RAP 3/14 pg. 42
1. How
old is the Earth?
2. How do we know?
TOPIC: Geologic
History
29.1 Fossils
29.2 Relative Time
29.3 Absolute Time
Paleontology

The study of life
that existed in
prehistoric times
I. Fossils -Preserved remains or traces
of once living organisms
A. Types
1. Original Remains
-preserved in its
entirety
Ex:
Wooly mammoths in ice
and soil
Insect in amber
2. Replaced remains
-Soft parts
replaced by
minerals
Ex: Petrified wood
3. Molds and Casts
-Mold: hollow depression
(cupcake pan)
-Cast: copy of original fossil
(cupcake)
4. Trace
-indirect evidence of
life
Ex:
trails
footprints
burrows
bite marks
Trace fossils
And one more example...
coprolite
5. Carbonaceous Remains
-thin carbon film
Ex. Fern print
What do we know about this animal?
Fossil Formation
Fossil formed?
1
2
3
4
5
6
7
Type
No
Why/Why not?
not deep enough
Yes
Original Remains frozen
No
too deep
No
too much pressure
Yes
replaced remains
Yes
mould cast
Yes
trace
fused with minerals
minerals filled in
uplifted
Fossil Formation

Burying Bodies

Making Fossils
Who dung it?
RAP
1.
2.
3/15
pg. 42
What type of rock do most fossils
occur in?
What is one method of preserving
a fossil?
B
3. In the picture, which
letter represents the
mold? Which letter
represents the cast?
A
II. Relative Time- placing events in
the sequence in which they occurred
**Does not identify actual dates **
A. Principle of Uniform
Processes
1. processes that occurred in past
produce same results as today
B. Principle of Superposition

Deposited sediments are
compressed into layers, or
strata
1. Oldest layer at bottom;
youngest at top
Deciphering relative time is like
detective work...
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
Sequences on pg. 44
3
2
1
4
1
2
3
C. Principle of Cross-Cutting
Relationships
1. Igneous intrusion is always younger
than rock it intrudes
4
2
1
3
5
Gaps in Relative Time
D. Unconformity
1. indicates where layers of rock are
missing
a. Exist because
-Lack of deposition
-Erosion
Example
E. Index Fossils
1. Four characteristics
a. Easily recognizable
b. Abundant
c. Widespread in occurrence
d. Existed only for a brief period
When did I live?
Who’s On First?: Period 6 Groups
Lab Table 1
Lab Table 4
Shannon
Jake
Jenny
Mitchell
Lab Table 2
With Ms. Renwick
Devin
Group 7
Jordan C.
Chrissy
Kaitlin Bennett
Kaitlyn Bishop
Dylan Wimer
Chase
Brice
Lab Table 5
Lab Table 3
Ashtyn
Alex
Daniel
Dylan S.
Trey
Anandhu
Davon
Lab Table 6
Josue
Yony
Ben
Luke
Group 8
Zach
Robert
Jordan B.
Put it all together...


Decoding geologic history...
And…
RAP 3/19 pg. 42
1. Select the fossil
you think would be
the best index
fossil and explain
why.
B
A
D
C
2. Put the following
events in the
correct order.
F. Correlation
1. Matching of rock layers from one
area to another
Practice!
unconformity
Section 1
Section 2
What
AnWhich
unconformity
islayers
the
sequence
(buried
ofsame?
rock
erosional
Of
E and
F,the
which
islayer
oldest?
layers
are
from
surface)
oldest
is to
represented
youngest? by the interface
between which two layers?
III. Absolute Time
A. Use ACTUAL dates to order events
B. Methods
1. Tree rings
2. Varves- layer of sediment
representing summer and winter
RAP 3/20
1.
2.
pg. 46
What is the difference between relative and
absolute time?
What type of fossil shows evidence of an
organism but not the organism itself?
RAP 3/21
pg. 46
**QUIZ ON FOSSILS, Relative and
Absolute Time
1.
2.
3.
What is the
relative age of
layer C and D?
What two things
can cause an
unconformity?
Which letter to the
right represents
an unconformity?
3. Radiometric dating
Compares ratio of
naturally occurring
unstable isotope
(parent) and its
decay products
(daughter)
Pb
Ur


Parent = original element
Daughter = the product of the decay
• Example: uranium-238 decays to lead-206
i. Half-life
-Time it takes for half
the radioactive atoms
to decay
=1 half-life
=2 half-lives
In each half-life, the amount of
atoms gets cut in half.
=parent
=daughter
One half-life.
=parent
=daughter
Two half-lives.
=parent
=daughter
Three half-lives.
=parent
=daughter
Four half-lives.
=parent
=daughter
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
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?
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**
RAP
1.
3/22
pg. 46
What is the definition of a half life?
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% parent
0% daughter
0 half lives
One half-life.
50% parent
50% daughter
1 half-life
Two half-lives.
25% parent
75% daughter
2 half-lives
Three half-lives.
12.5% parent
87.5% daughter
3 half-lives
Four half-lives.
6.25% parent
93.75% daughter
4 half-lives

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
Radiocarbon limitations


Only for once living organisms
Half-life=5730 years meaning the limit to
dating is about 70,000 years
17190
RAP 3/14
pg. 45
1.
2.
3.
RAP 3/13 pg. 43
1.
2.
3.
What is a half life?
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?
How do varves help geologists
understand the past?
Quiz

Relative Time, Fossils and Absolute Time
Web quest
Let’s get a little more
complicated…