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How do geologists know how old a rock is? (and liquid stone)

6 May 2012

At http://geology.utah.gov/surveynotes/gladasked/gladage.htm … Mark Milligan tells us the why and wherefore in an easy to understand style. Relative Dating is the easy part as it follows the sequence of superposition – each bed laid down like a layer cake. However, even this follows certain assumptions. These include they were originally laid down near horizontal and were not overturned after deposition. This is augmented by fossil succession – groups of fossil animals and plants occur in a similar layer cake formation. This leads to the dating of sediments by fossil content independent of geochronology. Next, there is crosscutting – igneous insertions or faults younger than the units they cut across, and inclusions, other rocks captured by younger igneous rock or sediments. However, there are also Gaps in the geological record, known as Unconformities. These apply where erosion has removed previously deposited material. These sometimes amount to large chunks of geological time. For example, across large swathes of southern England the basic geology has eroded away right down to the Jurassic era.

Absolute Dating, on the other hand, is based on nuclear decay rates of radioactive isotopes. Igneous rocks are most suited to this dating method whereas metamorphic rocks generally yield the age of the metamorphism – and not the formation of the actual rock. Most ancient sedimentary rocks cannot be dated radiometrically but superposition and crosscutting are used to place them in strick time limits (if bordered by radiometrically dated igneous rocks). Well, that puts paid to liquid stone preservinhg a nuclear clock date in the water and minerals in which the sediments were formed. However, sediments from 50,000 years ago to the present can be dated by C14 methodology.

At http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/clkroc.html we have Clocks in Rocks – which goes through all the methodology. At www.allaboutcreation.org/sedimentary-rocks-faq.htm 'can we date sedimentary rocks using radiometric dating techniques?' is the other side of the uniformitarian argument but admits straightaway that this cannot be done 'directly' as when rocks are in liquid form their radiometric clock resets itself – and that is the answer I was looking for. In other words, the sediments have been reformed from older pieces of rock, gravel, silt or sand and these particles won't give you the age of the sedimentation process. In addition, the redistribution process upsets the conditions necessary to achieve accurate results via radiometric dating – which conclusively answers the query in the last post, Liquid Stone.

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