Dynamic Earth Class 2 12 January 2006. Any Questions?
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Transcript of Dynamic Earth Class 2 12 January 2006. Any Questions?
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Dynamic EarthDynamic Earth
Class 2Class 2
12 January 200612 January 2006
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Any Questions?Any Questions?
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UniformitarianiUniformitarianismsm
The present is the key to the past
— — James James HuttonHuttonGeologic processes that we see Geologic processes that we see in operation today have in operation today have worked much the same way worked much the same way over geologic time — over geologic time — however, rates and intensity however, rates and intensity of processes may have of processes may have changed.changed.
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Early estimates of Earth’s ageEarly estimates of Earth’s age 1650: James Ussher1650: James Ussher -- -- Age: 6000 yearsAge: 6000 years
(F(From reconstruction of the Biblerom reconstruction of the Bible))
By the mid-19th Century, By the mid-19th Century, tthe age of the he age of the Earth was still only an educated guess, Earth was still only an educated guess, but the general feeling was that the but the general feeling was that the Earth was at least 500 million years old, Earth was at least 500 million years old, and probably much older.and probably much older.
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Radiometric DatingRadiometric Dating Use of radioactive decay to determine Use of radioactive decay to determine
the age of a rockthe age of a rock First proposed ~1896-1902First proposed ~1896-1902
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IsotopeIsotopessDifferent forms of the sameDifferent forms of the same
element containing the sameelement containing the same
number of protons, but varyingnumber of protons, but varying
numbers of neutronsnumbers of neutrons
i.e.i.e.::
235235U, U, 238238UU 8787Sr, Sr, 8686SrSr 1414C, C, 1212CC
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Naturally Occurring Isotopes Naturally Occurring Isotopes of Carbonof Carbon
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Pro
duct
ion a
nd
Pro
duct
ion a
nd
D
eca
yD
eca
y o
f R
ad
ioca
rbon
of
Rad
ioca
rbon
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Electron Capture
Beta Decay
Alpha Decay
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Radioactive Radioactive Decay of Decay of
Rubidium to Rubidium to StrontiumStrontium
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Radiometric DatingRadiometric Dating Key principle: Key principle: Half LifeHalf Life = time required for = time required for
1/2 of the nuclei in a sample to decay1/2 of the nuclei in a sample to decay
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Radioactivity and Absolute Radioactivity and Absolute TimeTime
Decay of parent atoms
Growth of daughter atoms
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Radioactive Radioactive Decay of Decay of
Uranium 238Uranium 238toto
Lead 206 Lead 206
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Isotopic Isotopic datingdating Radioactive elements (parents) decay to Radioactive elements (parents) decay to
nonradioactive (stable) elements nonradioactive (stable) elements (daughters)(daughters)
The rate at which this decay occurs is The rate at which this decay occurs is constant and knownconstant and known
Therefore, if we know the rate of decay Therefore, if we know the rate of decay and the amount present of parent and and the amount present of parent and daughter, we can calculate how long this daughter, we can calculate how long this reaction has been proceedingreaction has been proceeding
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Major Radioactive Elements Used Major Radioactive Elements Used in Isotopic Datingin Isotopic Dating
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Geologically useful decay Geologically useful decay schemesschemes
235U 207Pb 4.5 x 109
238U 206Pb 0.71 x 109
40K 40Ar 1.25 x 109
87Rb 87Sr 47 x 109
14C 14N 5730
Parent Daughter Half-life (years)
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Radiometric DatingRadiometric Dating Shows that the earth is much older than Shows that the earth is much older than
people had previously suspectedpeople had previously suspected Earth formed about 4.6 billion years Earth formed about 4.6 billion years
agoago
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Relative vs Absolute AgeRelative vs Absolute Age Usually geologists first establish Usually geologists first establish
relative ages then try to get absolute relative ages then try to get absolute age datesage dates
Determining Determining relativerelative age relies on a age relies on a number of geologic principles that number of geologic principles that were developed during the 17were developed during the 17thth to to early 19early 19thth centuries centuries
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Principle of SuperpositionPrinciple of Superposition In a sequence of undisturbed layered
rocks, the oldest rocks are on the bottom
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Layer 1
Layer 2
Layer 3
Layer 4
Principle of SuperpositionPrinciple of SuperpositionSedimentary rocks are deposited in a Sedimentary rocks are deposited in a
