Transcript File - Mrs. K. Smith

Age of Fossils
Essential Standard 8.E.2.1
Unit A, 2.2
Warm-up
• How do ice cores tell scientists about
Earth’s changing atmospheric makeup?
• How do fossils form?
• Take out your homework, pg. 51
Vocabulary
•
•
•
•
Relative age
Law of Superposition
Index fossil
Absolute age
Rocks provide Earth’s
Timeline
I. Relative Age
•The age of an event or
object relative to other
events or objects.
•It does NOT tell actual
age.
How old is Agnes? Mary was
born first, then Agnes, then John.
II. Law of Superposition
• the older beds
(layers of
rock) are on
the bottom
and the newer
are on the top.
Which layer is older the
shale or sandstone?
Fault- Break in the layers of
rock (rocks must be older than
the fault).
Folded or Overturned Rock- Rock
layers that were disturbed by plate
movements, can reverse order of age.
III. Igneous
Intrusions
• When magma intrudes other
rocks, it must be younger
than the rock affected.
IV. Unconformity
• When a layer of rock is missing from a
sequence. Usually due to erosion.
• Can represent thousands to millions
of years of missing time.
• G, D, F, E, Fold, Erosion, C, H, B, A, Erosion, Fault
• D, C, I, H, Fold, Erosion, B, A, F, Erosion, G, E, Erosion
Place events in order…
• Q, O, N, M,L, P, Tilt, Erosion, H, I, J, Erosion, K
V. Index Fossils
• Fossil associated with a
particular span of geologic time
• Would not extend far vertically in
layers.
• Are widespread (found across
the entire globe)
VI. Absolute Age
• Tells the actual age of an
event or object.
• Uses radioactive dating.
• Elements that form rock decay
and become a different
element, this is measured in
half lives.
I. Half Life
• time required for half of the atoms in a
sample of radioactive element to
decay.
• How many years will it take for two half-lives to
pass?
• What % of C14 will remain at this point?
Answer
If you can see in a rock sample
14
there is one quarter C and
14
three quarters N , the sample
must have gone through two
half-lives. 5,700 x 2 = 11,400
The sample is 11,400 yrs old.
EXAMPLE
You Try…
If you can see in a rock sample
th
14
there is 1/16 of C
remaining, the sample must
have gone through ________
half-lives and is ___________
yrs old.
Half-Life
Fraction of C14
Remaining
Number of years
0
1
0
1
1/2
5,700
2
1/4
11,400
3
1/8
17,100
4
1/16
22,800
Radioactive
Element
Rubidium-87
Half-life
Decay Product
48.6 billion years
Strontium-87
Potassium-40
8.4 billion years
Argon-40
Uranium-238
4.5 billion years
Lead-206
Uranium-235
0.7 billion years
Lead-207
Carbon-14
5730 years
Nitrogen 14
1. Which of the following elements has the longest half? Which
has the shortest?
2. Which of the elements turns into lead-206?
3. If a sample had half of its original uranium-235 left, about how
old would the sample be?
4. Why do you think scientists might use so many different
kinds of radioactive minerals to date rocks?
Assignment/Homework
•
•
•
•
Complete the worksheet on half lives.
Read 53-59, questions 1-6.
Quiz next class
TEST ON 19TH & 20TH
Bring Skittles to next class for a half life
activity, will need 100 Skittles.