science essay
Maracucholandia
ESC1000_Chapter8.pdf
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Geologic Time Chapter 8
Contents
1. Thinking About Time 2. The History of (Relative) Time 3. Geologic Time 4. Numerical Time 5. Rates of Change
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Geologic Time
• © Corbis
• How long has this landscape looked like this? How can you tell? Will people see this if they hike here in 80 years?
What time scale is illustrated in these examples? How well does this relate to geological time/geological forces?
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History of Relative Time 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?
Important principles:
• Superposition – rocks at the bottom are the oldest.
• Horizontal – Rocks deposit horizontally
• Cross-cutting relationships – older rocks may be cut by younger rocks or features.
• Inclusion – Younger rocks may incorporate pieces of older rocks.
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Superposition and Horizontality
Cross-cutting
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Cross Cutting Relationships
History of Relative Dating
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).
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History of Relative Dating
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.
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?
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Fossils
• 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?
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Proterozoic = “earlier life” Phanerozoic = “life revealed” Fossils are rare in pre-Phanerozoic rocks.
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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.
Biodiversity
“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?
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Look at the graph – do you see any patterns? Do they make sense?
Mass Extictions
Fossils found in rocks deposited before a mass extinction event are substantially different from those found in rocks from after the event. A ceratopsian, one of the many dinosaurs that went extinct at the end of the Cretaceous.
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Major mass extinctions throughout geologic history Cretaceous-Tertiary (K-T) extinction (tilde 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 a large comet/asteroid impact event. In all, about 75% of all species were destroyed.
Permian-Triassic (P-T) extinction (tilde 251 Ma): Killed off tilde 96% of marine species and 70% of land species. Often called “the great dying.” Its causes are still being debated.
Transitional Fossil
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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 = tilde 100 million years old.
• Flaw – did not take into account the formation of chemical sedimentary rocks which removes salt from the oceans.
Numerical Time
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 = tilde100 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.
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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).
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Unstable original isotope = parent Stable new isotope = daughter
Numerical Time
• Addition of an electron neutralizes positive charge of one proton changing it to a neutron.
• Loss of an electron gives one neutron a positive charge changing it to a proton.
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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
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.
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Numerical Time
The ratio of parent isotopes to daughter atoms tells us how many half-lives have passed, and therefore tells us age!
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.
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Table for Solving Decay Problems
life)HalfIsotopeoneforTimeof(LengthT#AgeSample 1/2
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0 Daughter %
Parent % Ratioformed %Daughter
Remaining
parent % elapsedT # 1/2
CHECKPOINT QUESTION
1) 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
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CHECKPOINT QUESTION
2) 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
CHECKPOINT QUESTION
3) 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
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Sedimentary rock ages are determined using a combination of relative time and numerical ages.
Great River Canyonlands 1878
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.
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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- Catastrophism
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
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