these ranges, and all others,
the climate of the surrounding area, especially to leeward, must have
been altered greatly. Many extensive salt deposits found now in fairly
humid regions, for example, the Pennsylvanian and Permian deposits of
Kansas and Oklahoma, were probably laid down in times of local aridity
due to the cutting off of moisture-bearing winds by the mountains of
Llanoria in Louisiana and Texas. Hence such deposits do not necessarily
indicate periods of widespread and profound aridity.

When the causes of ancient glaciation were first considered by
geologists, about the middle of the nineteenth century, it was usually
assumed that the glaciated areas had been elevated to great heights, and
thus rendered cold enough to permit the accumulation of glaciers. The
many glaciers occurring in the Alps of central Europe where glaciology
arose doubtless suggested this explanation. However, it is now known
that most of the ancient glaciation was not of the alpine type, and
there is adequate proof that the glacial periods cannot be explained as
due directly and solely to uplift. Nevertheless, upheavals of the lands
are among the most important factors in controlling climate, and
variations in the height of the lands have doubtless assisted in
producing climate oscillations, especially those of long duration.
Moreover, the progressive increase in the height of the lands has
presumably played a part in fostering local and zonal diversity in
contrast with the relative uniformity of earlier geological times.

IV. The contraction of the earth has been accompanied by volcanic
activity as well as by changes in the extent, distribution, and altitude
of the lands. The probable part played by volcanic dust as a
contributory factor in producing short sudden climatic variations has
already been discussed. There is, however, another though probably less
important respect in which volcanic activity may have had at least a
slight climatic significance. The oldest known rocks, those of the
Archean era, contain so much igneous matter that many students have
assumed that they show that the entire earth was once liquid. It is now
considered that they merely indicate igneous activity of great
magnitude. In the later part of Proterozoic time, during the second
quarter of the earth's history according to Schuchert's estimate, there
were again vast outflowings of lava. In the Lake Superior district, for
example, a thickness of more than a mile accumulated over a large area,
and lavas are common in many areas where rocks of this age are known.
The next quarter of the earth's history elapsed without any
correspondingly great outflows so far as is known, though several lesser
ones occurred. Toward the end of the last quarter, and hence quite
recently from the geological standpoint, another period of outflows,
perhaps as noteworthy as that of the Proterozoic, occurred in the
Cretaceous and Tertiary.

The climatic effects of such extensive lava flows would be essentially
as follows: In the first place so long as the lavas were hot they would
set up a local system of convection with inflowing winds. This would
interfere at least a little with the general winds of the area. Again,
where the lava flowed out into water, or where rain fell upon hot lava,
there would be rapid evaporation which would increase the rainfall. Then
after the lava had cooled, it would still influence climate a trifle in
so far as its color was notably darker or lighter than that of the
average surface. Dark surfaces absorb solar heat and become relatively
warm when the sun shines upon them. Dark objects likewise radiate heat
more rapidly than light-colored objects. Hence they cool more rapidly at
night, and in the winter. As most lavas are relatively dark they
increase the average diurnal range of temperature. Hence even after they
are cool they increase the climatic diversity of the land.

The amount of heat given to the atmosphere by an extensive lava flow,
though large according to human standards, is small compared with the
amount received from the sun by a like area, except during the first few
weeks or months before the lava has formed a thick crust. Furthermore,
probably only a small fraction of any large series of flows occurred in
a given century or millennium. Moreover, even the largest lava flows
covered an area of only a few hundredths of one per cent of the earth's
surface. Nevertheless, the conditions which modify climate are so
complicated that it would be rash to state that this amount of
additional heat has been of no climatic significance. Like the
proverbial "straw that broke the camel's back," the changes it would
surely produce in local convection, atmospheric pressure, and the
direction of the wind may have helped to shift the paths of storms and
to produce other complications which were of appreciable climatic

V. The last point which we shall consider in connection with the effect
of the earth's interior upon climate is internal heat. The heat given
off by lavas is merely a small part of that which is emitted by the
earth as a whole. In the earliest part of geological history enough heat
may have escaped from the interior of the earth to exert a profound
influence on the climate. Knowlton,[92] as we have seen, has recently
built up an elaborate theory on this assumption. At present, however,
accurate measurements show that the escape of heat is so slight that it
has no appreciable influence except in a few volcanic areas. It is
estimated to raise the average temperature of the earth's surface less
than 0.1 deg.C.[93]

In order to contribute enough heat to raise the surface temperature
1 deg.C., the temperature gradient from the interior of the earth to the
surface would need to be ten times as great as now, for the rate of
conduction varies directly with the gradient. If the gradient were ten
times as great as now, the rocks at a depth of two and one-half miles
would be so hot as to be almost liquid according to Barrell's[94]
estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate
that such high temperatures have not prevailed at such slight depths
since the Proterozoic. Furthermore, the fact that the climate was cold
enough to permit glaciation early in the Proterozoic era and at from one
to three other times before the opening of the Paleozoic suggests that
the rate of escape of heat was not rapid even in the first half of the
earth's recorded history. Yet even if the general escape of heat has
never been large since the beginning of the better-known part of
geological history, it was presumably greater in early times than at

If there actually has been an appreciable decrease in the amount of heat
given out by the earth's interior, its effects would agree with the
observed conditions of the geological record. It would help to explain
the relative mildness of zonal, seasonal, and local contrasts of climate
in early geological times, but it would not help to explain the long
oscillations from era to era which appear to have been of much greater
importance. Those oscillations, so far as we can yet judge, may have
been due in part to solar changes, but in large measure they seem to be
explained by variations in the extent, distribution, and altitude of the
lands. Such variations appear to be the inevitable result of the earth's


[Footnote 70: Chas. Schuchert: The Earth's Changing Surface and Climate
during Geologic Time; in Lull: The Evolution of the Earth and Its
Inhabitants, 1918, p. 55.]

[Footnote 71: Quoted by J. Cornet: Cours de Geologie, 1920, p. 330.]

[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage
of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.]

[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919,
pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80,
p. 308.]

[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan.,
1920, Vol. 80, p. 309.]

[Footnote 75: E. W. Brown: personal communication.]

[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous
Spheroid; in Contributions to the Fundamental Problems of Geology, by
T. C. Chamberlin, et al., Carnegie Inst. of Wash., No. 107, 1909.]

[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.]

[Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate;
in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]

[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.]

[Footnote 80: Chas. Schuchert: loc. cit., p. 78.]

[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of
Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: loc. cit.]

[Footnote 82: Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p.
982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
Am., Vol. 20, pp. 427-606; reference on p. 499.]

[Footnote 83: The general subject of the climatic significance of
continentality is discussed by C. E. P. Brooks: continentality and
Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct.,

[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The
Climatic Factor; Carnegie Institution, 1914, p. 286.]

[Footnote 85: A. de Lapparent: Traite de Geologie, 1906.]

[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.]

[Footnote 87: M. M. Metcalf: Upon an important method of studying
problems of relationship and of geographical distribution; Proceedings
National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.]

[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull.
Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others:
Outlines of Geologic History, 1910.]

[Footnote 89: Chas. Schuchert: The Earth's Changing Surface and Climate;
in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.]

[Footnote 90: A. J. Henry: The Decrease of Precipitation with Altitude;
Monthly Weather Review, Vol. 47, 1919, pp. 33-41.]

[Footnote 91: Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p.
511; and also A. J. Henry and others: Weather Forecasting in the United
States, 1913.]

[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull.
Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.]

[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911,
p. 63.]

[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.]

Next: Post-glacial Crustal Movements And Climatic Changes

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