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The Age of the Earth
The age of the earth has aroused the interest of scientists, clergy, and laymen. The first scientists to attack the problem were physicists, basing their estimates on assumptions that are not now generally accepted. G. H. Darwin calculated that 57 million years had elapsed since the moon was separated from the earth, and Lord Kelvin estimated that 20 - 40 million years were needed for the earth to cool from a molten condition to its present temperature. Although these estimates were much greater than the 6,000 years decided upon some two hundred years earlier from a Biblical study, geologists thought the earth was much older than 50 or 60 million years. In 1899 the physicist Joly calculated the age of the ocean from the amount of sodium contained in its waters. Sodium is dissolved from rocks during weathering and carried by streams to the ocean. Multiplying the volume of water in the ocean by the percentage of sodium in solution, the total amount of sodium in the ocean is determined as 16 quadrillion tons. Dividing this enormous quantity by the annual load of sodium contributed by streams gives the number of years required to deposit the sodium at the present rate. This calculation has been checked by Clark and by Knopi with the resulting figure in round numbers of 1,000,000,000 years for the age of the ocean. This is to be regarded as a minimum age for the earth, because all the sodium carried by streams is not now in the ocean and the rate of deposition has not been constant. The great beds of rock salt (sodium chloride), now stored as sedimentary rocks on land, were derived by evaporation of salt once in the ocean. The annual contribution of sodium by streams is higher at present than it was in past geological periods, for sodium is now released from sedimentary rocks more easily than it was from the silicates of igneous rocks before sedimentary beds of salt were common. Also, man mines and uses tons of salt that are added annually to the streams. These considerations indicate that the ocean and the earth have been in existence much longer than 1,000,000,000 years, but there is no quantitative method of deciding how much the figure should be increased.
Geologists have attempted to estimate the length of geologic time from the deposition of sedimentary rocks. This method of measuring time was recognized about 450 B.C. by the Greek historian Herodotus after observing deposition by the Nile and realizing that its delta was the result of repetitions of that process. Schuchert has assembled fifteen such estimates of the age of the earth ranging from 3 to 1,584 million years with the majority falling near 100 million years. These are based upon the known thicknesses of sedimentary rocks and the average time required to deposit one foot of sediment. The thicknesses as well as the rates of deposition used by geologists in making these estimates vary. Recently Schuchert has compiled for North America the known maximum thicknesses of sedimentary rocks deposited since the beginning of Cambrian time and found them to be 259,000 feet, about 50 miles. This thickness may be increased as other information accumulates, but the real difficulty with the method is to decide on a representative rate of deposition, because modern streams vary considerably in the amount of sediment deposited. In past geological periods the amount deposited may have varied even more, depending on the height of the continents above sea level, the kind of sediment transported, and other factors. But even if we knew exact values for the thickness of PreCambrian and PostCambrian rocks and for the average rate of deposition, the figure so obtained would not give us the full length of time involved. At many localities the rocks are separated by periods of erosion called unconformities, during which the continents stood so high that the products of erosion were carried beyond the limits of the present continents and "lost intervals" of unknown duration were recorded in the depositional record. It is also recognized that underwater breaks or diastems caused by solution due to acids in sea water and erosion by submarine currents may have reduced the original thickness of some formations. Geologists appreciated these limitations and hoped that a method would be discovered which would yield convincing evidence of the vast time recorded in rocks.
Unexpected help came from physicists studying the radioactive behavior of certain heavy elements such as uranium, thorium, and actinium. These elements disintegrate with the evolution of heat and give off particles at a constant rate that is not affected by high temperatures and great pressures. Helium gas is liberated, radium is one of the intermediate products, and the stable end product is lead with an atomic weight different from ordinary lead. Eight stages have been established in the radium disintegration series, in which elements of lower atomic weights are formed at a rate which has been carefully measured. Thus, uranium with an atomic weight of 238 is progressively changed by the loss of positively charged helium atoms each having an atomic weight of 4 until there is formed a stable product, uranium lead with an atomic weight of 206. Knowing the uranium-lead ratio and the rate at which atomic disintegration proceeds, it is possible to determine the time when the uranium mineral crystallized and the age of the rock containing it. By this method the oldest rock, which is of Archeozoic age, is 1,850,000,000 years old, while those of the latest Cambrian are 450,000,000 years old. Allowing time for the deposition of the early Cambrian formations, the beginning of the Paleozoic is estimated in round numbers at 500,000,000 years ago. This method dates the oldest intrusive rock thus far found to contain radioactive minerals. But even older rocks occur on the earth's surface, for they existed when these intrusions penetrated them. How much time should be assigned to them, we have no accurate way of judging. Recently attention has centered upon the radio activity of the isotopes of potassium, which disintegrate into calcium with an atomic weight of 40 instead of 40.08 of ordinary calcium. On this basis A. K. Brewer has calculated the age of the earth at not more than 2,500,000,000 years, but there is some question that this method has the same order of accuracy as the uranium-lead method. Geologists are satisfied with the time values now allotted by physicists for the long intervals of mountain-making, erosion, and deposition by which the earth gradually reached its present condition.
