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Stratigraphy (study of rock layers)
A tug-of-war as rocks got sorted into geological periods in the new science of stratigraphy
en.wikipedia.org/wiki/Image:Geological_map_of_Great_Britain.jpg
MurchisonSedgewick
Two Kinds of AgesRelative - Know Order of Events But Not
Dates• Civil War Happened Before W.W.II • Bedrock in Wisconsin Formed Before The
Glaciers Came Absolute - Know Dates• Civil War 1861-1865 • World War II 1939-1945 • Glaciers Left Wisconsin About 11,000 Years
Ago
RELATIVE DATING FIRST
Relative Dating: Putting geologic events into proper order (oldest to youngest), but without absolute ages. We use a number of principles and laws to do
this
STENO’S PRINCIPLES OF RELATIVE DATING (1669)
Principles of Relative Dating
• Principle of horizontality
• Principle of superposition
• Principle of crosscutting relations
• Principle of Inclusion
• Principle of faunal succession
Law of Original Horzontality Sedimentary units and lava flows are deposited horizontally.
Horizontally bedded sedimentary strata as seen from the North Rim of the Grand Canyon illustrating the immensity of geologic time. It took hundreds of millions of years for these strata to be deposited as layers of sediment that were eventually converted into rock.
THE GRAND CANYON
Law of Superposition
the layer below is older than the layer above.
THE GRAND CANYON
Kaibab Limestone
Toroweap Formation
Coconino Sandstone
Hermit Shale
Supai Group
Law of Cross-cutting Relationships
A rock is younger than any rock across which it cuts.
An basalt dike cutting through granite. The basalt dike is younger than the granite. (Photo taken on Cadillac Mountain, Bar Harbor, Maine by E. L. Crisp, August, 2005).
PRINCIPLE OF INCLUSION
• The Principle of Inclusion: a rock body that contains inclusions of preexisting rocks is younger than the rocks from which the inclusions came from.
• Which is older, a cake or the ingredients that are used to make the cake?
index fossil short-lived organism; points to narrow range
of geologic timefossil assemblage
group of fossilsassociatedtogether
Law of Faunal Succession
• Index fossils are used to correlate age-equivalent strata via the Principle of Faunal Succession.
• Index fossils have the following characteristics:– Short geologic time range.– Wide geographic distribution– Abundant– Easily recognizable
PRINCIPLE OF FAUNAL SUCCESSION
• Although rocks may be correlated based on physical correlation and superposition, this can only be done in a limited area where beds can be traced from one area to another. Also if we are correlating over a large area (from region to region, or continent to continent), it is unlikely that we can use physical correlation because rock types will change.
• To correlate over large regions and to correlate age-equivalent strata, geologists must use fossils. The use of fossils to correlate sedimentary strata is based on the work of William Smith (1812), the first to accurately state and use the Principle of Fossil Succession.
• The Principle of Faunal Succession states the assemblages of fossils succeed themselves in a definite and determinable order and the age of sedimentary strata can be determined by their contained fossils.
PRINCIPLE OF FOSSIL SUCCESSION
• The Principle of Fossil Succession is based on the following:– Life has varied through time. Of course this
implies that evolutionary change has occurred over time.
– Because biologic diversity has varied over time, fossil assemblages are different in successivley younger strata.
– The relative ages of fossil assemblages can be determined by superposition.
