RELATIVE-AGE DATING

Chapter 16 Lesson 2Pages 574-580

NEW VOCABULARY

Relative Age – the age of rocks and geologic features compared with other rocks and features nearby.

Superposition – the principle that in undisturbed rock layers, the oldest rocks are on the bottom.

Inclusion – a piece of an older rock that becomes part of a new rock.

Unconformity – a surface where rock has eroded away, producing a break, or gap, in the rock record.

Correlation – matching rocks and fossils from separate locations.

Index Fossil – Fossils that represent species that existed on Earth for a short length of time, were abundant, and inhabited many locations.

FIND RELATIVE AGE

A. B

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D.

FIND THE RELATIVE AGE

FIND RELATIVE AGE

RELATIVE AGES OF ROCKS Geologists, or scientists that study Earth and rocks, have developed a set of principles to compare the ages of rock layers.

They use these principles to organize the layers of rock according to their relative age.

Relative age is the age of rocks and geologic features compared with other rocks and features nearby.

*How might you define your relative age?

PRINCIPLES OF RELATIVE AGE DATING1. Superposition

2. Original Horizontality

3. Lateral Continuity

4. Inclusion

5. Cross-cutting Relationships

RELATIVE AGE AND SUPERPOSITION Monday

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RELATIVE AGE AND SUPERPOSITION Monday

Tuesday

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RELATIVE AGE AND SUPERPOSITION Monday

Tuesday

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RELATIVE AGE AND SUPERPOSITION Monday

Tuesday

Wednesday

Thursday

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RELATIVE AGE AND SUPERPOSITION Monday

Tuesday

Wednesday

Thursday

Friday

RELATIVE AGE AND SUPERPOSITION

How is this pile of clothes organized?

Where are the oldest clothes found in the pile?

Monday

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SUPERPOSITION

Superposition is the principle that in undisturbed rock layers, the oldest rocks are on the bottom.

Unless some force disturbs the layers after they were deposited, each layer of rocks is younger than the layer below it.

ORIGINAL HORIZONTALITY

The second principle of relative-age dating is original horizontality.

According to this principle, most rock-forming materials are deposited in horizontal layers.

Sometimes rock layers are deformed or disturbed after they form.

For example, the layers might be tilted or folded.

Even though they might be tilted, all the layers were originally deposited horizontally.

LATERAL CONTINUITY

Another principle of relative-age dating is that sediments are deposited in large continuous sheets in all lateral directions

The sheets, or layers, continue until they thin out or meet a barrier.

A river might erode the layers. But their placements do not change

CROSS-CUTTING RELATIONSHIPS Sometimes, forces within Earth cause rock formations to break, or fracture.

When rocks move along a fracture line, the fracture is called a fault

Faults and dikes cut across existing rock

According to the principle of cross-cutting relationships, if one geologic feature cuts across another, the feature that is cuts across is older.

A dike is a sheet of rock that formed in a crack in a pre-existing rock. Dikes are sometimes referred to as vertical intrusions.

INCLUSION Occasionally when rocks form they contain pieces of other rocks.

This can happen when part of an existing rock breaks off and falls into soft sediment or flowing magma

When the sediment or magma becomes rock, the broken pieces become a part of it.

A pieces of an older rock that becomes part of a new rock is called an inclusion.

According to the principle of inclusions, if one rock contains pieces of another rock, the rock containing the pieces is younger than the pieces.

UNCONFORMITIES

After rocks form, they are sometimes uplifted and exposed at Earth’s surface When rocks are exposed, wind and rain start to weather and erode them. These eroded areas represent a gap in the rock record Often, new rock layers are deposited on top of old, eroded layers. When this happens, an unconformity occurs. An unconformity is a surface where rock has eroded away, producing a break, or gap, in the rock record.

An unconformity is not a hollow gap in the rock.

It is a surface on a layer of eroded rocks where younger rocks have been deposited.

However, an unconformity does represent a gap in time.

It could represent a few hundred years, a million years, or even a billion years

UNCONFORMITIES

There are three types of unconformities.DisconformityAngular UnconformityNonconformity

DISCONFORMITY

Younger sedimentary layers are deposited on top of older, horizontal sedimentary layers that have been eroded

NONCONFORMITY

Younger sedimentary layers are deposited on older igneous or metamorphic rock layers that have been eroded.

ANGULAR UNCONFORMITY

Sedimentary layers are deposited on top of titled or folded sedimentary layers that have been eroded.

CORRELATION

When geologists are faced with unconformities, gaps in the rock record, they fill in the gaps by matching rock layers or fossils from separate locations.

Matching rocks and fossils from separate locations is called correlation.

MATCHING ROCK LAYERS

Another word for correlation is connection.

Sometimes it is possible to connect rock layers simply by walking along rock formations and looking for similarities.

At other times, soil might cover the rocks, or rocks might be eroded away

In these cases, geologists correlate rocks by matching exposed rock layers in different locations

Through correlation, geologists have established a historical record for part of the southwestern United States.

INDEX FOSSILS If scientists want to learn the relative ages of rock formations that are very far apart or on different continents, they often use fossils.

If two or more rock formations contain fossils of about the same age, scientists can infer that the formations are also about the same age.

Not all fossils are useful in determining the relative ages of rock layers Fossils of species that lived on Earth for hundreds of millions of years are not helpful for identifying when a rock was formed

INDEX FOSSILS

Scientists use fossil that existed for only a short time in many different areas on Earth, like trilobitesThese are called index fossils

When an index fossil is found in rock layers at different locations, geologists can infer that the layers are of similar age