Assumptions used in radiometric dating

Would he have concluded that the fossil date for the sediments was wrong? Would he have thought that the radiometric dating method was flawed? Instead of questioning the method, he would say that the radiometric date was not recording the time that the rock solidified.

He may suggest that the rock contained crystals (called xenocrysts) that formed long before the rock solidified and that these crystals gave an older date.

In other words, the age should lie between 197.2 million years and 203.6 million years.

However, this error is not the real error on the date.

In fact, he would have been equally happy with any date a bit less than 200 million years or a bit more than 30 million years.

They would all have fitted nicely into the field relationships that he had observed and his interpretation of them.

The geologist may have found some fossils in Sedimentary Rocks A and discovered that they are similar to fossils found in some other rocks in the region.

He assumes therefore that Sedimentary Rocks A are the same age as the other rocks in the region, which have already been dated by other geologists.

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What would our geologist think if the date from the lab were less than 30 million years, say 10.1 ± 1.8 million years? Or he may decide that the rock had been affected by a localized heating event—one strong enough to disturb the chemicals, but not strong enough to be visible in the field.Let us imagine that the date reported by the lab was 150.7 ± 2.8 million years.Our geologist would be very happy with this result.It is clear that the sedimentary rock was deposited and folded before the dyke was squeezed into place.By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.

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