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If you know when the top or bottom layers were formed - you know just about when was the middle one formed.
This is "relative dating", but it doesn't give you an exact age.
Geologists also have radiometric methods for absolute dating based on radioactive decay of certain are present.
The age is based on the half-life of the isotopes (their rate of decay over time).
If, for example, we cannot measure precisely the amount of sand in the lower bulb, but we have good reason to think that our estimate of it must be within 10% of the true value, then we also have a good reason to think that we can give the time it's been running to within 10%.
Or again, if there may have been sand in the lower bulb when it started running, but we have a good reason to think that there can't have been very much, then we also have a good reason to think that the figure which we get for the time can't be very wrong; and so on.
A method of dating which was accurate to within a billion years either way would still technically speaking be a method of absolute dating, it would just be a very bad one.
Suppose that we wanted to find out how long it has been since an hourglass was set running by measuring the amount of sand in the lower bulb.
By contrast, absolute dating allows us to assign dates to geological features.
Relative dating is a scientific process of evaluation used to determine the relative order of past events, but does not determine the absolute age of an object.
The circumstances of the object may allow one to say that one object is older than another without being able to assign a particular age to the objects.
To do so successfully, we would need to assure ourselves of the following conditions: Given these conditions, we can find out how long the hourglass has been running.
If we wish to use a geological process as the geological equivalent of an hourglass, we would want to have similar conditions: we would like to find some quantity which we can measure reliably (corresponding to the condition that we can measure the amount of sand in the lower bulb of the hourglass); which increases or decreases from a known quantity (corresponding to the lower bulb of the hourglass being empty when it starts running) at a known rate (corresponding to knowing the rate of flow of sand); and so forth.Short Answer: The term relative dating is distinguished from absolute dating to make it clear that one does not get a specific estimate of the age of an object from relative dating, but one does get such an estimate of true age from absolute dating.