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Geology is essentially a history of the earth and its inhabitants. It treats of rocks and of the agencies and processes which have been involved in their formation, and from the rocks and their structures it attempts to make out the various stages through which the earth and the living things which have dwelt upon it have passed. It is one of the broadest of the sciences, and brings under consideration certain phases of other sciences, particularly astronomy, physics, chemistry, zoology, and botany.
Subdivisions. So broad a science has many subdivisions. That phase which treats of the outer relations of the earth is Cosmic or Astronomic Geology; that which treats of the constituent parts of the earth and its material is Geognosy, of which the most important branch is Petrology, the science of rocks. That phase which deals with the structural arrangement of the rocks is Geotectonic, or Structural Geology; that which deals with the forces involved in geologic processes is Dynamic Geology; that which treats of the face of the earth, or topographic form, is Physiographic Geology; that which concerns itself with the fossils that have been preserved in the rocks, and with the faunas and floras that have lived in the past, constitutes Paleontologic Geology, or Paleontology. The treatment of the succession of events is Historical Geology, which is [p. 2] worked out chiefly from the succession of beds of rock formed in the progress of the ages.
Besides these general subdivisions, there are special applications of geologic knowledge which give rise to other terms. Thus Economic Geology is concerned with the industrial applications of geologic knowledge, and Mining Geology, which is a sub-section of economic geology, with the application of geologic facts and principles to mining. Other similar subdivisions might be mentioned.
Dominant processes. Three sets of processes, now in operation "oh the surface of the earth, have given rise to most of its surface features. These processes have been designated Diastrophism, Vulcanism (or Volcanism), and Gradation. Diastrophism includes all movements of the outer parts of the lithosphere, whether slow or rapid, gentle or violent, slight or extensive. Many parts of the land, especially along coasts, are known to be sinking slowly relative to the sea-level, while other parts are known to be rising. The fact that sediments originally deposited beneath the sea now exist. in some places, at great elevations, together with the fact that certain areas which were once land are now beneath the sea, proves that similar changes have taken place in the past. Earthquakes are another illustration of diastrophism. Vulcanism includes all processes concerned with the movements of lava and other volcanic products, whether extruded at the surface or not. Vulcanism and diastrophism may be closely associated, for local movements arc often associated with volcanic eruptions. Gradation includes all those processes which tend to bring the surface of the lithosphere to a common level. Gradational processes belong to two categories — those which level down, degradation, and those which level up, aggradation. The transportation of material from the land, whether by rain, rivers, glaciers, waves, or winds, is degradation, and the deposition of material, whether on the land or in the is aggradation. Degradation affects primarily the higher parts of the lithosphere, while aggradation affects primarily the lower.
The earth as a planet. Though supremely important to us, the earth is but one of the minor planets which revolve about the [p. 3] sun. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mercury, Venus, and Mars, are smaller. There are a host of asteroids, but all together they do not equal the mass of the smallest planet. The average mass of the planets is more than fifty times that of the earth, and Jupiter, the largest, has more than three hundred times the mass of the earth. The earth’s position is in no sense distinguished. It is neither the outer nor the inner, nor even the middle planet. Even in the inner group of four to which it belongs, it is neither the outermost nor the innermost member, though in this group it is the largest. Its average distance from the sun is about 92.9 million miles, and its period of revolution is 365 1/4 days, a period longer than that of any of the inner planets, and shorter than that of any of the outer ones. Its period of rotation is not very different from that of Mars, but is much shorter than that of the larger planets whose periods of rotation are known. The plane of the earth’s revolution approximates the planes of revolution of all the other planets. The orbit of the earth, like the orbits of the other planets, is an ellipse, and its eccentricity is about . The inclination of the earth’s axis, nearly 23 1/2° (23° 27’), is less than that of some planets, and more than that of others.
Its satellite. The earth is peculiar in having one unusually large satellite, which has a mass of its own. The larger planets have several satellites whose combined mass exceeds that of the moon, and perhaps in some few cases the individual satellites may be larger than the moon; but no other is gxr of the size of the planet about which it revolves. There is little doubt that the moon has played an important part in the history of the earth. It is the chief cause of oceanic tides, and the tides are efficient in the wear of the shores of the oceans and in the distribution of marine sediments. Tides have probably been of importance in the earth’s history ever since the ocean came into existence.
