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HISTORICAL GEOLOGY!GEOLOGY 106!

Research is formalized curiosity. (Zora Neale Hurston)!

ACCRETIONARY ORIGIN OF THE EARTH!

The Earth is a planetary body, formed during formation of the solar system from nebular clouds.!!Gas and dust in nebulae aggregate into rotating disks that produce a star, with a disk of particles around it that condense and aggregate into zones that generate planets.!!The SOLAR NEBULA THEORY of planet formation.!

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ACCRETIONARY ORIGIN

OF SOLAR

SYSTEM!The Earth and all components of our solar system originate from nebular cloud of gas and dust in Milky Way galaxy !

ORIGIN OF SOLAR SYSTEM!Sun &planets form 4.56 b.y.ago!!

Formed in nebula cloud of dust and gas!!- Sun is a small mass star!

!

Earth, Sun, & planets form at same time! - all have similar rotation & all orbit in same plane

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SOLAR NEBULA THEORY!1) Nebular cloud contract to central ball with 90% mass! & turbulent, rotating outer disk!!!

2) Solids condense in nebula! - heavy elements condense to refractory! rocks in inner area (high temps)! - lighter elements condense to ices in! outer areas (low temp) !

! - snow line at 2.7 astronomical units (AU)!!

3) Sun forms: gravitational contraction heats and starts! thermonuclear fusion reactions, igniting in an

explosive blast drives light materials out of inner areas of solar nebula!

4) Planets accrete from solids! - Terrestrial - Jovian!!5) Magnetic fields of Sun & nebula interact and ! slow Sun's rotation!!6) Residual material present in outer edge of Solar System! ! - Kuiper belt beyond Neptune with small icy planetoids! like Pluto; have irregular orbits;! some captured by Jovian planets as moons!! - Oort cloud in outer zone with comets; their! orbits easily disturbed and changed!

SOLAR NEBULA THEORY!

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PLANETARY ACCRETION!

Planets form by accretion of particulates and gases. The changes from A to C illustrate the formation of a zonal structure for the Earth.

ACCRETION OF THE EARTH!Earth formed by accretion of rocky materials (chondrules)! condensed in inner parts of solar nebula! - rich in metals (Fe, Mg, Al, Ni, Ca, Na, K) ! much more abundant than elsewhere in galaxy! - contains 0.03 weight% water (rock & hydro)!! - contains small amount of gases:! H2 & He gases nearly lacking!

! Earth's gravity cannot retain them!! Neon, argon, krypton & xenon are rare compared to!

!abundance in cosmos!!Earth:cosmic ratio!!Neon - 1:5,000,000,000!!Xenon - 1:5,000,000!

!These details are important for determining very early history of Earth

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ORIGIN OF HEAVY ELEMENTS!

Heavy elements formed by !thermonuclear fusion reactions in interior of stars !!

A series of fusion reactions:! H - He (main energy source)! He - C at higher temps! C cycle produces N & O, F, Ne, Na, Mg, P, S, and Fe!Fe fusion at extreme temps!!Heavier elements formed by neutron capture; they are!the waste products of fusion reaction!The galaxy has become enriched in heavy elements during last 13 billion years because of fusion reactions in stars!

Why is solar system enriched in heavy elements?!

NUCLEAR FUSION IN STARS!Iron (56Fe) is the end product of normal fusion in big stars!(with some cobalt and nickel produced as well)!!!Heavier elements formed during nova explosions

Why is Solar System enriched in heavy elements?

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NOVAMechanism for distributing heavy elements to

nebula !Nova & supernova explosions of stars blast gas into space, spreading nuclear waste through galaxy!!Enrich nebular clouds with heavy elements.!(Otherwise, solar system would be all ice/gas planets)!!Heavy elements may also form in some nebulae by high energy radiation of lighter elements!

Original atmosphere of Earth lost by solar blasting!!Result in loss of neon, argon, krypton & xenon !!

Replacement atmosphere formed by volcanic emissions (N2, CO2, H2O, HCl)!!!

