Chapter 8: Geologic Time
1.
The Art of Time
2.
The History of Relative Time
3.
Geologic Time
4.
Numerical Time
5.
Rates of Change
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Geologic Time
How long has this landscape looked like this? How can you
tell? Will your grandchildren see this if they hike here in 80
years?
The Good Earth/Chapter 8: Geologic Time
The Art of Time
How would you create a piece of art to illustrate the passage of time?
How do you think the Earth itself illustrates the passage of time?
What time scale is illustrated in the example above? How well does this
relate to geological time/geological forces?
The Good Earth/Chapter 8: Geologic Time
Go back to the Table of Contents
Go to the next section: The History of
(Relative) Time
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Paradigm shift: 17th century – science was a baby and geology as a discipline did not
exist. Today, hypothesis testing method supports a geologic (scientific) age for the
earth as opposed to a biblical age.
Structures such as the oldest Egyptian pyramids (2650-2150 B.C.) and the Great
Wall of China (688 B.C.) fall within a historical timeline that humans can relate to,
while geological events may seem to have happened before time existed!
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
A
B
The Grand Canyon – rock layers record thousands of millions of
years of geologic history.
• Relative Time = which
came first, second…
− Grand Canyon –
excellent model
− Which do you think
happened first – the
deposition of the rocks,
or the cutting through of
those rocks by the river.
Why?
At which location on the
picture above, A or B, are
the rocks younger?
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Red rock units
Tan rock units
-More complicated histories are
represented by multiple events.
Important principles:
Superposition – rocks at the bottom are
the oldest.
Cross-cutting relationships – older rocks
may be cut by younger rocks or features.
Inclusion – Younger rocks may incorporate
pieces of older rocks.
-Above: Explain the history of the
rocks using the events deposition,
erosion, and tilting. Refer to the
diagram at left for help. (Hint: there
are 4 major events)
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
18th century - James Hutton watched the landscape of his farmland
and invented our modern concept of geologic time.
Observation:
The landscape remained unchanged with
the passage of time.
Deductions:
1) The same slow-acting geological processes that operate today
have operated in the past, meaning it takes a long time to
influence the Earth’s surface significantly (Uniformitarianism).
2) All land should be worn flat (erosion) unless some process
renews the landscape by forming new mountains (cyclical
change).
- he called these eroded surfaces, representing gaps in time,
unconformities.
Controversial resulting message – Earth must be much older than
the commonly accepted age of 6,000 years.
The Good Earth/Chapter 8: Geologic Time
Checkpoint 8.2
Examine the following image of rock layers and answer
Questions 1 and 2 about relative time.
1. Which statement is most
accurate?
E
A
C
F
a. D is older than B
b. E is older than A
B
D
c. F is older than C
The Good Earth/Chapter 8: Geologic Time
Checkpoint 8.2
Examine the following image of rock layers and answer
Questions 1 and 2 about relative time.
2.
E
A
a. After A was deposited
C
F
B
When did the tilting of the
layers occur?
D
b. Between deposition of
layers E and A
c. Before B was
deposited
d. Between deposition of
layers C and E
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Grand Canyon Rock
Sequence:
-Rocks at base are older
than rocks at top
(superposition).
-Examine lowest units –
which is older, the schist
or the granite? Why?
-Schist – metamorphic –
thought to have been the
root of an ancient
mountain belt or volcanic
arc. How did the
schist/granite get
exposed at the surface?
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Sandstone, shale, limestone progression
indicative of passive margin (rising sea level).
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
How can we tell that the volcanism is younger
than formation of the sedimentary rocks?
The Good Earth/Chapter 8: Geologic Time
Checkpoint 8.6
The Power of Fossils
Geologists can correlate sedimentary rocks by comparing the
fossils found within the rocks
Fossils found in many rock layers
(long lived species) are difficult to
match to layers in other regions.
