Unit 7: Geologic History
I Introduction
A. The geologic history of the Earth can be determined by studying rocks. This tells us about
1. changes in climate
2. changes in living things
(text p.334)
B. We can explain the past using processes occurring today.
This view is called Uniformitarianism
Uniform = the same
“The present is the key to the past.”
Ex. How did the Grand Canyon form?
What people used to think:

“Noah’s flood!”
This view is called Catastrophism
Using uniformitarianism:
Answer: Long, slow weathering and erosion by the Colorado River
What killed the dinosaurs?
Catastrophism makes a comeback-
YouTube - Chicxulub impact visualization
II How to tell the Age of a Rock
A. Relative Dating : determines the age of a rock compared to other rocks.
1. Superposition : the oldest sedimentary rock layer, or strata, is found on the bottom, the youngest are on top.


Most sed. rock strata form from sediments deposited in water.
 Ex. Where this is happening today-
Great South Bay: image Great South Bay
2. Sed. Rock Strata are originally formed in horizontal layers .
(text p.338)
The Principle of Superposition
[Diagram]
3. Folds, Faults and Cracks in the rocks always occurred after the strata formed and are younger. (text p.338)
The Principle of Original Horizontality
Folded Layers
Faulted Layers
[Diagram]

4. Igneous Intrusions and Extrusions are younger than the rock layers they cut through. (text p.339)
[Diagram]
The Principle of Cross-Cutting Relationships igneous intrusion - Google Image Search
5. Fragments of rock found inside another rock are always older than the rock of which they are a part.
[Diagram]
(text p.339)
6. Fossils , the remains or traces of living things, give clues to the relative age of sed. strata. (Text p.344)
B. Unconformities : form when rock layers are eroded away.

New layers may form on this erosion surface.

This forms a gap in the rock record.

[Diagram]see p. 340
Analyzing Sequences
Formation of an Unconformity great unconformity - Google Image Search
[ Lab: sequence of events ]
C. Correlation : means matching up rock layers great distances apart.
(see text p.341)
Grand Canyon Correlation image
1.
Some New Vocabulary

Outcrop : an exposure of rocks at the
Earths surface.

Formation : a single uniform layer of rock.
2. Techniques for Correlating Rock
Formations:

 “Walking the Outcrop” : visually matching up formations while traveling along outcrop.

Rock Similarity : formations are matched up based on similar appearance.
[Diagram] correlation by rock similarity

Volcanic Ash Layers : form when ash from a volcano spreads over a wide area and is preserved in the rocks. spread of volcanic ash
This ash layer can be used as a
“time marker”.
[Diagram]
Ash layer 1
Ash layers in ice of glacier
Ash layer 2 volcanic ash layer 3 volcanic ash layer 4
3. Index Fossils are the best method of

rock correlation.

Formations are matched up by similar fossils. (see text p.346) image index fossils ]
(see esrt p.8-9)

What makes a good index fossil?
Animal or plant lived over a wide area
(large geographic distribution)
Animal or plant was very numerous.
(only a small fraction of living things are
turned into fossils.)
Animal or plant lived only during one time period.
If these conditions are met, the fossil will be widespread, but limited to only one specific rock layer.
[Diagram]

Review Powerpoint: rock correlation ppt.
YouTube - A look at the vernal equinox
[Lab: Rock Correlation]
D. Absolute Dating : finding the age of rocks in years before the present (BP).
1. There are about 90 types of atoms that make up all matter. These are called elements .
(see inside back cover of text)
2. Some elements naturally transform into other elements.
This is called Radioactive Decay.
[diagram]
3. Radioactive decay occurs at a predictable rate that is not changed by temperature, pressure, or chemicals.

4. Half-Life is the amount of time needed for ½ or 50% of the isotopes in a rock to change into another element.
Applet: Decay
[Diagram: radioactive decay graph]
(text p.348)
5. By measuring the amounts of an isotope and its decay product the age of a rock can be calculated.
YouTube - Radioactive Decay explained
Example: Isotope A decays into B. The half-life is 1 million years.
Rock age
(yrs.)
0
Number of half lives old
Amounts of
A and B
1 million
2 million

