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Flashcards in Geological Time Scale Deck (31)
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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?



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?



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

1. P has dual meaning for pressure wave (compression) or "primary wave" (fast, arrives first).
2. lower energy than other seismic waves
3. 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)
2. Higher energy, subtle shaking
3. Propagates only through solids


Three things about Love waves

1. Surface wave
2. Moves back and forth (shear wave)
3. Arrives after S-wave, commonly the most damaging


Three things about Rayleigh waves

1. Surface waves
2. Ground beneath wave deforms elestically in a circular motion
3. 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.