Today
1
The Age of the Earth
2
Geologic Time
• Exams next week
Early Christian Scholars
About 6000 years
3
Early geologists
> 6000 years required to deposit rock layers
James Hutton
William Smith
Charles Lyell
The Grand Canyon3
1
In groups…
4
Estimate the rate that sediment is deposited in
oceans.
Think about the depth of San Francisco Bay and
the rate that sediment might be deposited in the
bay. Is it …
Centimeters/year
• Inches / year
Meters/year
• Feet / year
Kilometers/year
• Miles / year
1 cm per year = 1 km per 100,000 years
Estimate erosion rates
5
Estimate the rate that rivers cut down into rocks.
Think about erosion in the foothills or the Sierra
Nevada. Is it …
Millimeters/year
Centimeters/year
• Inches / year
Meters/year
• Feet / year
Kilometers/year
• Miles / year
1 cm per year = 1 km per 100,000 years
6
Estimate the Age of
the Grand Canyon
About 1.6 kilometers deep (1 mile)
How long would it take to accumulate sediment?
How long would it take to cut down into the sediment?
Do you think you probably overestimated or
underestimated the age of the Grand Canyon?
Why?
2
James Hutton
7
> 6000 years required to deposit rock layers
James Hutton
William Smith
Charles Lyell
John Joly
8
19th century Irish geologist
How long would it take to make the oceans salty?
90 million years (90 MA)
Lord Kelvin
9
19th century physicist
How long would it take for a molten earth to cool?
100-200 MA
Kelvin did not know about radioactivity.
Energy from radioactive decay slows Earth’s colling.
3
Age of the Earth
Currently Accepted Scientific Estimate
10
4.6 billion years (GA, or giga-annum)
Based on radiometric age dating
Stay tuned later today
How long is 4.6 billion years?
How long is 4.6 billion years?
11
12
I have a measuring tape that is 100 feet long. It
represents 4.6 billion years. How many years are
represented by 1 inch?
4,600,000,000 years/ 1200 inches =
3,833,333 years per inch
Lets call it 3.8 million years / inch
4
The Principle of Uniformitarianism
13
The physical, chemical and biological laws that
operate today have also operated in the past.
Informally: “The present is the key to the past.”
In pairs…
14
List several common experiences in which you (or
others) assume uniformitarianism.
For example: I assume that when I step on the brake
pedal my car will stop.
Dating Rocks and Geologic Events
15
Relative Dating
Place rocks in proper sequence of formation relative to
one another.
Absolute Dating
Assign an age of formation to a rock.
5
16
Relative Age Dating
Unit A is older than
unit B
How do I know?
Unit F is younger
than unit D
How do I know?
Unit D is younger
than Units C and E
K
G
E
F
D
C
How do I know?
B
A
Unit G is younger
than Unit F
How do I know?
Relative Age Dating
17
Superposition
Original horizontality
Cross-cutting relationships
Inclusions
Unconformities
Lateral continuity
Figure 9.5
Law of Superposition
Youngest
18
Pebbly Sandstone
Limestone
Shale
Oldest
Sandstone
6
Principle of Original Horizontality
19
e
ston
and
S
bly
Pebstone
e
m
i
L
le
Sha
ne
dsto
San
Something happened to tilt these beds.
Principle of Cross-Cutting Relationships
20
Pebbly Sandstone
Limestone
Shale
Sandstone
Igneous dike
Principle of Inclusions
21
Oldest or youngest?
7
Types of Unconformities
22
• Angular Unconformities
• Disconformities
• Nonconformities
Each of these imply a break in deposition and
erosion because of uplift then new deposition
caused by subsidence.
Angular Unconformity
There are geologic
events that occurred
between the
formation of F and
the deposition of G
which are not
represented in these
rocks.
23
K
G
E
F
D
C
B
How do I know?
Disconformity
A
24
Pebbly Sandstone
Limestone
Shale
Sandstone
This contact is a disconformity.
8
25
Nonconformity
This contact between older igneous rocks and younger
sedimentary rocks is a nonconformity.
(a nonconformity can also occur between
metamorphic and sedimentary rocks)
Sedimentary rock
Place these geologic units in order from
oldest to youngest.
26
Be prepared to explain your reasoning.
