Dynamic EarthDynamic Earth

Class 2Class 2

12 January 200612 January 2006

Any Questions?Any Questions?

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.

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.

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

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

Naturally Occurring Isotopes Naturally Occurring Isotopes of Carbonof Carbon

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

Electron Capture

Beta Decay

Alpha Decay

Radioactive Radioactive Decay of Decay of

Rubidium to Rubidium to StrontiumStrontium

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

Radioactivity and Absolute Radioactivity and Absolute TimeTime

Decay of parent atoms

Growth of daughter atoms

Radioactive Radioactive Decay of Decay of

Uranium 238Uranium 238toto

Lead 206 Lead 206

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

Major Radioactive Elements Used Major Radioactive Elements Used in Isotopic Datingin Isotopic Dating

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)

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

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

Principle of SuperpositionPrinciple of Superposition In a sequence of undisturbed layered

rocks, the oldest rocks are on the bottom

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

Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Principle of SuperpositionPrinciple of Superposition

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

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

Principles of original Principles of original horizontality and superpositionhorizontality and superposition

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

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

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

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

Sedimentation of Beds A-D Sedimentation of Beds A-D Beneath the SeaBeneath the Sea

Uplift and Exposure of D to Uplift and Exposure of D to ErosionErosion

Continued Erosion Removes Continued Erosion Removes D and Exposes C to ErosionD and Exposes C to Erosion

Unconformity:Unconformity:a buried surface of erosiona buried surface of erosion

Subsidence and Subsidence and Sedimentation of E over CSedimentation of E over C

Formation of a UnconformityFormation of a Unconformity

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

South rim of the Grand Canyon

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

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

The Great Unconformity of the Grand CanyonThe Great Unconformity of the Grand Canyon

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

Siccar Point, ScotlandSiccar Point, Scotland

Buried and tilted erosional surface

Summary of Geologic EventsSummary of Geologic Eventsin a small areain a small area

Relative Geologic DatingRelative Geologic Dating

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.

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

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

Principle of Lateral Principle of Lateral ContinuityContinuity

Layered rocks are deposited in

continuous contact

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

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

Radiometric Radiometric dates dates

provide provide absolute absolute

ages to the ages to the Geologic Geologic ColumnColumn

Bracketing agesBracketing ages

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

Earth’s Magnetic

Field

Magnetization of Magnetization of MagnetiteMagnetite

Lavas Lavas record record

magnetimagnetic c

reversalreversalss

Magnetic Magnetic reversals reversals

over the past over the past 20 million 20 million

yearsyears

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

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

The Geologic The Geologic Column Column

and and Time ScaleTime Scale

TTueuesdaysday Geologic Time Scale (continued)Geologic Time Scale (continued) Homework #1 is due next Thursday, Homework #1 is due next Thursday,

January 19January 19