rap 3/14 pg. 42 1. how old is the earth? 2. how do we know?
TRANSCRIPT
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RAP 3/14 pg. 42
1.How old is the Earth?
2.How do we know?
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TOPIC: Geologic History
29.1 Fossils
29.2 Relative Time
29.3 Absolute Time
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Paleontology
The study of life that existed in prehistoric times
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I. Fossils -Preserved remains or traces of once living organisms
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1. Original Remains
-preserved in its entirety
Ex:
Wooly mammoths in ice and soil
Insect in amber
A. Types
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2. Replaced remains
-Soft parts replaced by minerals
Ex: Petrified wood
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3. Molds and Casts-Mold: hollow depression
(cupcake pan)
-Cast: copy of original fossil
(cupcake)
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4. Trace
-indirect evidence of life
Ex: trails
footprints
bite marks
burrows
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Trace fossils
And one more example...
coprolite
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5. Carbonaceous Remains
-thin carbon film
Ex. Fern print
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What do we know about this animal?
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Fossil Formation
Fossil formed? Type Why/Why not?
1
4
32
56
7
No not deep enough
Yes Original Remains frozen
No too deep
No too much pressure
Yes replaced remains fused with minerals
Yes mould cast minerals filled in
Yes trace uplifted
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Fossil Formation
Burying Bodies
Making Fossils
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Who dung it?
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RAP 3/15 pg. 42
1. What type of rock do most fossils occur in?
2. What is one method of preserving a fossil?
3. In the picture, which letter represents the mold? Which letter represents the cast?
A B
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II. Relative Time- placing events in the sequence in which they occurred
**Does not identify actual **dates
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A. Principle of Uniform Processes
1. processes that occurred in past produce same results as today
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B. Principle of Superposition
Deposited sediments are compressed into layers, or strata
1. Oldest layer at ; youngest at
bottomtop
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Deciphering relative time is like detective work...
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Who Dunnit?! Someone took the last cookie in the cookie jar last night.
The last person to leave is the culprit!
Clues: The Butler walks to work The Handyman rides a
bike The Cook rides a
motorcycle The Maid drives a car The Nephew has a
seeing-eye dog
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1
3
2
Sequences on pg. 44
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1
2
3
4
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C. Principle of Cross-Cutting Relationships
1. intrusion is always than rock it intrudesIgneous younger
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1
2
3
4
5
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Gaps in Relative Time
D. Unconformity
1. indicates where layers of rock are missing
a. Exist because
-Lack of deposition
-Erosion
Example
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1. Four characteristics
a. Easily recognizable
b. Abundant
c. Widespread in occurrence
d. Existed only for a brief period
E. Index Fossils
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When did I live?
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Who’s On First?: Period 6 GroupsLab Table 1Shannon
Jake
Jenny
Lab Table 2Chase
Brice
Lab Table 3Alex
Daniel
Dylan S.
Lab Table 4
Mitchell
Devin
Jordan C.
Kaitlin Bennett
Lab Table 5
Davon
Ashtyn
Trey
Lab Table 6
Josue
Yony
Ben
Luke
With Ms. Renwick
Group 7
Chrissy
Kaitlyn Bishop
Dylan Wimer
Anandhu
Group 8
Zach
Robert
Jordan B.
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Put it all together...
Decoding geologic history... And…
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RAP 3/19 pg. 42
1. Select the fossil you think would be the best index fossil and explain why.
2. Put the following events in the correct order.
A
D
CB
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F. Correlation
1. Matching of rock layers from one area to another
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Practice!
unconformity
Section 1 Section 2
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Which layers are the same?Of layers E and F, which is oldest?What is the sequence of rock layer from oldest to youngest?An unconformity (buried erosional surface) is represented by the interface between which two layers?
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III. Absolute Time
A. Use ACTUAL dates to order events
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B. Methods1. Tree rings
2. Varves- layer of sediment representing summer and winter
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RAP 3/20 pg. 46
1. What is the difference between relative and absolute time?
2. What type of fossil shows evidence of an organism but not the organism itself?
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RAP 3/21 pg. 46**QUIZ ON FOSSILS, Relative and Absolute Time
1. What is the relative age of layer C and D?
2. What two things can cause an unconformity?
3. Which letter to the right represents an unconformity?
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3. Radiometric dating
Compares ratio of naturally occurring unstable isotope (parent) and its decay products (daughter)
Pb
Ur
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Parent = original element Daughter = the product of the decay
• Example: uranium-238 decays to lead-206
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i. Half-life
-Time it takes for half the radioactive atoms to decay
=1 half-life
=2 half-lives
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In each half-life, the amount of atoms gets cut in half.
=parent
=daughter
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One half-life.
=parent
=daughter
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Two half-lives.
=parent
=daughter
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Three half-lives.
=parent
=daughter
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Four half-lives.
=parent
=daughter
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Don’t worry about the last atom. You start with so many trillions that you never really get there. (It will just decay and then they’re all gone.)
=parent
=daughter
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About how many students would have had to sit down if we started with twice as many students? What about if we only had half as many in this class? What does that tell you about how the quantity of "radioactive isotopes" affects the number that decay?Can you predict which of you is going to be the first to sit down? Why or why not?
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Keys to radioactivity
**Decay at constant rates regardless of time or climate**
**Decay begins as soon as rock crystallizes or organism dies**
**Ratio of amount of radioactive element left to the amount of stable product is used to determine the absolute age**
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RAP 3/22 pg. 46
1. What is the definition of a half life?
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Since you don’t know how many atoms you started with, a ratio between parent to daughter will tell you how many half-lives have gone by.
100% parent0% daughter0 half lives
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One half-life.
50% parent50% daughter1 half-life
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Two half-lives.
25% parent75% daughter2 half-lives
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Three half-lives.
12.5% parent87.5% daughter3 half-lives
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Four half-lives.
6.25% parent93.75% daughter4 half-lives
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Half-lives range from a fraction of a second to billions of years• Protactinium-234 has
half-life of ~1 min
• Uranium-238 has half-life of 4.5 by
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Radiocarbon limitations
Only for once living organisms Half-life=5730 years meaning the limit to
dating is about 70,000 years
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17190
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RAP 3/14 pg. 45
1.
2.
3.
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RAP 3/13 pg. 431. What is a half life?
2. You find a piece of wood and use carbon dating to determine the age. The ratio of parent to daughter is 1:1. How many half lives have passed?
3. How do varves help geologists understand the past?
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Quiz
Relative Time, Fossils and Absolute Time
Web quest
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Let’s get a little more complicated…