1. EARTH HISTORY
Chapter 6 review

2. THREE FUNDAMENTAL PRINCIPLES
1. UNIFORMITARIANISM:
-The processes that shape the Earth today are the same processes that occurred in the geologic past
“The present is the key to the past”

3. 2. ORIGINAL HORIZONTALITY
-Sedimentary rock layers are ALWAYS laid down in horizontal layers, until some other process alters them

4. 3. SUPERPOSITION
-The bottom layer of horizontal sedimentary rock layers is always the oldest, UNLESS the layers have been overturned or have had older rock formed on top of it
*The lower layers must first be in place before the next layer can be deposited

5. YOUNGEST
OLDEST

6. Igneous Extrusions & Intrusions
When molten rock flows on the Earth’s surface forming an igneous rock

7. -Causes surrounding layers to metamorphose (Contact metamorphism)
Intrusion:
Occurs when magma squeezes between layers of pre-existing rock beneath the Earth’s surface
-Causes surrounding layers to metamorphose (Contact metamorphism)

8. Folds and Faults are always younger than the original
rock layers
Fold:
-bends in rock layers produced by movements of the earth’s crust
Faults:
-breaks in the rock where shifting of rock layers has occurred, often associated with earthquakes

9. Fossils
-Preserved remains or trace evidence of a plant or animal living in the past
Fossils reveal
clues to ancient
environments
Mrs. Sharp

10. By comparing index fossils in various
Index Fossils: Fossils that are found geographically widespread and lived for
a short period of time
By comparing index fossils in various
locations on Earth, it is possible to
correlate (match) the relative ages
of the rocks in which they appear
“Geographically
widespread” means the
index fossils are found
in more than one rock column
Lived for a “short
period of time” means
the index fossils are found in only one layer

11. HOW CAN ROCKS
BE CORRELATED?

12. What is Correlation?
When geologists try to match rock outcrops in different locations to see if they formed at the same time.
How to correlate (match) rock layers:
Similarity of rock types
Matching index fossils
Volcanic ash layers used as time
markers

13. Relative Age of rocks: (sequence)
The age of a rock layer in
comparison to its surrounding layer
-Uses rock similarity, fossil
evidence, and volcanic time markers
to determine its order of occurrence
Absolute Age of rocks: (true age)
The age of a rock layer in years
-Uses radiometric dating to determine age--radioactive decay

14. Radiometric Dating of Rocks
Radioactive decay is the process by
which the natural breakdown of
unstable atoms of an element occurs, releasing particles and energy (heat), and changes that element’s atoms into a
new element
Example: it takes 4.5 billion years for uranium 238 to change into lead 206

15. Radioactive decay is NOT AFFECTED by temperature (heat) or pressure!
occurs at a constant
rate known as half-life.

16. ESRT pg. 1 Radioactive Decay Data chart
HALF-LIFE:
Half-life is the rate (time) it takes for one-half of the amount of original material to decay
If we know the half-life of a radioactive material, the age of the material can be determined by measuring the amount of decayed material in the sample.
ESRT pg. 1 Radioactive Decay Data chart

17. Some radioactive substances have a Short half-life: Carbon 14
-Good for dating recent organic
remains (between 1,000-50,0000 yrs.)
238
206
Uranium
decays to Lead
Long half-life: Uranium238
-Good for dating much older rocks
(a very long half-life) - it takes 4.6 billion years for uranium to decay to lead

18. Example:
The amount of Carbon-14 remaining in a fossil is 0.5 grams. How old is the fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  _____  _____)
1.How many half-lives did the sample
undergo?
2.Multiply this by the half-life for
Carbon-14 (ESRT)

19. Example:
The amount of Carbon-14 remaining in a fossil is 0.5 grams. How old is the fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo?
2.Multiply this by the half-life for
Carbon-14 (ESRT) Answer: 2 x 5,700 = 11,400 yrs.

20. Example:
The amount of Carbon-14 remaining in a fossil is 0.5 grams. How old is the fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? 2
2.Multiply this by the half-life for
Carbon-14 (ESRT)

21. Example:
The amount of Carbon-14 remaining in a fossil is 0.5 grams. How old is the fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? 2
2.Multiply this by the half-life for
Carbon-14 (ESRT)

22. organism shows the original amount of Carbon-14 was 2.0 grams.
Example:
The amount of Carbon-14 remaining in a fossil is 0.5 grams. How old is the fossil? An equal sample of an existing
organism shows the original amount of
Carbon-14 was 2.0 grams.
(2.0g  1.0g  0.5g)
1.How many half-lives did the sample
undergo? (how many times did it decay)
2 half-lives
2.Multiply this number by the half-life for
Carbon-14 (ESRT) Answer: 2 x 5,700 yrs. = 11,400 yrs.