The Western United States Part 4

Note the effect produced by drops of water falling upon a hot stove.

Each one, as it strikes, is partly changed to steam with a slight explosive sound. The result is similar when water is turned into the hot and nearly empty boiler of a steam-engine--an explosion is sure to follow.

When the pressure of steam suddenly formed within the earth is too great, a volcanic explosion takes place at some point where the overlying rocks are weakest, probably on or near one of the lines of fracture about which we have been speaking. The explosion is accompanied by thundering noises, tremblings of the earth, and the hurling of rock and molten lava into the air. That the rocks of the earth's crust are elastic is shown by the rebounding of a pebble thrown against a large boulder. If a file be drawn across the edge of a sheet of tin upon which sand has been sprinkled, the tin vibrates over its whole extent, as is shown by the jumping of the sand grains. Because of like elasticity in the materials which make up the surface of the earth, the vibrations produced by an explosion are carried through the solid earth for hundreds of miles.

[Ill.u.s.tration: FIG. 20.--EARTHQUAKE FISSURES NEAR MONO LAKE, CALIFORNIA]

The records of earthquakes show that they are much more violent and occur oftener where the crust of the earth is being disturbed by folding. We have seen that there are two main causes of earthquakes: the slipping of portions of the earth past each other along a fissure, and the contact of water with very hot rocks far below the surface.

It is probable that the earthquakes which occur so often in the western portion of the United States are due to the first of these causes. The numerous extinct volcanoes show that at one period this region was frequently shaken by explosive eruptions.

[Ill.u.s.tration: FIG. 21.--THE WASATCH RANGE

From Salt Lake City]

Mono Lake (see Fig. 42, page 99), at the eastern base of the Sierra Nevada Range, has been a centre for explosive eruptions, which were extremely violent at one time. The islands which rise in the lake are shattered, while Black Point, upon the northern sh.o.r.e, has been uplifted by an explosion from beneath, which split the rocks apart and formed deep fissures.

It is an interesting fact that in the Cordilleran region the mountains have been increasing in height in very recent years. We might almost say that they are growing to-day. In this region, then, we can actually see how mountains are made; we do not have to depend upon descriptions of the manner in which they are supposed to have been made thousands of years ago.

[Ill.u.s.tration: FIG. 22.--BLUFF FORMED BY AN EARTHQUAKE

At the foot of the Wasatch Range, Utah]

Any good map will show that the mountains of the Cordilleran region have in general a north and south direction. Their direction was determined by fissures formed long ago in the crust of the earth.

Movements have continued to take place along many of these fissures up to the present time, and probably will continue for some time to come.

In order to become better acquainted with these remarkable mountains, let us examine some of them, taking first the Wasatch Range in eastern Utah. The range has an elevation of nearly eleven thousand feet, rising gradually upon the eastern side, but presenting a bold and picturesque front upon the west, toward the plain of Great Salt Lake. A short drive from Salt Lake City brings us to the foot of the range, at the mouth of Little Cottonwood Canon.

A peculiar bluff which extends for a number of miles along the base of the mountains at once attracts our attention. The steep face of the bluff, which is from fifty to seventy-five feet high, appears to have been formed by a rising of the land upon the side next the mountains, or a dropping upon the valley side. There are reasons for believing that the formation of the bluff was due to the occurrence of an earthquake some time within the last century.

The bluff is closely related to the mighty mountains behind it. It was formed by the last of a series of movements in the earth which raised the great block known as the Wasatch Range to an elevation of six thousand feet above the plains at its base. Is it to be wondered at that disturbances of the earth which result in the erection of mountains of such height are frequently so severe as to destroy the strongest buildings?

Now let us go westward across the various parallel ranges of the Great Basin to Owens Valley at the eastern base of the Sierra Nevada mountains. This is the highest and longest continuous mountain range in the United States. For a distance of more than one hundred miles its elevation is from twelve thousand to over fourteen thousand feet.

