http://talc.geo.umn.edu/courses/100i/1001kirkby/ss6.html Page I of II 3/17/2006 Geochronology study of 'Earth time' time scale of geologic processes & methods of measuring time Historical Geology study of the physical and biological evolution of the Earth & its life Geologists use two different chronologies: relative and absolute time. RELATIVE TIME is concerned with the sequence in which events occurred. ABSOLUTE TIME llq It ~) is age in chronological units (years, days, seconds) How much time? ~4.6 billion years It ~) " Just the Basics... Relative Time
ss4 httn';/t~ k P'~O Ilmn~rlll/collrses/l 00 1/1 001 kirkhv/sso.htm I Page 2 of II 3/1 7/2006 Rock se uences are main record of Earth's history. Geometric relationships of rock units can be used to interpret the order in which they formed. Surfaces between rock units can also represent significant periods of time. Correlating between local sequences can be much more difficult than establishing local sequences. Relative Time constructing a sequence of events using geometric relationships of.!:.?~s, fossils, and ~her techniques (paleomagneti~m) much of stratigraphy is based on observation of geometric relationships and common sense ORIGINAL HORIZONTALITY: most sedimentary rocks were originally deposited under the influence of gravity, so formed as relatively flat-lying layers ORIGINAL CONTINUITY: rock units can be correlated between areas, even if exposures are not continuous STRATIGRAPHIC SUPERPOSITION: unless a sequence has been overturned or faulte overli older layers) CROSS-C TTING RELATIONSHIPS:
ss4 Page 3 of 11 if a rock truncates a layer, it is younger than the truncated layer; if a rock contains fragments (clasts) of a different layer, it is younger than the eroded layer surfaces between rock units also r,epresent time to accurately interpret past, need to be able to recognize surfaces that represent significant amounts of time. these are called UNCONFORMITIES Unconformities include: I) surfaces between sequences of layered rocks with different orie~ represent periods of uplift, deformation and erosion.' 2) surfaces between edimentary rock ( nd coarse-grain~i heous or high-grade metamorphic rock. again represent periods of uplift and erosion as well as 3) other surfaces between parallel strata from significantly different time periods) D JJ I. ~ ~'Z.1 easily overlooked, unless you have a good fossil record N)~ ~"q CORRELATION It is one thing to establish the sequence of events in a single area, quite another to compare different areas Continuous, global exposures do not exist. Regional or global correlation are largely based on key beds and, for last 540 million years, fossils. 1'.:1' KEY BEDS ')"... \) ~-r http://talc.,geo.umn.edu/courses/l 001/1001 kirkby/ss6.html 3/17/2006
ss4 Page 4 of 11 http://talc.geo.umn.edu/courses/l 00 1/1 001 kirkby/ss6.html 3/17/2006 are thin, distinct, wides read beds that represent short periods of time......--. (ex's.- volcanic ash beds, iridium-rich clays from meteorites ~ FOSS L SUCCESSION Life has changed (evolved) through time so similar fossil assemblages equal similar age but fossil record is also a product of environment as well as time so how can you correlate fossils between North America and Africa? between land and seafloor? 200+ years effort -> composite GEOLOGIC COLUMN Just the Basics... Absolute time radioactive decay can be used to determine the age of some materials, because decay process is unaffected by external factors. radioactive decay is a function of probability, not set rate. apparent dates may be altered, so check dates by using multiple methods understanding what a date means is as important as determining a date. Absolute time
ss4 Page 5 of 11 http://talc.geo.umn.edu/coursesl100 1/1 00 1 kirkby/ss6.html 3/17/2006 early attempts to quantify the Earth's age failed due to social misconceptions & lack of knowledge underestimated the earth's age but appeared to support an age of ~ 100 Ma contemporaneous efforts, not independent investigations Combined Sedimentary Column estimated Earth's age by combining thickness of thickest known sections of sedimentary rock for each time interval and then dividing the total thickness by the rate at which sediments are currently being deposited but underestimated extent of unconformities more importantly, didn't include Precambrian rock! Ocean's Salt Content estimated Earth's age by dividing total amount of dissolved salts in oceans by the rate at which rivers presently transport salt to ocean but didn't know at time t - Cooling Rate ---- ecycled through ocean system Estimated Earth's age by calculating time it would take Earth to cool from an original molten state (relict heat) but didn't know that Earth had a second source of ongoing heat generation (radioactive decay)
http://talc.,geo.umn.edu/courses/l 001/1001 kirkby/ss6.html 3/17/2006 ss4 Page 6 of 11 Our best modern estimates of dating are based on RADIOACTIVE DECAY oldest radioactive dates: 4.1 billion for sedimentary grain, 3.9 billion for igneous rocks. b lunar studies s ggest the earth is 4.6 billion years old. Elements are defined by the number of protons in their nucleus (atomic number), but their atomic mass is determined by both protons and neutrons. ISOTOPES are atoms with the same number of protons (same element) but different numbers of neutrons. Many elements have naturally occurring isotopes. Most of these are stable, but a few are unstable. Unstable isotopes have a tendency to transform to more stable configurations. This process has been called RADIOACTIVE 'DECAY' The original isotope is called the parent, the product is often called the daughter isotope. 1) frequency of radioactive decaydiffers for each isotope. 2) decay process is intrinsic to nucleus and is NOT affected by changes in the chemical and physical environment.
