Transcript lecture08x
Geologic Time Marble demo Some Index Fossils Foam strata Determining geological ages • Relative ages – placing rocks and geologic events in their proper sequence, oldest to youngest. • Absolute dates – define the actual numerical age of a particular geologic event. For example, large dinosaurs died out 65 mya. The Lavas along Rt 22 and Rt 78 were deposited about 205 mya. First principle of relative dating • Law of superposition • Developed by Nicolaus Steno in 1669 • In an undeformed sequence of sedimentary or volcanic rocks the oldest rocks are at the base; the youngest are at the top Superposition illustrated by strata in the Grand Canyon 2nd principle of relative dating • Principle of original horizontality • Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting) 3rd principle of relative dating • Principle of cross-cutting relationships If a rock layer is cut by a fault or igneous intrusion, the rock that is cut must be older than the layer that cuts it. Igneous Dike 3rd principle of relative dating • Principle of cross-cutting relationships (example 2) Fault cross-cuts limestone and shale Unconformities (loss of rock record) • An unconformity is a break in the rock record produced by erosion and/or nondeposition • Types of unconformities – Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost – Angular unconformity – tilted rocks overlain by flat-lying rocks – Disconformity – strata on either side of the unconformity are parallel (but time is lost) Layered sedimentary rocks (a) 8_9 Nonconformity Metamorphic rock Igneous intrusive rock (b) Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks (c) Disconformity Brachiopod (290 million years old) Trilobite (490 million years old) Horizontal younger sediments over tilted older sediments Cambrian Tapeats sandstone over Precambrian Unkar Group What type of unconformity is this? Grand Canyon in Arizona Formation of an angular unconformity Development of a Nonconformity An intrusion occurs The overburden is eroded away Pennsylvanian sandstone over Precambrian granite is a nonconformity Sea level rises, new sediment is deposited Nonconformity in the Grand Canyon - Sediments deposited over Schist Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist Correlation of rock layers • Matching strata of similar ages in different regions is called correlation Correlation of strata in southwestern United States Sections are incomplete Match with fossils and lithology Because of sea-level changes Fossils are more reliable than sequences However, falling sea level of sediment facies is useful for worldwide correlation. Why? Sauk Sequence WEST Transgression Middle Cambrian http://www.geo-tools.com/trilobites.htm Lower Cambrian http://www.wmrs.edu/projects/trilobites/images/trilo7-2.jpg Note how western BAS is older than eastern BAS EAST http://www.unh.edu/esci/wmsmith.htm Correlation of rock layers with fossils Correlation often relies upon fossils • Principle of fossil succession (Wm. Smith) http://www.csun.ed u/~psk17793/ES9 CP/ES9%20fossils .htm – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it • Index Fossils - useful for correlation – Existed for a relatively brief time – Were widespread and common •Most fossils are just impressions. A few may have small amounts of some original tissue How impression fossils form (the most common type) 8_10 Shells settle on ocean floor Cast forms when mold is filled in with mineral water Rock broken to reveal fossil cast Shells buried in sediment Mold, or cavity, forms when original shell material is dissolved Rock broken to reveal external mold of shell Fossil Succession shown with Index Fossils Determining the ages of rocks using overlap of fossils Dinosaurs and flowering plant fossils occur in these sediments, therefore sediments “A” are Cretaceous B contains trilobites, Cambrian to Permian, and Ginkgo leaves, Permian to Recent, therefore B is Permian Overlapping fossil ranges are the data used in Biostratigraphy – These are NOT Index Fossils- WHY? correlation with fossils Two Measured Sections Note the use of overlapping fossil ranges in two distant outcrops, even though the sediment facies are different. In the diagram, the presence of three species of index fossils is shown for two measured sections. The limestone marked A on the left is the same age as a. sandstone 1 b. mudstone 2 c. shale 3 d. sandstone 4 Correlation with index fossils in spite of unconformities Highway cut A Correlation Highway cut B Especially useful in well cores conodonts diatoms pollen are microfossils foraminifera radiolaria http://www.ucl.ac.uk/GeolSci/micropal/ Magnetostratigraphy dated with fossils or with absolute (radiometric) dating Geologic time scale • The geologic time scale – a “calendar” of Earth history • Subdivides geologic history into units • Originally created using relative dates • Structure of the geologic time scale •Eon, Era, Period, Epoch Geologic Timescale Eons Phanerozoic PreCambrian Divisions based on fossils Eon, Era, Period, Epoch Eras Origin of Period Names Geologic time scale • Structure of the geologic time scale • Names of the eons – Phanerozoic (“visible life”) – the most recent eon, began about 545 million years ago – PreCambrian (Cryptozoic) • PreCambrian subdivisions: • Proterozoic – begins 2.5 billion years ago • Archean – begins 3.8 bya • Hadean – the oldest eon begins 4.6 bya Read from bottom to top – Oldest to Youngest Geologic time scale • Precambrian time • Nearly 4 billion years prior to the Cambrian