Read an Excerpt
By Frank H. T. Rhodes, Raymond Perlman
St. Martin's PressCopyright © 1972 St. Martin's Press
All rights reserved.
GEOLOGY AND OURSELVES
Geology is the study of the earth. As a science, it is a newcomer in comparison with, say, astronomy. Whereas geology is only about 200 years old, astronomy was actively studied by the Egyptians as long as 4,000 years ago. Yet speculation about the earth and its activities must be as old as the human race. Surely, primitive people were familiar with such natural disasters as earthquakes and volcanic eruptions.
Gradually, human society became more dependent upon the earth in increasingly complex ways. Today, behind the insulation of our modern living conditions, civilization remains basically dependent upon our knowledge of the earth. All our minerals come from the earth's crust. Water supply, agriculture, and land use also depend upon sound geologic information.
Geology stimulates the mind. It makes use of almost all other sciences and gives much to them in return. It is the basis of modern society.
THE BRANCH OF GEOLOGY emphasized here is physical geology. Other Golden Guides of this series, Rocks and Minerals and Fossils, deal with the branches of mineralogy, petrology, and paleontology.
PHYSICAL GEOLOGY is the overall study of the earth, embracing most other branches of geology but stressing the dynamic and structural aspects. It includes a study of landscape development, the earth's interior, the nature of mountains, and the composition of rocks and minerals.
HISTORICAL GEOLOGY is the study of the history of earth and its inhabitants. It traces ancient geographies and the evolution of life.
ECONOMIC GEOLOGY is geology applied to the search for and exploitation of mineral resources such as metallic ores, fuels, and water.
STRUCTURAL GEOLOGY (tectonics) is the study of earth structures and their relationship to the forces that produce the structures.
GEOPHYSICS is the study of the earth's physical properties. It includes the study of earthquakes (seismology) and methods of mineral and oil exploration.
PHYSICAL OCEANOGRAPHY is closely related to geology and is concerned with the seas, major ocean basins, seafloors, and the crust beneath them.
THE SIZE AND SHAPE OF THE EARTH were not always calculated accurately. Most ancient peoples thought the earth was flat, but there are many simple proofs that the earth is a sphere. For instance, as a ship approaches from over the horizon, masts or funnels are visible. As the ship comes closer, more of its lower parts come into view. Final proof, of course, was provided by circumnavigating the globe and by photographs taken from spacecraft.
The Greek geographer and astronomer Eratosthenes was probably the first (about 225 B.C.) to measure successfully the circumference of the earth. The basis for his calculations was the measurement of the elevation of the sun from two different points on the globe. Two simultaneous observations were made, one from Alexandria, Egypt (Point B, seen here), and the other from a site on the Nile near the present Aswan Dam (Point A). At the latter point, a good vertical sighting could be made, as the sun was known to shine directly down a well at noon on the longest day (June 23) of the year.
Eratosthenes reasoned that if the earth were round, the noonday sun could not appear in the same position in the sky as seen by two widely separated observers. He compared the angular displacement of the sun (Y) with the distance between the two ground sites, A and B.
RECENT DATA from orbiting earth satellites have confirmed that the earth is actually slightly flattened at the poles. It is an oblate spheroid, the polar circumference being 27 miles less than at the equator. The following measurements are currently accepted:
Avg. diameter 7,918 mi.
Avg. radius 3,959 mi.
Avg. circumference 24,900 mi.
LARGE AS THE EARTH IS, it is minute in comparison with the universe, where distances are measured in light years — the distance light, moving at 180,000 miles per second, travels in a year. This is about 6 billion miles or 10 million, million kilometers. Using these units of measurements, the moon is 1.25 light seconds from the earth, the sun is 8 light minutes from the earth, and the nearest star is 4 light years from the earth. Our galaxy is 80,000 light years in diameter. The most distant galaxies are 8 billion light years from earth. It is estimated that there are at least 400 million galaxies "visible" from earth using radio telescopes and similar means of detection. Galaxies are either elliptical or spiral in shape.
ERATOSTHENES measured the distance (X) between Points A and B as 5,000 stadia (about 575 miles). Although the observer at Point A saw the sun directly overhead at noon, the observer at B found the sun was inclined at an angle of 7° 12' (Y) to the vertical. Since a reading of 7° 12' corresponds to one-fiftieth of a full circle (360°), Eratosthenes reasoned that the measured ground distance of 5,000 stadia must represent one-fiftieth of the earth's circumference. He calculated the entire circumference to be about 28,750 miles.
THE EARTH'S SURFACE, for the purpose of measurements, is commonly assumed to be uniform, for mountains, valleys, and ocean deeps, great as some are, are relatively insignificant features in comparison with the diameter of the earth.
But mountains are not insignificant to humans. They play a major role in controlling the climate of the continents; they have profoundly influenced the patterns of human migration and settlement.
Mountain ranges, with very few exceptions, are narrow, arcuate belts, thousands of miles in length, generally developed on the margins of the ancient cores or shields of the continents. They consist of great thicknesses of sedimentary and volcanic rocks, many of them of marine origin. Their intense folding and faulting are evidence of enormous compressive forces.
