Ch 3: Geological Time Flashcards
Isotopic (radiometric) ages
Dates with numbers (absolute dates). As exact as you can get..
Atoms are built from subatomic particles:
Protons, Neutrons, Electrons.
Nucleus includes ___ and is surrounded by an ____
Protons and neutrons, electron cloud
Electron cloud determiens
chemical properties
Neutrons
neutrons help bind nucleus together, the number of neutrons can vary resulting in isotopes
Radioactive decay
unstable isotopes break down.
n –> p + e. Loss of 2(n+p).
Emit particles and energy
Radioactive decay enables
measurement of geologic time due to its predictable rate of decay. (assuming closed system)
Half-Life
Time for half of the parent atoms of an isotope to decay into its daughter. Measures decay rate.
Half-Life formula
lamda=ln(2)/t(1/2)
Requirements for age determination
half life or decay constant of the isotope, original amount of parent and daughter isotope in the sample, final amount of parent/daughter isotope in the sample, need to know approx. age of sample.
Limitations of half-life
mineral grains must have formed at the same time as the rock. Can’t date sedimentary rocks
Stratigraphy
relative dating techniques for sedimentary rocks. Uses relative ages of adjacent layers: order of strata, correlation
Order of strata
- Principle of original horizontality
2. Principle of superposition
Principle of original horizontality
sedimentary strata are initially horizontal
Principle of superposition
in an undisturbed succession, higher rocks are younger.
Correlation by Lithology
Compare successions of layers
Correlations:
1) Lithology
2) Fossils
Correlation by Fossils
Compare similar fossils or fossil assemblages
Time scale
- Correlation gives relative timing of strata, was used to develop a geological time scale
- Eras divided into periods
- Subsequently numerical ages were added using isotopic methods
Cross-cutting relationships
Shows relative ages of features: igneous intrusive relationships, faults, unconformities, inclusions.
Igneous intrusive relationships
intrusion must be younger than host rock. i.e. Magmatic intrusions
Faults
faults must be younger than surrounding rocks
Unconformities:
Ancient Erosional Surface - Time gap or hiatus: angular unconformity, disconformity, nonconformity
Angular unconformity
different orientations above and below
Disconformity
Strata have same orientation above and below
Nonconformity
Unit below is not sedimentary
Earth History
Birth (4.6 Ga) - Hadean (4 Ga) - Archean (2.5-4 Ga) - Proterozoic (543 Ma -2.5 Ga) - Phanerozoic (0- 543 Ma)
Hadean Eon
Period before earliest known rocks, heating from radioactive decay melted most of early Earth. Intense meteorite bombardment
Archean Eon
Starts with earliest known rocks (Acasta Gneiss). Interior of Earth was hotter than now. Atmopshere mainly CO2, N2. First evidence of life around 3.5Ga.
First evidence of Life
Bacteria in the Warrawoona formation in Australia, simple life only
Proterozoic Eon
Defined at 2.5Ga. Tectonic system similar to present day. Single-celled organisms became abundant. Increasing oxygen.
Life during proterozoic
Mainly soft body organisms, photosynthesis was happening, evolution of a backbone by the end of proterozoic
Super continent
Rhodinia formed during Proterozoic and then split up at the end
Phanerozoic:
Paleozoic (248Ma-543Ma) - Mesozoic (65Ma-248Ma) - Cenozoic (0-65Ma)
Phanerozoic - Paleozoic era
Starts with first animal shell fossils (545 Ma), rapid increase in animal diversity, land colonized by plants followed by first land animals.
Paleozoic ends with ____
permian extinction, a major extinction.
Phanerozoic - Mesozoic era
Starts at a major extinction event, age of dinosaurs on land, giants ruled the sea, land and sky
Mesozoic:
Triassic Period - Jurassic Period - Cretaceous Period
Phanerozoic - Mesozoic - Triassic
Time of transition, Pangaea altered climate and circulation patterns. Diversification of the survivors of the Permian extinction.
Phanerozoic - Mesozoic - Jurassic
Big herbivores and nasty carnivores. Pangaea splits.
Phanerozoic - Mesozoic - Cretaceous
First fossils of: insects groups, modern mammal and bird groups, flowering plants. Pangaea continues to separate, a few new dinosaurs appeared.
Phanerozoic - Cenozoic era
Starts at major extinction event (65 Ma) A 10km in diameter hits Earth.
First humans (humanoid)
7 Ma
Why did smaller organisms survive the cretaceous period?
Fast reproduction rates, easier to evolve quicker, have higher offspring numbers.
