Our Rock-Mass Earth—Designed for Life
WE LIVE on the surface of an immense ball-shaped spaceship of rock and metal. If you could dig below the land surface to the depth of about twenty miles (32 kilometers) you would find nothing but lifeless rock in what is called the earth’s mantle. The crust, on which we live, made mostly of nonmetallic elements, is therefore a very thin skin on top of sextillions of tons of material we never see except for some molten rock that comes up through the crust due to volcanic action. Yet all of this is essential so that we may have a place to live.
How do geologists come to their conclusions about the earth’s composition? Actually, they have methods that probe the inside of our planet to provide a tentative description of earth’s interior, but they admit that the picture may not be accurate. No one has yet been able to dig into the mantle, even under the oceans where the crust is thinnest, only about three to five miles (5 to 8 kilometers) thick. A plan to do this was made during the International Geophysical Year (July 1957 to December 1958). You have probably read about this, the so-called “Mohole” project. “Moho” is an abbreviated term for the boundary between the crust and the mantle below. This attempt to find out the composition of the crust and just what constituted the top part of the mantle failed because of the enormous cost involved and the lack of expertness to do the job.
Earthquakes “X-ray” the Earth
Strange to say, it is earthquakes that have been most helpful in determining the structure of what lies under the earth’s surface. Knowing what is there, in turn, aids us in explaining things that make earth’s crust livable for us. The study of earthquakes is called “seismology.”
Seismologists have discovered that there are several kinds of vibrations, or waves, created during an earthquake. These waves radiate in all directions from the epicenter, the focus or place of origin of the quake. As the waves pass through the earth they curve with a curvature opposite to that of the earth’s surface, and seismograph stations miles away receive and register the waves. There are three kinds of waves: (1) the main wave, traveling along the crust, (2) a primary (“P”) wave (a push-pull type wave) and (3) a secondary (“S”) wave (a transverse wave). Both of these latter waves travel through the earth. The “P” wave is deflected about 1,800 miles (2,900 kilometers) down. The “S” wave is completely eliminated beyond this depth. This happens because they evidently meet some kind of barrier at the lower boundary of the mantle where it meets the outer core below. Another beam of the “P” wave continues through the earth’s center, though the “S” wave goes no deeper than 1,800 miles.
A Partially Liquid Core
Why is the “S” wave stopped at that 1,800-mile depth? Why also is there a deflection of one beam of the “P” wave and a slowing down of the other beam? Evidently because the outer core beneath earth’s mantle is liquid. This phenomenon can be illustrated by the fact that a hard metal object (for example, a bell) will carry a vibration better than will a soft object. The “S” wave cannot travel through a liquid and the “P” wave is either deflected or slowed down considerably. This slowing down lasts for about 1,350 miles (2,175 kilometers). The outer core, then, extending about 1,350 miles deeper toward the center of the earth, seems to be liquid, or behaves like liquid. In the great pressure and heat (about 4,000 degrees Fahrenheit [2,200 degrees Celsius] at the top of the outer core and about 9,000° F. [5,000° C.] at the bottom, where the central or inner core begins) the rock of the outer core may well be in a melted, liquid state.
Of what are the outer core and the central core composed? A study of meteorites coming to the earth from outer space suggests that these cores of the earth are mainly iron, alloyed with nickel. The earthquake wave traveling through the central core speeds up, denoting that it is solid, likely, for the most part, being extremely dense and hard.
The Crust and Its Underlying Mantle
We on the crust of the earth need a solid, stable place to live, which the earth indeed provides. But we need more than rock composed mainly of metal. These other needs the Creator provided in the earth’s crust, along with its atmosphere. The crust is made up of much lighter rock containing many elements, particularly oxygen and silicon, as well as aluminum, iron, calcium, sodium, potassium and magnesium, in order of their abundance. Oxygen and silicon, both nonmetallic elements, make up about three fourths of the total crust by weight and 93 percent by volume. Then there are water and atmosphere, which work together to break down the rocky crust into soil. But the Creator did not restrict the accomplishment of this task to mechanical means. He created the lowly earthworm, which does more for man than he can imagine.
The soil, particularly, provides elements to grow and sustain plant life, the basic food of all animal life on earth. (Gen. 1:29, 30) The other materials found in the crust, both organic and inorganic, are useful for construction, machinery, fuel, chemistry, medicine, and so forth, in countless applications.