Radiometric dating: Discovery
Henri Becquerel (1852-1908)
• In 1896, Discovery of radioactivity paved the way for the precise dating of events in the geological record
Pioneers
Arthur Holmes (1890-1965)
Ernest Rutherford (1871-1937)
• Rutherford figured out a technique to date the age of rocks in 1904• Holmes developed this kind of ‘radiometric dating’ still further.• In 1913 Holmes dated some rocks from Ceylon to 1600 million years
Oldest Rock
• Oldest rocks on Earth are the Acasta Gniess of northern Canada • 4030 million years old
© NASA
Acasta Gneiss
Zircon mineral
Oldest Grain
www.geology.wisc.edu/zircon/Earliest%20Piece/Images/5.jpg
4404 Mazircon grain
© NASA
• Ancient mineral grain found at Jack Hills, Australia • Mineral grain eroded from first crust and then deposited in a new rock
• Dates the Earth’s first crust to around 4404 million years
earliest life
cyanobacteria: primitive single-celled organisms found in Australia and dated at 3.7 billion years old
radiometric dating
as minerals crystallize in magma; they trap atoms of radioactive isotopes in their crystal structures
radioactive isotopes will decay immediately and continuously
as time passes, rock contains less parent and more daughter
uses continuous decay to measure time since rock formedonly possible since late 1890’s -- radioactivity discovered in 1896
• The rate of decay from parent to daughter isotope depends on its half life. The half life is the amount of time needed for half the parent isotope to decay to daughter isotope
Half life: 0
Half life: 1
Half life: 2
Linear
Exponential
HALF-LIFE
Radiometric Dating: Half-Life
have nuclei that spontaneously decay
daughterparent
radioactive isotopes
-- emit or capture subatomic particles
parent: decaying radioactive isotopedaughter: decay daughter
example: Uranium 238 decay to Lead 206 (stable)several steps(each has its own half-life)
most common dating systems
• uranium-thorium-lead dating (previous example)U-238, U-235, Th-232
each of these decays through a series of steps to Pb
U-238 to Pb-206 half-life = 4.5 byU-235 to Pb-207 half-life = 713 myTh-232 to Pb-208 half-life = 14.1 my
• potassium-argon dating
K-40 to Ar-40 half-life = 1.3 by
…argon is a gas--may escape (ages too young--daughter missing)
• rubidium-strontium dating
Rb-87 to Sr-87 half-life = 47 by
carbon-14 to nitrogen- 14 datingC-14 to N-14 half-life about 5700 years
Used to date artifacts (human remains or evidence thereof)
Only accurate to about 40,000 years
Formula for radioactive decayAmt remaining = initial amt (1/2)n
n = number of half-lives.
Or you can use the arrow method
Half-life Examples
Fluorine-21 has a half-life of 12.00 seconds. If you start with 25.00 g of fluorine-21, how many grams would remain after 60.0 s?
GIVEN:
t½ = 5.00 s
mi = 25.00 g
mf = ?
total time = 60.00 s
WORK:
n = 60.00s ÷ 5.00s =12
mf = mi (½)5
mf = (25 g)(0.5)5
mf = 0.78125 g
Fluorine-21 has a half-life of 12.00 seconds. If you start with 25 g of fluorine-21, how many grams would remain
after 60.00 s?
GIVEN:
t½ = 5.00 s
mi = 25 g
mf = ?
total time = 60.00 s
WORK:
n = 60.00s / 12.00s = 5 = number of half-lives
Go 5 arrows.
11/21/41/81/161/32
1/32(25g) = 0.78125 g remains
Half-life problems
Nitrogen-13 emits beta radiation and decays to carbon-13 with a half-life of 10 minutes. If you start with 2.00g of Nitrogen-13 how much will remain after 40 minutes.
• The half-life of oxygen-19 is 30 seconds, What fraction of the original isotope will remain after 4 min?
• If it takes 11.1 days for 40.00 grams of radon-222 to decay down to 5.00 grams, what is the half-life of radon-222?
Radon-222 has a half-life of 3.8 days, and decays to produce Polonium-218.
(a) If you started with an 8.00 gram sample of radon-222,
and 19 days goes by, how much radon-222 will be left?
(b) How long will it take for the 8.00g of radon-222 to be
reduced to .125 gram? Remember the half-life is 3.8 days.
(c) If you start with 8.00g, what percentage remains after 7.6 days?
(d) If the ratio of parent isotope to daughter isotope is 1:7, how many half-lives have passed?
basic geochronological assumptions• decay constants constant through geological time
igneous rocks are most reliable for dating…metamorphism may cause loss of daughter products……sedimentary rocks will give ages of source rocks…
• system closed to adding or subtracting of parent/daughter
-- good reasons to believe this is correct from nuclear physics
-- measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements
-- isotopic system and type of mineral (rock) are important
-- careful procedure is essential to correct analysis
