Chapter 4

Fig. 8.—An outcrop of stratified crystalline limestone (or marble) exhibiting two small sharp folds—a syncline on the left and an anticline on the right—near Lenox, Mass, These folds developed during the great mountain-making disturbance at the end of the Ordovician period fully 20,000,000 years ago. (After Dale, U. S. Geological Survey.)

From the standpoint of our consideration of slow earth-crust movements, it is important to bear in mind that lateral pressure in the zone of flowage has not only notably deformed rocks, but that, as a result of the buckling forces, given rock masses have, in many cases, been notably shifted downward or upward—mainly upward—from their original positions. Folded strata carrying shells of sea animals are commonly found thousands of feet above sea level in many of the great mountain ranges of the world. During the process of folding on a large scale the crust of the earth is very appreciably shortened at right angles to the directionof applied pressure, due to squeezing or bending of the strata. In the case of the Appalachian mountains of Pennsylvania it has been estimated that such shortening amounts to about twenty-six miles or, in other words, that the strata originally spread out horizontally across an area whose width was about 100 miles have been squeezed or folded into an area whose width is twenty-six miles less.

Fig. 9.—Structure section showing the profile of the mountains and relations of rocks below the surface near Livingston, Montana. The strata were crowded together until they bent into great sharply defined folds at the time of the Rocky Mountain Revolution several million years ago. Then the rocks broke along the fault fracture and the mass on the right was shoved over upon the mass on the left. (After U. S. Geological Survey.)

We shall now turn to a consideration of sudden earth movements and some of their effects, including earthquakes. Mention has already been made of the fact that, when pressures and strains are set up in the outer portion (“zone of fracture”) of the earth’s crust, the rocks yield mainly by breaking or fracturing because the rocks not being under a great load of overlying material are there brittle. The earth’s crust has been fractured on small and large scales in many places during the long space of geologic time. Where one block of earth’s crust has slipped or moved past another along a fracture wehave what is called a “fault.” Such displacements of rock masses vary in amount from less than an inch to some miles, and they constitute one of the most important features of the architecture of the outer portion of the earth. There are two types of faults fundamentally different as to cause. In one type (so-called “normal fault”) the rocks suddenly yield to a force of tension; a fracture develops and the earth block on one side of the fracture or fault drops with reference to that on the other. In the other type (so-called “thrust faults”) the rocks yield suddenly to a force of compression or lateral thrust, and one block of earth is pushed or thrust partly over another along the surface of fracture or fault. (SeePlate 8.)

Faults range in length up to hundreds of miles, those from one to twenty miles in length being very common. Where an earth block has been displaced thousands of feet along a fault surface, it is not to be understood that the whole displacement resulted from a single movement, but rather from a series of sudden movements separated by greater or less intervals of time. Each sudden movement along a fault surface produces a vibration of the earth near by. Many such sudden movements are known to have caused violent earthquakes. Displacements of twenty to fifty feet, as a result of single movements, are definitely known to have taken place in various regions during the last fifty years; and rarely, if ever, has any sudden displacement of as much as several hundred feet occurred. Cliffs and steep slopes very commonly result from faulting, but, because of the long lapse of time required for the repeated movements in the case of great faults, the cliffs or steep slopes begin to wear back and becomemore or less subdued long before the last of the movements take place. In regions where movements along great faults have long since ceased, the original steep slopes may be completely obliterated by erosion.

Fig. 10.—Vertical sections through strata illustrating common kinds of faults: a, “normal faults” where one mass simply sinks below another; b, a “thrust fault” where one mass is shoved over another. (After U. S. Geological Survey.)

How does the geologist determine the actual amount of displacement, especially in the case of a large fault in stratified rocks? First, the various formations of the region, where unaffected by faulting, are carefully studied, especially in regard to the character and thickness of each, and their relative geologic ages or normal order as they were deposited one layer above the other. Then, in the simple case of a normal-fault surface at right angles to horizontal strata, it is only necessary to find out what two formations or parts of formations come together along the fault fracture, and the actual amount of displacement is readily determined. Where strata and normal fault surfaces lie at various angles, and also in thrust faults, those angles must be determined in addition to the data above named. In many mining regions, where valuable deposits are affected by faulting, accurate knowledge of the direction and amount of displacements of faults is of great economic importance.