The material underlying the crust causes constant changes in the crust by volcanic action. Convection currents beneath the ocean floor are thought to be the cause of the mid-Atlantic ridge, a mountain range about 2,000 feet (600 meters) high. Some geologists believe that the continents now bordering the Atlantic Ocean were once joined together and then gradually split apart, with the mid-Atlantic ridge centrally located between the two.
There is a theory that the water in the oceans was produced from the mantle. The reason for this idea is that the upper part of the mantle contains a mineral called serpentine, which is a combination of the mineral olivine and water. The mantle is apparently the source of the water. Someone may say, ‘But look at the tremendous volume of water in the oceans.’ True, but when we realize that all our oceans are in the crust, and that in volume the mantle is more than fifty times as great as the crust, it is seen to be easily possible.
The earth’s crust, therefore, appears to rest upon a very hot mantle, which seems to be, at least in some areas, in a plastic state, the crust, in effect, “floating” upon the mantle. In the outer five miles (8 kilometers) of the earth’s crust the temperature rises approximately 1° F. (5/9° C.) every 60 feet (18 meters) in depth. Pressures are also greater the deeper we go. Water that seeps down until it reaches very hot rocks in earth’s interior becomes superheated to 290° F. (145° C.) and, when released, flashes instantly into steam. Thus we have the geysers, such as Old Faithful in Yellowstone Park, United States, which throws thousands of gallons of water more than 100 feet (30 meters) into the air. Hot springs are formed from some of this subterranean water, and they are important to man in hot baths, laundries, heating of buildings, and as sources of steam for generation of electricity.
Pressures of gas in the crust help in bringing artesian water and oil to the surface and also provide a source of natural gas for fuel. Deep in the mantle molten rock is formed. This is called “magma” (from a Greek word meaning “dough”). It may come upward under great pressure through openings or fissures and accumulate in the crust, then burst forth in volcanic activity. Great quantities of rocks, molten lava, steam, dust and ash, as well as a number of poisonous gases, are in this way thrown out. This, while destructive, brings material to the top that enriches the crust.
So the earth under us is far from a dead, inactive “pile” of rock. If dirt and rock could be heaped up by men in mountainous proportions it would be far from the fine structure that the Creator has brought about. A man-made mountain would be a mere “heap.” But consider only one marvelous feature of a mountain made by God, namely, the supply of sparkling fresh water flowing from many springs on its slopes. What a “plumbing system”! Also, we find hot springs, soda springs, sulphur and iron water and other types of water often beneficial to health.
The Magnetic Field
Another factor apparently governed greatly by the earth’s heavy core is the earth’s magnetic field. It is thought that electrical currents in the outer core are the main cause of the magnetic field. This field surrounds the earth just as a magnetic field surrounds a bar magnet. It is essential to life, for it shields the earth from destructive radiation coming from outer space. It has to do with radio transmission, and doubtless has other yet undiscovered beneficial effects on life. Charged particles from the sun and space tend to follow the magnetic field to produce the spectacular auroral displays. Sailors and travelers have long relied on earth’s magnetism for compass directions.
The earth’s magnetic fields undergo slow changes, due to unknown causes. The location of the magnetic poles varies from time to time. (There is a difference between the “geomagnetic” north pole, which is the northern end of the earth’s magnetic field, and the north “magnetic” pole toward which the compass points. The same principle is true as to the south geomagnetic and magnetic poles.) The actual geographical North Pole (the north end of the rotational axis of the earth) is at present several hundred miles from the geomagnetic and the magnetic north poles. The actual South Pole is likewise not at the same place as the south geomagnetic and magnetic poles.
Studies of permanently magnetized rocks in various parts of the earth seem to provide evidence that, in the far, far distant past, the magnetic poles “wandered” over the earth, reversing positions several times. The reason is unknown, as are many other factors about our marvelous earth. Right now the inside of the earth presents as many mysteries as do some things about universal space. This, in fact, highlights the words of the prophet about the Creator and Sovereign, Jehovah God:
“Who has measured the waters in the mere hollow of his hand, and taken the proportions of the heavens themselves with a mere span and included in a measure the dust of the earth, or weighed with an indicator the mountains, and the hills in the scales? . . . Look! The nations are as a drop from a bucket; and as the film of dust on the scales they have been accounted. Look! He lifts the islands themselves as mere fine dust. . . . There is One who is dwelling above the circle of the earth, the dwellers in which are as grasshoppers, the One who is stretching out the heavens just as a fine gauze, who spreads them out like a tent in which to dwell.”—Isa. 40:12-22.
[Diagram on page 17]
(For fully formatted text, see publication)
3 to 20 Miles