Liquid water: (a unique feature of the Earth)!Earth was cool enough for water vapor to condense and form oceans.!(

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THERMAL EVOLUTION!Earth is gradually cooling!!Early Earth hot because of gravitational contraction and meteoroid bombardment, !but most heat produced by radioactive decay!!Solid Earth:!Internal heat is driving force for Earth interior processes!- early melting allowed separation (by smelting) of metals and floating of light crustal magmas!- continues to drive movement of crustal plates (plate tectonics) and mantle plumes !!!Fluid Earth:!Solar heat: drives flow of air and water!Surface heat budget is mostly from solar radiation!

ZONAL STRUCTURE OF EARTH!Initial heating of Earth to 2000C !!Earth melted - volatiles lost (degassing)!!Melting and separation of immiscible materials!!Iron & nickel melt sink to center!

!to form CORE!!Ca, K, and Na compounds float to top!

!to form CRUST !(also much O, Si, Al, & U)!!Residual Fe & Mg silicates and metal oxides!

! form MANTLE

Zonal Structure of Earth

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ZONAL STRUCTURE OF EARTH!

MANTLE:Lithosphere: brittleAsthenosphere: weak plastic

! (partly melted)Mesosphere: strong plastic!

CRUST: too thin to show on this diagram!

Continental crust: Low density layer floating on mantle ! ! (covers 30% of Earth surface)

Oceanic crust: Chilled outer margin of mantle rock!

PROPERTIES OF LAYERS!

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FORMATION OF CORE!Core formation dominant event of the early Earth.!!

Core of Earth form within 30 million years of Earth origin.!!Smelting of iron from rocky solids released liquid iron.!!Moved most metals from mantle to center of Earth.!(Iron and siderophile (=metal-loving) elements.)!Composed of Fe with 5-10% Ni and 2-5% C,S,O and Si.!!

Density of liquid (outer) core lower than the density!of pure Fe+Ni.!!

Settling of iron to center of Earth releases a tremendous!amount of heat energy, accelerating process of melting.!

CONTINENTAL CRUST AND CRUSTAL EVOLUTION!

Melting of planets!4.5-4.4 b.y.!major meteor bombardment continue to 3.8 b.y.!!

Crust segregation!- lithosphere form by 4.4 b.y., with water-laden surface!

(no magma oceans)!- form felsic (diorite, granite) continental crust!-plate tectonic movement starts, probably! small size plates!-mantle plumes very active!

Form cratons !by 3.0 b.y. (2/3 of continental crust formed)!Areas of stabilized continental crust! -crust formed earlier, but was tectonically disturbed!Start modern style plate tectonics!

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NORTH AMERICAN CRATON!

CRATONS!Cratons are areas of continental crust underlain by a 'root' of upper mantle.!- fixed to upper mantle!- stable zone extend to 200 km deep!- mostly metamorphic & igneous rock units!!Upper mantle of melt-depleted peridotite (depleted in Ca, Al, Fe, which go readily into melt); makes it more bouyant and rigid than surrounding mantle!!Thought to be crust formed over mantle plumes!Shield areas - are exposed area of cratons! have plain, flat surfaces (from long erosion)!!Contain the oldest rocks: 3.8-4.3 b.y. (in Canada, Australia)!

Continents!Units of stable continental crust!

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AGE OF EARTH!Oldest rocks in solar system (meteorites, lunar rocks) ! 4.56 b.y.!!Oldest rocks on Earth are 4.28 b.y.!(some zircon crystals in metamorphics are ! 4.36 b.y.)!!!Use RADIOMETRIC DATING to determine age!

Geologic dating!

GEOLOGIC TIME SCALE!

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METHODS FOR AGE DATING!Numerical age dating - quantitative measures of time!(e.g., radioactive decay) that involve time-dependent processes; no addition or loss of parent or decay material in system!!Relative age dating - identify unique intervals of ! geologic time (e.g., dating with fossils)!!Inferential age dating - determine age by comparison to known sequence of events (e.g., magnetostratigraphy)!

RADIOMETRIC DATING!Numerical age determination of materials containing! radioactive isotopes.!!

Concept: Naturally occurring radioactive atoms change! to other atoms by spontaneous decay. !!

The decay process occurs in a time-predictable manner.!!

!examples: !parent - daughter!! ! !14C6 ! !14N7!

!! ! !40K19 ! !40Ar18!