DIVISIONS OF GEOLOGICAL TIME
The rocks of the accessible part of the earth are divided into five major divisions or eras, which are in the order of decreasing age, Archeozoic, Proterozoic, Paleozoic, Mesozoic, and Cenozoic. Superposition is the criterion of age. Each rock is considered younger than the one on which it rests, provided there is no structural evidence to the contrary, such as overturning or thrust faulting. As one looks at a tall building there is no doubt in the mind of the observer that the top story was erected after the one on which it rests and is younger than it in order of time. So it is in stratigraphy in which strata are arranged in an orderly sequence based upon their relative positions. Certainly the igneous and metamorphic rocks at the bottom of the Grand Canyon are the oldest rocks exposed along the Colorado River in Arizona and each successively higher formation is relatively younger than the one beneath it. The rocks of the Mississippi Valley are inclined at various angles so that successively younger rocks overlap from Minnesota to the Gulf of Mexico. Strata are arranged in recognizable groups by geologists utilizing a principle announced by William Smith in 1799. While surveying in England Smith discovered that fossil shells of one geological formation were different from those above and below. Once the vertical range and sequence of fossils are established the relative position of each formation can be determined by its fossil content. By examining the succession of rocks in various parts of the world it was found that the restriction of certain life forms to definite intervals of deposition was world wide and occurred always in the same order. Apparently certain organisms lived in the ocean or on the land for a time, then became extinct and were succeeded by new forms that were usually higher in their development than the ones whose places they inherited. Thus, the name assigned to each era implies the stage of development of life on the earth during the interval in which the rocks accumulated. The eras are subdivided into periods, which are grouped together in to indicate the highest forms of life during that interval. As the rocks of increasingly younger periods are examined higher types of life appear in the proper order, invertebrates, fish, amphibians, reptiles, mammals, man. From this it is evident that certain fossil forms limited to a definite vertical range may be used as index fossils of that division of geological time. Also, in this table are given for each era estimates of the beginning, duration, and thickness of sediments, based largely upon a report of a Committee of the National Research Council on the Age of the Earth. At the close of and within each era widespread mountain-making disturbances or revolutions took place, which changed the distribution of land and sea and affected directly or indirectly the life of the sea and the land. The close of the Paleozoic era brought with it the rise of the Appalachian Mountains. It has been estimated that only 3 per cent of the Paleozoic forms of life survived and lived on into the Mesozoic era. The birth of the Rocky Mountains at the close of the Mesozoic was accompanied by widespread destruction of reptilian life. Faunal successions responded noticeably to crustal disturbances.
UNCONFORMITIES. In subdividing rocks geologists have been guided by the periods of erosion resulting from extensive mountain construction. Uplift of the continents causes the shallow seas to withdraw from land thereby deepening the ocean and allowing erosion to start on the evacuated land areas. Since all the oceans are connected, sea level throughout the world was affected in many instances, leaving a record of crustal movements in the depositional history of each of the continents. At many places the rocks of one era are separated from those of another by unconformities or erosion intervals, in which miles of rocks were eroded from the crests of folds before sedimentation was resumed on the truncated edges of the mountain structure. There are four stages in the development of an angular unconformity, so named because there is an angular difference between the bedding of the lower series and that of the overlying series. If the series above and below an unconformity consist of marine formations, four movements of the area relative to sea level took place. In stage 1 the sandstones and shales comprise a conformable marine series, which was laid down by continuous deposition with the bedding of one formation conforming to the next. We have seen that the deposition of 24,000 feet of sediment requires repeated sinking of the area below sea level. In stage 2 the region was folded and elevated above sea level, so that erosion could take place. Since erosion starts as soon as the land develops an effective slope for corrosion, there is no proof that this structure ever stood 24,000 feet high. But, the evidence is clear that 24,000 feet were eroded to produce the flat surface, shown in stage 3. In order that the over lying marine series could be deposited the area had to be again submerged below sea level. Since the region now stands above sea level, a fourth movement is necessary. In some cases crustal movement does not tilt or fold the beds, but merely elevates horizontal strata so that erosion removes material and leaves an irregular surface on which sedimentation may be resumed with the deposition of an overlying formation parallel to the first. An erosion interval between parallel formations is a disconformity. But not all unconformities and disconformities are confined to the close of eras. Local deformation and uplift caused erosion between formations within the same era and within the same period. In the Grand Canyon region Devonian rocks rest on the eroded surface of Cambrian formations. At other North American and European localities Ordovician and Silurian rocks occupy this interval, so that the disconformity within the Paleozoic era at this locality represents two whole periods. It is only by carefully tracing the sequence of rocks of one region into another that the immensity of geological time can be appreciated from stratigraphy.
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