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# Relative Dating

Read the following story carefully. Then determine the correct order for the sentences. Be aware that the order has been mixed up!

I got out bread, peanut butter and jelly.
I ate the sandwich.
I put the two pieces of bread together.
I spread a layer of jelly on another piece of bread.
I was hungry.
I spread a layer of peanut butter on a piece bread.

How did you determine which sentence to put where? If you did it like many people, you based your sentences on what you know normally happens when you make a lunch. This is similar to what geologists do to determine age.

The process of putting things in a "correct order" based on experience is called relative dating. Relative dating is determining whether an object is older or younger than other objects or events. It doesn’t give an exact age but a relative (or comparison) age. This means that you know which events came first, but not the exact time when they happened.

Imagine that throughout your life you’ve taken many pictures. Rather than putting them into a photo album they are stacked up in a box. Where are the oldest pictures, by year, going to be in your stack? At the bottom or the top? What about the newer pictures? Where are they in the stack? Rock layers are just like the stack of pictures. The layers at the bottom are the oldest. Each layer will be successively younger or more recent. This is called superposition. Superposition is the principle that says younger rocks lie above older rocks in an undisturbed sequence.

Scientists use the fossils of animals to help determine relative age. Certain groups of fossil animals and plants occur in the geologic record in a specific order. If a scientist finds one of those fossils, they can then assume the age of the rock based on the age of the fossil.

Often igneous intrusions will cut across layers of rock. The principle of cross-cutting relationships states that the intrusions are younger than the rock they cut. Therefore, the igneous intrusion may be below a layer that is older than it.

The USGS has a good introduction of Relative Dating to have you practice putting historical events in the correct order. Rock layers represent  "historical events," so a rock layer is like a page in the book of Earth's geologic history.

The following activity lets you be the geologist. There are five different pictures of one rock sequence throughout time. Number the pictures from one to five with one being the oldest and five being the youngest. Below each picture, choose the process that is taking place in the box above. (Hint- For help numbering the boxes, refer back to the last three paragraphs.) Write the term that describes the event that happened in the space below the drawing.

Examine the drawing below and answer the questions:

Analysis:

1. Is rock layer "A" younger or older than the other rocks? What evidence explains your answer?
2. Is the fault (dotted line labeled "L") older or younger than layer D? Explain your answer.
3. List the rock layers in order from oldest to youngest. (include the lines "K and L").

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