Dating Rock Layers

How to tell how old the layers of rock are.

What is Age?

There are two kinds:

-Absolute Age – The number of years since the rock formed. (150 million years old, 10 thousand years old.)

– Much harder to figure out. Often impossible.

What is Age?

– Relative Age – The age compared to the ages of other rocks. (older than this rock, younger than that rock.)

– Much easier to figure out. Can be deduced from clues on the Earth.

What are the ways to tell RELATIVE AGERELATIVE AGE.

1. Law of Superposition

2. Extrusion & Intrusion

3. Faults

4. Unconformities

5. Uniformitarianism

What is the Law of Superposition?

In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Age and Superposition

What is Extrusion?

• When magma cools on the surface of the Earth it’s called an EXTRUSION.

• Extrusions are always YOUNGER than the rock it is sitting on.

• For extrusion think exit or outside.

What is an Intrusion?

• When magma cools beneath the surface of the Earth it’s called an INTRUSION.

• Intrusions are always YOUNGER than the rocks around it.

• For intrusion think inside.

Here’s an Intrusion

Extrusions? Intrusions?

First of all, which rock layer is the oldest?What is this called?

INTRUSION

EXTRUSIONF

G

C

D

A

B

E

More rock layers are deposited on top of the extrusion.

What is a Fault?

• A BREAK in the Earth’s crust.

• A fault is always YOUNGER than the rock it cuts through.

• Faults move over time, making determining the age of the layers confusing.

Here’s a Fault

FaultsThis crack in the rock layers is called a fault.

Faults move over time causing the rock layers to move.

Faults

As more time goes by, more rock layers are added on top. Making aging the rock layers difficult.

Faults

A

A

B

B

C

C

DE

Geologists must then label each of the rock layers. This can be tough because they weren’t around to see the whole process occur.

What is an Unconformity?

• The surface where new rock layers meet a much older rock surface beneath them.

• An unconformity shows where some rock layers have been lost because of erosion.

Unconformity

Uniformitarianism

• Geologic process that happened in the past can be explained by current geologic processes.

• Processes such as volcanism and erosion that go on today happened in a similar way in the past.

• Because geologic process tend to happen at a slow rate, this means that Earth must be very old.

Summary

• Relative vs. Absolute Age• Determining Relative Age:

1. Law of Superposition – on top is younger

2. Intrusions – magma inside Earth.

3. Extrusions – magma on top of Earth.

4. Faults – breaks in the crust

5. Unconformities – signs of erosion.

6. Uniformitarianism – processes repeat and happen slowly showing extreme age of Earth.

Human Impact

How humans have changed their geological and biological

environment.

Human Impact

• Humans impact their environment in a number of different ways including the following:

1. Urbanization

2. Deforestation

3. Desertification

4. Changing Water Flow

5. Pollution

What is Urbanization?

• The growth of urban areas (cities) caused by people moving into cities.

• When cities increase in size, the population of rural areas near the city may decrease.

What is Deforestation?

• The removal of trees and other vegetation from an area.

• Urbanization can cause deforestation when forests are replaced with buildings.

What is Desertification?

• The process by which land becomes more desert like and unable to support life.

• Urbanization can also lead to desertification.

What happens when Water Flow is Changed?

• Digging irrigation canals to get water to crops changes the flow of rivers.

• Irrigation arid areas (deserts) changes the ecology of those areas.

• Building dams disrupts water flow and affects the ecology of the land and water.

What is Pollution?

• Contamination of air, water or land by pollutants from human and natural sources.

Summary

• Human Impact:1. Urbanization – building cities

2. Deforestation – cutting down trees

3. Desertification – turning land into deserts

4. Changing Water Flow – changes environment with increase or decrease of water

5. Pollution – contaminating environment