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

Petroleum and coal are only found in rocks young enough to have had abundant life to produce petroleum and coal .
The Good Earth/Chapter 8: Geologic Time

Diamonds are only found in very old parts of earth.

Paradigm shift: 17 th 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 Geologic Time – Places sedimentary rocks in a sequence of old to young rocks. Radiometric (absolute or numeric) Time – Uses radioactive isotopes to determine the time since new minerals formed in a rock. Usually used in igneous rocks.

B A The Grand Canyon – rock layers record thousands of millions of years of geologic history.
• In undeformed sedimentary rocks, the oldest rock is on the bottom.
− Grand Canyon – excellent model − Which do you think happened first – the deposition of rocks at location “A” or “B”. Why?
At which location on the picture left, A or B, are the rocks younger?
The Good Earth/Chapter 8: Geologic Time

Red rock units Tan rock units
Sedimentary rock deposited underwater is horizontal. Sedimentary rock deposited on land CAN be non-horizontal.
The Good Earth/Chapter 8: Geologic Time

Red rock units Tan rock units
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.
-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) The Good Earth/Chapter 8: Geologic Time

18 th 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

A B
Examine the following image of rock layers and answer Questions 1 and 2 about relative time.
C D E F 1. Which statement is most accurate?
a. D is older than B b. E is older than A c. F is older than C The Good Earth/Chapter 8: Geologic Time

A B
Examine the following image of rock layers and answer Questions 1 and 2 about relative time.
C D E F 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 The Good Earth/Chapter 8: Geologic Time

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

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

Sedimentary rock layers are used to determine earth history.
Rock erosion erases that record.
Places where ancient rocks were eroded are called unconformities.
Three types of unconformities.
The Good Earth/Chapter 8: Geologic Time

There is an angle between the sedimentary rocks below the erosion surface and the rocks deposited on top of the erosion surface.
The Good Earth/Chapter 8: Geologic Time

The sedimentary rocks are parallel on either side of the erosion surface.
The Good Earth/Chapter 8: Geologic Time

Sedimentary rocks have been deposited on igneous or metamorphic rocks.
The Good Earth/Chapter 8: Geologic 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

Sandstone, shale, limestone progression indicative of passive margin (rising sea level).
The Good Earth/Chapter 8: Geologic Time

How can we tell that the volcanism is younger than formation of the sedimentary rocks?
The Good Earth/Chapter 8: Geologic Time

Actual Geologic Time: Clocks in the Rocks Biblical Calculation
Archbishop Ussher of Northern Ireland calculated the age of earth as being 4004 B.C. based on careful reading of the Bible. In 1997 Earth was 6000 years old.
Many complications.
Egyptian cat mummies vs. fossilized cats required much more time for cat evolution.

Actual Geologic Time: Clocks in the Rocks Evolution and Fossils
Charles Lyell used the rate of fossil change from first fossils to present with known changes during the Ice Age. 80 million years since the beginning of the Cenozoic. Not too far off at 65 million years.

Actual Geologic Time: Clocks in the Rocks Sediment Deposition Rate
Estimates of how fast sediment accumulates at the mouths of rivers with a comparison of known sedimentary rock thicknesses gave an estimate of a million to over a billion years.

Actual Geologic Time: Clocks in the Rocks Ocean Salinity
• Sir Edmund Halley (of the comet) estimated the age of the earth based on the amount of salts in the oceans and how much salt is transported into the oceans by streams and rivers. Assumed the original ocean was pure water, no loss of ocean salinity by precipitation, or adhesion of salts to ocean clay minerals.
• John Joly estimated 90 million years for the accumulation of ocean salts. Important in that this demonstrated an earth age much older than a few thousand years.

Ocean Salinity

Actual Geologic Time: Clocks in the Rocks Earth Cooling Rate
Lord William Kelvin (of Kelvin temperature fame; 273.14°C = 0°K) calculated how long it would take earth to cool from an original molten state. He calculated an age of between 24-40 million years using black-body radiation. Just a little off.
Didn’t know about the production of heat from radioactive decay.