Mountains are not limited to the land. The ocean floor has even more relief than the continents. Most of the continental margins extend as continental shelves to a depth of about 600 feet below the level of the sea, beyond which the seafloor (continental slope) plunges abruptly down (see here).
The ocean floor adjacent to some islands and continents has long, deep trenches, the deepest of which, off the Philippines, is about 6 ½ miles deep (here). A worldwide network of mid-oceanic ridges, of mountainous proportions, encircles the earth. This network has geophysical and geologic characteristics that suggest it occupies a unique role in earth dynamics — that along these ridges new seafloor is constantly being created.
THE EARTH'S CRUST, derived from the denser, underlying mantle (see here), consists of two kinds of rock. The continental crust differs from the oceanic crust in being lighter (2.7 gm./cc. compared with 3.0), thicker (35 km-70 km versus 6 km), older (up to 3.5 billion years versus a maximum of 200 million years), chemically different, and much more complex in structure. These differences reflect the different modes of formation of the two kinds of crust (see here).CHAPTER 2
THE EARTH IN SPACE
EARTH is one of nine planets revolving in nearly circular (elliptical) orbits around our star, the sun. Earth is the third planet out from the sun and the fifth largest planet in our solar system, having a diameter of about 7,918 miles. It completes one orbit around the sun in about 365 ¼ days, the length of time that gives us our unit of time called a year.
PLANETS vary in size, composition, and orbit. Mercury, with a diameter of 3,112 miles, is the planet nearest to the sun. It orbits the sun in just three earth months. Jupiter, about ten times the diameter of Earth (83,000 miles), is the largest planet and fifth in distance from the sun, taking about 11 ¾ earth years to orbit the sun. Pluto, the most distant planet, takes about 247 ¾ earth years to orbit around the sun. The inner planets have densities, and probably compositions similar to Earth's; outer planets are gaseous, liquid, or frozen hydrogen and other gases.
THE SUN, an average-sized star, makes up about 99 percent of the mass of the solar system. Its size may be illustrated by visualizing it as a marble. At this scale, the earth would be the size of a small grain of sand one yard away. Pluto would be a rather smaller grain 40 yards away.
SATELLITES revolve around seven planets. Including the earth's moon, there are 61 satellites altogether; Mars has 2, Jupiter 16, Saturn 18, Uranus 15, Neptune 8, and Pluto 1.
COMETS are among the oldest members of the solar system. They orbit the sun in extremely long, elliptical orbits. As comets approach the sun, their tails begin to glow from friction with the solar wind.
THE MOON, earth's natural satellite, has about ¼ the diameter, 1/81 the weight, and the density of our planet. The moon completes one orbit around the earth every 27? days. It takes about the same length of time, 29 ½ days, to rotate on its own axis; hence, the same side, with an 18% variation, always faces us. The moon's surface, cratered by meteorite impact, consists of dark areas (maria) which are separated by lighter mountainous areas (terrae). Terrae are part of the original crust, formed about 4.5 billion years ago; maria are basins, excavated by meteorite falls, filled by basaltic lavas formed from 3 to 4 billon years ago.
ASTEROIDS, the so-called minor planets, are rocky, airless, barren, irregularly shaped objects that range from less than a mile to about 480 miles in diameter. Most of the asteroids that have been charted travel in elongated orbits between Mars and Jupiter. The great width of this zone suggests that the asteroids may be remnants of a disintegrated planet formerly having occupied this space.
METEORS, loosely called shooting stars, are smaller than asteroids, some being the size of grains of dust. Millions daily race into the earth's atmosphere, where friction heats them to incandescence. Most meteors disintegrate to dust, but fragments of larger meteors sometimes reach the earth's surface as "meteorites." About 30 elements, closely matching those of the earth, have been identified in meteorites.
THE EARTH'S MOTIONS determine the daily phenomenon of day and night and the yearly phenomenon of seasonal changes. The earth revolves around the sun in a slightly elliptical orbit and also rotates on its own axis. Since the earth's axis is tilted about 23 ½° with respect to the plane of the orbit, each hemisphere receives more light and heat from the sun during one half of the year than during the other half. The season in which a hemisphere is most directly tilted toward the sun is summer. Where the tilt is away from the sun, the season is winter.
RELATIVE MOTIONS OF THE EARTH
REVOLUTION is earth's motion about the sun in a 600-million-mile orbit, as it completes one orbit about every 365 ¼ days traveling at 66,000 mph.
ROTATION is a whirling motion of the earth on its own axis once in about every 24 hours at a speed of about 1,000 mph at the equator.
NUTATION is a daily circular motion at each of the earth's poles about 40 ft. in diameter.
PRECESSION is a motion at the poles describing one complete circle every 26,000 years due to axis tilt, caused by gravitational action of the sun and moon.