Dependence on the sun. By far the most important external relation of the earth is its dependence on the sun, of which it is a mere satellite. Its mass is less than that of the sun, upon which it depends for nearly all its heat and light, and, through these, for nearly all of the activities that have given character to [p. 4] its history. A little heat and light are derived from other bodies, and an important source of energy is found in the interior of the earth itself; yet all of these are so far subordinate to the great flood of energy which comes from the sun that they are quite inconsequential. The dependence of the earth on the sun has been intimate throughout its past. history, and its future is locked up with the destiny of that great luminary.Geology in its broadest phases can therefore scarcely be separated from the study of the sun; but this falls in the province of the astronomer rather than the geologist.
Meteorites. There are a multitude of small bodies passing through space in varying directions and with varying velocities, and occasionally encountering the earth to which they add their substance. Some of these meteorites revolve about the sun like minute planets, but some of them come from such directions and with such velocities as to show that they do not belong to the sun’s family. Some consist almost wholly of metal, chiefly iron alloyed with a little nickel (holosiderites) ; some consist of metal and rock intimately mixed (syssiderites and sporadosiderites) ; and some consist wholly of rock (asiderites) . It is now thought that the meteorites throw some light, perhaps important light, on the early history of the earth. They are therefore of interest to the geologist. The amount of material added to the earth by their infall is now relatively slight compared with the whole body of the earth; but their contributions in the distant past may perhaps have been greater.
The constitution of the earth. The materials of the earth fall into three grand divisions: (1) The atmosphere, (2) the hydrosphere (water sphere), and (3) the lithosphere (rock sphere).
1. The Atmosphere
Though the study of the atmosphere constitutes the science of meteorology, the atmosphere is a part of the earth, and as a part of the earth, it falls within the province of geology. It is an intimate mixture of (1) all those substances that cannot take a liquid or solid state at the temperatures and pressures which prevail at the earth’s [p. 5] surface, together with (2) such transient vapors as the various substances of the earth throw off. The first class form the permanent gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen- about 21 parts, carbon dioxide about .03 part, together with small quantities of argon, and several other rare constituents. Chief among the second class is water vapor, which varies greatly in amount from time to time and from place to place. Here too, belong the gases which issue from volcanoes, and a great variety of volatile organic substances. Dust and other matter suspended in the air are usually regarded as impurities rather than constituents of the atmosphere; but they are of great importance because they affect its temperature and luminosity, and they facilitate the condensation of moisture.
Mass and extent. The total mass of the atmosphere is estimated to be tne mass of the earth. It exerts a pressure of about fifteen pounds per square inch at the sea-level. Its density decreases upward, but its actual height is not known. There is no direct evidence of its existence above a few hundred miles, but there are theoretical grounds for believing that it extends out very much farther.
Geologic activity. The atmosphere is the most mobile and active of the three great subdivisions of the earth. Its direct and indirect effects on water and rocks are so great that it must be regarded as one of the great agents of change. It acts chemically upon the rock substance of the earth, causing hardening of the rock in some cases, but more often causing disintegration, by means of which rock is reduced to soil-like material, and prepared for removal by winds and waters. When in motion, the atmosphere acts mechanically on the surface of the land, transporting dust and sand. Its greatest function, however, is in furnishing the conditions for water action. Rains, streams, glaciers, and all the various forms of moving water upon land, are dependent in one way or another on the atmosphere. On the ocean, too, wave action is dependent largely on the winds. Streams and waves, which are the most familiar agents of geologic change, are therefore to be credited as much to the atmosphere as to the hydrosphere.