Radioactivity Provides a Way to Date Rocks (A Hot Date!)
The discovery of natural radioactivity by Henri Becquerel allowed geologists to determine the time it has been since new minerals formed in a rock.
Atoms are composed of a nucleus in the center with an electron cloud surrounding the nucleus.
Isotopes are elements with different mass numbers caused by a different number of neutrons.

What Occurs When Atoms Decay?
• When there are too many neutrons in the nucleus it becomes unstable and radioactively decays .
– Alpha decay – an alpha particle is ejected and the mass reduced by four (4). Two protons and two neutrons.
– Beta decay – a high velocity electron is ejected from the nucleus when a neutron decays to a proton/electron pair.
– Gamma radiation – high energy electromagnetic wave given off.

Why Radioactivity Lets Us Date Ancient Rocks with Confidence
• The rate of radioactive decay is independent of temperature, pressure and chemical environment.
• Statistical process given by the half life. The time it takes one half of the parent isotope to decay to its stable daughter isotope – Parent isotope - .
– Daughter isotope - .

Why Radioactivity Lets Us Date Ancient Rocks with Confidence
Some minerals can contain only certain elements based on their size and bonding coordination. (Testable hypothesis) – At time zero there are only parent isotopes in the mineral. After a long period of time some of these decay.
– Measure the quantity of parent and daughter isotopes using a mass spectrometer.
– Graph results. A straight line implies a good date.

Mass Spectrometer

Why Igneous Rocks Give the Most Trustworthy Dates
• These rocks date the time since new minerals formed in a rock.
– Best rocks where new minerals form are the Igneous Rocks.
– Sedimentary rocks have rarely been used if we know that the minerals grew during the formation of the rock.
• Can date the age of source rocks. Stone Mountain. Bedford Canyon Formation.
– Metamorphic rocks can be used sometimes if they are high temperature rocks where many new minerals are formed. • Some minerals can contain only certain elements based on their size and bonding coordination. (Testable) – At time zero there are only parent isotopes in the mineral. After a long period of time some of these decay.
– Measure the quantity of parent and daughter isotopes using a mass spectrometer.
– Graph results. A straight line implies a good date.

Half Life (not the game)

Important Radioactive Isotopes
• The time it takes one half of the original parent isotope to decay to its stable daughter isotope.

The Potassium-Argon Method
• K 40 decays to Ar 40 . Half-life = 1.251 billion years.
• A major problem is that Ar is a gas and can be lost from the system which would give a too young age.

The Rubidium-Strontium Method
• Rb 87 decays to Sr 87 . Half-life = 48.8 billion years .

How Carbon-14 Enters the Environment
• Cosmic ray strikes N 14 neutron and making C 14 .
converting a proton to a • Carbon reacts with O 2 to make CO 2 .

How Carbon-14 Enters the Environment
• C 14 O 2 then is incorporated into sugar 14 C 6 H 12 O 6 + O 2

How Carbon-14 Dating Works
• The living plant or animal always has a constant concentration of C 14 while alive. After death no new C 14 enters system and C 14 only decays allowing geologists to determine the time since death.

Fission Track Dating
• Natural radioactive decay sometimes produces high-energy particles from uranium and thorium decay. These particles cause considerable visible destruction near the site of the original radioactive parent. We can count these and estimate the age of a rock.

Using Dating Methods
• We can bracket rock layers using both relative and radiometric dating methods.

How Old Is Earth?
Geologists have evidence that the Earth’s age is 4.5 – 4.6 billion years old. There are no rocks of this age in Earth’s crust. The oldest evidence of minerals is 4.36 billion years, a mantled zircon.
Lunar samples and some meteorites are 4.5 billion years old.

The Goosenecks of the San Juan River
Geologic Time Scale • “Middle Life”.
• Triassic: named for the tri-fold division of rocks.
• Jurassic: named for the Jura Mountains between Switzerland and France.
• Cretaceous: Latin “chalk” found in England, France, Holland and Belgium.