OUR SOLAR SYSTEM revolves around the center of our Milky Way Galaxy. Our portion of the Milky Way makes one revolution each 200 million years.
GALAXIES seem to be receding from the earth at speeds proportional to their distances.
THE SUN is the source of almost all energy on earth. Solar heat creates most wind and also causes evaporation from the oceans and other bodies of water, resulting in precipitation. Rain fills rivers and reservoirs, and makes hydroelectric power possible. Coal and petroleum are fossil remains of plants and animals that, when living, required sunlight. In one hour the earth receives solar energy equivalent to the energy contained in more than 20 billion tons of coal, and this is only half of one billionth of the sun's total radiation.
Just a star of average size, the sun is yet so vast that it could contain over a million earths. Its diameter, 864,000 miles, is over 100 times that of the earth. It is a gaseous mass with such high temperatures (11,000° F at the surface, perhaps 325,000,000°F at the center) that the gases are incandescent. As a huge nuclear furnace, the sun converts hydrogen to helium, simultaneously changing four million tons of matter into energy each second.
THE MILKY WAY, like many other galaxies, is a whirling spiral with a central lens-shaped disc that stretches into spiral arms. Most of its 100 billion stars are located in the disc. The Milky Way's diameter is about 80,000 light years; its thickness, about 6,500 light years. (A light year is the distance light travels in one year at a velocity of 186,000 mi. per sec., or a total of about 6 trillion miles.)
GALAXIES are huge concentrations of stars. Within the universe, there are innumerable galaxies, many resembling our own Milky Way. Sometimes called extragalactic nebulae or island universes, these star systems are mostly visible only by telescope. Only the great spiral nebula Andromeda and the two irregular nebulae known as the Magellanic Clouds can be seen with the naked eye. Telescopic inspection reveals galaxies at the furthermost limits of the observable universe. All of these gigantic spiral systems seem to be of comparable size and rotating rapidly. Nearly 50 percent appear to be isolated in space, but many galaxies belong to multiple systems containing two or more extragalactic nebulae. Our galaxy is a member of the Local Group, which contains about a dozen other galaxies. Some are elliptical in shape, others irregular. Galaxies may contain up to hundreds of billions of stars and have diameters of up to 160,000 light years. Galaxies are separated from one another by great spaces, usually of about 3 million light years.
Many galaxies rotate on their own axes, but all galaxies move bodily through space at speeds of up to 100 miles a second. In addition to this, the whole universe seems to be expanding, moving away from us at great speeds. Our nearest galaxy, in Andromeda, is 2.2 million light years away.
About 100 million galaxies are known, each containing many billions of stars. Others undoubtedly lie beyond the reach of our telescopes. It seems very probable that many of the stars the galaxies contain have planetary systems similar to our own. It has been estimated that there may be as many as 10 of these. Chances of life occuring on other planets would, therefore, seem very high, although it may not bear an exact resemblance to life on earth.
THE CHEMICAL ELEMENTS are the simplest components of the universe and cannot be broken down by chemical means. Ninety-two occur naturally on earth, 70 in the sun. They develop from thermonuclear fusion within the stars, in which the elementary particles of the lightest elements (hydrogen and helium) are transformed into heavier elements.
THE ORIGIN OF THE UNIVERSE is unknown, but all the bodies in the universe seem to be retreating from a common point, their speeds becoming greater as they get farther away. This gave rise to the expanding-universe theory, which holds that all matter was once concentrated in a very small area. Only neutrons could exist in such a compact core. According to this theory, at some moment in time — at least 5 billion years ago — expansion began, the chemical elements were formed, and turbulent cells of hot gases probably originated. The latter separated into galaxies, within which other turbulent clouds formed, and these ultimately condensed to give stars. Proponents refer to this as the "Big Bang" theory, a term descriptive of the initial event, perhaps as long as 10-15 billion years ago.
THE ORIGIN OF THE SOLAR SYSTEM is not fully understood, but the similar ages of its components (Moon, meteorites, Earth at about 4.5-4.6 billion years) and the similar orbits, rotation, and direction of movement around the sun, all suggest a single origin. The theory currently most popular suggests that it formed from a cloud of cold gas, ice, and a little dust, which began slowly to rotate and contract. Continuing rotation and contraction of this disc-shaped cloud led to condensation and thermonuclear fusion — perhaps triggered by a nearby supernova, from which stars such as the sun were formed. Collision of scattered materials in the disc gradually led to the formation of bodies — planetismals — which became protoplanets. The growing heat of the sun probably evaporated off the light elements from the inner planets (now represented by the dense, rocky "terrestrial" planets—Mercury, Venus, Earth, Mars, and the Moon). The outer planets, because of their greater distance from the sun, were less affected and retained their lighter hydrogen, helium, and water composition. Perhaps they formed from mini-solar-planet systems within the larger disc. This composition may well reflect that of the parent gas cloud.
Excerpted from Geology by Frank H. T. Rhodes, Raymond Perlman. Copyright © 1972 St. Martin's Press. Excerpted by permission of St. Martin's Press.
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