Psst! I Heard Edwards Dates Really Old Rocks

I was not even a thought in the 1970s, but I've heard it was a pretty good time to be a rock. People took you as their pets, and I'll bet Professor Lawrence Edwards had a couple Pet Rocks back in the day.

Family Portrait: From left to right: Momma Igneous, Baby Sedimentary, and Poppa Metamorphic.
Family Portrait: From left to right: Momma Igneous, Baby Sedimentary, and Poppa Metamorphic.Courtesy Wikimedia Commons

You see, Edwards is an isotope geochemist, which sounds just about as awesome as it is: he studies the teeny tiny radioactive elements in rocks. These elements help Edwards date rocks. No, that doesn't mean he wines and dines them. Quite the opposite! Edwards developed a sneaky way to figure out how old they are (and let me tell you, nobody wants to be reminded of their age when they're hundreds of thousands of years old).

Edwards' method is similar to carbon-14 dating, only way better. In certain kinds of rocks, Edwards can date rocks as old as 500,000 years compared to carbon-14's measly 50,000 years. That's a whole order of magnitude older! Here's how Edwards' method works: Scientists know that half of any quantity of uranium decays into thorium every 245,500 years. Edwards uses a mass spectrometer to measure the ratio of two radioactive elements -- uranium and thorium. Then, Edwards compares the present ratio of uranium to thorium to what scientists would expect from the half-life decay and bada-bing, bada-boom! Simply genius.

Why am I getting all hyped up over some old rocks? Because they're helping us learn more about ourselves and the tenuous place we hold in this world. For example, Edwards has used his super-special method to trace the strength of monsoon seasons in China. Turns out weak monsoon seasons correlate with the fall of several historical dynasties, and strong monsoons correlate with climatic warming in Europe. Edwards calls this work,

"the best-dated climate record covering this time period."

Your Comments, Thoughts, Questions, Ideas

Anonymous's picture
Anonymous says:


posted on Sun, 01/02/2011 - 4:51pm
Anonymous's picture
Anonymous says:

Very interesting! Cleverly written! I want to know more. I look forward to sharing this article with the kids at school! (Cute picture too. I remember those!)

posted on Thu, 01/06/2011 - 5:54pm
Anonymous's picture
Anonymous says:

So about The "Poppa Metamorphic" What is the exact name of this particular black, very dense, smooth rock? I ask because i found a very similar rock recently and its much heavier than it looks and its magnetic...

posted on Fri, 12/30/2011 - 1:09am
mdr's picture
mdr says:

I'm not sure what Poppa Metamorphic in the picture is. He looks like a piece of basalt (an igneous rock) to me.

But I'm guessing what you found is a piece of iron ore composed of magnetite which, as the name suggests, is a highly magnetic mineral. Magnetite is often black in color and very dense. It's often found in banded iron formations (BIF), which are really sedimentary in origin, but are sometime considered meta-sedimentary due to the effects of undergoing some metamorphism. The Magnetic Rock Trail along the Gunflint Trail in northern Minnesota contains many outcrops of iron formations loaded with magnetite.

posted on Fri, 12/30/2011 - 12:23pm
KelsiDayle's picture
KelsiDayle says:

Whew! Thanks for answering Anonymous' question, mdr. I'm afraid my own geology is pretty poor.

posted on Tue, 01/03/2012 - 3:40pm
well.... My name is Cookie 101's picture
well.... My name is Cookie 101 says:

well what is so ewww i dont hink any of this is ewwwww! <3>

posted on Tue, 01/03/2012 - 5:02pm

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