PEBBLES
BAMBAM
BETTY
BARNEY
FRED
WILMA
Igneous Dike
27
Correlation
Figure 8.8
9
28
Fossils
Remains or traces of prehistoric life.
Reveal history of life on earth.
Help correlate rock units.
Help age-date rocks.
29
Types of Fossils
Casts
Impressions
Types of Fossils
30
Foot Print
Insect in Amber
10
Types of Fossils
31
Carbon Impression
Petrified Wood
Fossil Preservation
32
Why don’t we have a good record of fossil jelly
Soft parts decay easily.
fish?
Why do we a better fossil record of marine
animals than land animals? Marine critters get
buried more easily.
Why do we have a better record of animals than
Plants
don’t
have
any bones or
plants?
hard parts that are easily preserved.
What circumstances favor the preservation of
fossils?
THEY CONTAIN HARD PARTS
THEY ARE BURIED QUICKLY
33
There are
Exceptions
Fossilized
Dinosaur Heart
discovered in
the rib cage of
dinosaur.
11
Thescelosaurs
Fossil Correlation
34
35
“Fossil organisms succeed one another in a
definite and determinable order, and therefore any
time period can be recognized by its fossil
content.”
Principle of fossil correlation
William Smith
36
Age of Rock Layers
Older → Younger
Fossil Succession
http://pubs.usgs.gov/gip/fossils/succession.html
12
37
Correlation is usually done with fossil assemblages
I have a picture with the following 3 people in it:
Albert Einstein (1879-1955)
Martin Luther King, Jr. (1929-1968)
Hillary Rodham Clinton (1950-present)
When was the picture taken?
Describe the advantages of using fossil
assemblages to constrain correlations and relative
dates.
38
Index Fossils
Geographically widespread
Short life span
Particularly valuable for age dating
Geologic
Time Scale
39
Cenozoic
65
Mesozoic
248
Paleozoic
540
Precambrian
13
Cenozoic
2ma
65ma
Triassic
248ma
Permian
Paleozoic
Early humans
40
Tertiary
K-T Mass Extinction
Cretaceous
First flowering plants
Jurassic
Mesozoic
540ma
Quaternary
First birds, first mammals
First dinosaurs
Permo-Triassic Mass Extinction
Pennsylvanian
First reptiles, coal swamps
Mississippian
Coal swamps
Devonian
First amphibians
Silurian
Ordovician
Cambrian
First land plants; Appalachians
Precambrian
First fish
Earliest hard shell animals
Blue-green algae
See Figure 8.14
4600ma
The KT Extinctions
41
About 70% of life on earth went extinct
All animals larger than dogs
Duration of extinction debated
Few years to about 1 MA
Rapid in either case
Causes?
Asteroid or comet impact
Increased volcanic activity
Decrease in
Solar Radiation
Asteroid Impact Theory
42
http://rainbow.ldeo.columbia.edu/courses/v1001/23.html
14
Impact Theory
43
Iridium
Rare in Earth’s Crust
Abundant in meteorites
and asteroids
High levels of Iridium
at the KT boundary
world-wide
Particularly abundant
near impact craters
Additional Supporting Evidence
44
Ash layers cover some
Iridium anomalies
Shock quartz
Layered quartz created by
explosively compressing quartz
Other geologic features now
recognized as associated with
explosive compression
The Mother of All Craters
45
Chicxulub
Yucatan Peninsula
200 miles across
> 1 mile deep
Impact energy
equivalent to a
Magnitude 12
earthquake!
15
Chicxulub Impact Crater
46
http://www.tufts.edu/as/wright_center/impact/impactc.html
47
http://impact.arc.nasa.gov/gallery/ac91-0193.jpg
Why would a huge asteroid impact cause
mass extinctions?
48
Decreased solar radiation
Many plants die
Temperatures decrease
• Many species cannot adapt
Mostly large ones
Extensive fires
Acid rain
Blacken skies
further
Tidal waves
Severe weather
http://www.ucmp.berkeley.edu/diapsids/extinction.html
16
Absolute Age Dating
49
Assign numerical dates to rocks or geologic
events.
A Thought Experiment
50
A warehouse is filled with inflated red balloons;
The balloons are defective.
Each day 1/2 of the inflated balloons pop.
A bad guy sneaks into the warehouse and steals a truck
load of balloons;
Just inflated ones.
Sometime later the police catch the bad guy with a truck
load of balloons.
