chapter 8: geologic time copyright © the mcgraw-hill companies, inc. permission required for...

Chapter 8: Geologic Time

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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The Art of Time

The History of Relative Time

Geologic Time

Numerical Time

Rates of Change

The Good Earth/Chapter 8: 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?

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?

Go back to the Table of Contents

Go to the next section: The History of (Relative) 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!


• 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?


The Grand Canyon – rock layers record thousands of millions of years of geologic history.

At which location on the picture above, A or B, are the rocks younger?

A

B



-More complicated histories are represented by multiple events.

-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)

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.

Tan rock units

Red rock units

The History of (Relative) Time18th 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.

Checkpoint 8.2


Examine the following image of rock layers and answer Questions 1 and 2 about relative time.

A

B

C

D

E 1. Which statement is most accurate?

a. D is older than B

b. E is older than A

c. F is older than C

F


Examine the following image of rock layers and answer Questions 1 and 2 about relative time.

A

B

C

D

E

2. When did the tilting of the layers occur?

a. After A was deposited

b. Between deposition of layers E and A

c. Before B was deposited

d. Between deposition of layers C and E

F

Checkpoint 8.2



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?



Sandstone, shale, limestone progression indicative of passive margin (rising sea level).



How can we tell that the volcanism is younger than formation of the sedimentary rocks?


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?

Checkpoint 8.6


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.


Go to the next section: Geologic Time


Geologic Time


Proterozoic = “earlier life”

Phanerozoic = “life revealed”

Fossils are rare in pre-Phanerozoic rocks.

(ancient life)

(middle life)

(new life)

Geologic Time

Q: Why does this matter?

Cambrian explosion (542-488 Ma)

Explosion of organisms with hard skeletons at beginning of Cambrian


A: Hard parts can be easily preserved as fossils.

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?

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.

Geologic Time


Look at the graph – do you see any patterns?

Do they make sense?

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.

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?

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)


Go to the next section: Numerical 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.

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.


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.

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

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.

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.

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?

Numerical Time


The ratio of parent isotopes to daughter atoms tells us how many half-lives have passed, and therefore tells us age!

These ratios don’t change, regardless of isotope.

Very little parent remains

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.

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 dividingby 2

100-%Parent

Sample Age = #T1/2 x (Length of Time for one Isotope Half life)


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. Uranium 235

B. Carbon 14

C. Uranium 238

D. All of the above



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

c. correct for the sedimentary rock

Note: can you explain your answer?


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



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 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


Numerical Time


Sedimentary rock ages are determined using a combination of relative time and numerical ages.


Go to the next section: Rates of Change


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)

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.”

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

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



List some examples of events that can influence the Earth. Say whether they are high-magnitude or low-magnitude events and why.



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.

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 End



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