layer-cake fashion:layer-cake fashion:
Each layer is older than the one Each layer is older than the one above and younger than the one above and younger than the one
belowbelow
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Oldest rocksOldest rocks
Youngest rocksYoungest rocks
Principle of SuperpositionPrinciple of Superposition
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Principle of SuperpositionPrinciple of Superposition Although this is really obvious, it was Although this is really obvious, it was
not stated until 1669not stated until 1669 This principle generally applies to This principle generally applies to
volcanic rocks as well as sedimentary volcanic rocks as well as sedimentary rocks rocks
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Principle of Original Principle of Original HorizontalityHorizontality
Layers of sediment are deposited in a Layers of sediment are deposited in a horizontal or nearly horizontal horizontal or nearly horizontal position parallel to the Earth’s surfaceposition parallel to the Earth’s surface
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Principles of original Principles of original horizontality and superpositionhorizontality and superposition
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Principle of Original Principle of Original HorizontalityHorizontality
Note that original horizontality is not Note that original horizontality is not strictly applicable to volcanic rocks strictly applicable to volcanic rocks bbecause they are often deposited on ecause they are often deposited on slopesslopes
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Principle of Cross-cutting Principle of Cross-cutting RelationshipsRelationships
Something (such as a dike or fault) Something (such as a dike or fault) that cuts across a layer must be that cuts across a layer must be younger than the layeryounger than the layer
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Layers of rock are said to Layers of rock are said to be be conformableconformable when they when they
are found to have been are found to have been deposited essentially deposited essentially without interruptionwithout interruption
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Unconformity Unconformity Results from interruption of deposition Results from interruption of deposition Represents a long period of time during Represents a long period of time during
which there either was no deposition, or which there either was no deposition, or earlier deposited material was eroded earlier deposited material was eroded awayaway
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Sedimentation of Beds A-D Sedimentation of Beds A-D Beneath the SeaBeneath the Sea
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Uplift and Exposure of D to Uplift and Exposure of D to ErosionErosion
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Continued Erosion Removes Continued Erosion Removes D and Exposes C to ErosionD and Exposes C to Erosion
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Unconformity:Unconformity:a buried surface of erosiona buried surface of erosion
Subsidence and Subsidence and Sedimentation of E over CSedimentation of E over C
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Formation of a UnconformityFormation of a Unconformity
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Unconformity Unconformity Often not easy to recognize if the Often not easy to recognize if the
layers are all parallellayers are all parallel Much easier to recognize when there is Much easier to recognize when there is
a period of folding of the rocks before a a period of folding of the rocks before a period of erosion and renewed period of erosion and renewed depositiondeposition
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South rim of the Grand Canyon
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South rim of the Grand Canyon
250 million years old250 million years old
550 million years old550 million years old1.7 billion years old1.7 billion years old
Paleozoic StrataPaleozoic Strata
PrecambrianPrecambrian
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South rim of the Grand Canyon
250 million years old250 million years old
550 million years old550 million years old
Unconformity1.7 billion years old1.7 billion years old
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The Great Unconformity of the Grand CanyonThe Great Unconformity of the Grand Canyon
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Siccar Point, ScotSiccar Point, Scotlandland: Hutton’s Classic : Hutton’s Classic UnconformityUnconformity -- -- Old Red Sandstone (~345 Old Red Sandstone (~345 mymy) ) overlies overlies rocks that arerocks that are ~425 million years old ~425 million years old
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Siccar Point, ScotlandSiccar Point, Scotland
Buried and tilted erosional surface
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Summary of Geologic EventsSummary of Geologic Eventsin a small areain a small area
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Relative Geologic DatingRelative Geologic Dating
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These methods work well in small areas These methods work well in small areas where we can see the relationships where we can see the relationships between rock layers. between rock layers.