A few examples of normal faults from well-known districts will now be briefly described. The whole eastern front of the central and southern Sierra Nevada Range of California is a great, steep fault slope, from a few thousand to ten thousand or more feet high and hundreds of miles long, of such recent geologic age that it has been only moderately affected by erosion. In fact, it is well known that the southern two-thirds of the range is a great tilted fault block, the total displacement having resulted from repeated sudden movements since about the middle of the present geologic era. A great fault also extends along the eastern base of the great Wasatch Range of Utah and the steep slope thousands of feet high is a fault scarp only slightly modified by erosion. Renewed movements along this profound fault have very recently taken place as proved by the presence of fresh fault scarps in loose deposits which have accumulated across the mouths of some of the canyons, as, for example, near Ogden. In fact, practically all of the north-south ranges of the Great Basin from Utah to California are essentially a series of tilted fault blocks. Another great fault, less conspicuous from the topographic standpoint, is hundreds of miles long in the Coast Range Mountains of California. At the time of the San Francisco earthquake of 1906 there was a renewed sudden movement along this great fracture. The eastern one-half of the Adirondack Mountains of New York is literally a mosaic of hundreds of fault blocks. Many of these faults are from two to thirty miles long and they commonly show displacements of from a few hundred to 2,000 or more feet. A glance at the geological map (in colors) of thevicinity of the great copper mines at Bisbee, Arizona, shows most of that region to contain a network of normal faults which separate it into a mosaic of fault blocks. In each of the examples of faults just given a block of earth has sunk relative to the other, or in other words, each is a “normal fault.”

We shall now turn to some large scale cases of faults in which great masses of earth have been pushed one over another—so-called “thrust faults.” In the southern Appalachian Range, and especially well exhibited in the vicinity of Rome, Georgia, one portion of the mountain mass has literally been shoved over another, at a low angle over a fault surface many miles long, for fully seven miles westward. Both the tremendous weight of rock material actually translated and the number of sudden movements required in the operation stagger the imagination. It is safe to say that during the long time of this great operation violent earthquakes were not uncommon. In the Rocky Mountains of the northern United States and southern Canada there is the greatest known thrust fault on the continent. It is hundreds of miles long, and the actual displacement is commonly at least several miles. In the Glacier National Park of Montana it has been established that the front range portion of the Rockies has actually been pushed at least seven miles, and possibly as much as twenty miles, eastward over a fault surface, and out upon the Great Plains. In some cases rocks of the Prepaleozoic Age have there been pushed upon rocks of the late Mesozoic Age, thus locally upsetting the geologic column.

Fig. 11.—East-west profile and vertical structure sections fifty-two miles long in the Mohawk Valley region of New York, showing numerous tilted fault blocks which notably influence the topography. Vertical scale exaggerated. The rocks are Prepaleozoic and early Paleozoic in age. (Modified by the author after Darton, New York State Museum.)

The Wasatch Range of Utah, in addition to the great normal fault along its western base, contains a remarkable system of thrust faults. In the region now occupied by the Wasatch Mountains a number of parallel (thrust) faults were developed close together and the broken pieces of the earth’s crust between them were pushed up, the rocks on one side of each crack riding up over those on the other side until a great mountain range was formed where once lay a plain. In the Ogden Canyon one great earth block of Prepaleozoic (Algonkian) Age has been shoved thousands of feet over late Paleozoic (Carboniferous) rock, which latter has in turn been thrust over early Paleozoic (Cambrian) rock. This thrust faulting was accomplished before the development of the geologically recent normal fault along the western base of the range.

Fig. 12.—Vertical (structure) section through a part of the earth’s crust several miles long in Ogden Canyon, Utah, showing the system of great thrust faults. Prepaleozoic (Algonkian) rocks have been pushed far over upon late Paleozoic (Carboniferous) strata, which latter have in turn been shoved over early Paleozoic (Cambrian) strata, etc. (After U. S. Geological Survey.)