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A thermal blanket. The atmosphere is a thermal blanket to the rest of the earth. In its absence the heat of the sun would reach the surface with much greater intensity than now, and it would be radiated back from the surface almost as rapidly as received. During the night, there would be a degree of cold far greater than that now known to any part of the earth. In passing through the atmosphere, certain portions of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is transformed into vibrations of lower intensity which are then more effectively retained by the atmosphere. The air thus distributes and equalizes the temperature. The two constituents of the atmosphere which are most efficient in this work are water vapor and carbon dioxide, and the climate of the earth is believed to have been very greatly affected by the varying amounts of these constituents in the atmosphere, as well as by variation in the total mass of the atmosphere.
The function of the atmosphere in sustaining life and promoting all that depends on life is obvious.
1. The Hydrosphere
The water which lies upon the surface of the solid earth, is about part of the earth’s mass. Were the solid part of the earth perfectly spheroidal, this amount of water would constitute a universal ocean a little less than two miles deep. Owing to the unevenness of the surface of the lithosphere, the water is chiefly gathered in great basins or troughs, occupying nearly three-fourths (72%) of the earth’s surface. These basins are all connected. so that anything which changes the level of the water in one, changes the level in all.
Oceanic dimensions. The surface area of the ocean is estimated at 143,259,300 square miles. The area of the true oceanic basins is only about 133,000,000 square miles, but these basins are somewhat more than full, and the ocean water laps up on the cont incut al shelves to the extent of more than 10,000,000 square miles. !f about 600 feet of the upper part of the ocean were removed, the true ocean basins would be just full. Beneath about 20% of the ocean area, the bottom sinks to depths of between 6,000 [p. 7] and 12,000 feet; under about 53% it sinks to depths of between 12,000 and 18,000 feet; and under 4% it ranges from 18,000 feet down to about 30,000.
Besides the ocean, the hydrosphere includes all the water which constitutes the surface streams and lakes, together with that which permeates the pores and fissures of the outer part of the solid earth. The water of the earth becomes a hydro_sphere_ only when the ground water is considered. All other waters of the earth are small in amount, compared with the great mass of the ocean.
Geologic activity. Of all geological agents, water is the most obvious and apparently the greatest, though its efficiency is affected by many conditions, especially the relief of the land, and temperature. Through the agency of rainfall, surface streams, underground waters, and waves, the hydrosphere is constantly modifying the surface of the lithosphere, and at the same time carrying sediment from the land and depositing it in the various basins. The hydrosphere thereby becomes the great agency for the degradation of the land and the building up of the basin bottoms. It is therefore both destructive and constructive in its action. The beds of sediment which it lays down follow one another in orderly succession, each later one lying above an earlier. In this way, they form a time record. And as relics of the life of each age become more or less embedded in the sediments, they furnish the means of following the history of life from age to age. The historical record of geology is therefore very largely dependent upon the fact that the waters have buried in systematic order, relics of the life of successive ages.
The special processes of the hydrosphere will be the subject of discussion hereafter. Suffice it here to recognize its great function in the constant degradation of the land, and in the deposition of the derived material in orderly succession in the basins.
3. The Lithosphere
The atmosphere and hydrosphere are envelopes or shells, rather than true spheres, though both penetrate the lithosphere to some extent. The lithosphere, on the other hand, is an oblate spheroid with a polar diameter of 7,899.7 miles, and an equatorial diameter [p. 8] of about 26.8 miles more. Its equatorial circumference is 24,902 miles, its meridional circumference 24,860 miles, and its surface area about 196,940,700 square miles. Its average specific gravity is about 5.57. The oblateness of the spheroid is the result of the rotation of the earth. Computations seem to indicate that the equatorial bulge is very nearly what it would be if the earth were in a liquid condition. From this the inference has been drawn that the earth was in that condition when it assumed its present form. It is thought by others, however, that the plasticity of the earth is such that it would assume this form under the influence of rotation at the present rate, even if the interior is solid.
