Geol106-1 - HISTORICAL GEOLOGY GEOLOGY 106 Observe the...

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Unformatted text preview: HISTORICAL GEOLOGY GEOLOGY 106 Observe the Earth and it shall teach thee. (Job 12:8) ORIGIN OF THE EARTH ACCRETIONARY ORIGIN OF THE EARTH begins with BIG BANG THEORY of origin of universe that produces matter then, SOLAR NEBULA THEORY of solar system origin leads to formation of planets 1 STAR FORMATION The BIG BANG THEORY explains radial movement of galaxies from central point and points to origin of matter at 13 billion years ago (red shift) Explosive origin produces nuclear particles, atoms (mostly Hydrogen), and energy - determines amount of hydrogen & helium in universe - radiant energy still present as 5°K background radiation Millions years after explosion, matter aggregates into stars, nebulae, and galaxies, including nebular clouds that form stars and planetary systems. Our Sun is a late generation star - Later generation stellar systems are more enriched in heavy elements Why is Solar System enriched in heavy elements? SECONDARY ENRICHMENT OF HEAVY ELEMENTS Why is solar system enriched in 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 “Waste products” of fusion are heavy elements 2 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 NOVA Nova & supernova explosions of stars blast gas into space, spreading nuclear waste through galaxy Enrich nebular clouds with heavy elements. Heavy elements may also form in some nebulae by high energy radiation of lighter elements 3 ACCRET IONARY ORIGIN OF SOLAR SYSTEM 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 4 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 4) Sun produces explosive blast driving light materials out of inner areas of nebula SOLAR NEBULA THEORY 5) Magnetic fields of Sun & nebula interact and slow Sun's rotation 6) Planets accrete from solids - Terrestrial - Jovian 7) 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 planet as moons - Oort cloud in outermost zone contains comets; orbits easily disturbed and changed 5 PLANETARY ACCRETION ACCRETION OF THE EARTH Rock: Earth formed by accretion of refractory materials - metals (Fe, Mg, Al, Ni, Ca, Na, K) much more abundant than elsewhere in galaxy contains 0.03 weight% water (rock & hydro) Gases: H2 & He gases nearly lacking - Earth's gravity cannot retain Neon, argon, krypton & xenon rare compared to cosmos Earth:cosmic ratio Neon - 1:5,000,000,000 Xenon - 1:5,000,000 6 EARLY EARTH ATMOSPHERE Original atmosphere of Earth lost by solar blasting 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. (<100° C on surface; <75 ° C at top troposphere) HCl (acid) would dissolve in water to form acidic oceans and rain, that in turn would rapidly react with exposed rocks (weathering). Resulting chemistry produce ocean water similar to modern oceans. Zonal Structure of Earth HEAT FLOW Driving force for Earth interior processes Internal heat: drives motion of mantle and crust (mantle plumes; plate tectonics) Internal heat result of: gravitational contraction radioactive decay meteoroid bombardment Radioactive decay: main source of longterm internal heat 4 b.y. ago: 50% of Earth heat from radioactive decay; now only 10% Fluid Earth: Solar heat: drives flow of air and water Earth heat budget now mostly from solar radiation 7 ZONAL STRUCTURE OF EARTH Initial heating of Earth to 2000°C 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 MANTLE: Lithosphere: brittle Asthenosphere: weak plastic (partly melted) Mesosphere: strong plastic CRUST: too thin to show on this diagram 8 PROPERTIES OF LAYERS Continental crust: Low density layer floating on mantle (covers 30% of Earth surface) Oceanic crust: Chilled outer margin of mantle rock FORMATION OF CORE Core formation dominant event of the early Earth. Core of Earth form within 50 million years of Earth origin. Moved most metals from mantle to center of Earth. (Iron and "siderophile" elements.) Composed of Fe with 5% Ni and 2-10% S. 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. (enough to melt entire Earth twice) 9 CONTINENTAL CRUST Melting of planets 4.5-4.4 b.y. Probably no magma oceans on Earth Crust segregation - lithosphere form by 4.4 b.y., with water-laden surface form felsic (diorite, granite) continental crust plate tectonic movement starts mantle plumes dominant small size tectonic plates Geologic dating AGE OF EARTH Oldest rocks in solar system (meteorites, lunar rocks) 4.56 b.y. Oldest rocks on Earth are 4.03 b.y. (but some zircon crystals in metamorphics are 4.4 b.y.) Use RADIOMETRIC DATING to determine age 10 GEOLOGIC TIME SCALE GEOLOGIC TIME SCALE 11 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 14C 14N → 6 7 40K 238U 92 → 19 → 234Th 40Ar 18 90 → 206Pb 82 Nearly all elements have radioactive isotopes. 