Thursday, July 28, 2011

A term in geology: principle of superposition

The principle that in a series of stratified sedimentary rocks the lowest stratum is the oldest, is known in geology as the principle of superposition [1].Sometimes referred to as law of superposition, this principle is part of a set of three principles in stratigraphy, which were formulated in the seventeenth century by the Danish anatomist and geologist Nicolaus Steno. Steno's insight derived from his experience as an anatomist as well as his observations and knowledge of the growth of crystals, erosion of land and exposure of marine fossils in the mountains surrounding Florence,Tuscany.

The stratigraphic principle is a feature of the geologic field trip Principles of Historical Geology, which beautifully demonstrates these principles in an online slide-show file. My favorite narrative, explaining the superposition principle, is that given by Alan Cutler in his book The Seashell on the Mountaintop [2]:

The backbone of his [Steno's] system was a simple but tremendously powerful idea. Recognizing that the layers of rock that entombed fossil shells were made by the gradual accumulation of sediment, he realized that each layer embodied a span of time in the past. He saw no way to measure the number of years or centuries involved, and was loathe to speculate, but it was clear that the layers on top of the other formed an unambiguous sequence. The lowest layer had been formed first, the highest last. Depending on their fossils and their sediments, the layers recorded the succession of seas, rivers, lakes, and soils that once had covered the land. Geologists call Steno's insight the “principle of superposition.” It means that, layer by layer, the history of the world is written in stone.   

Cutler compares the significance of Steno's results with that of Galileo's breakthrough: while the latter opened up space by observation and discovery with a telescope, Steno provided an approach to study the past of planet earth—although only in relative terms of time until the invention of absolute dating methods such as techniques based on isotopes and radioactivity. 

[1] The Free Dictionary Thesaurus: principle of superposition [].
[2] Alan Cutler: The Seashell on the Mountaintop. Dutton (Penguin Group), New York, 2003; page 14.

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