Any sudden movement of part of the crust of the earth, due to a natural cause, produces a trembling or shaking called an earthquake. Though earthquakes are generally classed among the most terrifying of all natural phenomena, those which have occurred during human historic times have had scarcely any geological or topographical effects of real consequence on the face of the earth. Locally, the effects may be notable and the destruction of life and property may be great. The earth may belocally cracked and rent, small fault scarps may develop, landslides and avalanches may result from the shaking of the earth, buildings may be demolished, and sea waves may be rolled upon the land. On the other hand, many earthquakes, called “tremors,” are too slight to be noticed by people, though they are recorded by specially constructed instruments called “seismographs.” We have already stated that actual sudden displacements causing earthquakes have amounted to twenty or even fifty feet right along fault fractures, but during the vibrations or quakings, which are often so destructively sent out into the neighboring country, the earth’s surface rarely actually moves more than a small fraction of an inch. Because of the suddenness of the movement objects on the surface may be moved inches or even feet. Violent shocks may last one or two minutes and cause the whole earth to tremble, though at distant points only seismographs record the movement. It is probably true that some part of the earth is shaking all the time.

Studies during the last fifty years have made it certain that the main cause of earthquakes is the sudden slipping of earth blocks past each other along fault fractures, the sudden slipping furnishing the impulse which sends out the vibrations into the surrounding more or less elastic crust of the earth. The low rumbling to roaring sound, which sometimes immediately precedes an earthquake, is probably due to the grinding of the rocks together below the surface.

Earthquakes generally accompany volcanic outbursts of the violent or explosive type, and in such cases subterranean explosions cause both the eruptions and the quakings of the earth.It is well known that the principal volcanic districts or belts of the earth are also the belts of most frequent earthquakes, but this does not mean that volcanic action causes most of the earthquakes. Active volcanoes and earthquakes are so commonly associated in the same belts because those belts no doubt represent portions of the crust which are now most actively yielding to the forces directly resulting from the shrinkage of the earth. Within the volcanic belts many earthquakes take place unaccompanied by any volcanic action, and many others take place far from volcanoes. Some earthquakes have been caused by the impact of great landslides or avalanches, or by the sudden caving in of underground openings.

Brief descriptions of a few typical carefully studied earthquakes during recent years will serve to make the main features of earthquakes still clearer to the reader.

The violent Japanese earthquake of 1891 was caused by the sinking of a block of earth forty miles long from two to thirty feet below that on the other side of a fault fracture. There was also considerable horizontal shifting, and cracks developed in the adjacent region. A distinct fault scarp, fifteen to twenty feet high, developed, and in some cases extended right across cultivated fields.

Fig. 13.—Map of the United States, showing the large areas over which three of the greatest of our earthquakes were actually felt by people. These earthquakes were recorded in many parts of the world by delicate instruments: New Madrid, 1811; Charleston, 1886; San Francisco, 1906.

Fig. 14.—Sketch map showing the trace of the great fault fracture along which a renewed sudden movement of as much as twenty feet took place to cause the San Francisco earthquake of 1906. (After U. S. Geological Survey.)

The San Francisco earthquake of 1906 was produced by renewed movement along the great fault which extends lengthwise through the Coast Range Mountains for several hundred miles. It is literally correct to say that, for 250 miles along this great earth fracture, one part of the Coast Range instantaneously slipped from two to twenty-two feet past the other. More or less of the movement extendedat least several thousand feet down into the earth. In this case both sides slipped and the movement was very largely horizontal rather than vertical. The land on the east side of the fault moved south and that on the west side moved north, the amount diminishing away from the fault on each side so that some miles out the actual crustal movement was only a few inches. When one thinks of the tremendous volumes of earth material involved in this shifting of the earth’s crust, is it any wonder that such destructive earthquake waves were produced? Many buildings were wrecked, several hundred people were killed, the disastrous San Francisco fire resulted, water mains were broken, and fences and roads crossed by the fault were dislocated as much as fifteen to twenty feet.

Plate 5.—Swift Current Valley in Glacier National Park, Montana.This was once a deep V-shaped canyon carved out (eroded) by stream action. Then a great valley glacier slowly plowed its way through it during the Ice Age and, by ice erosion, the present nearly straight U-shaped canyon has resulted. (Photo by Campbell, U. S. Geological Survey.)

Plate 6.—View in the Yosemite Valley from Near the Western Entrance.The great rock called “El Capitan,” on the left rises 3,500 feet above the river, and Bridal Veil Falls on the right is 620 feet high. All the rock is granite, the nearly vertical walls of which have resulted from the action of a great glacier which plowed its way through the valley during the Ice Age; the valley walls have been cut back by the removal of large vertical joint blocks. The flat bottom of the valley has resulted from the filling with sediment of a postglacial lake in the valley. (Photo by F. N. Kneeland, Northampton, Mass.)