ss4 Page 7 of 11 htto://talc.geo.umn.edu/courses/l 001/1 001 kirkbv/ss6.html 3/17/2006 3) decay process is a function of probability, not a set rate HALF-LIVES are used to measure decay A half-life is the time necessary for the number of parent atoms to be reduced by half (ranges from seconds to billions of years). So, if you know original composition, present composition, and half-life, age can be easily calculated provided no material is lost or added! heating & chemical alteration may allow material to be gained or lost and throw dates off - even though they do not affect decay process how can we tell if dates altered? best dates are those that can be confirmed by multiple methods Note - it is as important to realize what dates mean as to arrive at dates themselves! Uranium-Lead Dating nearly all uranium deposits contain~u235 and Th232 each is radioactive and decays to a different Pb isotope (each at a different rate) in addition a fourth Pb isotope is non-radioactive its concentration in lead-bearing materials doesn't change so ages of uranium-bearing materials can be cross-checked by comparing 3 decay processes long half-live so useful range is 10 MY to 4.6 BY
ss4 Page 8 of 11 htto:l/talc. Q:eo.umn.edu/courses/1 001/1 001 kirkbv/ss6.html 3/17/2006 basis of estimates for age of Earth fro ~nd meteorite sam les POTASSIUM - ARGON DATING 3 potassium isotopes occur in potassium-bearing minerals (K39, K40, K41), only K40 is radioactive of any population of parent K40 atoms, 12% will change to Ar40 88% will change to Ca40 so if you measure the amount of either Ar40 or Ca40 you can estimate age. Ar40 is used because it is an inert (noble) gas that does not easily fonn chemical bonds. in a magma, K40 decays to argon,but the argon freely escapes as magma cools, it fonns solid minerals In the crystal, argon gas produced from K40 decay is trapped in the mineral's crystal lattice. So amount of argon gas relative to remaining K40 dates time since mineral cooled However, if mineral is heated to high temperatures, the crystal lattice expands, and argon gas can escape. when mineral cools, argon gas wjfl again be trapped and begin to accumulate in crystal lattice so metamorphism c n 'reset' apparent radiometric dates ev n ough it does NOT affect decay process K-Ar dating only gives the time since the mineral last cooled below its confinement temperature, --...
htto:!/talc.geo.umn.edu/courses/1 001/1 00 1 kirkbv/ss6.html 3/17/2006 ss4 Page 9 of 11 this can be original cooling from magma or later cooling after high temperature metamorphism to test which, Rubidium-Strontium dating often run on same samples Rubidium - Strontium Dating rubidium often substitutes for potassium in crystal lattice of potassium-bearing minerals but Rd87 decays to Sr97which is a non-gas daughter, so it is not lost as crystal lattice expands during heating Rd87- Sr97 dates provide useful cross-check of K-Ar dates & combination of two processes ca e1 date metamorphic events CARBO -14 DATING rs-;l k. > r'''t-' -<..I-r. used to date materials from oncejiviog orgoli:lisms or samples of atmosphere gases trapped as bubbles precipitated minerals (including ice) cosmic radiation ofn14 in the atmosphere, produces C14 C 14 is unstable and decays back into N 14. So unlike other methods cannot compare daughter to parent Instead compare C 14 to stable C 12 because half-life ofc14 is 5,370 years but mixing of atmosphere is only days to weeks ratio of C 14 to C 12 in the atmosphere is relatively constant. Living organisms incorporate Cl4 and CI2 in the same ratio but at death this incorporation stops C12 is stabl,but C14 continues to de a so ratio of two changes after death
http://talc.geo.umn.edu/courses/lool/loolkirkbv/ss6.html 3/17/2006 ss4 Page 10 of 11 Amount of C 14 left in organic tissue can be used to date the time of organism's death (wood, plants, bone, cloth). even gas, hardened amber or pre,5ipitated minerals can be dated but what if ratio of C 14 to C 12was different in past? (it has varied slightly!) historic records a"nd tree rings ~an be used to check radiocarbon dates - because there is a difference, dates are given in radiocarbon years, not calendar years C14 dating is great for human record, but as half-life of C 14 is short it can only be used fo last 50,000 to 70,000 ye s - so its geological applications are limited ~\V ~::\>-~6- ~ Dendrochronology (tree rings) c~ ~\ufl ~ \ll- in temperate climates tree rings show seasonal alterations./'~y'~ ~ "... by overlapping living and fossil material, local sequence can be established to 9000+ years but sequence must be established in each area - climate influenced Paleomagnetic Dating Iron-bearing materials can record magnetic field at time they formed
ss4 Page 11 of 11 httn //t::llr OPrl llmn pr!n/rrlllr<;p<;1100ji100j brkhv/<;<;f\ html '\/17/?OOf\ Polarity of magnetic field has reversed itself numerous times Seafloor rock recorded reversal pattern for last 170 million years since magnetic reversals do NOT occur at set intervals, but fonn an irregular pattern. other records of magnetic reversals can be matched up to the seafloor pattern to narrow age (similar to tree rings). since the ocean floor can be dated by other radiometric methods As we do not have a continuous seafloor record older than 170 million years, this method cannot be used for older time periods paleomagnetism can be reset if rock is heated so method cannot be used with highly metamorphosed rock