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Name: Patty
Status: Educator
Grade: K-3
Location: CA
Country: United States
Date: Summer 2010


Question:
How can you tell how old Earth is?



Replies:
Patty,

Certain radioactive elements transmute in a series or a chain. One element becomes another becomes another. For example: Uranium becomes Thorium becomes Radium becomes Actinium . . . and so on until it ultimately becomes Lead. The time for any concentration of Uranium to become half the original concentration (known as half-life) as it become Radium is very well known. Likewise the half-life of Radium (as it becomes Actinium) is also well-known. Thus, combining all these half-lives, the half-life of the process from Uranium to Lead becomes also quite well known and established to be 704 million years.

Thus if we find a sample of rock (preferably large whole rock that has not been broken or pulverized - this way the transmutation elements will be next to each other) and we find some Uranium, Radium, Actinium ... Lead in it - all next to each other, then we can surmise that the system initially started as Uranium which over time, steadily, became Lead. If we then analyze the concentrations of each of these elements, we can determine how far back in time this sample was pure Uranium. We can then determine how old that rock is because the entrapment of these elements, all next to each other, could have only happened when the rock was formed.

This process only tells us the age of the rock. It is conceivable that the Earth is much older than that rock, that the rock formed much later than the Earth itself. However, if we investigate enough rocks all over the Earth, and we find similar ages of rock, then it become less likely that all these rocks independently formed at the same time but at a different time from the Earth's formation. It is more likely that these rocks containing Uranium formed when the Earth formed.

Using this process and analysis we've found that the Earth is 4.54 billion years old.

Greg (Roberto Gregorius)
Canisius College



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