12 DECAY PROCESSES Alpha decay - release of alpha particle (4∝2) is same as release of a helium nucleus without any electrons - atomic number decreases by 2; atomic weight decreases by 4 example: 235U 231Th → 92 90 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: 14C → 6 14N 7 DECAY PROCESSES 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: 40K 19 → 40Ar 18 Spontaneous fission - disintegration of a heavy nucleus into large fragments example: 238U 92 → Ba56 + Kr36 + 3 n 13 DECAY PROCESSES RADIOACTIVE DECAY 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 daughter product 1/2 of parent to decay to Half-lives range from milliseconds to billions of years 1) Uranium-235 → Lead decay series 0.7 b.y. 2) Uranium-238 → Lead decay series 4.5 b.y. 3) Potassium → Argon electron capture 4) Carbon-14 (Radiocarbon) β emission 1.3 b.y. 5568 years 14 ISOTOPIC HALFLIFE U235 DECAY SERIES In 235U 92 decay series: Isotope particle emitted U-235 Th-231 Pa-231 Ac-227 Th-227 Ra-223 Rn-219 Po-215 Pb-211 Bi-211 Po-207 Pb-207 α β α β α α α α β α β stable half life of isotope 7.13 x108 years 25.6 hours 3.43 x104 years (34,300 years) 13.5 years 18.9 days 11.2 days 3.917 seconds 1.83 x 10-3 seconds 36.1 minutes 2.16 minutes 4.76 minutes Sum of half-lives = 0.7 billion years 15 U238 DECAY SERIES URANIUM SERIES METHODS Decay of U-235 & U-238 to isotopes of lead Compare ages of co-occurring U-235→Pb-207 and U-238 →Pb-206; both ages should be the same (be concordant) [some migration of lead atoms may occur] Also, Decay of intermediate isotopes Thorium 230: in decay series of U-238→Pb-206 Protactinium 231: in decay series U-235→Pb-207 In water, secreted in shell, thus isolating it from uranium. Forms closed system, with Th and Pa as parent isotopes. Half-lives of 75,000 years and 33,000 years. Dating limit about 300,000 years 16 K-40→Ar-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 method: a modified, more precise method. 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) 17 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. FISSION radiation damage dating method) TRACK DATING (a numerical 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. 18 COSMOGENIC decay clock method) DATING NUCLIDE (A numerical 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 β emission 26Al β+ (positron) emission 32Si β emission 36Cl β emission 53Mn electron capture half-life 1,510,000 years half-life 716,000 years half-life 172 years half-life 301,000 years 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. DENDROCHRONOLOGY Age dating using annual growth bands Growth bands record environmental variation in addition to age 19 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) Other dating methods used to verify assumed ages. 20 EARTH’S MAGNETIC FIELD P L E I S T O C E N E 1n BRUNHES NEOGENE RECORD 0.78 1 Jaramillo 1r 2n 2 2r MATUYAMA Olduvai Reunion Record of reversals in direction of Earth’s magnetic field during last 5 million years (changes in polarity of the dipole field) 2.58 GAUSS 3 P L I O C E N E 2An Kaena Mammoth Black shows times of normal polarity; white shows times of reversed polarity 3.58 4 2Ar 4.18 3n GILBERT Cochiti Nunivak Sidufjall 5 M I O C 5.23 3r Thvera 21 Geologic Time Scale 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 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. 22 GEOLOGIC TIME SCALE 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. 23 EARTH FORMED OF LAYERS general belief that whole Earth formed of layers G. Arduino, 1759: proposed names for layers Tertiary (or Alluvial) loose materials in low areas, with young fossils Secondary layered, fossiliferous rocks in mountains; tilted Primary hard crystalline rocks in centers of mountains This was the basis for GEOLOGICAL TIME SCALE - A method of defining units of geologic time NAMING GEOLOGIC TIME UNITS Compiled in early 1800's, using biotic succession . Story: A. Sedgewick & R. Murchison,1835-1841 Murchison trip to eastern Europe tested units (to see if they were useful “worldwide”). On trip, saw and proposed Permian. Personal quarrel and feud over early Paleozoic units. 