Week 6 Lecture: Age Dating

Question 1

What are the forms of age dating?

Answer 1

• Relative
• Radiometric

Question 2

What geologic time period are we in?

Answer 2

Phanerozoic

Question 3

What are isotopes?

Answer 3

atoms of one element with same number of protons, but with different number of neutrons.
(Sum of protons and neutrons is the atomic mass)

Question 4

What age dating method uses radioactive elements?

Answer 4

Absolute Dating

Question 5

Describe the radioactive decay process

Answer 5

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

Question 6

What is the charge for alpha, beta, and gamma particles/rays

Answer 6

Alpha (+2)
Beta (-1)
Gamma: neutral (0)

Question 7

List the ionizing effect and danger of each radioactive particle

Answer 7

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)

Question 8

What are elements that emit ionizing radiation?

Answer 8

Radionuclides

Question 9

What are radionuclides that decary in more than one step?

Answer 9

series radionuclides

Question 10

How is the decay rate of a radionuclide measured?

Answer 10

In terms of half-life, or the time required for half of radioactive atoms present to decay

Question 11

Do all atoms of a radioactive element decay at once?

Answer 11

No, only a portion decay at any given time

Question 12

What is true about the decay rate / half-life of any particular radiogenic isotope?

Answer 12

The decay rate is constant

Question 13

How does Radiometric Dating work?

Answer 13

• known half-life (rate of decay)
• measure present amount of parent isotope
• measure present amount of daughter isotope
• age can be calculated

Question 14

Which is a common isotope used for radiometric dating with a very long half life and what is that half life?

Answer 14

Zircon - 4.5 billion years

Question 15

Who developed the Principle of Uniformitarianism and how did he do that?

Answer 15

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”

Question 16

What form of age dating uses chronological history or sequence of events?

Answer 16

Relative Dating

Question 17

How does Relative Dating differ from Absolute Dating?

Answer 17

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

Question 18

What is the Principle of Original Horizontality?

Answer 18

• Sedimentary layers deposit in relatively uniform horizontal sheets
• Any deformational event must be younger than age of deformed rock layers

Question 19

What is the Principle of Lateral Continuity?

Answer 19

Layers of sediment commonly extend laterally in all directions for a significant distance

Question 20

What is the Principle of Superposition?

Answer 20

Sedimentary layers go from oldest rocks on the bottom to youngest on the top

Question 21

What is the Principle of Fossil Succession?

Answer 21

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)

Question 22

What is the Principle of Inclusion?

Answer 22

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

Question 23

What is the Principle of Crosscutting Relations?

Answer 23

A rock layer (dyke) may cut through a number of layers (which means its younger than those layers)

Question 24

List 4 Primary Geological Contacts

Answer 24

• Bedding contact
• Intrusive contact
• Metamorphic contact
• Erosional contact (unconformities)

Question 25

List a Secondary Geological Contact

Answer 25

Fault

Question 26

What are the 3 Uncomformities?

Answer 26

• Angular Unconformity
• Disconformity
• Nonconformity

Question 27

What is an Angular Unconformity?

Answer 27

Where a compressed folded layer on the bottom is eroded away and covered by a new layer

Question 28

What is a Disconformity?

Answer 28

An uncomformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition

Question 29

What is a Nonconformity?

Answer 29

Eroded metamorphic or igneous rocks are overlaid with sedimentary rocks

Question 30

What is Geologic Correlation?

Answer 30

Where sections that are apart (such as across a valley) can be matched with their similar layers to determine age or a sequence