Geological Time Scale Flashcards
What is a commonly used radioisotope to date rocks, and what is its half life?
Uranium 238, 4.5 Ga
What is closure temperature?
Below a certain temperature, diffusion of isotopes will no longer be possible in and out of the crystal.
What is the half life of Carbon-14
5,730 years
How many half lives is Carbon 14 good for?`
10 (60000 years)
How many isotopes does carbon have?
3
How is Carbon 14 formed
interaction of cosmic-ray neutrons with N14 in the upper atmosphere to form 14CO2
Describe the basic conventional method of Carbon-14 dating
1) Collect sample (usually ~20-100 g needed) 2) Burn it to release CO2. Collect CO2 3) Measure beta emissions from CO2 in a shielded (steel-plated) Geiger counter. Repeat measurement & compare.
4) Use known decay rate curve to obtain 14C age.
What dating methods can be used for Phanerozoic strata?
Relative and Absolute dating, since there is an abundance of fossils
What is the only dating method used in Precambrina strata, and why?
Absolute dating. Why? Faunal record is scanty. As such, stratigraphic boundaries are purely numerical; they are defined using radiometric means.
Where and how old is the oldest dated rock on the planet?
Jack Hills meta-sediment unit in Australia, dated at 4.4 Ga
Where does the age of 4.6 Ga of the earth come from?
The age of meteors that have the same composition as earth
What is earths radius?
6400km
What is the deepest well on earth?
12.3 km
Define a fault
A planar surface in the ground across which movement has occurred
What causes earthquakes
fracturing of the lithosphere following a period of elastic deformation and buildup of energy.
What is the release of stored up energy of earthquakes called?
Seismic waves
Define the epicenter
The point on the earths surface that is directly above the earthquake focus
Define the focus
The point where the earthquake actually originates.
Define body waves
travel through the Earth, arrive first, can commonly barely be felt, and have a higher frequency.
Define surface waves
Surface waves travel along the Earth’s surface, arrive after P-waves, cause almost all of the destruction, and have a lower frequency.
What two kinds of body waves are there?
P-waves and S-waves
What two kinds of surface waves are there?
Love waves and Rayleigh waves
Define three things about a P-wave
- P has dual meaning for pressure wave (compression) or “primary wave” (fast, arrives first).
- lower energy than other seismic waves
- Propagates through solids, liquids, and gases
Define three things about a S-wave
S has dual meaning: “shear wave” (transverse) or secondary wave (arrives after Pwave)
- Higher energy, subtle shaking
- Propagates only through solids
Three things about Love waves
- Surface wave
- Moves back and forth (shear wave)
- Arrives after S-wave, commonly the most damaging
Three things about Rayleigh waves
- Surface waves
- Ground beneath wave deforms elestically in a circular motion
- Arrives after S-wave, destructive
Dating the geological record using radioactivity is possible because
a) nucleosynthesis generated some radioactive elements with very long half lives.
b) radioactive elements decay regularly.
c) the ratio of parent to daughter isotopes can be measured accurately and precisely in the lab.
What are isotopes
Elements of the same atomic number with different numbers of neutrons
Boundaries between major subdivisions of the fossil-bearing portion of the geological record are generally associated with
a) globally correlatable changes.
b) mass extinction events.
c) the appearance and/or dissapearance of key index fossils.
Why do we use chondrites to date the earth?
a) Chondrites are thought to have formed at about the same time as the Earth as the solar system cooled.
b) The Earth’s surface is continuously being recycled, so old rocks are scarce.
c) Minerals in the old rocks that do exist have typically had their closure temperatures breached following initial their crystallization.
If carbon 14 dating cannot be used after 60000 years, what two options are then used?
K40 or U238