[Ill.u.s.tration: FIG. 23.--EASTERN FACE OF THE SIERRA NEVADA MOUNTAINS

Formed by a great fracture in the earth's crust]

Owens Valley was in 1872 the centre of one of the most severe and extensive earthquakes ever recorded in the United States. The little village of Lone Pine, situated in the valley below Mount Whitney, was utterly demolished, twenty people were killed and many injured.

A portion of the valley near the village sank so low that the water flowed in and formed a lake above it. The land was so shaken up that the fields of one man were thrust into those of his neighbor.

For a distance of several hundred miles to the north along the base of the mountains the earth was fractured, and bluffs from ten to forty feet high were formed as a result either of the dropping of the surface of the valley upon the eastern side, or of the raising of the mountains upon the west.

This slipping of the earth which gave rise to the earthquake bluffs was the most recent of a long series of similar events which have raised the precipitous eastern wall of the Sierra Nevada mountains to a height of two miles above Owens Valley. If you will go out into the centre of the valley and look west toward the mountains, you will see three bluffs or scarps. The first, which is twenty feet high, was made at the time of the last earthquake; the second, known as the Alabama Hills and rising about four hundred feet, was formed at an earlier time; the third, rising back of the others, is that of the main Sierra.

Similar cliffs appear at the bases of other ranges of mountains in the Great Basin. Springs abound along these fractures in the earth, for the surface waters have an opportunity to collect wherever the rocks are broken. Numerous fertile valleys mark the line of earthquake movements, for the broken rocks and abundant springs favor rapid erosion.

Among the Coast Ranges of California there appears a series of fractures in the earth which form a line nearly four hundred miles long. They extend from a point near San Bernardino in a northwesterly direction to the neighborhood of San Francisco. Severe earthquakes have taken place along this line since the country was settled.

The pressure and grinding of the earth upon opposite sides of the fissures has formed long low ridges of earth. Small valleys have been blocked, and the old stage road from Los Angeles to Bakersfield, which followed the course of the fissures for a number of miles, has been almost obliterated.

[Ill.u.s.tration: FIG. 24.--ELIZABETH LAKE, CALIFORNIA

Occupying a valley primarily due to earthquake movements]

Hundreds of cliffs and mountain scarps throughout the West have come into existence as the results of movements such as we have been describing. Where the disturbances have been recent the mountains are bold and picturesque. Those produced in earlier times are in many instances so worn away that it is difficult to tell with certainty how they were made.

THE LAST VOLCANIC ERUPTIONS IN THE UNITED STATES

There are more volcanoes in our country than is generally supposed.

Some are very small and some rank among the greatest of mountain peaks, but all together there are many hundreds, perhaps thousands, of them. At present they are all silent and apparently dead. We are accustomed to speak of them as extinct volcanoes, but of this we must not be too sure.

They stand dark and cold, giving no clue to the nature of the forces which made them, except perhaps by the presence of an occasional hot spring and the appearance of the rocks of which they are composed.

The slag-like character of these rocks we have learned to a.s.sociate with intense heat. Some of these volcanoes are very old and have been nearly worn away; others are new and almost as perfect as when they were first made.

Where shall we go to find these volcanoes? Are there any upon the Atlantic coast or neighboring highlands? Though you may travel over all that portion of our country, you will find none, although you will discover in places, as for instance in the palisades of the Hudson, lavas which came from very ancient volcanoes, worn down so long ago that their very sites are lost to view.

If we search the Mississippi basin we find there even fewer traces of volcanic action than upon the eastern highlands. The greater portion of the vast area embraced by the Mississippi River and its tributaries has had a very uneventful history, although at times earthquakes may have occurred and the sky may have been darkened by ashes from eruptions in distant parts of the earth.

