4.6 billion yrs old
Radiation measurements
The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist, opened new vistas in science. In 1905, the British physicist Lord Rutherford--after defining the structure of the atom -- made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. B. Boltwood, a radiochemist at Yale University, published a list of geologic ages based on radioactivity. Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years.
The next 40 years was a period of expanding research on the nature and behavior of atoms, leading to the development of nuclear fission and fusion as energy sources. A byproduct of this atomic research has been the development and continuing refinement of the various methods and techniques used to measure the age of Earth materials. Precise dating has been accomplished since 1950.
A chemical element consists of atoms with a specific number of protons in their nuclei but different atomic weights owing to variations in the number of neutrons. Atoms of the same element with differing atomic weights are called isotopes. Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). The rate of decay is conveniently expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay. Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their radioactivity within a few days or years. Some isotopes, however, decay slowly, and several of these are used as geologic clocks.
Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures are complex. The numbers of parent and daughter isotopes in each specimen are determined by various kinds of analytical methods. The principal difficulty lies in measuring precisely very small amounts of isotopes.
The potassium-argon method can be used on rocks as young as a few thousand years as well as on the oldest rocks known. Potassium is found in most rock-forming minerals, the half-life of its radioactive isotope potassium-40 is such that measurable quantities of argon (daughter) have accumulated in potassium-bearing minerals of nearly all ages, and the amounts of potassium and argon isotopes can be measured accurately, even in very small quantities. Where feasible, two or more methods of analysis are used on the same specimen of rock to confirm the results.
Another important atomic clock used for dating purposes is based on the radioactive decay of the isotope carbon-14, which has a half-life of 5,730 years. Carbon-14 is produced continuously in the Earth's upper atmosphere as a result of the bombardment of nitrogen by neutrons from cosmic rays. This newly formed radiocarbon becomes uniformly mixed with the nonradioactive carbon in the carbon dioxide of the air, and it eventually finds its way into all living plants and animals. In effect, all carbon in living organisms contains a constant proportion of radiocarbon to nonradioactive carbon. After the death of the organism, the amount of radiocarbon gradually decreases as it reverts to nitrogen-14 by radioactive decay. By measuring the amount of radioactivity remaining in organic materials, the amount of carbon-14 in the materials can be calculated and the time of death can be determined. For example, if carbon from a sample of wood is found to contain only half as much carbon-14 as that from a living plant, the estimated age of the old wood would be 5,730 years.
The radiocarbon clock has become an extremely useful and efficient tool in dating the important episodes in the recent prehistory and history of man, but because of the relatively short half-life of carbon-14, the clock can be used for dating events that have taken place only within the past 50,000 years.
The following is a group of rocks and materials that have dated by various atomic clock methods:
[TABLE]
[TR]
[TD]Sample[/TD]
[TD]Approximate Age in Years[/TD]
[/TR]
[TR]
[TD]Cloth wrappings from a mummified bullSamples taken from a pyramid in Dashur, Egypt. This date agrees with the age of the pyramid as estimated from historical records[/TD]
[TD]2,050[/TD]
[/TR]
[TR]
[TD]CharcoalSample, recovered from bed of ash near Crater Lake, Oregon, is from a tree burned in the violent eruption of Mount Mazama which created Crater Lake. This eruption blanketed several States with ash, providing geologists with an excellent time zone.[/TD]
[TD]6,640[/TD]
[/TR]
[TR]
[TD]CharcoalSample collected from the "Marmes Man" site in southeastern Washington. This rock shelter is believed to be among the oldest known inhabited sites in North America[/TD]
[TD]10,130[/TD]
[/TR]
[TR]
[TD]Spruce woodSample from the Two Creeks forest bed near Milwaukee, Wisconsin, dates one of the last advances of the continental ice sheet into the United States.[/TD]
[TD]11,640[/TD]
[/TR]
[TR]
[TD]Bishop TuffSamples collected from volcanic ash and pumice that overlie glacial debris in Owens Valley, California. This volcanic episode provides an important reference datum in the glacial history of North America.[/TD]
[TD]700,000[/TD]
[/TR]
[TR]
[TD]Volcanic ashSamples collected from strata in Olduvai Gorge, East Africa, which sandwich the fossil remains of Zinjanthropus and Homo habilis -- possible precursors of modern man.[/TD]
[TD]1,750,000[/TD]
[/TR]
[TR]
[TD]Monzonite.Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon. Utah.[/TD]
[TD]37,500,000[/TD]
[/TR]
[TR]
[TD]Quartz monzoniteSamples collected from Half Dome, Yosemite National Park, California.[/TD]
[TD]80,000,000[/TD]
[/TR]
[TR]
[TD]Conway GraniteSamples collected from Redstone Quarry in the White Mountains of New Hampshire.[/TD]
[TD]180,000,000[/TD]
[/TR]
[TR]
[TD]RhyoliteSamples collected from Mount Rogers, the highest point in Virginia.[/TD]
[TD]820,000,000[/TD]
[/TR]
[TR]
[TD]Pikes Peak GraniteSamples collected on top of Pikes Peak, Colorado.[/TD]
[TD]1,030,000,000[/TD]
[/TR]
[TR]
[TD]GneissSamples from outcrops in the Karelian area of eastern Finland are believed to represent the oldest rocks in the Baltic region.[/TD]
[TD]2,700,000,0000[/TD]
[/TR]
[TR]
[TD]The Old GraniteSamples from outcrops in the Transvaal, South Africa. These rocks intrude even older rocks that have not been dated.[/TD]
[TD]3,200,000,000[/TD]
[/TR]
[/TABLE]
The
Fossil records
As geologists continued to reconstruct the Earth's geologic history in the 1700s and early 1800s, they quickly recognized that the distribution of fossils within this history was not random -- fossils occurred in a consistent order. This was true at a regional, and even a global scale. Furthermore, fossil organisms were more unique than rock types, and much more varied, offering the potential for a much more precise subdivision of the stratigraphy and events within it.
