Magmas produced by the melting of older crust can be identified because their zircons commonly contain inherited older cores. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found. Where the crust is under tension, as in Iceland, great fissures develop.
It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced. The principles for relative age dating described above require no special equipment and can be applied by anyone on a local or regional scale. The sequence of a layered sedimentary series is easily defined because deposition always proceeds from the bottom to the top.
In some cases, they spread between the layers of near-horizontal sedimentary or volcanic units to form bodies called sills. Dikes that cross fault boundaries may even be found. This information has also helped determine the age of the Earth itself. The lateral variation in sediment within a stratum is known as sedimentary facies. Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.
Magmas produced in this way are regarded as recycled crust, whereas others extracted by partial melting of the mantle below are considered primary. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. Local melting may occur, and certain minerals suitable for precise isotopic dating may form both in the melt and in the host rock. Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age. The portion that remains in a fissure below the surface usually forms a vertical black tubular body known as a dike or dyke.
Analytical methods are now available to date both growth stages, even though each part may weigh only a few millionths of a gram see below Correlation. Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents. In addition, the redeposition process upsets the conditions necessary to achieve accurate results through radiometric dating. To determine the ages of these specimens, scientists need an isotope with a very long half-life. This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil.
Some method of correlating rock units must be found. Absolute dating allows rock units formed at the same time to be identified and reassembled into ancient mountain belts, which in many cases have been disassociated by subsequent tectonic processes. Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record. Dikes do not always continue upward in a simple fashion. Episodes of continental collision can be dated by isolating new zircons formed as the buried rocks underwent local melting.
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