Transcript Relative Age - (www.ramsey.k12.nj.us).

Geological Time
The Earth’s Age
a. Up until the 1700s Earth’s
age was estimated to be
6,000 years old
b. Today: Earth’s age is
estimated to be 4.6 billion
years old.
c. Determined by absolute
dating or radiometric
isotopes
Importance of Rock Record
• Paleoenvironment & Climate
Was this place a swamp? Coral reef?
Desert? Tropical forest? Covered in
ice?
• Rates of Climate Change
Has Earth rapidly warmed or cooled
before? What’s Earth’s normal?
• Document Evolution
Fossil record
• Major Events: Meteroid impact;
Mountain building (uplift); Rifting;
Glaciation
Match each of the following:
Weathering
Erosion
Deposition
1. The process of chemically or physically
breaking down rock into smaller pieces
1. The process of wearing down rock and
carrying it away to a new location
2. Sediment is laid down in a new location
Sedimentary Rocks
a. Weathering and
erosion breaks
down rocks into
sediments
b. Sediments are
deposited,
compacted and
cemented to form
sedimentary rocks
Relative Dating: Putting rocks
in chronological order
a. Relative Age: the rock’s
ages compared to the ages of
other rocks above or below it
in a sequence of rock layers
b. Allows you to determine
the sequence of events
c. No Specific Dates
Relative Age
1. Law of Superposition
a. A sedimentary rock
layer is older than
the layer above;
younger than layer
below
b. Undisturbed
c. Sediments are
deposited on top of
existing layers and
lithified.
Relative Age
2. Principle of Horizontality
a. Sedimentary rock
layers started out
HORIZONTAL.
b. If layers are TILTED
or CURVED,
tectonics deformed
them (Mt. Building or
Faulting)
Relative Age
3. Crosscutting Relationships
a. If a fault or igneous
intrusion cuts across a
layer … it happened
after that layer
Relative Age
4. Unconformities
a. Breaks in geologic record = Missing Time
b. Deposition stopped or Rock layers were
removed (usually after uplift and erosion)
c. Types of Unconformities
Look for erosional surfaces; tilted layers; or igneous
intrusions
Left: Nonconformity = Igneous or metamorphic rock is uplifted, exposed, and
eroded. Sed layers deposited on top.
Middle: Angular Unconformity = layers are folded or tilted, then eroded. New
layers sed layers deposited on top.
Right: Disconformity = Horizontal layers are uplifted and eroded. New sed.
Layers deposited on top.
• Law of superposition
• Principle of
horizontality
• Crosscutting
relationships
• Unconformities
What is a fossil?
a. The remains or traces of living thing
preserved from the past
b. For a fossil to form, the remains of a living thing
must be buried quickly by sediment
c. Hard shells, bones and teeth have a better chance
of being preserved
Relative Age: Index Fossils
Fossils that narrow age of
rock to a geologic period
or era (millions of years)
Requirements:
a. Abundant - found in many
regions
b. Lived during “short” ,
specific span of time
c. Distinguishing features
Relative Age: Index Fossils
Example: Ammonite fossils in layer 4 formed in
rocks 108 - 206 mya
Absolute Age: Radiometric Dating
a. Uses Radioactive
Isotopes
b. Compares relative
amounts of
parent:daughter
c. Gives specific age of
rock
Atoms
• The smallest particle of
matter
• Central region= nucleus
• Nucleus= protons +
neutrons
• The number of Protons
(+) determine
characteristics of an
atom
• The number of
neutrons can vary
Absolute Age: Radiometric Dating
a. Isotopes = Atoms of the same element with different # of
neutrons.
Ex: 12 C (6 protons + 6 neutrons), 14 C (6 protons + 8 neutrons)
b. Radioactive Isotopes = Atoms that have nuclei that break apart
(unstable) naturally. This Release energy & particles
Absolute Age: Radioactive Decay
c. Unstable PARENT Isotope breaks down to stable
DAUGHTER Isotope (& releases energy)
d. Decay happens at a constant rate (not changed by Temp.,
Pressure, or environmental conditions).
Absolute Dating: Radiometric
Decay
Absolute Age: Half Life
a. The time it takes for 1/2 the
mass of PARENT -->
DAUGHTER.
 Half life of 14C = 5,730
years
 100 g 14 C -----> 50g 14 C +
50g 14N after 5,730 years
Half- Life of U 238
= 4.5 billion years
Absolute Dating: Half Life
Complete the chart below
Time
Parent
isotope (g)
Daughter
isotope (g)
Rock
cyrstallizes
(forms)
100
0
1 half-life
2 half - lives
25
15 million
25% (1/4)
87. 5
6.25
Time
(Years)
0
50% (1/2)
3 half-lives
4 half-lives
Remaining
Parent
45 million
6.25%
(1/16)
Complete the chart below
Time
Parent
isotope (g)
Daughter
isotope (g)
Remaining
Parent
Time
(Years)
Rock
cyrstallizes
(forms)
100
0
100%
0
1 half-life
50
50
50% (1/2)
15 million
2 half - lives
25
75
25% (1/4)
30 million
3 half-lives
12. 5
87. 5
12. 5% (1/8)
45 million
4 half-lives
6.25
93.75
6.25%
(1/16)
60 million
Absolute Dating: Carbon Dating
a. Used for dating organic matter found in younger rocks (<
70,000 years)
b. Organic means anything that contains carbon and comes
from living things
c. Wood, bones, shells
d. 14 C is incoporated into plants via photosynthesis (plants
take in CO2 from air)
e. Alive - Organisms have constant ratio of 12C: 14C
f. Dead - 14C decays and 14N increases
Answers to Quick Lab p.196
1. Parent Isotope
2. Daughter Isotopes
created by decay
3. 20 seconds
4. After 3 intervals:
12.5%
After 6 intervals: 1.
5%
After 9 intervals:
0.195%
5. No new parent (paper)
added or removed; cut at
constant rate (half-life)