!! !238U92! !234Th90 206Pb82!

!Nearly all elements have radioactive isotopes.!

Radiometric Dating

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DECAY PROCESSES!Alpha decay - release of alpha particle (42) is same as!release of a helium nucleus without any electrons!!

!- atomic number decreases by 2;!! atomic weight decreases by 4!

!

example: !235U92 ! !231Th90 !!Beta emission - release of an electron !

! (but it comes from the nucleus)! [neutron can be visualized as proton plus electron]!!

! !n = p+ + e-!!- atomic number goes up by 1;!! atomic weight unchanged (0)!

!

example: !14C6 ! !14N7!

TEKS review

Electron capture - electron is captured by a proton,!turning it into a neutron!

! !p+ + e- n!!

! - atomic number goes down one!! [atomic weight unchanged]!

!

example: !40K19 ! !40Ar18!!!Spontaneous fission - disintegration of !a heavy nucleus into large fragments;!usually two fragments with masses near!95 and 137!!

example: 238U92 Ba56 + Kr36 + 3 n

DECAY PROCESSES!TEKS review

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DECAY PROCESSES!TEKS review

ISOTOPIC HALFLIFE!

The amount of time needed for half of the parent isotope to decay is always the same, regardless of how much of a parent sample remains during the decay process.

TEKS review

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Decay of radioactive isotopes governed by law:!!atomic disintegration = decay X atoms!

!unit time ! constant present!!Decay constant (l) : the fraction of atoms decaying per unit time!!

Half-life: time needed for 1/2 of parent to decay to daughter product!!

Half-lives different for every radioactive isotope!!!

1) Uranium-238 Lead! decay series ! 4.5 b.y. !!

2) Uranium-235 Lead! decay series ! 0.7 b.y.!!

3) Potassium Argon ! electron capture! 1.3 b.y.!!

4) Carbon-14 (Radiocarbon) emission ! 5568 years!

RADIOACTIVE DECAY!

U235 DECAY SERIES!In 235U92 decay series:!!Isotope ! particle emitted ! !half life of isotope! ! !!

! U-235 ! ! ! ! 7.13 x108 years!! Th-231 ! ! ! 25.6 hours!! Pa-231! ! ! ! 3.43 x104 years (34,300 years)!! Ac-227! ! ! ! 13.5 years ! !!! Th-227 ! ! ! 18.9 days !! Ra-223 ! ! ! 11.2 days!! Rn-219 ! ! ! 3.917 seconds!! Po-215 ! ! ! 1.83 x 10-3 seconds!! Pb-211 ! ! ! 36.1 minutes!! Bi-211! ! ! ! 2.16 minutes!! Po-207 ! ! ! 4.76 minutes!! Pb-207 !stable!

!! !Sum of half-lives = 0.7 billion years!

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U238 DECAY SERIES!

U/Pb DATING METHOD!Decay of U-235 & U-238 to isotopes of lead!!

Measure amounts of uranium and lead isotopes and compare ages calculated from co-occurring U-235Pb-207 and U-238Pb-206 decay; !both ages should be the same (be concordant)!! [some migration of lead atoms can occur in rocks]!!Accurate dating depends on maintaining a closed system for parent and decay product!!

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CLOSED SYSTEM!Def: A material (mineral, rock, etc.) that has not lost or added parent or decay components (radioisotopes, etc.) after formation!!!A valid age date depends on meeting the condition that there was no addition or loss of parent or decay product in the system!!Loss of decay isotope leads to calculating a younger age than the time of rock formation.!

CLOSURE TEMPERATURES!

Dating different minerals with with different methods can determine age of formation, cooling history and ages of deformation in a rock!!A tool for documenting tectonic history!

Examples of temperature boundary limits for maintaining a closed system!!Closed system maintained below temperatures shown!

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K-40Ar-40 DATING METHOD!! !(An accumulation clock method)!! !Potassium contains 0.01% 40K isotope!

!

Method best for dating volcanic rock and minerals.!!

1) Pre-existing argon lost from magma.!2) At cooling, new (radiogenic) argon is retained !

!in mineral or rock!3) As rock cools below closure temperature it forms !

!closed system.!!

Age: the time since material began to retain argon.!!

