Lecture 3 - Geochronology

Question 1

types of dating

Relative Dating

Answer 1

• using the principle of stratigraphy
• biochronology (faunal succession)

Question 2

types of dating

Absolute Dating

Answer 2

• radiometric dating
• electron methods

Question 3

types of dating

Paleomagnetism

Answer 3

• method for linking up to absolute dating methods

Question 4

strata

Answer 4

layers of rock

Question 5

stratigraphic laws

original horizontality

Answer 5

rock layers originally layed down horizontally
- any divergence must come through later other processes
- particles that make up strata come from water which spread horizontally (i.e. water spilling from a glass)

Question 6

stratigraphic laws

lateral continuity

Answer 6

chemically identical beds are a part of the same layer of rock bed (even if moved later on)

Question 7

stratigraphic laws

law of superposition

Answer 7

older layers on bottom, younger layers on top
- tectonic plate movements can “mess up” the deposition of strata

Question 8

stratigraphic laws

bioturbation

law of superposition

Answer 8

any movement of sediment by biological organisms

Question 9

stratigraphic laws

cross-cutting relations

Answer 9

cross-cutting layers can only cut across existing layers
- normal faults (vertical movement) can move sections of multiple beds and the area of contact might fill up with other sediments

Question 10

stratigraphic laws

law of faunal succession

Answer 10

can use biological organisms present in the fossil bed to determine age of strata

Question 11

stratigraphic laws

index fossils

law of faunal succession

Answer 11

organisms that are more specific to a certain time period/relatively constrained times - more indicative of the age of the strata

Question 12

absolute dating

radiometric methods

Answer 12

relies on the natural occurrence of radioactive isotopes

Question 13

absolute dating

radioactive decay 14C

Answer 13

• 14C decays to 14N
• output of one electron
• half-life: 5730 years
• after 50k years, amount of carbon is negligible

Question 14

absolute dating

Potassium-Argon Dating

Answer 14

based on decay of 40K to 40 Ar
- Ar gas escapes liquid magma but starts accumuluating as magma cools/hardens
- Ar clock is “zero-ed” out at time of eruption
- half-life: 1.3 billion years
- limited by age of earth
- useful for volcanic rocks billions of years ago

Question 15

absolute dating

what is tephra

Answer 15

bed formed of volcanic ashfalls which contains glass and minerals = can be directly dated

Question 16

absolute dating

electron dating methods

Answer 16

• thermoluminescence
• optical stimulating luminescence
• electron spring resonance
• natural radiation causes electrons to break free, accumulutes in “natural” traps in minerals

Question 17

magnetostratigraphy

magnetostratigraphy

Answer 17

uses polarity reversals of the Earth to date rocks

earth is magnetic because of it’s core

Question 18

magnetostratigraphy

what are reversals

Answer 18

periodical movement of earth’s magnetism

Question 19

magnetostratigraphy

process of magnetic field recording

Answer 19

• when lava is hot, iron containing molecules spin all over but as they cool, start to align iwth earth’s magentism
• completely aligned with N of the earth at that time’s magnetic field

Question 20

magnetostratigraphy

magnetic bar code

Answer 20

pattern of reverse and normal polarity helping link unknown sections to known timescales
- kind of random, unpredictable