3 million
From ESRT p. 1 and text p.349
Isotope Changes into
Carbon-14
(C 14 )
Half-Life
(years)
Nitrogen-14 (N 14 ) 5,700
Potassium-40
(K 40 )
Uranium-238
(U 238 )
Argon-40 (Ar 40 ) or
Calcium-40 (Ca 40 )
1.3 Billion
Lead-206 (Pb 206 ) 4.5 Billion
Rubidium-87
(Rb 87 )
Strontium-87 (Sr 87 ) 49 Billion
YouTube lesson- Page 1-Radioactive Decay
6. C-14 is best for dating recent (last
50,000 years), organic (once alive) remains . (text p.350)
Examples: Wood, bones, charcoal
YouTube - Radiocarbon Dating

Carbon 14 Dating Calculator
7. K-40 and U-238 are best for dating most Earth rocks.
8. Rb-87 would be used to date the very oldest Earth rocks, moon rocks, and meteorites.
YouTube - Radiometric Dating
YouTube lesson - Radioactive Decay
III The Geologic Time Scale
A. The Earth is incredibly old by human standards.
Earliest Human-like fossils about 2 MYA.
0.04% of Earth History
Earliest Human Civilization about 8,000 Y.A.
0.00013% of Earth History
B. The Geologic Time Scale is divided into
time periods based on changes in fossils.

(See ESRT p.8 and 9, Text p.ES40-41)
ESRT
The earth is about 4.6 Billion years old. If you started counting to 4.6 billion, saying one number each second, counting 24 hours a day, 7 days a week, you did not eat, sleep, or anything else but count:
It would take you about 11.5 days to reach
1 million .
Almost 4 months to reach 10 million.
About 3.5 years to reach 100 million
35 years to reach 1 billion and 161 years to reach 4.6 billion!
C. Largest Time units are Eons
1. Precambrian spans from the origin of the Earth to 544 MYA (88% of Earth history)
How did life first begin on Earth? One theory:
YouTube - Hydrothermal Vents


Living things lacked bones, shells, or teeth so fossils are very rare.

Life on Earth begins and oxygen atmosphere forms.
See ESRT for other major events.
Snapshot of Life #1: Oldest Microfossils-
3.6 Billion years (Maybe)- Apex Chert-
Australia: apex chert - Google Search
Snapshot of life #2: Gunflint Chert- 1.9
Billion years (Canada) gunflint chert - Google Search
Past lives: Chronicles of Canadian Paleontology - Gunflint Chert fossil stromatolites living stromatolites- sharks bay
Snapshot of Life #3: Ediacaran Fauna- 560
Million years
Ediacaran Fauna - Google Search recreation of seabottom -560 MYA
2. Phanerozoic spans from 544 MYA to the present day (12% of Earth history).


Fossils are abundant because living things developed hard parts.
Snapshot of Life #4: Burgess Shale- 505
MYA (Canada)
Burgess Shale trilobites another strange fossil from the Burgess Shale
Burgess Shale seabottom
1. Paleozoic Era (6% of Earth History)
End marked by greatest extinction event in
Earth history- 90% of all life disappears.
See ESRT for major events
2. Mesozoic Era (4% of Earth History)
See ESRT for major events
YouTube - Greatest Discoveries: Origin and Evolution of Life Pt.1
(start
1:47)
YouTube - Greatest Discoveries: Origin and Evolution of Life Pt.2
(end
3:00)
3. Cenozoic Era (2% of Earth History)
See ESRT for major events

RB p.264 #1-15
YouTube - Our 4.6 billion years old planet. The big picture
Evolution of Life in 60 Seconds § SEEDMAGAZINE.COM
IV Fossils and Evolution
A. Fossils show a gradual change in the features of living things. horse evolution
Life has developed from less complex
(bacteria, algae) to more complex (trees, whales, humans).
B. In the 1860’s Charles Darwin developed the
Theory of Evolution by Natural Selection.

This explains how the features of living things have changed through time.
YouTube - Greatest Discoveries: Origin and Evolution of Life Pt.1
Ex. Dogs/Wolves,
New Finding Puts Origins of Dogs in Middle East - NYTimes.com
Lions/Tigers
, liger - Google Image Search

Human/Neanderthals, neanderthal - Google Image Search
Human Skin Color

It also explains how new types of living things (species) arise.
Natural_Selection.asf
C. Over geologic time many types of living
things have disappeared (become extinct ).
Text p.390-diagram

Mass Extinction Events have occurred many times including the end of the
Paleozoic and the Mesozoic Era’s.
RB p.273 #1-15
[complete, discuss]