Geologic Time U235-713my Geologic Time Using the graph and tables answer the fol

Question

Geologic Time

U235-713my

Geologic Time Using the graph and tables answer the following questions 1. A mass spectrometer analysis of zircon crystals found in a vertical dike that cuts across several layers of tuff revealed that 25% of the original uranium-235 remains. Both the tuff layers and the dike are overlain by a shale layer that is not cut by the dike. Assume that the dike is the same age as the zircon crystals a) What is the absolute age of the dike? b) What is the age of the tuff layers? C) What is the age of the shale layer? 100 Daughter 50. a 25 Parent o 12.5 m 6.25 , 5 Half Lives Elapsed

Explanation / Answer

Uranium-235 has a half-life of 713 million years. The amount of unanium left in the dike is 25% and hence 75% of Uranium is used which is equivalent to 2 half lives or 2 x 713 million years = 1,426 million years

a) Absoulute age of dike is 2 half lives or 2 x 713 million years = 1,426 million years

b) As per the Principle of Cross-cutting relationships we can say that dike is younger than the tuff or tuff layer is older than dike therefore age of tuff is more than 1,426 million years old.

c) Now the shale layer is not been intruded by dike and hence is younger than the age of dike or tuff therefore it is less than 1,426 million years old

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