Index fossils: species that existed
for a relatively short period of
geologic time and found over
large geographic areas are the
best for precise correlations.
Which of the fossils in the
diagram at left (1,2, or 3) would
make the best index fossil? Why?
The Good Earth/Chapter 8: Geologic Time
The History of (Relative) Time
Fossils of the Grand Canyon support the
geologic interpretations
Although they do not preserve the body of an organism, tracks are
important trace fossils that tell us something about the organisms
that left them behind.
The Good Earth/Chapter 8: Geologic Time
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The Good Earth/Chapter 8: Geologic Time
Geologic Time
(new life)
(middle life)
(ancient life)
Proterozoic = “earlier life”
Phanerozoic = “life revealed”
Fossils are rare in prePhanerozoic rocks.
The Good Earth/Chapter 8: Geologic Time
Geologic Time
Cambrian explosion (542-488 Ma)
Explosion of organisms with hard skeletons
at beginning of Cambrian
Q: Why does this matter?
A: Hard parts can be easily
preserved as fossils.
The Good Earth/Chapter 8: Geologic Time
Geologic Time Checkpoint 8.10
Carefully examine the relative
positions of the lettered arrows
in the following diagram and
answer the questions:
1) Which letter corresponds
most closely to the first
appearance in the rock record
of abundant fossils?
2) Which letter corresponds
most closely to the extinction
of the dinosaurs?
The Good Earth/Chapter 8: Geologic Time
Geologic Time
“The majority of all species that have
lived on Earth are now extinct”
All major phyla were derived by the Cambrian.
The diversity of organisms has increased through time.
Q: How would an extinction affect
biodiversity?
A: Biodiversity decreases after a major
extinction event.
The Good Earth/Chapter 8: Geologic Time
Geologic Time
Look at the
graph – do
you see any
patterns?
Do they
make
sense?
The Good Earth/Chapter 8: Geologic Time
Geologic Time
Mass extinctions = events in which large numbers
of species die.
Fossils found in
rocks deposited
before a mass
extinction event are
substantially
different from those
found in rocks from
after the event.
The extinct mastodon, a smaller cousin of the
mammoth.
The Good Earth/Chapter 8: Geologic Time
Geologic Time
Major mass extinctions throughout
geologic history
Cretaceous-Tertiary (K-T) extinction (~ 65 Ma): No dinosaurs (except
perhaps birds) survived the event. Mammals were able to expand and
become the dominant group. WHY? The cause is believed to be impact
on earth by a large comet/asteroid. In all, about 75% of all species were
destroyed.
Permian-Triassic (P-T) extinction (~ 251 Ma): Killed off ~96% of marine
species and 70% of land species. Often called “the great dying.”
Studying extinction events can shed light on 1) the cause of the extinction,
and 2) the response of different types of organisms to such events.
Why might some species go extinct while others don’t?
The Good Earth/Chapter 8: Geologic Time
Geologic Time Concept Survey
How might each of the following events have
contributed to global changes that could
have caused extinctions?
A. Assembly of the supercontinent
Pangea and creation of a single
worldwide ocean
B. Thousands of volcanic eruptions over
a period of one million years in
northeastern Russia
C. An impact event (comet/asteroid)
The Good Earth/Chapter 8: Geologic Time
Go back to the Table of Contents
Go to the next section: Numerical Time
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Early methods for determining the age of the Earth
were flawed: yielded ages too recent
Salinity of oceans – salt delivered to oceans from the continents through
streams. Mass of salt in oceans/amount of salt contributed to oceans each
year by streams = age of Earth. Age estimate = ~100 million years old.
-Flaw – did not take into account the formation of chemical sedimentary rocks
which removes salt from the oceans.
Conductive cooling of earth – knowing Earth’s volume and properties of
rocks, can calculate how long it would take for earth to cool from molten state to
present state. Age estimate = ~100 million years old.