What happens when we want to tell the What happens when we want to tell the relative ages of the strata on Oahu with relative ages of the strata on Oahu with respect to strata on Maui? respect to strata on Maui?
We have to figure out some way to We have to figure out some way to correlatecorrelate the layers of interest. the layers of interest.
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Process used to tie separated strata togetherProcess used to tie separated strata together
Based on matching physical features such asBased on matching physical features such as Physical continuityPhysical continuity - trace of rock unit - trace of rock unit Similar rock typesSimilar rock types - marker beds, coal - marker beds, coal
seams, rare minerals, odd colorseams, rare minerals, odd color
CorrelationCorrelation
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CorrelationCorrelation Within sedimentary layers there are Within sedimentary layers there are
often the remains of small animals often the remains of small animals ((fossilsfossils))
Fossils are quite useful for correlating Fossils are quite useful for correlating between two sections that are not between two sections that are not laterally continuouslaterally continuous
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Principle of Lateral Principle of Lateral ContinuityContinuity
Layered rocks are deposited in
continuous contact
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CorrelationCorrelation Fossils have evolved through time, so Fossils have evolved through time, so
when we find a fossil of the same type in when we find a fossil of the same type in two different areas, we are pretty sure two different areas, we are pretty sure that the rocks are about the same agethat the rocks are about the same age
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TThis technique is not very useful in his technique is not very useful in HawaiiHawaii -- -- Why?Why?
Fossils helpful in sedimentary rocks, Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocksbut usually no fossils in volcanic rocks
Sometimes small amounts of sediment Sometimes small amounts of sediment between layers of volcanic rock (such between layers of volcanic rock (such layers might have fossils), but most layers might have fossils), but most rocks in Hawaii do not have fossilsrocks in Hawaii do not have fossils
CorrelationCorrelation
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Radiometric Radiometric dates dates
provide provide absolute absolute
ages to the ages to the Geologic Geologic ColumnColumn
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Bracketing agesBracketing ages
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MagnetostratigraphyMagnetostratigraphy Technique that works best in volcanic Technique that works best in volcanic
rocksrocks Time scale based on polarity reversal of Time scale based on polarity reversal of
Earth's magnetic fieldEarth's magnetic field Major problem is that Earth's magnetic Major problem is that Earth's magnetic
field has been constant for the past field has been constant for the past 700,000 yrs (no reversals), so this 700,000 yrs (no reversals), so this doesn't work for very young rocksdoesn't work for very young rocks
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Earth’s Magnetic
Field
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Magnetization of Magnetization of MagnetiteMagnetite
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Lavas Lavas record record
magnetimagnetic c
reversalreversalss
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Magnetic Magnetic reversals reversals
over the past over the past 20 million 20 million
yearsyears
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Magnetic time scale Magnetic time scale 0-700,000 -- Normal0-700,000 -- Normal 700,000 - 2.5 my -- Reversed700,000 - 2.5 my -- Reversed > 2.5 my -- Normal> 2.5 my -- Normal Ko`olau lavas mostly reversed in Ko`olau lavas mostly reversed in
polarity, so they must be older than polarity, so they must be older than 700,000 yrs, but younger than 2.5 my700,000 yrs, but younger than 2.5 my
Lavas on Kaua`i and in Wai`anae Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they Range show normal polarity, so they must be older than 2.5 mymust be older than 2.5 my
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The Geologic time scaleThe Geologic time scale Divisions in the worldwide Divisions in the worldwide
stratigraphic column based on stratigraphic column based on variations in preserved fossilsvariations in preserved fossils
Built using a combination of Built using a combination of stratigraphic relationships, cross-stratigraphic relationships, cross-cutting relationships, and absolute cutting relationships, and absolute (isotopic) ages(isotopic) ages
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The Geologic The Geologic Column Column
and and Time ScaleTime Scale
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TTueuesdaysday Geologic Time Scale (continued)Geologic Time Scale (continued) Homework #1 is due next Thursday, Homework #1 is due next Thursday,
January 19January 19