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The Relative Age of Rocks

Objectives:

Explain the law of superposition
Explain how geologists determine the relative age of rocks
Evaluate the usefulness of index fossils.

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Geologic Time Scale

The Geologic Time Scale is a record of the life forms and geologic events in Earth’s history.

Why do we not talk in terms of calendars when we talk about the Earth’s geologic history?

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Relative vs. Absolute age

What is the difference between these two statements:

I am older than my sister.
I am 42 years old.

Geologists use both of these ways to express the age of a rock and any fossil found in it.

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Relative age: The age of a rock compared to the ages of other rocks.

Absolute age: The number of years since a rock was formed.

Everyone and everything can be given a relative and absolute age.

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The Position of Rock Layers

Scientists have a hard time determining the absolute age of rocks. Why do you think that is so?
The law of superposition states that in horizontal sedimentary rock layers the youngest layer is at the top.
How can the law of superposition help us determine the relative age of a sedimentary rock layer?

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Law of superposition - activity

In your formative, notice the question
Assuming that CT is the oldest layer, position the cards from oldest to youngest.
Be ready to explain your reasoning.

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Determining Relative Age

There are other clues besides the position of rock layers.

Cross-Cutting Principle: When something cuts across a body of rock, that “something” is younger than the rock it cuts across.

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Determining Relative Age

Clues from igneous rocks:

Lava that hardens on the surface is called an extrusion. An extrusion is always younger than the rocks below it.
Lava that cools and hardens below the surface is called an intrusion. An intrusion is always younger than the rock layers that surround it.

WHY?

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Clues from faults: A fault is always younger than the rock it cuts through

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What happens when parts of a layer have eroded away or been otherwise lost?

Uncomformity: When new rock layers are much younger than the layers beneath it.

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Using Fossils to Date Rocks

To date rock layers, geologists first give a relative age to a layer of rock at one location. Then they can give the same age to matching layers of rock at other locations.

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Example using Silly Bandz

Points of agreement:

Silly Bandz were popular between July-Oct 2010.
Silly Bandz could be bought anywhere during that time.
By Dec 2010, it had become very difficult to get Silly Bandz.
Using most people’s standards, by December 2010 it had become “silly” to wear Silly Bandz.

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Now let’s assume that I open my daughter’s dresser today and find a pile of Silly Bandz. When can I assume that she put them there? *

Let’s travel 3 years into the future. My daughter is finally cleaning out her dresser. She finds a couple of Silly Bandz sandwiched between a purple shirt (bottom) and a blue shirt (top).

What is the relative age of the purple shirt? How do you know? *

Finally, let’s imagine that Hannah finds an undated letter from Annie with a brand new packet of Silly Bandz. Can she accurately estimate its date? Why or why not. *

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Back to geology, index fossils (Silly Bandz) help geologists match rock layers, and helping to determine the relative ages of the rocks in which they occur.

To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed only briefly. Why?*

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Use the following information to arrange the fossil cards in order.

Pelecypods appeared 500 million years ago.

Trilobites disappeared 250 million years ago.