- Flaws – did not yet know about radioactive decay and the resulting
contribution of heat. Nor was the theory of plate tectonics yet proposed, and
calculations were made assuming heat was diffused uniformly across the
earth’s surface.
The Good Earth/Chapter 8: Geologic Time
Numerical Time Checkpoint 8.13
Between 1860 and 1920 geologists
attempted to estimate the Earth’s age by
how long it would take for the thickest
sequences of sedimentary rocks to form.
Geologists examined sequences of rocks
for each geologic period. From the
estimated rates for the formation of these
units, different scientists estimated ages for
Earth ranging from 3 million years to 15
billion years. Explain why these estimates
varied over such a wide range.
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Unstable isotopes held the key to the numerical age
of the Earth!
Isotopes – atoms of the same element with
different numbers of neutrons.
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Radioactive decay – our clock for planet Earth
Protons (positively charged) repel each other. This repulsion is balanced
by neutrons acting as a buffer, but in some isotopes the repulsion is too
great = unstable isotopes.
An unstable nucleus may spontaneously change to a more stable form
through radioactive decay.
Radioactive decay releases energy (heat).
Unstable original isotope = parent
Stable new isotope = daughter
The Good Earth/Chapter 8: Geologic Time
Numerical Time
a. Addition of an
electron neutralizes
positive charge of one
proton changing it to a
neutron.
b. Loss of an electron
gives one neutron a
positive charge
changing it to a
proton.
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Ages calculated using radioactive decay tell us
when the minerals in a rock first solidified from
a molten state or formed through
metamorphism.
Radioactive ages do not tell us when a
sedimentary rock was deposited.
Oldest rocks on Earth ~ 4 billion years old.
This is when our crust began to form (solidify)
from the molten state.
Age of Earth, 4.6 billion years old, comes from
radiometric ages of meteorites and moon rocks.
The Good Earth/Chapter 8: Geologic Time
Numerical Time Concept Survey
Half-life = the time it takes for half of the parent isotopes to convert to daughter
atoms.
Isotopes have characteristic half-lives. In other words, the length of the
half-life for a given isotope is always the same.
Experiment: Take out a sheet of paper. This is your parent isotope.
The initial ratio of parent to daughter is 1:0, or 100% parent and 0%
daughter. Every 5 seconds, tear it in half and set the new piece aside
into a pile of daughter atoms. Each time you tear a piece in half you
are reproducing radioactive decay. Keep on tearing until you cannot
tear it anymore.
How many pieces do you have in your daughter pile after 15 seconds?
What is the ratio of parent to daughter after 15 seconds?
How many pieces of daughter did you end up with?
What was the half-life of your fictitious isotope?
The Good Earth/Chapter 8: Geologic Time
Numerical Time
These ratios don’t
change, regardless of
isotope.
The ratio of parent isotopes to daughter atoms tells us how
many half-lives have passed, and therefore tells us age!
Very little parent
remains
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Isotopes with longer half-lives are better for dating older rocks
(daughter has had time to accumulate).
Isotopes with short half-lives are only useful for dating younger
rocks, as almost all parent will have decayed over a relatively
short period of time.
The Good Earth/Chapter 8: Geologic Time
100
0
N/A
50
50
1:1
25
75
1:3
12.5
87.5
1:7
6.25
93.75
1:15
Keep dividing
by 2
100-%Parent
Sample Age = #T1/2 x (Length of Time for one
Isotope Half life)
The Good Earth/Chapter 8: Geologic Time
Numerical Time Concept Survey
Which of the isotopes listed in the chart
would be most useful for dating rocks that
formed shortly after the Earth formed?
A.
B.
C.
D.