During the great earthquake on the coast of Alaska in 1899 notable changes took place along the shore for some miles, one portion having suddenly risen as much as forty-seven feet, while another portion sank below sea level.

Fig. 15.—Map showing the principal earthquake regions of the world.

In 1886 the earthquake centering near Charleston, S. C., was preceded by rumbling and roaring noises and the slight quaking increased to violent shaking which lasted more than a minute. Eight minutes later a rather violent earthquake shock took place, followed during the next ten or twelve hours by less severe shocks. Most buildings in the city werewrecked or more or less badly damaged, and some people were killed. The shocks were so violent that the quaking was actually felt by people over an area of more than 2,000,000 square miles, the disturbance having spread at the rate of about 150 miles per minute. Near Charleston openings and fissures were formed through which sand and muddy water were ejected, but the cause of the disturbance was most likely slipping of the old very hard rocks below the loose deposits of the Coastal Plain.

From 1811 to 1813 a series of violent earthquakes developed in the general vicinity of New Madrid, Missouri. In an area of over 2,000 square miles, now called the “sunk country,” many portions suddenly sank giving rise to small fault scarps or cliffs, and various lake basins were formed. Development of a fissure caused a local change in the course of the Mississippi River.

In 1897, Assam, India, was shaken by an earthquake of unusual magnitude, which lasted 21/2minutes. An area of 150,000 square miles was disastrously shaken, and the shocks were distinctly felt over an area of 750,000 square miles. A number of notable fault scarps developed, the movement on one having been thirty-five feet.

CHAPTER VIII

VOLCANOES AND IGNEOUS ROCKS

NOT only because of the great power and terrifying grandeur of violent eruptions, but also because of their destruction of life and property, volcanoes stand out in the popular mind as among the most real and important of all geological phenomena. But great volcanic outbursts, like violent earthquakes, are in truth only outward, sensible, relatively minor manifestations of the tremendous earth-changing forces below the surface. They are far less important as geological agencies than the mighty interior forces which cause parts of continents to be slowly upraised and the rocks folded, or even than the incessant action of streams whereby the lands are cut down. Even as an igneous agency, volcanoes are notably less important than the development and shifting of molten materials within the earth’s crust. Volcanic action is, however, not only conspicuous, but it is also of real significance as a means of changing the earth, such action having taken place since very early known geologic time. After bringing out the main facts and principles of volcanoes, aided by descriptions of specific eruptions, we shall turn to a consideration of igneous activity within the earth’s crust.

Mount Vesuvius in Italy is perhaps the most famous active volcano in the world. Its eruptions havebeen more or less carefully studied for a longer time than any other. The eruption in the year 79 A. D. was really a tremendous explosion causing a large part of the old crater to be blown away, and sending immense volumes of rock fragments, mostly finely divided (so-called “ashes”) into the air which completely buried the small city of Pompeii. Water from the great clouds of condensing steam, mixed with “ashes,” formed muddy floods which overwhelmed Herculaneum. Little or no lava was erupted. Since that time the crater has been more or less active and the present cone, 4,000 feet high, has been built up. During the last fifty years the greatest eruptions took place in 1872 and 1906, when, streams of molten rock flowed down the sides of the mountain.

Fig. 16.—Map showing the distribution of active and recently active volcanoes of the world.

One of the greatest volcanic explosions ever recorded was that of the island of Krakatoa, between Sumatra and Java, in 1883. The greater part of theisland was blown away and there was water 1,000 feet deep, where just before the island stood hundreds of feet high. About a cubic mile of rock material was sent into the air mostly in the form of fine dust—some of it for seventeen miles—and completely hid the sun, causing total darkness during the eruption. Dust fell over an area of several hundred thousand square miles. Several days after the explosion ships more than 1,000 miles away were dust covered. Such enormous quantities of a light porous lava called “pumice” fell and floated upon the sea that navigation was badly obstructed many miles from the volcano. Extremely fine dust gradually spread through the whole earth’s atmosphere, causing the extraordinary red sunsets for several months. The sound of the explosion was heard for hundreds of miles. Great sea waves 50 to 100 feet high were stirred up and they swept inland for several miles over the low-lying coast lands of neighboring Java and Sumatra, overwhelming hundreds of villages and drowning tens of thousands of people.