period (.545 billion= 545 million) • Long time units because the events of Precambrian history are not know in detail – few fossils, most rock modified • Immense space of time (Earth is ~ 4.6 by) • PreCambrian (4.6 -0.545)/4.6 ~ 88% Geologic time scale - Eras • Structure of the geologic time scale • Era – subdivision of an eon • Eras of the Phanerozoic eon – Cenozoic (“recent life”) begins ~ 65 mya – Mesozoic (“middle life”) begins ~ 248 mya – Paleozoic (“ancient life”) begins ~ 545 mya • Eras are subdivided into periods • Periods are subdivided into epochs • Eon>Era>Period>Epoch Using radioactivity in dating • Importance of radiometric dating • Allows us to calibrate geologic timescale • Determines geologic history • Confirms idea that geologic time is immense Radiometric Age Determinations of the Earth • The oldest known minerals ever found on Earth include some from NW Australia. The containing rock (a conglomerate) is about 3.0 billion years old. The rock contains detrital grains of the mineral zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons. •Similar dates are known from Yellow Knife Lake, NWT, Canada Radiometric Age Determinations of the Earth • The age of the Earth is thought to be about 4.54 (roughly 4.6) billion years • Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time. Recall Isotopes • The number of protons in an atom's nucleus is called its atomic number –defines “element” • Mass protons + neutrons called atomic mass or, more loosely, atomic weight • The number of neutrons can vary • Atoms of the same element with different numbers of neutrons are called isotopes. Some are radioactive Radioactive parent nucleus pp p p p p Decay process 8_19 pp pp p p p (a) Alpha decay p p p p Atomic mass decreases by 4; atomic number decreases by 2 p p Proton Neutron pp Daughter nucleus pp p p p p Alpha particle Emission of 2 protons and 2 neutrons (alpha particle) pp p p p p p Atomic mass not changed much; atomic number increases by 1 because Neutron becomes proton Beta particle (b) Beta decay An electron (beta particle) is ejected from the nucleus pp p p p p p pp p p p p p Beta particle (c) Electron capture pp p p p p Atomic mass unchanged; atomic number decreases by 1 electron combines with a proton to form a neutron Using radioactivity in dating • Parent – an unstable radioactive isotope. Parent atom fits in crystal. • Daughter product – stable isotopes resulting from decay of parent. Doesn’t fit, so none present when crystal formed. • Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product A radioactive decay curve 1/2 = 50% parent: 1 half-life has passed 1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed 1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed Marble Demo Used up around Sparta, Byram – Zircons in pegmatites, rule of cross cutting Used around here, East Africa, Biotite etc. in volcanics U-238 decays to Pb-206 How do we actually “date” a rock? 1. Collect sample 2. Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids 3. Pass through extreme heat, electric and magnetic fields. 4. Measure parent/daughter ratio of target isotopes - mass spectrometer Dating sediments without fossils Wasatch Fm. younger than 66 my Mancos Shale and Mesa Verde Fm. older than 66 my Morrison Fm older than 160 my Radiometric Dating with Igneous Rocks Or Bracket between fossiliferous layers Even better: we get lucky. A layer we need to date is between two datable beds So we have and upper and lower bound on the age of this limestone: Basalt Lava flow 2 200 mya We can bracket this limestone’s age between 209 and 200 mya Lava flow 1 209 mya Mineral crystal Dating a crystal 1 Mineral crystal formed in igneous rock Parent atoms 2 Daughter atoms Igneous rock buried beneath younger rocks; daughter atoms formed by normal decay (3) We calculate age based on half-life But IF: 8_22b Heat Resets the clock 3 Deep burial and metamorphism during mountain building causes daughter atoms to escape from crystal Rock looks as if it just formed: it looks young Age found dates from metamorphic event 4 After mountain building ends, accumulation of daughter atoms in crystal resumes Easily recognized, useful in studying metamorphism Dating with carbon-14 (Carbon Dating) • Half-life only 5730 years • Used to date organisms • Carbon-14 is produced in the upper atmosphere • Useful tool for geologists who study very recent Earth history Carbon-14 Atoms split into smaller8_24 particles, among them neutrons Cosmic rays bombard atmospheric atoms Neutrons strike nitrogen atoms Nitrogen atoms lose a proton and becomes carbon-14 C-14 mixes with atmospheric oxygen to produce CO2 C-14 absorbed by living organisms CO2 taken up by plants CO2 dissolved in water C-14 intake ceases when organism dies; C-14 concentration decreases Years of age 500 Tree Rings 8_27 Annual-ring similarities show correlation Current year 50 400 100 150 200 Tree growth rings A B C D A Sediment layers with tree logs to be collected for dendrochronology B C D Buried tree logs Lakes south of the glacier track its advance - NH 8_28 Dating with Lake Varves Very little or no runoff Heavy runoff into lake Ice Summer Turbid water Summer layer (coarse, thick, and light-colored) Winter Clear water Winter layer (fine, thin, and dark-colored) Modern Lakes, just count back from present. Fossil pollen track climate. Examples of fossil pollen http://bcornet.tripod.com/Cornet94/Cornet94.htm http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html Hanneke Bos End of Geologic Time Lecture