About 75 years ago, the best guess for the age of Earth was about 100 million years before the present. About 50 years ago, the best estimate was approximately 2 to 3 billion years. Now, the best estimate is that the earth was formed 4.75 billion years ago. In cosmic time, it may have taken another 14 billion years for the gases in the universe to coalesce into a liquid or plastic form. Scientific research and technological advancement improves with time, but how can such a huge amount of time be put into an understandable perspective? The archaeologist D. J. Mahony made an analogy with a walk down the avenue of time into the past, covering a thousand years at each step. The first step would take us back to the battle of Hastings, the second to the beginning of the Christian era, the third to Homeric Troy, the fourth to Abraham, and the seventh to the earliest traditional history of Babylon and Egypt. About a quarter mile would lead to the origin of the oldest stone tools found in Europe. To continue until we encountered the most ancient fossil organisms would mean a journey of more than 250 miles.
James Hutton, in the 18th century, was a farmer, a doctor, and, many believe, the father of modern geology. He perceived, from what he viewed and understood of the soils and the rocks, that the formation of the earth took a much longer period to happen than that postulated by Archbishop Ussher in the 17th century, who proposed that the earth was created on the evening of October 22, 4004 BCE. Charles Lyell, in the 19th century, using earlier works as well as his own observations, indicated that the earth was not only of great age but that its processes in the present could be used to illustrate the changes of the past. Early in the 20th century, Alfred Wegener presented the concept of continental drift, now known as plate tectonics, to his peers and to the world. These concepts by Hutton, Lyell, and Wegener were not readily accepted. But now we know that Pangea was truly a single continent about 200 million years ago and that that continent has become no fewer than six: Africa, Antarctica, Australia, Eurasia, North America, and South America. The geologic processes that we observe today have been recurring again and again. Some of these events and processes can be viewed in the geologic timescale in Table 1.
The study of Earth’s origin is a work in progress. A multitude of geologists and other scientists for the past 500 hundred years or so have built the body of knowledge we have currently. Today, scientists are using new technologies to piece together how the earth was formed. It some ways, it is like building a structure from the top down, or as Hutton put it more than 200 years ago, “The present is the key to the past.” Speculation concerning Earth’s origin and its formation runs rampant. This continues today regarding Earth’s formation, but with developing technologies, new, vivid, and more exact evidence has begun to appear.
Astronomers, seismologists, physicists, chemists, and biologists, among others, are accumulating the evidence necessary to reveal the history of Earth and its age. The interior of the earth, its core, consists of hot, molten material. The core is surrounded by a
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mantle of more viscous material with variations in its fluidity. On top is the less hefty crust of the earth in two parts: the lower magnesium-silicate layer and the upper alumino-silicate layer, or the present landmasses. The crust consists of rigid materials such as silica (SiO2) which is almost 95% of the total volume, followed by aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg), all of which add up to 98.59% by weight. In the core, below the crust, heat is generated and con- vected through the mantle. The heated material pushes forth and tends to weaken the crust, slowly moving it laterally, sometimes allowing molten material to ooze out, as in sea-floor spreading. This process of deformation has continued through time.
Landmasses built by volcanism, faulting and folding, and upheavals related to the collision of continents become exposed to the atmosphere, and they contribute sediments that are carried to lower elevations by moving water, moving air, moving ice, and currents and tides in bodies of water. Deformation of the crust has occurred repeatedly, producing a complex crustal surface.
Geological Eras
The Precambrian is divided into eon: the Archeozoic, which is characterized by primeval conditions, and the Proterozoic, considered to be the very beginning of primitive life. However, additional distinct eras during the Precambrian may emerge with the greater understanding afforded by newer technologies. During the Precambrian eon, considerable deformation has been noted with no fewer than three major orogenies and subsequent long periods of erosion and deposition, in addition to two ice ages. Every continent has Precambrian shields, many of which contain a wealth of minerals, including iron, copper, nickel, silver, and gold.
The Paleozoic era consists of the Cambrian through the Permian periods. This era begins with the appearance of trilobites and other marine fossils in vast shallow seas that came with climatic warming at the end of the Proterozoic ice age. The Ordovician was the greatest of all submergences, with reef systems and an abundance of marine fossils stretching from the tropics to the arctic regions of today. It must be remembered that the landmasses had different shapes and locations until about 200 million years ago. The rocks of this period offer gas and oil, as well as building stone such as marble, slate, limestone, and dolomite, and ores like hematite, lead, and zinc. The Silurian times were relatively calm as terrestrial life began, with the first air-breathing animals such as scorpions and millipedes, and marine life continued to develop. During the Devonian, which was relatively calm, some disturbances can be noted, as with other periods. Land plants flourished as did marine invertebrates; the ascendancy of the fishes began, including air-breathing fishes. The Mississippian and Pennsylvanian mark the millions of years of the carboniferous system with swamp vegetation, interspersed with sediments of clay, silt, and sand. The Permian marks a crisis in Earth history with major mountain building. Although deserts were widespread during this time, so were vast areas in the southern hemisphere covered with ice, and the decline of carboniferous floras can be noted.
The Mesozoic era includes the Triassic, Jurassic, and Cretaceous periods, during which the reptiles emerged. This was the age of dinosaurs measuring about 65 feet in length and more than 18 feet high at the hips. Also, smaller dinosaurs, birds, and mammals appeared in great quantities. The end of the era came with orogenic forces and a colder climate, with evidence of glaciers in Australia. It must be remembered that continents were not the same shapes or in the same locations as they are today.
The Cenozoic era is that of the modern world of the past 65 million years or so. The Mesozoic lasted about twice that long and the Paleozoic about five to six times longer. An appreciation for the magnitude and duration of the events of the Cenozoic era—including the birth of numerous volcanoes, the vast mountain ranges, the occurrence of earthquakes marking the faults, the array of hills, plateaus, and plains, the evidence of the great ice ages, and the broad expanses of oceans—brings into sharper focus the scale of time against which the existence of humankind can be understood.
Richard A. Stephenson
See also Darwin, Charles; Dating Techniques; Geology; Gosse, Philip Henry; Hutton, James; Lyell, Charles; Moon, Age of; Pangea; Sun, Age of; Time, Planetary; Wegener, Alfred
Further Readings
Ausich, W. L., & Lane, N. G. (1999). Life of the past (4th ed.). Upper Saddle River, NJ: Prentice Hall. Dunbar, C. O. (1955). Historical geology. New York: Wiley.
Mahony, D. J. (1943). The problem of antiquity of man in Australia. Memoirs of the National Museum, Melbourne, 13, 7.
Tarbuck, E. J., & Lutgens, F. K. (2008). Earth (9th ed.). Upper Saddle River, NJ: Prentice Hall.