Fig. 17.—The great hole left after the top of Mt. Katmai in southern Alaska was blown off in 1912 by one of the most tremendous volcanic explosions in the annals of human history. The water in the lake is hot. (After Griggs, National Geographic Magazine.)

One of the greatest explosions on record was that of Katmai volcano, several thousand feet high, on the coast of Alaska, in June, 1912. Not only was the top of the mountain completely blown off, but also a great crater pit, three miles wide across the top and several thousand feet deep, was developed in the stump of the former mountain. Volcanic dust fell to a depth of several feet within twenty-five to fifty miles of the mountain. Dust accumulated to a depth of nearly a foot in the village of Kodiak, 100 miles east of the mountain, where total darkness prevailed for more than two days. A lake of very hot water now occupies the bottom of the great new crater. The noise of the explosion was heard for at least 750 miles.

Fig. 18.—Diagrammatic vertical or structure section through a portion of the earth illustrating the common modes of occurrence of igneous rocks. P, deep-seated (plutonic) igneous rock; S, strata; D, dikes; M, mass of igneous rock forced between strata bending them upward; F, feeding channel of volcano; V, volcano; L, lava sheets. (By the author.)

One of the most frightful volcanic catastrophes of recent years was the eruption of Mont Pelée, island of Martinique, West Indies, in 1902. In this case, also, no lava was poured out, but violent explosions sent great clouds of very highly heated gases and vapors, mingled with incandescent dust, thousands of feet into the air.One of these great clouds rushed down the mountain at hurricane speed and destroyed the city of St. Pierre with its 30,000 inhabitants. After the main eruption a spine or core of hard rock began to rise out of the crater and it slowly grew to a height of 1,000 feet in several months, after which it began to crumble away. This spine probably represented nearly frozen lava which solidified as it was gradually forced out of the mountain.

Of special interest to us, though not of great importance is the only active volcano in the United States. In May, 1914, Mount Lassen (or Lassen Peak), a long inactive volcano in northern California, suddenly burst forth explosively and during the next several years hundreds of eruptions occurred. Little or no lava appeared, but great clouds of steam and dust often shot into the air from one to three miles above the top of the mountain, which lies over 10,000 feet above sea level. (Plate 10.) Great quantities of dust have accumulated for miles around the mountain. At this writing (October, 1920) the mountain is again active.

It should not be presumed, however, that all, or nearly all, volcanoes are of the explosive type. Others of the more quiet type are well exemplified by the two great Hawaiian volcanoes, Mauna Loa and Kilauea. Any but relatively very minor explosions rarely, if ever, occur, the product of such volcanoes being almost wholly lava, which flows down the mountainsides in molten streams. The Hawaiian Islands have, in fact, been almost entirely built up by successive eruptions of lava, the building-up process having begun well below sea level. Mauna Loa rises to nearly 14,000 feet above the sea, but,due to the fact that the streams of lava have spread so far, the mountain has an exceptionally low angle of slope which makes it difficult to realize that it is so high. Considering its submarine portion, Mauna Loa really rises nearly 30,000 feet above the sea floor. Although Kilauea lies nearly 4,000 feet above sea level on the flank of Mauna Loa, and only twenty miles distant from it, the two volcanoes are singularly independent in regard to their eruptions. Each mountain has a crater irregularly oval in shape, nearly three miles long, bounded by almost vertical walls of hard lava, in some cases arranged in terraces. The floors of the great crater pits are relatively level, and consist of black lava in which are lakes of molten and even boiling lava. The black lava floor is, in each case, only a frozen or hardened crust upon a great column of molten lava extending down into the mountain. Prior to an eruption of Mauna Loa the lava column rises hundreds of feet in the crater, but during recent years the lava seldom, if ever, flows out over the crater rim. Instead, it breaks through the mountainsides at various altitudes, the great flow of 1919 having started at an altitude of about 8,000 feet. This stream of liquid rock, fully one-half of a mile wide, flowed for weeks down the mountainside and into the ocean, the waters of which, in contact with the highly heated lava, were thrown into terrific commotion. In 1885 a stream of lava several miles wide flowed forty-five miles. In one case, lava traveled the first fifteen miles in two hours, but this is an unusually great rate of speed. Lava streams in general seldom move faster than one or two miles per hour, and as the liquid rock gradually cools and becomes more and more viscous, the speed diminishesto zero. Almost incredible volumes of steam emanate from streams of molten lava.

