Slide 1
Parent and daughter atoms can record a geological age
Radiometric dating uses radioactive parent atoms and their daughter products. Their measured relationship can reveal time since a geological system became closed.
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The dated event is not automatically the birth of an entire mountain or landscape. Depending on the mineral and isotope system, the result may record crystallization, cooling or a later disturbance.
Slide 2
Half-life supplies the decay timescale
Each radioactive isotope has a characteristic decay rate. The half-life is the time over which half a large population is expected to decay.
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Useful clocks span different timescales. A very short-lived parent may be exhausted in an ancient sample, while a very long-lived parent may produce too little daughter material for a young event to be measured precisely.
Slide 3
Daughter atoms may already have been present
A daughter isotope may already have been present when the clock began. Dating methods must account for that initial contribution.
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Some minerals preferentially include a parent while excluding its daughter during formation. Other methods use several related samples or isotope ratios to estimate the initial component instead of assuming it was zero.
Slide 4
Loss or gain of atoms can disturb the clock
Loss or gain of parent or daughter atoms can alter an apparent age. Heating, fluids or weathering can disturb a mineral’s isotopic record.
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Such disturbance can itself be scientifically useful if identified. Comparing minerals with different retention properties may reveal a cooling history rather than one simple formation date.
Slide 5
Other dates and field relationships test the interpretation
Multiple isotope systems and field relationships can test an interpretation. Agreement is strongest when the dates correspond to the same expected event.
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Sedimentary grains may be older than the layer that contains them. Dating a volcanic ash bed within the sequence can instead help bracket deposition, connecting a numerical clock to relative geological order.
Sources
- A Beginner’s Guide to Dating Rocks
US Geological Survey
- Geologic Time: Radiometric Time Scale
US Geological Survey