Law of superposition
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- A separate article is about the superposition principle of physics.
The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with stratigraphy. In its plainest form, that is: layers are arranged in a time sequence, with the oldest on the bottom and the youngest on the top, unless later processes disturb this arrangement. The law was first proposed in the 17th century by the Danish scientist Nicolas Steno.
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[edit] Development of the Law of Superposition
Assuming that all rocks and minerals had once been fluid, Steno reasoned that rock strata were formed when particles in a fluid such as water fell to the bottom. This process would leave horizontal layers. Thus Steno's principle of original horizontality states that rock layers form in the horizontal position, and any deviations from this horizontal position are due to the rocks being disturbed later.
There are exceptions to this case, because sediments may be deposited on slopes or gradients. These may be steep, locally, and can be up to several degrees. Nevertheless, the principle is essentially true.
Steno stated another, more general principle in this way:
- If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface.
In other words: a solid object will cause any solids that form around it later to conform to its own shape.
Steno was able to show by this reasoning that fossils and crystals must have solidified before the host rock that contains them was formed. If a "tongue stone" had grown within a rock, it would have been distorted by the surrounding rock, in much the same way that a tree root is distorted by growing into a crack in the earth. Instead, the "tongue stone" must have been buried in soft sediments which hardened later. Veins (mineral-filled cracks) and many crystals, on the other hand, must have formed after the surrounding rock was a solid, because they often did show irregularities of form caused by having to conform to the surrounding solid rock.
Finally, in the case of strata, layers on top of a set of strata conform to the shape of lower layers. . . and therefore, in a set of strata, the youngest layers must be those of the top layer, and the oldest must lie on the bottom.
From Steno's observation that rock strata form when particles fall out of suspension in a fluid, it then follows that the youngest stratum is on the top of a sequence. However, this principle also applies to other types of rocks that do not form with water, such as volcanic rocks which spread on older flows, by flow banding.
Steno realized that other geological processes could create apparent exceptions to his laws of superposition and horizontality . He reasoned that the formation of caves might remove part of a lower layer, and that the collapse of a cave might transport large pieces of an upper layer downwards. He recognized that rocks might be uplifted by subterranean forces. Geologists now recognize that tilting, folding, and faulting may also complicate the analysis of a stratigraphic sequence. Molten rock may force its way through surrounding rocks and may sometimes squeeze between older rock layers, also forming an exception to Steno's law. However, such anomalies leave physical evidence in the disturbed rocks; for example, faulted rock layers may be cracked, broken, or metamorphosed along the fault lines.
Steno's law is a statement of relative time, not absolute time: two rock layers, in principle, could form millions of years apart, or days apart.
[edit] The law of superposition and biblical mythology
Steno himself saw no difficulty in attributing the formation of most rocks to the flood mentioned in the Bible. However, he noticed that, of the two major rock types in the Apennine Mountains near Florence, Italy, the lower layers had no fossils, while the upper ones were rich in fossils. He suggested that the upper layers had formed in the flood, after the creation of life, while the lower ones had formed before life had existed. This was the first use of geology to try to distinguish different time periods in the Earth's history – an approach that would develop spectacularly in the work of later scientists.
The Law of Superposition is widely used in creation science to refute geological scientific arguments on the age of the Earth, especially by reference to catastrophism forming turbidites (considered to be evidence of the Great Biblical Flood), which often show exceptions to simplistic applications of the Law of Superposition, specifically the typical conception of a fining up sequence. Such arguments are naturally fallacious, because Steno did not recognise fining up sequences, and the Law of Superposition has, (as all scientific laws have), been modified to take into account modern conceptions and increased knowledge of the natural world.
Thrust faults were also unknown to Steno and his contemporaries and were not descried until the late 19th Century and early 20th century by Peach and Horne at Knockan Crag, Scotland, on the Moine Thrust Fault. Thrust faults can cause confusion with the Law of Superposition because they occur parallel to bedding and can be difficult to detect, thus creating situations where inexplicably, older strata can overly younger. Creation science often uses examples of thrust-faulted stratigraphic sections to disprove the ubiquitous applicability of the Law of Superposition, often at great logical error.
One further interesting argument proposed by creation scientists is the unconformable chronostratigraphy and repeated chronostratigraphic sections of the Arctic ice sheet and glaciers of Iceland as evidence of catastrophism and violation of Steno's Law of Superposition. Such features may, in fact, be a thrust–fault–hosted wihin the glacial pile. This is of great importance to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.
When combined with the related law of faunal succession, the law of superposition provides a very powerful tool for dating rocks and strata.
[edit] Superposition as modified by archaeological considerations
Superposition in Archaeology and especially in stratification use during excavation is slightly different as the processes involved in laying down archaeological strata are some what different form geological processes. Man made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes. Some archaeological strata (often termed as contexts or layers) are created by undercutting previous strata. An example would be that the silt backfill of a underground drain would form some time after the ground immediately above it. Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall. Superposition in archaeology]] requires a degree of interpretation to correctly identify chronological sequences and in this sense superposition in archaeology is more dynamic and multi- dimensional.
[edit] See also
- Principle of original horizontality
- Principle of lateral continuity
- Law of faunal succession
- Stratigraphy
- Structural geology
- Harris matrix
- Stratification (archeology)
[edit] References
- Hamblin, W.K. The Earth's Dynamic Systems, A Textbook in Physical Geology, by W. Kenneth Hamblin, BYU, Provo, UT, Illus. William L. Chesser, Dennis Tasa, (Burgess Publishing Company, Minneapolis, Minnesota), c 1978, pg. 115, "The Principle of Superposition and Original Horizontality;" pg. 116: The Principle of Faunal Succession, "The Principle of Crosscutting Relations;" pg 116-17: "The Principle of Inclusion," (as in the Steno discussion above).
- Principles of Archaeological Stratigraphy. 40 figs. 1 pl. 136 pp. London & New York: Academic Press. ISBN 0-12-326650-3