Subsequent heating allows argon gas to escape - so, !can date time of metamorphism of heated rocks.!!Although 89% of K-40 decays to Ca-40, this decay not often used for dating.

40Ar/39Ar DATING METHOD!This is the current methodology used with K-Ar dating:!!In addition to natural decay of K-40 to Ar-40, a sample is irradiated to convert K-39 to Ar-39 and both Ar decay isotopes are measured. The increased accuracy of measuring Ar isotopes in the same step is more accurate than measuring K and Ar separately. [a metal (K) and a gas (Ar) cannot be measured in the same step]!!A standard of known age is dated at the same time to provide necessary calibration for accurate age calculation.

Note: half-life is 1.3 b.y., but method used mostly for 10-200 Ma age rocks! Explain why

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C14 DATING METHOD!! ! !(A numerical decay clock method)!!

Half-life for C-14 is 5568 years!Useful limit for C-14 dating 60,000 years.![Useful range for age dating is about 10 half-lives of the isotope used.]!!

C-14 formed from N-14 in atmosphere,! by cosmic ray bombardment, via electron capture.!!

C-14 is then used by living organisms.!(Method dates once-living material).!!!!!!!

When organism alive, C-14 level nearly constant (continual ! intake of C-14). At death, C-14 level begins to decrease.!Measure C-14 remaining in sample to determine age.!!

Assume constant production of C-14.! (only minor fluctuations in past 60,000 years)

COSMOGENIC NUCLIDE DATING!Measures accumulation of new radioactive isotopes, !produced by exposure to cosmic radiation. Cosmic rays!affects materials within a few metres of ground surface.!!

Dates ice, lake sediments, and exposure surfaces on rock.!!Useful cosmogenic nuclides:! 10Be emission ! !half-life 1,510,000 years! 26Al! + (positron) emission !half-life 716,000 years! 32Si! emission ! !half-life ! 172 years! 36Cl! emission ! !half-life 301,000 years! 53Mn electron capture !half-life 3,700,000 years!!

Best for dating exposure surfaces is ratio of 26Al/10Be.!!

Dating limit for exposure surfaces is the saturation level, !when new nuclei balanced by loss.!Saturation level occurs after a few half-lives.!

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DATING METHODS!Uranium-235Lead-207! decay series 0.7 b.y.!!Uranium-238Lead-206! decay series 4.5 b.y.!!Thorium-232Lead-208! decay series 14.1 b.y.!!Rubidium-87Strontium87 ! ! 48.8 b.y.!!Samarium-147Neodymium143 ! 103 b.y.!!Potassium-40Argon-40 electron capture 1.3 b.y.!!Radiocarbon (Carbon-14) emission! 5568 years!

FISSION TRACK DATING!! !(a numerical radiation damage dating method)!

!

Heavy radioactive decay particles (fission fragments) !produce tunnel of damage in mineral or glass.!!

These fission tracks are up to 15 microns long.!!

Seen by acid etching a polished surface of material.!

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FISSION TRACK DATING!! !(a numerical radiation damage dating method)!

!

Heavy radioactive decay particles (fission fragments) !produce tunnel of damage in mineral or glass.!!

These fission tracks are up to 15 microns long.!!

Seen by acid etching a polished surface of material.!!

Age determined by: !1) counting number of fission tracks !per unit area! (= daughter product)!2) annealing surface to remove tracks!3) irradiate to induce fission of remaining radioactives!4) count number of new fission tracks (= parent material)!!

Method works well with zircon and glass.!!

Material must remain below annealing temperature.

DENDROCHRONOLOGY!Age dating using annual growth bands!!Growth bands record environmental variation in addition to age!

Inferential Dating

Specimen

Standard

A specimen is matched to a dated standard

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SCLEROCHRONOLOGY!Determining age, growth rate and environmental variation using periodic (annual, monthly, daily) growth increments in skeletons.!!

Corals, molluscs, brachiopods!

Bivalve growth bands! Coral growth banding!

Inferential Dating

MAGNETOSTRATIGRAPHY!Inferential dating method!

!Layered rocks contain history of Earth's magnetic field !through time.!!Earth's magnetic field is global in extent.!- changes polarity!- reversal occurs quickly - a global time horizon!- intervals between reversals have different duration!!Match local record against global standard record to !determine age. (is pattern matching wiggly lines)!!Other dating methods used to verify assumed ages.!

