Review – Relative Dating Principle of Original Horizontality Principle of Superposition Principle...

Review – Relative Dating

Principle of Original Horizontality

Principle of Superposition

Principle of Cross-Cutting Relationships

Inclusions

Principle of Faunal Successions

I. Principles of Relative Dating

Numerical Dating

Determining the age of a rock or the Earth’s surface in years

Based on some type of natural clock (e.g., annual tree or coral growth rings)

I. Principles of Numerical Dating

I. Isotope Dating—based on the rate of decay of radioactive isotopes within rocks

A. Structure of atoms—very small, one hundred-millionth of a centimeter

Atoms: smallest particles of an element that retain all of the element’s chemical properties

Nucleus: Protons, positive chargeNeutrons, neutral charge

Electrons orbit: if nucleus is a basketball, electrons up to 3 km away


I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks

1. Charge: Protons +Neutrons (neutral)Electrons -

Atoms always have the same # of protons and electrons so the charge is balanced.

2. Mass: Protons 1 AMU or atomic mass unit

= 1.67x10-24 gms Neutrons = 1 AMUElectrons = mass negligible



3. Atomic Number = number of protons

Atomic number determines an atoms identity

H=1, Mg=12, O=8, Fe=26

4. Atomic Mass = protons + neutrons

Can change in different atoms of the same element

5. Isotopes: atoms of the same element (same atomic #), with different # of neutrons


For example Oxygen (8 protons): 8 neutrons 16O9 neutrons 17O10 neutrons 18O


B. What happens during radioactive decay

1.Radioactive isotopes: have nuclei that spontaneous decay by emitting or capturing a variety of subatomic particles (protons, electrons, etc.)

Unstable isotopes stable isotopes


C. How it works (radioactive decay)

1. Decay rates of radioactive atoms are constant

2. Unaffected by temperature, pressure, water, etc.

3. Half Life: time it takes for half the atoms of the parent isotope to decay, ranges from tens of billions of years to thousandths of a second.

Percentage of parent atoms that decay in each half life is the same (50%)

The actual number of atoms that decay with each passing half-life continually decreases


Increase in daughter = decrease in parent

C. How it works (radioactive decay)

4. When mineral crystallizes (e.g., zircon mineral)

a. some amount of radioactive parent may get incorporated into the mineral structure

b. but daughter is different element and doesn’t “fit” in the structure (or inert and won’t combine), so at the time of formation there is 100% parent and 0% daughter


C.How it works (radioactive decay)

4. When mineral crystallizes (e.g., zircon mineral)

c. With a given time, the amount of parent slowly declines and amount of daughter increases within the mineral.

d. So we can go back and measure the ratio of parent to daughter (knowing the half life) and determine when the mineral crystallized.


Radioactive isotopes are incorporated in minerals and rocks in a variety of ways.

As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure.

At the time of crystallization, only parent isotopes are included in the mineral.

Radioactive parent isotopes then begin to decay producing daughter isotopes.


I. Principles of Numerical Dating ISOTOPE DATING uses this process to measure the amount of time elapsed since the mineral’s formation.

With time, the amount of parent isotope will decrease and the amount of daughter isotope will increase.

The DECAY RATE is constant and acts like a “clock”.

Decay rates are not affected by temperature, pressure, or chemical reaction with the parent isotope.

By measuring the ratio of parent to daughter isotopes in the mineral and comparing it with the rate of radioactive decay, we can determine the numerical age of a rock.

D. Dating Rocks

1.Igneous rocks – the best! Dates when the minerals formed

2. Metamorphic: during metamorphism ions can migrate, so dating tells us when metamorphism ended.

3. Sedimentary rocks: more errors because it dates the age of the individual pieces, gives maximum age










E. Sources of Error

Groundwater can bring in new ions and carry old ions away

Sample unfractured and unweathered rock, makes the age of the rock too young)

Very young rocks: not enough time to accumulate daughter isotopes to be measured accurately

So we date many different samples and make sure the ages agree

We date using different dating methods to make sure they agree


We pick dating methods based on approximate age of the rock and the rock composition

Old rocks – long half livesYoung rocks – short half livesComposition: have to use a dating method corresponding to what is in the

rock

II. Types of Isotope Dating

II. Types of Isotope Dating (page 252, table 8.1)

1. Uranium-thorium-lead (granite)

2. Rubidium-Strontiumplagioclase feldspar (igneous

and metamorphic rocks)

3. Potassium-Argonlots of minerals (plagioclase,

biotite, muscovite, amphibole)

Argon is inert gas and will not combine into any minerals



4. Carbon 14 (radiocarbon dating)

14C 14N5730 year ½ life

Useful between 100 and about 50,000 years old

Can date things that contain organic carbon (Used to be living): bones, shells, wood, charcoal, plants, paper, cloth, pollen, seeds)



4. Carbon 14 (radiocarbon dating)

Anything living takes in carbon in the form of CO2 (stable and unstable) by eating and drinking and photosynthesis

As long as the organism is living, it is taking in more 14C but when it dies, it’s 14C starts declining (12C remains constant)

Longer time since it died less 14C


III. Other Dating Techniques: Besides isotopic methods

A. Dendrochronology (Tree-ring dating)

Trees grow rings for each yearWe can count rings to get ages of trees

Pronounced changes in climate (i.e. drought) causes distinct patterns that can then be correlated between trees

Useful for dating: landslides, avalanches, or mudflows or wooden artifacts




Trees grow rings for each yearWe can count rings to get ages of trees

Pronounced changes in climate (i.e. drought) causes distinct patterns that can then be correlated between trees




B. Varve chronology (lake deposits)

Lakes produce annual layers of sediment much like tree rings

Spring & summer high sediment input thick, coarse, light-colored layers

Winter little to no sediment, especially when covered in ice—very fine stuff deposited which has been floating around for months dark, thin layers

Useful for dating: landslides into the lake


Origin of Lake Varves (Summer)

Origin of Lake Varves (Winter)


C. Lichenometry (dating lichen colonies)

Lichen—simple plant-like colonies the grow on exposed rock

For similar rocks and similar climate: the larger the lichen colony, the longer the time since the growth surface was exposed

Develop a growth curve based on measuring lichen of known age (tombstones, buildings) then extrapolate/interpolate to age of unknown rock

Useful for dating: glacial deposits, rockfalls, mudflows (expose new rock to surface)

I. Principles of Relative Dating

Review Principles of Absolute Dating

Radioactive isotopes are incorporated in minerals.

As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure.

At the time of crystallization, only parent isotopes are included in the mineral.

Radioactive parent isotopes then begin to decay producing daughter isotopes.

Isotope Dating: based on the rate of decay of radioactive isotopes within rocks

Carbon 14

14C 14NShort ½ life

Useful between 100 and about 50,000 years old

Can date things that contain organic carbon (Used to be living): bones, shells, wood, charcoal, plants, paper, cloth, pollen, seeds)

Review Principles of Absolute Dating

Isotope Dating: based on the rate of decay of radioactive isotopes within rocks




B. Varve chronology (lake deposits): Useful for dating: landslides into the lake

C. Lichenometry (dating lichen colonies): Useful for dating: glacial deposits, rockfalls, mudflows (expose new rock to surface)


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Review – Relative Dating Principle of Original Horizontality Principle of Superposition Principle...

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