Transcript Document

Geologic Time
The Geologic Time Scale
• Geologists have divided the history of the
Earth in time units based on fossil
evidence.
• The time units are part of the Geologic
Time Scale.
• The scale starts from Earth’s origin, 4.6
billion years ago, to present day.
Eons
The
Geologic
Time
Scale
Eras
Periods
Earth’s History Compared to a Football Field
Pages 8 and 9 of your Earth Science Reference Tables!!
Fossils
• A fossil is any evidence of earlier life
preserved in rock.
• Fossils are ONLY found in sedimentary
rocks!
• Why?
• Igneous rocks are cooled magma/lava and
metamorphic rocks have undergone too
much heat and pressure to preserve a
fossil.
Fossils
• Types of Fossils:
• 1. Original Remains – unchanged remains
of a plant or animal.
• 2. Amber – animal or plant caught in
sticky amber and amber hardens and
preserves it – think Jurassic Park!
• 3. Replaced Remains – soft parts of
animal have disappeared and the hard
parts are replaced by minerals. Ex. –
petrified wood
Fossils
• 4. Molds and Casts – a mold is a hollow
depression left in the rock from a shell or
bone. A cast is when the mold fills in with
material and makes a cast of the mold.
• 5. Trace Fossils – evidence of life other
than the actual remains of the plant or
animal. Ex. – trails, footprints, burrows,
etc.
Petrified Wood
Carbonized
Remains
Burrows
Footprints
Mold and Cast
Index Fossils
• Index fossils are very useful for dating
rocks.
• These fossils are used to correlate (matchup) rock layers from one geographic area
to another.
• This is very useful to geologists because
they can then determine when a specific
rock layer formed.
Fossil Correlation
Time when all three
generations coexist
grandpa
father
son
1900
time
2000
Fossil Correlation
Different species overlap in time
species A
Age of rock
containing these
3 fossils
species B
species C
5
4
3
Time (millions of years)
Fossil Correlation
Index Fossils
• To be considered a GOOD index fossil, it
must have these three things:
• 1. Be easily recognized
• 2. Be abundant (many of them found)
• 3. Be spread-out over a large geographic
area.
• Someday, we (humans) will make great
index fossils!!
Relative Dating of Rocks
• James Hutton proposed the principle of
uniformitarianism.
• This states that the processes occurring
on Earth today, have been occurring since
Earth first formed.
• In other words, “the present is the key to
the past”.
Geologic Principles
• Relative dating of rocks is
getting an approximate
age of the rock formation,
without being able to get
an EXACT age.
• In this picture of the
Grand Canyon, each
horizontal layer of rocks
represents a different
chunk of time.
Geologic Principles
• The first geologic principle is the Principle of
original horizontality. It states that sedimentary
rocks are always deposited in horizontal layers,
unless something has happened to fold or fault
them.
• The second principle goes along with the first.
Because sedimentary rocks are deposited
horizontally, the one on the bottom must be the
oldest and each layer on top of that gets
younger, with the youngest on the top.
• This is called the Principle of Superposition.
Geologic Principles
Original horizontality
rocks form horizontal layers
youngest
oldest
Superposition
oldest layers are on the bottom
Geologic Principles
• The third principle is the Principle of
Cross-cutting Relationships.
• This states that an igneous rock intrusion
or a fault must be younger than the rock it
cuts across.
• In other words, the pizza or cake has to be
baked before you can cut it!!
Igneous Intrusions
• There are 2 types
of igneous
intrusions.
• 1. Dyke – molten
rock that cuts
vertically into
rock layers.
• 2. Sill – A
horizontal sheet
of molten rock
that gets
squeezed
between 2 rock
layers.
Geologic Principles
Cross - cutting features are younger
canyons
intrusions
faults
Unconformities
• Earth’s surfaces are constantly being
eroded.
• Because of this, often there is a gap in the
rock record.
• These gaps are called unconformities.
