Stratigraphy
The stratigraphy (including stratigraphy ) is in the earth sciences the most important method for correlation and relative dating especially fossiliferous sedimentary rocks, but also of fossil- free volcanic rocks ( lava flows, volcanic ash ). Today it is divided into a number of sub-disciplines, quite accurately allows their combined use in conjunction with geochronology, the relative and absolute dating of rocks and thus a reconstruction of the Earth's history.
Objectives
The aim of the stratigraphy to time to arrange rock body on the basis of organic and inorganic matter contained therein relative and time to set spatially distant rock units relate to each other is ( correlation). Starting from local and regional sequences, these should be included in the internationally accepted global chronostratigraphic and geochronological units.
Importance
Stratigraphy, a branch of historical geology, the basis for the reconstruction of the geological history and the history of life on Earth. However, it is often also the solution of general geological questions.
In the 19th century it was recognized that one can apply this method on all other layers, including the elements contained in them. Thus, the stratigraphy was also applied to archeology.
Stratigraphic principle
The stratigraphic principle (also called " Stratigraphic Basic Law " or storage usually called ) is the basis of stratigraphy: Sedimentary layers in the footwall ( "down" ) are older than sedimentary strata in the hanging wall ( "up" ). This principle already recognized Nicolaus Steno in 1669. Tectonic processes, unusual depositional environments and intrusive bodies, this rule can break through but in some cases.
Methods of stratigraphy
Subdisciplines
- Chronostratigraphy: The relative time determination based on timestamps in rock bodies. These timestamps can be the first appearance or cessation of certain fossils, event horizons, geochemical markers and polarity change in the Earth's magnetic field. All other methods of stratigraphy are hooked into the stratigraphic sequence.
- Biostratigraphy: The relative time determination by fossils. Here are primarily the life and the first appearance or extinction of individual species, rarely used even by faunal. Other distinctions: Orthostratigraphie: Structure by binding specified index fossils
- Parastratigraphie: Structure by other index fossils
- Pedostratigraphie: Outline fossil soil horizons
Conjunction with geochronology
The geochronology is concerned with the absolute time determination of the geological past. It is in principle independent of the stratigraphy discipline, but the dating of rocks is only useful in conjunction with the stratigraphy.
Two examples of geochronological methods:
- Dendrochronology allowed under certain circumstances a year exact dating of the studied woods. It is, however, only in the geologically very short period of the past 12,000 years ( that is, only in the Holocene ) used. It is particularly in the archaeological stratigraphy is an important tool for dating the layers.
- The Warvenchronologie allows a year exact age determination in the ideal case. It is limited in Sweden in the last 10,000 years, in the Eifel on the last 23,000 years. In some lakes, however, are dating to 76,000 years before present succeeded.
Geological time scale
The various methods of stratigraphy have now found a very detailed relative time scale of Earth's history. However, in order to classify these relative periods of time in an absolute time scale, the methods of stratigraphy are not enough. The researchers of the late 18th and 19th century the first to use the idea that the Earth is not emerged within a few thousand years. They could initially only estimate the periods during which the earth and life is actually incurred on it.
In the 20th century made possible the discovery of radioactive decay processes by Henri Becquerel and the derived radiometric age determinations a method of absolute - time determination of rocks. The now developed different methods of radiometric age determinations now allow a relatively accurate absolute age determination of the events of earth's history. However, the accuracy decreases with increasing age of the rocks from more and more. However, the created by the different methods of stratigraphy relative time scale is still valid, as the various stratigraphic methods give much more accurate classification and division of the earth's history than just allow geochronology alone.