24 GEOLOGIC TIME SCALE UNITS Time units recognized by contained fossils - units of geologic time scale - are relative time units. These are chronostratigraphic units. Boundaries of units are time horizons. Hierarchy of units (in geologic time scale) Eras -groups of systems [Paleozoic, Mesozoic, Cenozoic] System (or Period) Series (Epoch) - subdivisions of systems Inferential age dating EUSTATIC SEALEVEL CURVES Changes in sealevel are global. Historic record of changes is eustatic sealevel curve. This curve charts transgressions and regressions of sedimentary cycles (T-R cycles). - reversals on curve are global-synchronous events which can be used to correlate stratigraphic sections Causes of eustatic changes in sealevel: 1) Storage of water on land in continental glaciers 2) Change in mid-ocean ridges by changes inmantle. 25 Inferential age dating SEALEVEL & TECTONICS SEQUENCE UNITS The sedimentary unit Unconformity-bound units containing a cycle of deposits; they extend from basin center to highlands Are T-R (transgressive-regressive) cycles: are the result of deposition during sealevel or baselevel change in a basin. Most are global; produced from eustatic changes Unconformities correspond to tectonic or climatic events; are datable Contain characteristic groups of deposits (system tracts): transgressive, highstand, regressive and lowstand sets of deposits 26 TRANSGRESSION & REGRESSION TRANSGRESSION: migration of a shoreline away from center of an ocean basin, covering land with water. Sedimentary environments (facies) shift landward; seaward facies overlie more landward facies. REGRESSION: migration of shoreline towards center of ocean basin, exposing more land. Sedimentary environments (facies) shift seaward; landward facies overlie more seaward facies. T-R CYCLES Transgression & regression occur because: 1) sealevel rises or falls 2) land sinks or rises 3) shorelines are filled in with sediment deposits Produce a sedimentary cycle (T-R cycle) with vertical trend of a) change in water depth, associated with b) change in sediment type 27 UNCONFORMITIES The boundaries of units Gaps in the rock record Def: An unconformity represents a break in deposition for a long time interval; shows relation between tectonism, erosion, and sedimentation. DISCONFORMITY: Surface created by erosion or non-deposition. NONCONFORMITY: Sediments overlie igneous/metamorphic rocks. ANGULAR UNCONFORMITY: Sediments overlie tilted strata. UNCONFORMITIES IN GRAND CANYON 28 FACIES The components of a sedimentary unit The way a sedimentary deposit is related to other sediments of the same age is the basis for facies. Facies: Lateral changes in sediment type of age-equivalent sedimentary deposits that result from deposition in different depositional environments. Different types of sediments are deposited in different depositional environments. Facies changes occur in predictable patterns along gradients: 1) elevation - depth 2) climate gradient FACIES The reason for defining facies In working with sediments, we want to know the age of the deposits and the environment in which they accumulated. There is a rich mosaic of depositional environments on the surface of the Earth and sediments contain indicators of those environments in the form of contrasting lithology, sedimentary structures, fossils and chemistry. The deposits of these many environments are known as facies. 29 WALTHER’S LAW Lateral and vertical relationships of facies. Definition: For sediments deposited during a transgression or regression, laterally adjacent facies will occur above or below the other in a conformable vertical sequence of deposits. Change in vertical trend similar to change in lateral trend. Used to predict type of sediment present in unsampled areas. Sediments SEDIMENTARY UNITS They occur in characteristic units Sediments contain a record of conditions at the Earth surface through time, but the record may be discontinuous. In the stack of sediments, the missing intervals occur at a surface called an unconformity. Unconformities of wide extent separate related groups of sediment that are called sequences. Sequence units contain sediments deposited in a mosaic of depositional environments. The sediments of adjacent depositional environments are called facies. When depositional environments (facies) are mapped in the sediment record, a picture of the Earth surface can be mapped. To do this requires determining the geologic age of the sediments. 30 ...
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This note was uploaded on 11/22/2010 for the course GEOL 106 taught by Professor Yancey during the Spring '08 term at Texas A&M.

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