In 1840 an outflow of lava took place from the side of Kilauea Mountain and ran into the sea. Since that time the floor of the great crater pit (quoting Professor W. H. Hobbs) “has been essentially a movable platform of frozen lava of unknown and doubtless variable thickness which has risen and descended (hundreds of feet) like the floor of an elevator car between its guiding ways. The floor has, however, never been complete, for one or more open lakes are always to be seen, that of Halemaumau, located near the southwestern margin, having been much the most persistent. Within the open lakes the boiling lava is apparently white hot at a depth of but a few inches below the surface, and in the overturnings of the mass these hotter portions are brought to the surface and appear as white streaks marking the redder surface portions. From time to time the surface freezes over, the cracks open and erupt at favored points along the fissures, sending up jets and fountains of lava, the material of which falls in pasty fragments that build up driblet cones. Small fluid clots are shot out, carrying threadlike lines of lava glass behind them, the well-known “Pelée’s hair.” Sometimes the open lakes build up congealed walls, rising above the general level of the pit, and from their rim the lava spills over in cascades to spread out upon the frozen floor.”

In some regions, like the Columbian Plateau of the northwestern United States and the Deccan of India, each covering about 200,000 square miles, vast quantities of lava have been poured out layer upon layer to depths of even thousands of feet. Distinctvolcanic cones or mountains in those regions are either absent or too scarce to look to as sources of so much lava. Such lava floods were probably mostly erupted from great fissures in the earth’s crust, the fluidity to spread many miles.

Some idea of the quantitative geological importance of volcanism may be conveyed to the reader when we assert that, according to a conservative estimate, fully one-half of a million cubic miles of molten rocks have been poured out upon the surface of the earth through volcanic action in relatively recent geological time! The Cascade Range with its lofty peaks, including Mount Shasta and Mount Rainier, each rising more than 14,000 feet above the sea, has been built up very largely by volcanic action during the last era of geologic time. Many other mountain peaks and various ranges have been similarly developed either wholly or in part. The great chain of Aleutian Islands extending hundreds of miles into the sea, is the scene of much volcanic activity where a great mountain range is now literally being born out of the sea by the processes of vulcanism.

Before this the reader has more than likely wondered about the source of the heat, vapors (mainly water), and power involved in volcanic action. Answers to these questions are closely tied up with the precise cause (or causes) of volcanic action which remains one of the most uncertain of the larger problems of geologic science. Before briefly discussing the causes, a few additional facts should be stated. First, in regard to the heat, a careful determination of the temperature of the molten lava of Kilauea in 1911 showed it to be 1,260 degrees Centigrade, or 2,300 degrees F. This is, however,a relatively low temperature, because many lavas from other regions show melting points all the way up to at least 2,000 degrees Centigrade (3,600 degrees F.). Water in the form of steam is quantitatively one of the greatest products of volcanoes. A fair idea of the tremendous volumes of water involved may be gained from the statement that a careful estimate shows that fully 460,000,000 gallons of water in the form of steam erupted from a single secondary cone of Mount Etna during a period of 100 days. Among other gases which are given off in greater or less amounts during volcanic activity are carbonic acid gas, sulphureted hydrogen, sulphur dioxide, and hydrochloric acid. Some idea of the power back of volcanoes may be gained not only from the tremendous explosions such as those above described, but also from the fact that the pressure necessary to raise the column of lava from sea level to the top of Mauna Loa (nearly 14,000 feet) is about 1,150 atmospheres, or about 17,000 pounds per square inch. The actual pressure must there be much greater because the lava is forced up from far below sea level.

A long-held idea that a relatively thin crust covers a molten interior, and that downward pressure of this crust due to earth contraction causes molten rocks to be forced out, has been too thoroughly disproved to now be at all seriously entertained. The fact that near-by volcanoes commonly erupt entirely independently, as in the case of Mauna Loa and Kilauea, shows that there can be no universal liquid beneath a relatively thin crust. Other arguments against liquidity of the earth’s interior are that the earth acts like a body nearly as rigid as steel against the powerful tide-producing forces, and that earthquakewaves which pass through the earth to a depth of at least 2,000 miles are the kind which require a solid medium for transmission.