Some popped, some inflated.
How can the police determine when the balloons were
stolen?
Defective Balloon Warehouse
51
17
One Day Later
Two Days Later
Three Days Later
52
53
54
18
A Thought Experiment
55
A warehouse is filled with inflated red balloons;
The balloons are defective.
Each day 1/2 of the inflated balloons pop.
A bad guy sneaks into the warehouse and steals a truck
load of balloons;
Just inflated ones.
Sometime later the police catch the bad guy with a truck
load of balloons.
Some popped, some inflated.
How can the police determine when the balloons were
stolen?
In This Thought Experiment
56
Inflated balloons represent radioactive atoms
trapped in igneous rock at the time it crystallizes.
Some of these get transformed to other atoms
(radioactive decay)
Popped balloons represent new atoms (daughter
products) that form when radioactive atoms
decay.
The rate that half of the inflated balloons pop
represents the radioactive decay rate
Half-life
(One-day for the balloons)
Absolute
Age Dating
Assign specific
(numerical)
age to a
geologic unit
of geologic
event.
57
Cenozoic
65
Mesozoic
248
Paleozoic
540
Precambrian
19
Quicktime Movies of Radioactive Decay
Example: Carbon-14
58
59
Atomic Number = 6
Atomic Mass
12C
– 6 neutrons
• Stable
13C
– 7 neutrons
• Stable
14C
– 8 neutrons
• Radioactive (unstable)
• Decays to Nitrogen 14 (14N)
14C
Atoms Are Like Inflated Balloons
60
14N Atoms Are Like Popped Ones
20
How would police
figure out when
balloons
were locked in truck
?
Need to know
61
How much 14C we have today
How much 14C we started with
How fast 14C decays
Carbon-14 (Radiocarbon) Dating
62
Electron Capture
14N
14C
14C
absorbed by living
organisms at nearly
constant ratio to 13C
(14C / 13C = constant)
Carbon-14 (Radiocarbon) Dating
63
Electron Capture
14N
14C
C no longer absorbed after
organisms die.
14C decays radioactively, 13C
doesn’t.
So 14C / 13C in the dead tree
and croaked frog decreases
over time.
21
Radiocarbon in the Atmosphere
64
If 14C/13C ratio decreases by a factor of 2 then half
of the 14C has decayed away.
What was the starting 14C ?
Compare tree-ring ages to 14C ages and back out the
starting 14C in last 4000 years.
65
Radiocarbon Dating
Need to know
How much 14C we have today
How much 14C we started with
How fast 14C decays
Decay Rates
66
Half-life
Time required for half of
the radioactive atoms to
decay.
Does not depend on
• Environmental factors
Does depend on
• Type of isotope
Figure 10.15
22
Half-life of common radioisotopes
67
Half-lives of some elements (Table 10.1)
Carbon-14 (C14)
T1/2=5730 years
Potassium-40 (K40) T1/2= 1.3 billion years
• Decays to Argon-40
Uranium-238 (U238) T1/2=4.5 billion years
Uranium-235 (U235) T1/2=0.7 billion years
• Decays to Lead-207
Dating Igneous Rocks
68
As magma crystallized it incorporates radio
isotopes into some crystalls
Feldspar usually contains some Uranium (U)
Feldspar does not contain lead (Pb)
• Rarely in magma
• No place for it in crystal structure
235U
Lead
Feldspar Crystal
How Old is this Rock?
69
In groups of 3...
A rock sample contains
• 200,000 atoms of 235U
• 600,000 atoms of 207Pb
How many atoms of 235U were there originally?
What is ratio (235U today)/ (235U originally)?
How many half-lives have elapsed?
How old is the rock? (T1/2=0.7 billion years)
23
Absolute Age Dates For Sedimentary Rocks
70
Why is it difficult to get radiometric age dates for
sedimentary rocks?
Combine w/
relative dating
techniques
Geologic
Time Scale
Can Old
Senators
Demand More
Political Power
Than Junior
Congressmen?
Tough
Question.
Quaternary
71
Cenozoic
Tertiary
Cretaceous
65
Mesozoic
Jurassic
Triassic
Pennsylvanian
Pennsylvanian
248
Mississippian
Devonian
Paleozoic
Silurian
Ordivician
Cambrian
540
Precambrian
24