Upload
brittany-bryan
View
239
Download
0
Tags:
Embed Size (px)
Citation preview
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. Principles of Numerical Dating
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
I. Principles of Numerical Dating
I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks
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
I. Principles of Numerical Dating
For example Oxygen (8 protons): 8 neutrons 16O9 neutrons 17O10 neutrons 18O
I. Isotope Dating: based on the rate of decay of radioactive isotopes within rocks
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
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
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.
I. Principles of Numerical Dating
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
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
I. Principles of Numerical Dating
C.How it works (radioactive decay)
I. Principles of Numerical Dating
C.How it works (radioactive decay)
I. Principles of Numerical Dating
C.How it works (radioactive decay)
I. Principles of Numerical Dating
C.How it works (radioactive decay)
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
II. Types of Isotope Dating
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)
I. Principles of Numerical Dating
II. Types of Isotope Dating
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
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
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
I. Principles of Numerical Dating
III. Other Dating Techniques: Besides isotopic methods
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
I. Principles of Numerical Dating
Origin of Lake Varves (Summer)
Origin of Lake Varves (Winter)
III. Other Dating Techniques: Besides isotopic methods
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
III. Other Dating Techniques: Besides isotopic methods
A. Dendrochronology (Tree-ring dating)
Useful for dating: landslides, avalanches, or mudflows or wooden artifacts
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)
I. Principles of Numerical Dating