1. Earth Science Chapter 6
I. Rocks of the crust provide clues to Earth’s past
By analyzing these clues we can infer events from the past

2. A. Principle of Uniformitarianism
1. Major assumption in geology
2. Events in the past occurred the same way that they are occurring today. Examples Include:
Weathering/erosion
Deposition
Volcanism
Plate tectonics

3. B. Catastrophism
Principle that states that geologic change (formation of mountains, canyons, seas) occurs suddenly during rare events
C. Modern Geology
1. view most changes as happening gradually (uniformitarianism), with some catastrophes creating change (catastrophism)

4. D. Geologic Time
Geologists have divided Earth’s history into time units based on the the fossil record

5. 1. A study of the fossil record shows
A great variety of plants, animals, and simpler life forms have lived on Earth in the past

6. II. Geologic Dating

7. A. There are two types of Geologic Dating
1. Absolute
Determines how many years old something is
2. Relative
Used to determine if one thing is younger or older than another

8. 3. Tools used to determine
Absolute Age
Relative Age
Using radiometric dating
Law of superposition
Use of index fossils
Correlation of rock layers

9. B. Relative Dating

10. 1. Law of Superposition
Sediments are laid down underwater in horizontal layers and form sedimentary rocks

11. Law of Superposition
In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest
Lower layers must be in place before younger rocks can be deposited on top of them
Exception: when something occurs to overturn layers

12. Grand Canyon

13. GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM

14. Law of Superposition Rock layers are older than folds found in them
Layers were there before they were folded

15. Folds/Tilted

16. Law of Superposition Rock layers are older than faults found in them
This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

17. Law of Superposition
Fossils are generally the same age as the rock layers in which they are found
Animal remains are deposited along with the sediments that will turn into sedimentary rocks

18. Law of Superposition
Igneous intrusions are younger than the rock that they cut through or flow out of

19. Igneous Intrusion - Cross Cutting

20. Unconformity
When a new rock layer is formed atop an eroded surface

22. Taconic Unconformity Upper Silurian Carbonates
August 18
Upper Silurian Carbonates
Tilted Ordovician Shales and Sandstones
We stopped along the side of the road where engineers had cut into the rock formations to make the road. The unconformity was clearly visible showing banded carbonate rock overlying tilted shales and sandstones.
unconformity
Taconic Unconformity
Allen

23. Practice: what happened here?
August 18
Practice: what happened here?
Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the next slide.
Allen

26. Correlation
Matching similar rock layers in different locations to see if they formed at the same time

27. f03_09_pg37
f03_09_pg37.jpg

28. 1. Which layers are the same?
2. Which layer is older E or F?
3. What is correct sequence of rock layers from oldest to youngest?
4. An unconformity is represented by the interface between which 2 layers?

29. Which fossil might be found in Devonian rock layers?

30. Volcanic Ash Falls
Can also be used to correlate rock layers over a large area
Ash is a good indicator because:
The ash from one explosion has distinct characteristics
ash can be deposited around the globe
The event occurs at one, geologically brief, time

31. August 18
Fossils
Naturally occurring preserved remains or impressions (physical evidence) of living things
Generally only hard parts get preserved
Bones, teeth, shells
Hard parts are replaced by naturally occurring minerals
Allen

32. Eurypterus
NY State Fossil
Silurian index
fossil

33. Fossils Types Ice-fossil is preserved in Tarfossil is preserved in
Amber-fossil is preserved in
Petrified-common with bone or wood, spaces are filled up with minerals or replaced by minerals

34. Trace Fossils Preserved evidence
August 18
Trace Fossils
Preserved evidence
Can be a mold (impression, mark, or cavity made by shell or other surface)
Ex: dinosaur footprint
can be a cast (forms when mold is filled in)
Ex: filled in footprint
Allen

36. Fossils Why don’t fossils exist in most igneous or metamorphic rock?
August 18
Fossils
Generally found in sedimentary rock layers
Why don’t fossils exist in most igneous or metamorphic rock?
Allen

37. August 18
Fossils
The living organism that made the fossil lived during the time the rock layer was forming
i.e. when the sediment was being deposited
Allen

38. Fossils Fossils can provide information about ancient environments
Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…

39. Fossils A study of the fossil record shows:
August 18
Fossils
A study of the fossil record shows:
A great variety of plants, animals and simpler life forms have lived on Earth in the past
That life forms have evolved through time
Most life forms of the geologic past have become extinct
Allen

40. Index Fossils Index Fossils--used to date rock layers (strata)
Fossils from creatures that existed for a geologically short period of time
Ex. less than 2.0 x 107
Fossils from creatures that had a wide geographic distribution

41. Which letter would make a good index fossil?
Location A
Location B
Location C
Rock layer 1 W
W Z
Rock layer 2
W Z Y Z
Rock layer 3
W X X
X Z
Which letter would make a good index fossil?

42. Absolute Dating

43. When an unstable radioactive element changes into a stable element
Radioactive Decay
When an unstable radioactive element changes into a stable element

44. Example Unstable Radioactive Isotope Stable Decay Product
Carbon 14 (C14)
Nitrogen 14 (N14)
Uranium 238 (U238)
Lead 206 (Pb206)
Parent
Daughter

45. Radiometric Dating
Half life: The time required for 1/2 of a parent material to break down to daughter material

46. Another look

47. Radiometric Dating
The age of objects can be determined by measuring the relative amounts of radioisotope and decay products

48. Radiometric Dating
The half lives of radioisotopes vary depending upon the isotope
Examples:
C14 = 5,770 years
U238 = 4,500,000,000 years
Rb87 = 47,000,000,000 years

49. Carbon 14 is used to date biological remains
Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies
Uranium 238 is used to date rocks
Larger half life

50. Geologic Time
Earth is about 4,600 million years old! (4.6 billion years!)
Rock Quarry in Utah has fossil bones from 150 million years ago!
150 million years is only 3% of the time Earth has existed
Grand Canyon- layers represent 2 billion years of Earth’s history

51. The geologic time scale:
Represents the billions of years that have passed in Earth’s history
Divides Earth’s 4.6 billion years into sections of time
Eons-largest divisions of time
Eras- includes 2 or more periods
Period- parts of an era
Epoch -subdivision of a period

52. Paleozoic Era [542 million to 251 million years ago]
first era that is well represented by fossils, includes marine life, but few land organisms
By the middle of this era, all of the modern plants groups appeared
By the end of the era, amiphibians and reptiles lived on land

53. Mesozoic Era-[ 251 million years ago]
Age of reptiles, dinosaurs dominated
Small mammals appear
Birds appeared late in this era, likely evolved from a type of dinosaur
At the end of this era, 15-20% of all species became extinct

54. Cenozoic Era [65.5 million years ago]
Age of the mammals; mammals flourished after mass extinction at the end of the Mesozoic era
Being warm blooded and bearing live young may have helped mammals survive the environmental changes that may have caused the extinction of the dinosaurs