Let us then briefly consider more plausible views regarding the cause of volcanic action. First of all we may be sure that the earth is highly heated inside. Measurements in many deep borings show that the temperature increases at the rate of about 1 degree F. for each 50 to 60 feet downward, to depths greater than a mile. Accordingly, on the basis of 1 degree rise in 50 feet, at depths of 20 to 35 miles, the temperature must be great enough (2,120 degrees to 3,590 degrees F.), to cause all ordinary rocks to meltif they were at the surface. At such depths, however, the downward pressure upon the rocks is so great that their melting points are notably raised, and there is every reason to believe that under ordinary conditions the rocks 20 to 35 miles down are not molten. If we adhere to the older (nebular) hypothesis of earth origin, the interior heat of the earth is left over from the cooling, once molten, earth. On the basis of another (planetesimal) hypothesis, the earth’s heat is due to the steady, powerful action of gravity causing the earth to contract. In any case, the earth is hot inside as proved by deep well records and igneous phenomena in general, and it is a contracting or shrinking body as proved by the many large scale zones of wrinkling or folding of rocks. If, then, highly heated solid rocks at reasonable distances down in any part of the earth are subjected to relief of pressure by an earth movement such as upward crumpling of the crust, or by readjustment of large fault blocks, such heated solid rocks would become molten. The very earth movement whichbrings about relief of pressure and melting may very reasonably be regarded as the power which forces some of the newly formed molten material higher up into the earth’s crust, and even out upon the surface. This view harmonizes with the well-known fact, already mentioned, that the main belts of active volcanoes are also the main belts of active earth movements, such as earthquakes.

Another source of power behind volcanic action is steam pressure. We have already mentioned the fact that vast amounts of water in the form of steam escape from volcanoes or even from streams of molten lava. The violent volcanic explosions are quite certainly all, or nearly all, direct results of sudden giving way of volcanoes to steam pressure which accumulates during greater or less periods of time, and with little or no possibility of escape, without rupturing the mountain. Steam alone, or combined with some of the other gases so common as volcanic products, may also aid in forcing out molten rock. What is the source of the steam and other gases or vapors? According to one view they were originally in the earth, while according to another view the water at least has been absorbed by the molten rocks from surface waters which worked their way downward. At least two arguments oppose the second hypothesis: first, that not a few volcanoes are really many miles from the sea or other bodies of water, while downward percolation of rain water would fall far short of supplying the tremendous quantities of water ejected, and second, any water taken up by molten rock must be absorbed within a very few miles of the surface because, as we have learned, farther down there are no openings large enough to permit thedownward passage of water, but, as a matter of fact, the very upper part of the earth’s crust is just the place where molten rocks begin to give up their water, often with terrific violence.

We may now turn to a consideration of the other very important kind of igneous activity, namely, the rise and transfer of molten materials within the earth’s crust, but not to the surface. The quantity of such deep-seated (so-called “plutonic”) igneous rock material which has been intruded into the earth’s crust within known geologic time, is far greater than that which has been forced to surface, that is the so-called “volcanic” material. The plutonic rocks are always thoroughly crystallized, and they are generally coarser grained than the volcanic rocks.

Where molten materials have been forced into cracks or fissures in the crust of the earth and there congealed, we have a very common mode of occurrence called “dikes” (Plate 9). In many regions often one set of dikes was formed, after which one or more succeeding injections from the same or different deep-seated bodies of molten rock took place, and some of the later dikes were forced to cut across earlier ones. Dikes of all lengths up to at least thirty miles, and of all widths up to many hundreds of feet, are known, but they are generally less than a mile long and not more than a few feet or rods wide. They have been intruded into all kinds of rock formations—igneous, sedimentary, and metamorphic. Dikes are common in many parts of the world and they often excite the interest of lay-men. They are wonderfully displayed along the southern coast of Maine.Plate 9shows small dikes where the molten material was forced from a largermass into a body of older dark rock. The Palisades of the Hudson River, just north of New York City, consists of a layer of igneous rock several hundred feet thick which, in the molten condition, was forced nearly horizontally between layers of sandstone millions of years ago, that is in the early Mesozoic era. The palisade or columnar structure was caused by cracking of the rock during the cooling and contraction. This is the explanation of most columnar structures of igneous rocks, exceptionally fine exhibitions being at the Giant’s Causeway in Ireland, and Devil’s Tower, Wyoming (Plate 10).

