Lecture 15 // Age Dating

Question 1

Isotopes

Answer 1

Atoms of the same element that have different mass numbers because they differ in the numbers of neutrons in the nucleus.
• Eg. Carbon-12 and Carbon-14

Question 2

Radioisotopes

Answer 2

Some isotopes are unstable and will spontaneously convert (decay) to more stable isotopes.

Question 3

Radioisotopic decay

Answer 3

The release of atomic particles and radiation (energy + heat).

• Eg. Carbon-14 decays by releasing a beta particle. (a proton is converted to a neutron and an electron in the nucleus, and the electron is ejected)
• The Carbon-14 thus loses a proton and becomes Nitrogen-14. (the atomic number is increased by one but the mass number remains the same)

Question 4

Parents & daughters isotopes

Answer 4

Radioisotopes are termed parents (eg. U238), the stable isotopes they convert into are 
 termed daughters (eg. Pb206).

Question 5

Mass spectrometer.

Answer 5

The proportions of parents and daughters within a sample of a mineral grain can be determined using a mass spectrometer.

Question 6

Isotopic decay

Answer 6

Isotopic decay occurs at a uniform, fixed rate (the decay rate goes unchanged over geologic time). If you know the rate and the proportion of parents and daughters, then you can determine the age.

Question 7

Half life

Answer 7

• The time required for half of the parents in a mineral grain to decay.
• Let’s say that the proportion of parents to daughters is 1:31. That means that for every 32 atoms, there are 1/32 parents and 31/32 daughters.
• Some radioisotopes have short half lives (eg. C-14 is 5730 years) but most are very long (eg. U-238 is 4.5 billion years, eg. K-40 is 1.25 billion years).
• C-14 is used to date organic matter up to 50,000 years old. The other methods are used to date the age of igneous and metamorphic rocks of any age.

Question 8

Determining the radioisotopic age of minerals within rock…

Answer 8

• Using a mass spectrometer to determine the ratio of radioactive parent isotopes and their stable daughter isotopes within a single mineral grain.
• Knowing the half-life of the radioactive parent isotope.
• Plugging in that information into a known formula.

Question 9

What do you find out if you

radioisotopic-age date…

Answer 9

Igneous rocks:
• The age of crystallization of that mineral from magma or lava.
Metamorphic rocks:
• The age or recrystallization (due to heat, pressure, and/or hot fluid activity).
Sedimentary rocks:
• The age of the source of that mineral grain (original source would have been either igneous rock or metamorphic rock)…NOT the age of deposition of the sediments!

Question 10

Determining a sequence of

geologic events.

Answer 10

Geologic events:
1. Deposition of strata
2. Deformation (faulting / folding / tilting)
3. Igneous intrusion (plutons
such as dikes and batholiths)
4. Metamorphism
5. Erosion

Strata = any material laid down on the Earth’s surface as layers. This includes all sediments as well as volcanic ash and lava flows.

Question 11

Geologists use seven basic stratigraphic principles in relative age dating.

Answer 11

1. Superposition
2. Original Horizontality
3. Original Lateral Continuity
4. Cross Cutting Relationships
5. Inclusions
6. Fossil Succession
7. Unconformities

Question 12

Superposition

Answer 12

Within most sequences of strata, the oldest layer is at the bottom, and the layers get progressively younger as you move upwards. However, this principle does not hold true in all instances.

Question 13

Original Horizontality

Answer 13

Strata is typically deposited in horizontal layers. If horizontally laid down strata is found otherwise, then deformation must have occurred following deposition.

Question 14

Original Lateral Continuity

Answer 14

Strata continues in all directions until it either thins out, grades into another sediment type, or comes up against a barrier. This principle is used when correlating beds from one location to another.

Question 15

Cross Cutting Relationships

Answer 15

A unit (or feature) that cuts across another must be younger than the rock(s) it cuts across.
• They must have already been there to get cut!
• Applies to intrusions
• Applies to faults

Question 16

Inclusions

Answer 16

Inclusions are pieces of rock that are found within other rock units. The pieces must have already existed in order to be included, hence they are older than the rock they are included in.

Question 17

Fossil Succession

Answer 17

Life has evolved over time, and certain time periods can be recognized by the suite of fossils contained within strata.

Eg. The trilobite genus Olenellus lived only during the Cambrian Period (488 - 542 Mya).

Question 18

Unconformities

Answer 18

Surfaces of erosion that separate much younger rock above from much older rock below.

Surface of significant erosion during a hiatus in deposition.

Question 19

3 types of unconformity

Answer 19

There are 3 types of unconformity – each named
based on the type of rock lying below the erosion
surface and if strata lies below then the
orientation is also considered.

1. Disconformity
2. Angular unconformity
3. Nonconformity

Question 20

Disconformity

Answer 20

Strata above and below the surface of erosion lie the same way.
Typically identified by vastly different aged fossils above & below, or by the disconformity cross-cutting old faults or intrusions.

Question 21

Angular unconformity

Answer 21

Strata above and below the surface of erosion lie at different angles

Question 22

Nonconformity

Answer 22

The rock that lies below the surface of erosion is NOT strata, it is either intrusive igneous or metamorphic rock.