Week 6 Lecture: Age Dating Flashcards
What are the forms of age dating?
- Relative
- Radiometric
What geologic time period are we in?
Phanerozoic
What are isotopes?
atoms of one element with same number of protons, but with different number of neutrons.
(Sum of protons and neutrons is the atomic mass)
What age dating method uses radioactive elements?
Absolute Dating
Describe the radioactive decay process
An unstable radioactive isotope over time decays into a radiogenic isotope (daughter element). The energy released in this process emits alpha, beta, or gamma particles/rays
What is the charge for alpha, beta, and gamma particles/rays
Alpha (+2)
Beta (-1)
Gamma: neutral (0)
List the ionizing effect and danger of each radioactive particle
Alpha: strong ionization and most dangerous in the body
Beta: weak ionization and most dangerous when outside the body
Gamma: very weak ionization and most dangerous when outside the body (needs lead to block)
What are elements that emit ionizing radiation?
Radionuclides
What are radionuclides that decary in more than one step?
series radionuclides
How is the decay rate of a radionuclide measured?
In terms of half-life, or the time required for half of radioactive atoms present to decay
Do all atoms of a radioactive element decay at once?
No, only a portion decay at any given time
What is true about the decay rate / half-life of any particular radiogenic isotope?
The decay rate is constant
How does Radiometric Dating work?
- known half-life (rate of decay)
- measure present amount of parent isotope
- measure present amount of daughter isotope
- age can be calculated
Which is a common isotope used for radiometric dating with a very long half life and what is that half life?
Zircon - 4.5 billion years
Who developed the Principle of Uniformitarianism and how did he do that?
James Hutton
- He noticed most sedimentary layers were deposited in horizontal stata from gradual process
- He concluded those layers took a very long time to form
- He proposed “Present is key to the past”
What form of age dating uses chronological history or sequence of events?
Relative Dating
How does Relative Dating differ from Absolute Dating?
Relative dating does not come up with an exact date, but rather uses features within strata to provide an age of a rock layer relative to the other layers surrounding it
What is the Principle of Original Horizontality?
- Sedimentary layers deposit in relatively uniform horizontal sheets
- Any deformational event must be younger than age of deformed rock layers
What is the Principle of Lateral Continuity?
Layers of sediment commonly extend laterally in all directions for a significant distance
What is the Principle of Superposition?
Sedimentary layers go from oldest rocks on the bottom to youngest on the top
What is the Principle of Fossil Succession?
Sequences of successive strata and their corresponding enclosed faunas (fossils) have been matched together to form a composite section detailing Earth’s history.
(Basically fossils in a rock layer help determine its age by matching to fossils in surrounding rock layers)
What is the Principle of Inclusion?
Older rocks may be enclosed in a younger rock layer due to events such as erosion occuring before the young layer was deposited on the old layer
What is the Principle of Crosscutting Relations?
A rock layer (dyke) may cut through a number of layers (which means its younger than those layers)
List 4 Primary Geological Contacts
- Bedding contact
- Intrusive contact
- Metamorphic contact
- Erosional contact (unconformities)
List a Secondary Geological Contact
Fault
What are the 3 Uncomformities?
- Angular Unconformity
- Disconformity
- Nonconformity
What is an Angular Unconformity?
Where a compressed folded layer on the bottom is eroded away and covered by a new layer
What is a Disconformity?
An uncomformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition
What is a Nonconformity?
Eroded metamorphic or igneous rocks are overlaid with sedimentary rocks
What is Geologic Correlation?
Where sections that are apart (such as across a valley) can be matched with their similar layers to determine age or a sequence