Inferential Dating

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EARTHS MAGNETIC FIELD!

Record of reversals in direction of Earths magnetic field during last ! 5 million years!(changes in polarity of the dipole field)!!Black shows times of normal polarity; white shows times of reversed polarity

Thvera

Sidufjall

Nunivak

Cochiti

MammothKaena

Reunion

Olduvai

Jaramillo

2Ar

2An

1r

1n

3.58

2.58

0.781

2

3

4

5

2n

2r

GAUSS

GILBERT

MATUYAMA

BRUNHES

3n

3r

4.18

5.23

PLEISTOCENE

PLIOCENE

MIOC

NEOGENE RECORD!

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MAGNETOSTRATIGRAPHICCORRELATION!

Magnetostratigraphy of Late Miocene La Vedova section, Italy!- global geomagnetic standard shown on right side of figure

ASTROCHRONOLOGY!An inferential dating method that uses Milankovich orbital variations of the Earth to subdivide time!

Cyclicity in Aptian sediments, central Italy: well developed 100 k.y. cycles!

Ice Ages climate cyclicity: change from dominant 41 k.y. cycles to 100 k.y. cycles

Inferential Dating

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MILANKOVICH ORBITAL VARIATIONS!

Produce variations in insolation of solar radiation. This type of variation has been determined for the past 40 million years!Can be extended back much further

Insolation - amount of solar radiation received on a surface

ASTROCHRONOLOGY CORRELATION!

Cyclicity in Aptian sediments, central Italy

Tuning of Corvi Beach sediments to Milankovich cycle insolation

The results of this correlation produce age determinations accurate to 100,000 year intervals of time

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STABLE ISOTOPE STRATIGRAPHY!Concentrations of stable isotopes vary with time.!

!- increase of radiogenic daughter products!!- variation in amounts stored oceans, mantle, etc.!

!

Sr stratigraphy! Increasing amounts of Sr87 added by radioactive decay!!

C13/C12 stratigraphy! Storage of organic carbon (photosynthesis uses one! isotope) in sediments changes isotope ratios of water,etc.!!

O18/O16 stratigraphy! Isotope ratios controlled by temperature, & ice volume;! can determine paleotemperatures!

Inferential Dating

STABLE ISOTOPE STRATIGRAPHY!

This is the best known example of stable isotope stratigraphy!!It shows events and trends during a time of longterm climate cooling

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RELATIVE DATING METHODS!PRINCIPLE OF BIOTIC SUCCESSION!!

Fossils and fossil assemblages change through a stratigraphic section and do not repeat!!PRINCIPLE OF SUPERPOSITION!!

In an undisturbed sequence of sediments, younger deposits occur at at the top and older at the bottom!!CROSS-CUTTING RELATIONS!!

A rock unit which intrudes another is younger than the surrounding rock unit!

Geologic Time Scale!

FOSSIL AGE RANGES!Relative Dating

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PRINCIPLE OF BIOTIC SUCCESSION!! ! William Smith, 1815!

!

Def: Assemblages of fossils change regularly through a stratigraphic section and do not repeat, so each stratigraphic unit contains a unique set of fossils.!!

Each species is limited to a part of the geologic column.!!The tool used for defining units of geologic time scale.!!Based on the !Principle of Organic Evolution.!!Def: Organic evolution is an irreversible process and each species is unique and different from all others and has a unique time range.!

PRINCIPLE OF SUPERPOSITION!Def: In an undisturbed sequence of sedimentary strata, the oldest strata lie at the bottom and the youngest strata lie at the top.!!Correlary 1 -- Principle of original horizontality!!

Sediment strata are deposited as horizontal layers.!!Correlary 2 -- Principle of original lateral continuity!!

Strata originally extended in all directions until they thinned to zero at the edge of original area (or basin) of deposition.!

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GEOLOGIC TIME SCALE!Note: the ages of unit boundaries on the geological time scale are continually being revised and change with better dating. For example, this scale has the Cret/Paleogene boundary at 65.4 Ma, but latest work shows it to be 66.0 Ma.