Dating by numbers

Most of these principles were formally proposed by Nicolaus Steno (Niels Steensen, Danish), in 1669, although some have an even older heritage that extends as far back as the authors of the Bible.A few principles were recognized and specified later.For example, the principle of superposition is based, fundamentally, on gravity.In order for a layer of material to be deposited, something has to be beneath it to support it.They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.In the last 200 or more years of their application, they are valid, but geologists do not assume they are.However, note that because of the "principle of cross-cutting relationships", careful examination of the contact between the cave infill and the surrounding rock will reveal the true relative age relationships, as will the "principle of inclusion" if fragments of the surrounding rock are found within the infill.Cave deposits also often have distinctive structures of their own (e.g., spelothems like stalactites and stalagmites), so it is not likely that someone could mistake them for a successional sequence of rock units. Each of them is a testable hypothesis about the relationships between rock units and their characteristics.

The principle of superposition therefore has a clear implication for the age of a vertical succession of strata.Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another.The most common rocks observed in this form are sedimentary rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).Despite this, the "principle of cross cutting relationships" can be used to determine the sequence of deposition, folds, and faults based on their intersections -- if folds and faults deform or cut across the sedimentary layers and surfaces, then they obviously came after deposition of the sediments.You can't deform a structure (e.g., bedding) that is not there yet!The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers.In such a situation, the "principle of superposition" is easily applied, and the strata towards the bottom are older, those towards the top are younger.Geochronologists do not claim that radiometric dating is foolproof (no scientific method is), but it does work reliably for most samples.It is these highly consistent and reliable samples, rather than the tricky ones, that have to be falsified for "young Earth" theories to have any scientific plausibility, not to mention the need to falsify huge amounts of evidence from other techniques.An early summary of them is found in Charles Lyell's .In no way are they meant to imply there are no exceptions.

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