• There are 3 types of unconformities.
1. Angular Unconformity
Rock layers above and below the unconformity are at an angle to each other
horizontal
layers
tilted layers
2. Disconformity
Rock layers above and below the unconformity are parallel to each other
horizontal
layers
horizontal
layers
3. Nonconformity
Rock layers above and below the unconformity are different rock types
sedimentary
igneous or
metamorphic
Sedimentary
Rocks
Nonconformity
Igneous
Rocks
Youngest
Relative age problems
10. F
9. A
F
W
A
8. W
L
7. P
Z
6. X
P
T
5. L
R
X
4. Z
C
OLD
igneous
metamorphic
all other layers are sedimentary
3. T
2. R
1. C
Oldest
Absolute Dating of Rocks
• The main method used to get the exact
age of a rock is called radioactive or
radiometric dating.
• This method studies the decay rates of
radioactive isotopes in order to determine
the age of the rock.
Radioactive Dating
• During radioactive decay, the original material or “parent
material” decays at a set rate into the new material or
“daughter material”
• As the number of parent material decreases, the number
of daughter material increases.
• The amount of time that it takes for exactly one-half of
the parent to turn into daughter is known as a half-life.
• For example – the half-life of Carbon-14 is 5,700 years.
Carbon-14 is the parent material and Nitrogen-14 is the
daughter material it decays into.
• Therefore, this means that it will take 5,700 years for
one-half of the Carbon-14 to turn into Nitrogen-14.
Radioactive Dating
• THE GOOD NEWS!!
• Half-lives are completely and totally set in
stone and DO NOT ever change!!
• The half-life of Cabon-14 is ALWAYS
5,700 years, no matter what!!
• ALSO – some common half-lives are given
free of charge on page 1 of your ESRT!
Radioactive Dating
• The smaller the half-life, the less useful the
radioactive isotope is for dating really old stuff.
• For example, Carbon-14 half life is only 5,700
years. Therefore, it can only accurately date
rocks no older than about 100,000 years old.
• Therefore, to date rocks that formed back at the
beginning of the Earth, 4.6 billion years ago, we
would use an isotope with a much larger halflife.
• Since Uranium-238 half-life is 4.5 billion years,
anything containing it has undergone 1 half-life
since the Earth first formed!
Example of Radiocarbon
decay (decay of Carbon-14)
Analogy: “decay” of Bit-O-Honey
“Original”
Bit-OHoney
(Parent
Material)
“Decayed” Bit-O-Honey
(Daughter material)
100
50
25
13
1
2
3
4
5
Number of parent isotopes
Number of daughter isotopes
6
20 of original 100 left
100
50
25
20
13
1
2 2.2
3
4
5
Number of parent isotopes
Number of daughter isotopes
6
Start with a million parent atoms…
1
0.5
0.25
0.13
PARENT=1,000,000
DAUGHTER=0
1
0.5
0.25
0.13
1
PARENT=500,000
DAUGHTER=500,000
1
0.5
0.25
0.13
1
2
PARENT=250,000
DAUGHTER=750,000
1
0.5
0.25
0.13
1
2
3
PARENT=125,000
DAUGHTER=875,000
1
0.5
0.25
0.13
1
2
3
4
PARENT=62,500
DAUGHTER=937,500
1
0.5
0.25
0.13
1
2
3
4
5
PARENT=31,250
DAUGHTER=968,750
1
0.5
0.25
0.13
1
2
3
4
5
6
PARENT=15,625
DAUGHTER=984,375
Let’s Try Some More…
Parent
Daughter
1
0
1/2
1/2
1st half-life
1/4
3/4
2nd half-life
1/8
7/8
3rd half-life
1/16
15/16
4th half-life
Let’s Try Some More…
C – 14
N - 14
1
0
50%
50%
1st half-life
25%
75%
2nd half-life
12.5%
87.5%
6.25%
93.75%
3rd half-life
4th half-life