Geological Time Scale

Question 1

What is a commonly used radioisotope to date rocks, and what is its half life?

Answer 1

Uranium 238, 4.5 Ga

Question 2

What is closure temperature?

Answer 2

Below a certain temperature, diffusion of isotopes will no longer be possible in and out of the crystal.

Question 3

What is the half life of Carbon-14

Answer 3

5,730 years

Question 4

How many half lives is Carbon 14 good for?`

Answer 4

10 (60000 years)

Question 5

How many isotopes does carbon have?

Answer 5

3

Question 6

How is Carbon 14 formed

Answer 6

interaction of cosmic-ray neutrons with N14 in the upper atmosphere to form 14CO2

Question 7

Describe the basic conventional method of Carbon-14 dating

Answer 7

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.

Question 8

What dating methods can be used for Phanerozoic strata?

Answer 8

Relative and Absolute dating, since there is an abundance of fossils

Question 9

What is the only dating method used in Precambrina strata, and why?

Answer 9

Absolute dating. Why? Faunal record is scanty. As such, stratigraphic boundaries are purely numerical; they are defined using radiometric means.

Question 10

Where and how old is the oldest dated rock on the planet?

Answer 10

Jack Hills meta-sediment unit in Australia, dated at 4.4 Ga

Question 11

Where does the age of 4.6 Ga of the earth come from?

Answer 11

The age of meteors that have the same composition as earth

Question 12

What is earths radius?

Answer 12

6400km

Question 13

What is the deepest well on earth?

Answer 13

12.3 km

Question 14

Define a fault

Answer 14

A planar surface in the ground across which movement has occurred

Question 15

What causes earthquakes

Answer 15

fracturing of the lithosphere following a period of elastic deformation and buildup of energy.

Question 16

What is the release of stored up energy of earthquakes called?

Answer 16

Seismic waves

Question 17

Define the epicenter

Answer 17

The point on the earths surface that is directly above the earthquake focus

Question 18

Define the focus

Answer 18

The point where the earthquake actually originates.

Question 19

Define body waves

Answer 19

travel through the Earth, arrive first, can commonly barely be felt, and have a higher frequency.

Question 20

Define surface waves

Answer 20

Surface waves travel along the Earth’s surface, arrive after P-waves, cause almost all of the destruction, and have a lower frequency.

Question 21

What two kinds of body waves are there?

Answer 21

P-waves and S-waves

Question 22

What two kinds of surface waves are there?

Answer 22

Love waves and Rayleigh waves

Question 23

Define three things about a P-wave

Answer 23

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

Question 24

Define three things about a S-wave

Answer 24

S has dual meaning: “shear wave” (transverse) or secondary wave (arrives after Pwave)

2. Higher energy, subtle shaking
3. Propagates only through solids

Question 25

Three things about Love waves

Answer 25

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

Question 26

Three things about Rayleigh waves

Answer 26

1. Surface waves
2. Ground beneath wave deforms elestically in a circular motion
3. Arrives after S-wave, destructive

Question 27

Dating the geological record using radioactivity is possible because

Answer 27

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.

Question 28

What are isotopes

Answer 28

Elements of the same atomic number with different numbers of neutrons

Question 29

Boundaries between major subdivisions of the fossil-bearing portion of the geological record are generally associated with

Answer 29

a) globally correlatable changes.
b) mass extinction events.
c) the appearance and/or dissapearance of key index fossils.

Question 30

Why do we use chondrites to date the earth?

Answer 30

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.

Question 31

If carbon 14 dating cannot be used after 60000 years, what two options are then used?

Answer 31

K40 or U238