1. Law of Superposition
Chapter 8 Lesson 2

2. Secrets of the Layers of Rock

3. Law of Superposition
In layers of horizontal sedimentary rock, each layer is older than the layer above it and younger than the layer below it
“Oldest on bottom, youngest on top”

4. Age of Rocks Pages 304-305 in Ch. 8
The relative age of a rock is its age compared to the ages of other rocks.
Ex: You are a 6th grader. Most 6th graders are about years old.
The absolute age of a rock is the number of years since the rock formed.
Ex: You are 11 years, 7 months old

5. Relative age Activity
As a class you are going to line yourselves up by your relative age
(from oldest by the door to youngest to the other side of the room)

6. Absolute age
Now you are going to line yourselves up by your relative age
(from oldest by the door to youngest to the other side of the room)
Ex: You are 11 years, 7 months old

7. Intrusions: Magma pushes bodies of rocks below the surface.
Fault: A break in the earths crust * They are always younger than the rock it is cutting through
Intrusions: Magma pushes bodies of rocks below the surface.
It cools and hardens into a mass of igneous rock
* Intrusions are always younger than the rock layers it around it
Extrusions:
Lava that hardens on the surface and forms igneous rock
*extrusions are always younger than the rocks below it

8. How Rock Layers Can Change
Pages in Ch. 8
Unconformities
A gap in the geological time record. It shows were rock layers have been lost due to erosion
Folding
Layers are overturned completely. The youngest layer may end up as the top layer

9. How do fossils help us to tell time?

10. Index Fossils
Fossils of widely distributed organisms that lived during only one short period.
Example: ammonite

11. Methods for Dating the Ages of Rocks:
Relative Age
Index Fossils
Extrusion (igneous)
Intrustion (igneous)
Law of Superposition
Folding
Uncomformity
*The age of rock compared to the age of other rocks
*Sedimentary rock ( usually)
Absolute Age
Radioactive decay
Radioactive dating
Half-life
Carbon-14 dating
*The number of years since the rock formed
*Igneous Rock

12. Radioactive Decay
Elements in rocks can break down, or decay, over time. Scientists study the amount of decay to discover the age of rocks.
During radioactive decay, the elements of one atom break down to form atoms of a different element.
This rate (or amount/speed) of decay is measured by the half-life (how long it takes half the atoms to decay).

13. Radioactive Dating Half-Life
The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay. What pattern do you see in the graph?

14. Radioactive Dating
Studying radioactive decay (breakdown of rocks) to determine the age of a rock.
That information helps scientists calculate how old a rock is.
In Carbon-14 dating, scientists see how much of the element carbon-14 is left in a rock. When carbon-14 decays, it turns into Nitrogen-14, so how much carbon-14 is left will tell scientists how long it has been decaying, therefore telling them the age.

15. Radioactive Dating
Elements Used in Radioactive Dating

16. Geologic Time
The Geologic Time Scale: A record of the geological events and the evolution of life forms as shown in the fossil records
The divisions of the geologic time scale are used to date events in Earth’s history.
It helps make it easier to understand how things have changed over time, or evolved

17. Eras
Divided time between the Precambraim Time and the present time into 3 long units
Periods
Eras are subdivided into units of geological time
* The Precambrian Time covers about 88% of Earth’s History and it ended 542 million years ago. Very few fossils have survived this time period

18. How has Earth Changed over time?
Natural processes such as weathering, erosion and plate tectonics have reshaped Earth’s surface.
The distribution of land and water on Earth has changed over time. Pangea is the large landmass that use to be on earth. Overtime, the landmass broke up into different smaller landmasses that have become our continents
Uniformitarianism – geologic processes that we have today we also had in the past.
This helps scientists understand why and how the Earth changed. The Earth is always changing or evolving. By studying Earth’s changes we see in the present, scientists can make inferences about the past. (like changes in rocks, weather, etc)

19. Geologic Time Pangea The Evolving Earth
The timeline shows how Earth has changed over time.
Pangea
The name of the large landmass when the continents were formed into one large landmass.