1. 4.2 The Relative Age Of Rocks
Earth’s History – Course 2 - Mrs. Bloch
The Relative Age Of Rocks
Essential Question: How do Scientist study Earth’s past?
How Old are Rock Layers?
How Can Rock Layers Change?
Pgs

2. My Planet Diary for Florida pg. 146
Florida Mines Many different kinds of rocks are mined in Florida. The state has one of the most active mining industries in the country. In addition to providing the minerals needed to pave roads and construct buildings, the quarries that the rocks come from also hold fossils. These fossils could be used to figure out how old the rocks surrounding them are.
Communicate: Rock quarries
can be used by both miners
and scientists. What other
natural resources have
multiple purposes?

3. How Old Are Rock Layers? Pg. 146
2 Ways to determine the age of a rock
Relative Age vs. Absolute Age
The Relative Age of a rock is its
age compared to the ages of other
rocks.
The Absolute Age (Numerical Age)
of a rock is the number of years that
have passed since the rock formed.

4. The Law Of Superposition
Rock Layers pg. 147
The Law Of Superposition
Geologists use the law of superposition to determine the relative ages of sedimentary rock layers.
According to 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.

5. Law of Superposition
The law of superposition states that the older layers of rock are on the bottom and the younger layers of rock are on the top.

6. Clues From Igneous Rock pg. 148
Geologists study extrusions, intrusions, faults, and index fossils to determine the relative ages of rocks.
An extrusion is formed by lava
that hardened on the Earth’s
surface and formed igneous rock.
An intrusion is magma,
Which cools and hardens into
a mass of igneous rock below
the Earth’s surface.
A fault is a break in Earth’s crust.
Forces inside Earth cause movement of the rock on opposite sides of the fault. A fault is always younger than the rock it cuts through. To find its relative age of the fault, geologist study the relative age of the youngest layer cut by the fault.

7. The Relative Age of Rocks
APPLY IT!! Pg. 148
The Relative Age of Rocks
The diagram shows rock layers found at a site. Identify the area on the diagram that shows an intrusion. What is the oldest layer of rock in the diagram?
Circle the area on the diagram that shows an intrusion.
Shade the oldest layer on the diagram.
Infer. What can you infer about the relative ages of areas B and E?

8. Scientists use index fossils to match rock layers.
The Relative Age of Rocks pg. 149
Index Fossils
Fossils that are widely distributed and that represent organisms that existed for a geologically short period of time could be used as index fossils Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
Scientists use index fossils to match rock layers.
Match the rock layers in locations 2, 3, and 4 with first area shown. Which fossils can be used as index fossils?

9. Assess Your Understanding pg. 149
1a. Explain In an area with several different rock layers, which is oldest? Explain. ______________________________________ ___________________________________________________ b. Infer How could a geologist match the rock layers in one area to rock layers found in another area? _____________________ I get it! Now I know that you can find the relative age of rocks by

10. How Can Rock Layers Change? Pg. 150
The geologic record of sedimentary rock layers is not complete. Most of our planet’s geologic record has disappeared through erosion. Gaps in the geologic record and folding can change the position in which rock layers appear. These changes make it more challenging for scientists to identify accurately the ages of some rocks. (motion along faults can also change rock layers line up) An unconformity is a gap in the geologic record. An unconformity shows where rock layers have disappeared due to erosion. Folding is when forces inside the Earth fold rock layers so much that the layers are turned over completely and the youngest layers may be on the bottom! Folding and faulting in the Earth's crust

11. The Relative Age of Rocks – pg. 150
Unconformities
and Folding
What are the oldest and youngest layers in the last two diagrams?
Identify the unconformity. Which is the part of the fold that is overturned?

12. Apply It! Pg. 151 What does the photo show?
(an unconformity / folding)
2. What evidence do you see for your answer?
_____________________________________
3. Challenge. What can you infer about the
History of this area?

13. Assess Your Understanding pg. 151
2a. List Name two ways rock layers can change. ___________________________________________________ b. Explain. How does folding change rock layers? ______________________________________________________________________________________________________ c. Draw Conclusions. Two locations include a layer of rock with a particular index fossil. In one location, the layer occurs in a higher position than in the other. What can you conclude about the history of the two areas.

14. Radioactive Dating
Marie Curie
In 1896, French scientists named Marie and Pierre Curie heard about experiments that had been done by another scientist, Henri Becquerel. Marie Curie later described what happened: