How paleontologists tell time
The first step is determining whether similar beds in separated outcrops can actually be traced laterally until they are seen to be part of the same original layer. Failing that, the repetition of a certain layered sequence e. Finally, the measurement of a host of rock properties may well be the ultimate key to correlation of separated outcrops. The more ways in which two rocks are physically alike, the more likely it is that the two formed at the same time. Only a partial listing of physical characteristics is necessary to indicate the breadth of approach in this area.
Such features as colour, ripple marks, mud cracks, raindrop imprints, and slump structures are directly observable in the field. Properties derived from laboratory study include 1 size, shape, surface appearance, and degree of sorting of mineral grains, 2 specific mineral types present and their abundances, 3 elemental composition of the rock as a whole and of individual mineral components, 4 type and abundance of cementing agent, and 5 density, radioactivity, and electrical-magnetic-optical properties of the rock as a whole.
With the development of miniaturized analytical equipment, evaluation of rock properties down a small drill hole has become possible. The technique, called well logging , involves lowering a small instrument down a drill hole on the end of a wire and making measurements continuously as the wire is played out in measured lengths. By this technique it is possible to detect depth variations in electrical resistivity, self-potential, and gamma-ray emission rate and to interpret such data in terms of continuity of the layering between holes.
Subsurface structures can thus be defined by the correlation of such properties. Field geologists always prize a layer that is so distinctive in appearance that a series of tests need not be made to establish its identity. Such a layer is called a key bed. In a large number of cases, key beds originated as volcanic ash. Besides being distinctive, a volcanic- ash layer has four other advantages for purposes of correlation: Correlation may be difficult or erroneous if several different ash eruptions occurred, and a layer deposited in one is correlated with that from another.
Even then, the correlation may be justified if the two ash deposits represent the same volcanic episode.
How paleontologists tell time
Much work has been undertaken to characterize ash layers both physically and chemically and so avoid incorrect correlations. Moreover, single or multigrain zircon fractions from the volcanic source are now being analyzed to provide precise absolute ages for the volcanic ash and the fossils in the adjacent units. The end product of correlation is a mental abstraction called the geologic column. In order to communicate the fine structure of this so-called column, it has been subdivided into smaller units. Lines are drawn on the basis of either significant changes in fossil forms or discontinuities in the rock record i.
In the upper part of the geologic column, where fossils abound, these rock systems and geologic periods are the basic units of rock and time. Lumping of periods results in eras, and splitting gives rise to epochs. In both cases, a threefold division into early—middle—late is often used, although those specific words are not always applied.
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Similarly, many periods are split into three epochs. However, formal names that are assigned to individual epochs appear irregularly throughout the geologic time scale. Over the interval from the Paleozoic to the present, nearly 40 epochs are recognized. This interval is represented by approximately formations, discrete layers thick enough and distinctive enough in lithology to merit delineation as units of the geologic column.
Also employed in subdivision is the zone concept, in which it is the fossils in the rocks rather than the lithologic character that defines minor stratigraphic boundaries. The basis of zone definition varies among geologists, some considering a zone to be all rocks containing a certain species usually an invertebrate , whereas others focus on special fossil assemblages. The lower part of the geologic column, where fossils are very scarce, was at one time viewed in the context of two eras of time, but subsequent mapping has shown the provincial bias in such a scheme. Consequently, the entire lower column is now considered a single unit, the Precambrian.
The results of isotopic dating are now providing finer Precambrian subdivisions that have worldwide applicability. The geologic column and the relative geologic time scale are sufficiently defined to fulfill the use originally envisioned for them—providing a framework within which to tell the story of Earth history. Mountains have been built and eroded away, seas have advanced and retreated, a myriad of life-forms has inhabited land and sea.
In all these happenings the geologic column and its associated time scale spell the difference between an unordered series of isolated events and the unfolding story of a changing Earth. Although relative ages can generally be established on a local scale, the events recorded in rocks from different locations can be integrated into a picture of regional or global scale only if their sequence in time is firmly established. The time that has elapsed since certain minerals formed can now be determined because of the presence of a small amount of natural radioactive atoms in their structures. Whereas studies using fossil dating began almost years ago, radioactivity itself was not discovered until , by French physicist Henri Becquerel , and it has only been from about that extensive efforts to date geologic materials have become common.
Methods of isotopic measurement continue to be refined today, and absolute dating has become an essential component of virtually all field-oriented geologic investigations see also isotope.
In the process of refining isotopic measurements, methods for low-contamination chemistry had to be developed, and it is significant that many such methods now in worldwide use resulted directly from work in geochronology. It has already been explained how different Earth processes create different rocks as part of what can be considered a giant rock forming and reforming cycle.
Attention has been called wherever possible to those rocks that contain minerals suitable for precise isotopic dating. It is important to remember that precise ages cannot be obtained for just any rock unit but that any unit can be dated relative to a datable unit. The following discussion will show why this is so, treating in some detail the analytic and geologic problems that have to be overcome if precise ages are to be determined.
As various dating methods are discussed, the great interdependence of the geologic and analytic components essential to geochronology should become evident. The field of isotope geology complements geochronology. Workers in isotope geology follow the migration of isotopes produced by radioactive decay through large- and small-scale geologic processes.
Isotopic tracers of this kind can be thought of as an invisible dye injected by nature into Earth systems that can be observed only with sophisticated instruments. Studying the movement or distribution of these isotopes can provide insights into the nature of geologic processes. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval.
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Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Correlation Principles and techniques Correlation is, as mentioned earlier, the technique of piecing together the informational content of separated outcrops.
Geologic column and its associated time scale The end product of correlation is a mental abstraction called the geologic column. The geological time scale is used by geologists and paleontologists to measure the history of the Earth and life. It is based on the fossils found in rocks of different ages and on radiometric dating of the rocks. Sedimentary rocks made from mud, sand, gravel or fossil shells and volcanic lava flows are laid down in layers or beds. They build up over time so that that the layers at the bottom of the pile are older than the ones at the top.
Geologists call this simple observation the Principle of Superposition, and it is most important way of working out the order of rocks in time. Ordering of rocks and the fossils that they contain in time from oldest to youngest is called relative age dating. Once the rocks are placed in order from oldest to youngest, we also know the relative ages of the fossils that we collect from them.
Relative age dating tells us which fossils are older and which fossils are younger. It does not tell us the age of the fossils. To get an age in years, we use radiometric dating of the rocks. Not every rock can be dated this way, but volcanic ash deposits are among those that can be dated.