Uranium 235
Carbon 14
Uranium 238
All of the above
The Good Earth/Chapter 8: Geologic Time
Numerical Time Checkpoint 8.14
1) Radioactive isotopes in clastic sedimentary
rocks always predict an age that is:
a. older than the sedimentary rock.
b. younger than the sedimentary rock
Note: can you explain
your answer?
c. correct for the sedimentary rock
The Good Earth/Chapter 8: Geologic Time
Numerical Time Checkpoint 8.14
2) The isotope of element X has 15 protons,
17 neutrons, and 15 electrons. The element
therefore has an atomic number of _____,
and a mass number of _____.
a. 15; 32
b. 17; 15
c. 17; 47
d. 15; 30
The Good Earth/Chapter 8: Geologic Time
Numerical Time Checkpoint 8.14
3) If radioactive decay began with 400,000
parent isotopes, how many would be left
after three half-lives?
a. 200,000
b. 100,000
c. 50,000
d. 25,000
The Good Earth/Chapter 8: Geologic Time
Numerical Time Checkpoint 8.15
The half-life of a radioactive isotope is 500 million
years. Scientists testing a rock sample discover that
the sample contains three times as many daughter
atoms as parent isotopes. What is the age of the
rock?
a. 500 million years
b. 1,500 million years
c. 1,000 million years
d. 2,500 million years
The Good Earth/Chapter 8: Geologic Time
Numerical Time
Sedimentary rock ages are determined using a
combination of relative time and numerical ages.
The Good Earth/Chapter 8: Geologic Time
Go back to the Table of Contents
Go to the next section: Rates of Change
The Good Earth/Chapter 8: Geologic Time
Rates of Change
Recall James Hutton’s suggestion that features on the Earth’s surface
were formed by the same slow processes that we see operating today.
This concept is known as uniformitarianism.
Green River, Canyonlands (1871)
Green River, Canyonlands (1968)
The Good Earth/Chapter 8: Geologic Time
Rates of Change
The concept of uniformitarianism
would suggest that the ancient
mudcracks (lower) formed under the
same conditions that form modern
mudcracks (above).
By understanding modern processes,
we can learn about processes that
occurred in the geological past.
“The present is the key to the past.”
The Good Earth/Chapter 8: Geologic Time
Rates of Change
•
Mountains and oceans – grand features that were hard
to explain.
•
With no rigorous scientific method, people explained
these features as the result of short, catastrophic
events.
Catastrophism: The Earth has been (and can be) affected by short
duration, sometimes violent events that may be global in nature.
Catastrophic events without precedents that cannot be explained by
physical or chemical processes are not science.
High-magnitude events – relatively rare, affect a large area
Low-magnitude events – frequent, more localized
The Good Earth/Chapter 8: Geologic Time
Rates of Change Checkpoint 8.18
Place each of the following events in the appropriate location on the timeline below,
according to either its frequency (how often) or length of time over which it occurs.
1. The time between large eruptions of the same volcano
2. A season (e.g. spring)
3. Time between great earthquakes on the San Andreas fault
4. Period required to form the Atlantic Ocean
5. Formation and decay of a tornado
6. Earth’s orbit around the sun
7. Length of orbit around the sun
8. Time between mass extinctions
9. Time required to carve the Grand Canyon
10. Growth of major U.S. cities
11. Formation and decay of a hurricane
The Good Earth/Chapter 8: Geologic Time
Rates of Change Checkpoint 8.19
List some examples of events that
can influence the Earth. Say
whether they are high-magnitude
or low-magnitude events and why.
The Good Earth/Chapter 8: Geologic Time
Rates of Change Checkpoint 8.19
We have just discussed Earth history
stretching back 4.6 billion years. Has
the history of life on Earth been more
affected by rare, high-magnitude
events or frequent, low-magnitude
processes? Justify your choice.
The Good Earth/Chapter 8: Geologic Time
Geologic Time Concept Map
Complete the
concept map to
evaluate your
understanding of
the interactions
between the earth
system and
geologic time.
Label as many
interactions as
you can using
information from
this chapter.
The Good Earth/Chapter 8: Geologic Time
The End
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The Good Earth/Chapter 8: Geologic Time