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Image that says Geologic Time. Image showing a large section of rock and its different layers.  Please have someone assist you with this.

How Old is That Rock?
How can you tell the age of a rock or to which geologic time period it belongs? One way is to look at any fossils the rock may contain. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. Another way is to use the "What's on top?" rule. When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks.

But these two methods only give the relative age of rocks--which are younger and which are older. How do we find out how old a rock is in years? Or how do we know how long ago a particular group of fossilized creatures lived?

The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. When rocks are formed, small amounts of radioactive elements usually get included. As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements. Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements are found in the rock.

Image showing the radioactive age dating of a rock.  Please have someone assist you with this.

Image of the Geologic Time Column.  Please have someone assist you with this.A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

The age of the rock in years can be found by measuring the rate at which a parent element decays and then measuring the ratio of parent element to daughter element in the rock. The ages in years of the different geological time periods are found by measuring the absolute ages of many rocks from all of the different periods. The absolute ages of some of the different geologic time periods are shown along the right side of the Staircase of Time.

The steps of the Staircase of Time are drawn to be almost the same size, so you might think that the time periods are the same length, but they are not. The absolute ages of rocks taken from the different time periods have shown that the time periods were of greatly differing lengths. Some were very short, like the Quaternary period (only 2 million years), while others were very long, like the Proterozoic Era (almost 2 billion years). According to absolute-age measurements, an accurate representation of the lengths of the major geologic time periods is shown in the time bar at right.

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Geologic Time
Cenozoic Era
Mesozoic Era
Paleozoic Era
The Precambrian Eon
Names on the Staircase of Time
Image of a star. How Old is That Rock?
Geologic Time Activity
What is a Million?
Finding an Event in Time
 
             
     
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Geologic Time | Cenozoic Era | Mesozoic Era | Paleozoic Era | The Precambrian Eon | The Staircase of Time | How Old is That Rock? | Geologic Time Activity | What is a Million? | Finding a Place in Time

Diversity | Adaptation | Plate Tectonics | Cycles | Spheres | Biomes | Geologic Time

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April 28, 2005

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