A type of occurrence not so common, but of special interest, is where a body of molten rock rising in nearly horizontal strata becomes cooler and therefore stiffer or more viscous and, losing its power to penetrate, forces its way between the layers causing the strata to be arched or domed over it. Sufficient removal of overlying material by erosion has revealed many fine examples of this type of occurrence.

Another type of interest is the volcanic neck, which is the core or plug filling the feeding channel of a volcano. In certain regions, like parts of Arizona and New Mexico, extinct volcanic mountains may be all cut away by erosion, except the central cores or necks which, both because they are more resistant and are last to be reached by erosion, stand out conspicuously as great towers on the landscape (Plate 9).

Most important of all from the quantitative standpoint, however, are the great bodies of igneous rocks, ranging up to many miles across, which, in a molten condition, were forced irregularly into the earth’s crust from unknown depths.

The common rock called granite belongs in this category of rocks, which are the best and most extensively developed of all igneous types. The roots or cores of great mountain ranges often consist of such rocks which are exposed to view only after removal of great thickness of overlying material. Immense areas of granite and other plutonic rocks of extra deep-seated origin are exposed, because of removal of overlying material by erosion, in southeastern Canada, the Adirondack Mountains, New England, the Piedmont Plateau of the Atlantic Coast, and in the Sierra Nevada Mountains. All the rock forming the lofty walls of Yosemite Valley is granite, which was forced into the earth’s crust in relatively late Mesozoic time, and which has since been laid bare by erosion.

CHAPTER IX

WATERS WITHIN THE EARTH

IT has been estimated that approximately 1,500 cubic miles of water fall upon the surface of the United States each year. About one-half of this goes back into the atmosphere by evaporation; about one-third of it flows away in surface streams; and the remaining one-sixth enters the crust of the earth. Considerable water which enters the earth returns to the surface as springs, by capillarity of soils and rocks, or by being drawn up into plants and evaporated. Some idea of the amount of ground water may be gleaned from the statement, based upon a careful estimate, that all the water in the rocks and soils of the first 100 feet below the surface of the United States would make a layer seventeen feet thick. In most humid regions the soils and loose rock formations are saturated with water at greater or less depths (usually less than 100 feet) below the surface. The surface of this saturated layer is called the ground-water level, or more familiarly the “water table.” The water table shifts up and down more or less according to variation in rainfall.

In addition to the water held in the loose rocks and soils near the earth’s surface, large quantities occur in definite layers (usually strata) of porous rocks which very commonly extend at various angles, hundreds or even thousands of feet into the earth. A very fine illustration of this principle is the caseof the Dakota sandstone formation of Nebraska. Almost anywhere across the State a well drilled through a bed of clay and into the porous sandstone layer encounters water. (Figure 19.) Another principle is also well illustrated, namely, that water in such a porous layer may actually travel hundreds of miles, water obtained from a well sunk to the Dakota sandstone having actually traveled under the surface of the State all the way from the eastern face of the Rocky Mountains, where rain and melting snow entered the upturned and exposed porous rock layer. Another good example is Iowa, where certain porous rock layers outcropping in the northwestern and northeastern corners of that and adjacent States gradually bend down under the State, reaching the greatest depths (up to 3,000 feet) far in the interior. From wells 3,000 feet deep near Boone, Iowa, it is, therefore, a fact that some of the water pumped out of the earth actually traveled underground all the way from beyond the corners of the State. This sort of travel of underground water is common in many parts of the world. It should be clearly understood that such water does not flow freely as in a pipe along subterranean passageways, but rather it slowly works its way between the grains of porous rock. Where such water moves distinctly downward, and the porous layer has both above and below it an impervious rock layer like shale or clay, it gradually gets under greater and greater pressure. In some cases such pressure has actually been found by deep drilling to be equivalent to that of a column of water several thousand feet high. The rate of motion of water in porous underground rock layers is very slow, data from various sources indicating a rate of speed of notmore than one-fifth of a mile a year in coarse porous sandstone, while in many rocks it cannot be more than ten to fifty feet per year.

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