The recognition of the utility of fossils for more precise "relative dating" is often attributed to William Smith, a canal engineer who observed the fossil succession while digging through the rocks of southern England. But scientists like Albert Oppel hit upon the same principles at about about the same time or earlier. In Smith's case, by using empirical observations of the fossil succession, he was able to propose a fine subdivision of the rocks and map out the formations of southern England in one of the earliest geological maps (1815). Other workers in the rest of Europe, and eventually the rest of the world, were able to compare directly to the same fossil succession in their areas, even when the rock types themselves varied at finer scale. For example, everywhere in the world, trilobites were found lower in the stratigraphy than marine reptiles. Dinosaurs were found after the first occurrence of land plants, insects, and amphibians. Spore-bearing land plants like ferns were always found before the occurrence of flowering plants. And so on.
The observation that fossils occur in a consistent succession is known as the "principle of faunal (and floral) succession". The study of the succession of fossils and its application to relative dating is known as "biostratigraphy". Each increment of time in the stratigraphy could be characterized by a particular assemblage of fossil organisms, formally termed a biostratigraphic "zone" by the German paleontologists Friedrich Quenstedt and Albert Oppel. These zones could then be traced over large regions, and eventually globally. Groups of zones were used to establish larger intervals of stratigraphy, known as geologic "stages" and geologic "systems". The time corresponding to most of these intervals of rock became known as geologic "ages" and "periods", respectively. By the end of the 1830s, most of the presently-used geologic periods had been established based on their fossil content and their observed relative position in the stratigraphy (e.g., Cambrian (1835), Ordovician (1879), Silurian (1835), Devonian (1839), Carboniferous (1822), Permian (1841), Triassic (1834), Jurassic (1829), Cretaceous (1823), Tertiary (1759), and Pleistocene (1839)). These terms were preceded by decades by other terms for various geologic subdivisions, and although there was subsequent debate over their exact boundaries (e.g., between the Cambrian and Silurian Periods, which was resolved by proposal of the Ordovician Period between them), the historical descriptions and fossil succession would be easily recognizable today.
By the 1830s, fossil succession had been studied to an increasing degree, such that the broad history of life on Earth was well understood, regardless of the debate over the names applied to portions of it, and where exactly to make the divisions. All paleontologists recognized unmistakable trends in morphology through time in the succession of fossil organisms. This observation led to attempts to explain the fossil succession by various mechanisms. Perhaps the best known example is Darwin's theory of evolution by natural selection. Note that chronologically, fossil succession was well and independently established long before Darwin's evolutionary theory was proposed in 1859. Fossil succession and the geologic time scale are constrained by the observed order of the stratigraphy -- basically geometry -- not by evolutionary theory.
Stratigraphic
In places where layers of rocks are contorted, the relative ages of the layers may be difficult to determine. View near Copiapo, Chile.
At the close of the 18th century, careful studies by scientists showed that rocks had diverse origins. Some rock layers, containing clearly identifiable fossil remains of fish and other forms of aquatic animal and plant life, originally formed in the ocean. Other layers, consisting of sand grains winnowed clean by the pounding surf, obviously formed as beach deposits that marked the shorelines of ancient seas. Certain layers are in the form of sand bars and gravel banks -- rock debris spread over the land by streams. Some rocks were once lava flows or beds of cinders and ash thrown out of ancient volcanoes; others are portions of large masses of once molten rock that cooled very slowly far beneath the Earth's surface. Other rocks were so transformed by heat and pressure during the heaving and buckling of the Earth's crust in periods of mountain building that their original features were obliterated.
Between the years of 1785 and 1800, James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world. Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages. He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time. Further, he proposed that wherever uncontorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger.
Today, such a proposal appears to be quite elementary but, nearly 200 years ago, it amounted to a major breakthrough in scientific reasoning by establishing a rational basis for relative time measurements. However, unlike tree-ring dating -- in which each ring is a measure of 1 year's growth -- no precise rate of deposition can be determined for most of the rock layers.