[Ill.u.s.tration: FIG. 25.--FISSURE IN THE LAVA, SHADOW MOUNTAIN

The groovings in the lava show that it was squeezed out in a half-solid condition]

It is in the country west of the Rockies, the region last to be explored and settled, that the objects of our search come to light.

Here are volcanoes and lava fields so extensive as almost to bury from sight the older surface of the earth. Some of them appear as if but yesterday they had been glowing with heat.

In the Cordilleran region Nature has carried on her work with a master hand. She has lifted the earth's crust to form a great plateau.

Portions of the plateau she has broken, projecting the fragments upward to form lofty mountains, while along the fissures thus created she has squeezed out fiery molten matter from the interior of the earth. This molten material has spread out in fields of lava or has piled itself about small openings, forming volcanic cones, which in some cases have overtopped the loftiest mountain ranges of the continent. It is believed that a number of these volcanic eruptions have occurred in the Cordilleran region of the United States since the discovery of America, and that one took place within the lifetime of many persons now living.

San Francisco Mountain, in northern Arizona, is the loftiest volcanic peak of a region dotted with volcanoes and lava flows. This great volcano, like most of its neighbors, has long been extinct, although a few miles to the eastward there appears a group of small but very new cones.

A ride of fifteen miles from the town of Flagstaff, across the forest-covered plateau, brings us to Shadow Mountain and the fields of lava and volcanic sand lying at its base. The mountain, throughout its height of over one thousand feet, is a conical aggregate of loose lapilli which give way under the feet and make climbing the peak very tiresome.

The lapilli and scoriae are slag-like fragments of lava which have been blown out of the throat of the volcano while in a hot or semi-molten condition. These fragments, as they fall back to the earth, collect about the opening and in time build up the volcano, or cinder cone, as such a mountain is frequently called. The finer particles, which have the appearance of dark sand, fall farther away and form a layer over the surface for some miles upon every side. These products of an explosive volcano are sometimes called cinders and ashes, because of their resemblance to the slag and refuse of furnaces.

In the case of the volcano which we are studying, the lapilli are so black that they give the cone the appearance of being darkened by the shadow of a cloud, and on this account the peak is named Shadow Mountain. As the days are usually bright here, the shadow effect is very striking.

[Ill.u.s.tration: FIG. 26.--EDGE OF LAVA FIELD, WITH PUMICE IN THE FOREGROUND

Near Shadow Mountain]

There are several smaller craters, east of the main one, which also threw out volcanic sand and lapilli. The surrounding hills are of volcanic origin, although very much older than Shadow Mountain.

These hills are covered with pine forests; but trees or plants have gained only slight hold upon the newer surfaces of the cinder cones, which present a picture of almost complete desolation.

There have been two other eruptions since the making of the cinder cones, and these were marked by flows of molten lava. Although the rough and rugged surface of the older flow has not yet begun to crumble and form soil, as it must do in time, yet a few trees are found here and there, reaching their roots down for the scanty nourishment to be drawn from the crevices of the rocks.

The last flow of lava, which was very small, ran into a depression in the other flow just described. This lava appears so fresh that we almost expect to find the rocks still warm. What a contrast between the wooded hillside adjoining, with its carpet of soft volcanic sand, and the jagged surface of the lava! Care must be taken in climbing over the lava, for the sharp points and angles are ever ready to tear one's shoes and hands. It cannot be many years since these hard, cold rocks formed a glowing ma.s.s of molten matter creeping quietly out of some hidden fissure which reached far down into the earth. The lava hardened as it became cold, just as does molten iron when led from the furnace to make a casting.

At one spot in the lava field stand the remains of rude stone houses built into caverns in the lava. About them are scattered pieces of broken pottery. These rude dwellings were probably occupied by some of the prehistoric people whose homes are also found along many of the streams, and in the caves of the plateau region. We can see no reason for their coming to this desolate place, so far from a water supply, unless it was that the rugged lava offered some protection from their enemies.

Now let us imagine ourselves transported to northern California.