Irregularities. It is only in a general view, however, that there is a close approximation to a perfect spheroidal surface. In detail there are very notable variations from it. The equatorial diameters are not exactly equal, and the continental protuberances are, on the average, some three miles above the bottoms of the oceans. The continental platforms and ocean basins do not correspond accurately with the present land and water surfaces, for about the continental land there are submerged borders, the continental shelves, beyond which the surface of the lithosphere descends rapidly to the depths of the ocean. The continental shelf belongs properly to the continent, but its outer edge is covered by 100 fathoms or less of water. If the upper 600 feet of the ocean were removed, the outlines of the land would correspond quite closely with the border of the true continental platforms. The agencies which produced the continental platforms and abysmal basins, and the great undulations, foldings, and volcanic extrusions of both, are yet subjects of debate.
It is customary to look upon the protrusions of the continents as the great features of the earth’s surface, but in reality the oceanic depressions are the master feature. Both in breadth and depth they much exceed the continental protrusions, and if the earth be regarded as a shrunken body, the settling of the ocean bottoms has doubtless been the greatest diastropic movement.
The following tables show the relative areas of the lithosphere
above, below, and between certain levels.[1]
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Per cent of Surface above | Per cent of Surface below | |
---|---|---|
Contour 24,000 feet above sea-level | 0.004 | 99.996 |
" . 18,000 " " " | 0.09 | 99.91 |
" 12,000 " " " | 0.7 | 99.3 |
" 6,000 " " " | 2.3 | 97.7 |
Sea-level | 27.7 | 72.3 |
Contour 6,000 feet below sea- level | 42.5 | 57.5 |
" 12,000 " " " | 57.3 | 42.7 |
" 18,000 " " " | 96.8 | 3.2 |
" 24,000 " " " | 99.93 | 0.07 |
Per cent | ||
— | — | |
More than 6,000 feet above sea-level | 2.3 | |
Between sea-level and 6,000 feet above | 25.5 | |
Between sea-level and 6,000 feet below | 14.8 | |
Between 6,000 and 12,000 feet below sea-level | 14 .8 | |
Between 12,000 and 18,000 feet below sea-level | 39.4 | |
Between 18,000 feet and 24,000 feet | 3.1 |
From these estimates it appears that if the surface were graded to a common level by cutting away the continental platforms and dumping the matter in the ocean basins, the average plane would lie somewhere near 9,000 feet below sea-level. The continental platform may be conceived as rising from this common plane rather than from the sea-level.
Epicontinental seas. Those shallow portions of the sea which lie upon the continental shelf, or extend into the interior of the continent, such as the Baltic Sea and Hudson Bay, may be called epicontinental seas, for they really lie upon the lower border of the continental platforms. Those detached bodies of water which occupy deep depressions in the surface are to be regarded as true abysmal seas. Such, for example, are the Mediterranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as low as many parts of the true ocean basin itself.
Diversities of surface. The bottoms of the oceanic basins are diversified by broad undulations which range through many thousands of feet, but they have not those irregularities of form that give variety to land surfaces. The ocean bottoms are also diversified by volcanic peaks, many of which rise to the surface and constitute isolated islands. From many of them, the solid surface slopes rapidly down to abysmal depths, so that many of the volcanic [p. 10] islands constitute peaks whose heights and slopes would seem extraordinary, if the ocean were removed.
The surface of the land is diversified in a similar way by broad undulations and volcanic peaks, and also by narrower wrinklings and foldings of the crust, and all of these irregularities have been carved into varied and picturesque forms by subaerial erosion. In this respect the surface of the land differs radically from the bed of the sea.
The outer part of the lithosphere is often called the crust of the earth. No definite lower limit can be assigned to the crust, and the former notion that it is the solid portion overlying a liquid part beneath, is now generally abandoned. The term crust as now used means the outer, cooler portion of the lithosphere. Its thickness is undefined, but it includes a shell several miles thick, at least, and perhaps a few score miles.
The interior. Concerning the great interior of the earth, little is known except by inference. From the weight of the earth,[2] it is inferred that its interior is much more dense than its surface. From its behavior under the attraction of other bodies, it is believed to be at least as rigid as steel, and its interior cannot, therefore, be liquid, in the usual sense of that term. From the phenomena of volcanoes, and from observations on temperature in deep borings, it is inferred that its interior is very hot. Further inferences concerning its character are less simply stated, and will be referred to later.