Absolute-Dating Methods-4 typesThe discovery of radioactivitydestroyed Kelvins argument for the age of Earthand provided a clock to measure Earths ageA. Radioactivity is the spontaneous decay of an atoms nucleus to a more stable formThe heat from radioactivity helps explain why the Earth is still warm insideRadioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic eventsB. Fission Track Measurements (0.4 to 1.5 MYA)C. Carbon 14 dating- less than 70,000 years oldD. Tree Ring dating- back as much as 14,000 years ago
Radioactive DecayThe different forms of an elements atoms with varying numbers of neutrons are called isotopes Different isotopes of the same element have different atomic mass numbersbut behave the same chemicallyRadioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different elementThree types of radioactive decay:In alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus.
Radioactive DecayIn beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton.In electron capture decay, a proton captures an electron and converts to a neutron.
Radioactive DecaySome isotopes undergo only one decay step before they become stable.Examples:rubidium 87 decays to strontium 87 by a single beta emissionpotassium 40 decays to argon 40 by a single electron captureBut other isotopes undergo several decay stepsExamples:uranium 235 decays to lead 207 by 7 alpha steps and 6 beta stepsuranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps
Uranium 238 decay
Concept of Half-LivesThe half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope - to decay to atoms of a new more stable daughter isotopeThe half-life of a specific radioactive isotope is constant and can be precisely measuredThe length of half-lives for different isotopes of different elements can vary from less than 1/billionth of a second to 49 billion yearsRadioactive decay is geometric not linear, so has a curved graph
Geometric Radioactive DecayIn radioactive decay, during each equal time unit one half-life, the proportion of parent atoms decreases by 1/2
Determining AgeBy measuring the parent/daughter ratio and knowing the half-life of the parentwhich has been determined in the laboratorygeologists can calculate the age of a sample containing the radioactive elementThe parent/daughter ratio is usually determined by a mass spectrometeran instrument that measures the proportions of atoms with different masses
Determining AgeFor example: If a rock has a parent/daughter ratio of 1:3 = a parent proportion of 25%, and the half-life is 57 million years, how old is the rock?25% means it is 2 half-lives old.the rock is 57 x 2 =114 million years old.
What Materials Can Be Dated?Most radiometric dates are obtained from igneous rocksAs magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atomsBecause they fit, some radioactive parents are included in the crystal structure of certain mineralsThe daughter atoms are different elements with different sizes and, therefore, do not generally fit into the same minerals as the parentsGeologists can use the crystals containing the parents atoms to date the time of crystallization
Igneous Crystallization Crystallization of magma separates parent atoms from previously formed daughtersThis resets the radiometric clock to zero.Then the parents gradually decay.
Not Sedimentary RocksGenerally, sedimentary rocks cannot be radiometrically datedbecause the date obtained would correspond to the time of crystallization of the mineral, when it formed in an igneous or metamorphic rock,not the time that it was deposited as a sedimentary particleException: dating the mineral glauconite,because it forms in certain marine environments as a reaction with clay during the formation of the sedimentary rock
Sources of UncertaintyIn glauconite, potassium 40 decays to argon 40because argon is a gas, it can easily escape from a mineralA closed system is needed for an accurate datethat is, neither parent nor daughter atoms can have been added or removed from the sample since crystallizationIf leakage of daughters has occurred it partially resets the radiometric clock and the age will be too youngIf parents escape, the date will be too old.The most reliable dates use multiple methods.
- Sources of UncertaintyDuring metamorphism, some of the daughter atoms may escapeleading to a date that is too young.However, if all of the daughters are forced out during metamorphism, then the date obtained would be the time of metamorphisma useful piece of information.Dating techniques are always improving.Presently measurement error is typically
Dating Metamorphisma. A mineral has just crystallized from magma.
b. As time passes, parent atoms decay to daughters.
c. Metamorphism drives the daughters out of the mineral as it recrystallizes.d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time.Dating the whole rock yields a date of 700 million years = time of crystallization.
Long-Lived Radioactive Isotope Pairs Used in DatingThe isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurableSuch isotopes include:Parents DaughtersHalf-Life (years)Uranium 238 Lead 206 4.5 billionUranium 234 Lead 207704 millionThorium 232 Lead 20814 billionRubidium 87 Strontium 87 48.8 billionPotassium 40 Argon 401.3 billion Most of these are useful for dating older rocks
2. Fission Track DatingUranium in a crystal will damage the crystal structure as it decaysThe damage can be seen as fission tracks under a microscope after etching the mineralThe age of the sample is related to the number of fission tracks and the amount of uraniumwith older samples having more tracksThis method is useful for samples between 1.5 and 0.04 million years old
3. Radiocarbon Dating MethodCarbon is found in all lifeIt has 3 isotopes carbon 12 and 13 are stable but carbon 14 is notCarbon 14 has a half-life of 5730 yearsCarbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once livingThe short half-life of carbon 14 makes it suitable for dating material < 70,000 years oldIt is not useful for most rocks, but is useful for archaeology and young geologic materials
Carbon 14 Carbon 14 is constantly forming in the upper atmosphereThe carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organismsWhile the organism lives it continues to take in carbon 14but when it dies the carbon 14 begins to decaywithout being replenishedThus, carbon 14 dating measures the time of death
4. Tree-Ring Dating MethodIn cross-dating, tree-ring patterns are used from different trees, with overlapping life spansThe age of a tree can be determined by counting the annual growth rings -in lower part of the stem (trunk)The width of the rings are related to climate and can be correlated from tree to tree-a procedure called cross-dating-The tree-ring time scale now extends back 14,000 years
Chp 9: EarthquakesWhat is an Earthquake?Definition: a sudden motion or trembling of the Earth caused by the abrupt release of slowly accumulated elastic energy in rocks.
A. Stress -The force that is exerted against an object; the rocks of the Earths crust are stressed by tectonic forces -Whenever an object is stressed it changes its size and shapeB. Strain- The deformation that results from stress 1. elastic deformation -Stress applied slowly -When stress its removed the object springs back to its original size and shape 2. All rocks deform elastically when tectonic stress is applied; the sudden release of that stored elastic energy when the rock fractures causes earthquakes 3. elastic limit: the limit beyond which the rock cannot deform elastically 4.plastic deformation: when a rock will not return to its original shape when the stress is released 5. brittle rupture-When a rock is deformed beyond the elastic limit, it may ruptureIt breaks sharply and the fracture becomes a permanent feature of the rock
Chapter 9-Earthquakes: Izmit, Turkey 1999: est 17,000 people died, 240,000 buildings damagedFig. 9-CO, p.240
Table 9-1, p.242
Chp 9: EarthquakesC. Deformation of Tectonic Plate 1. Segmented lithosphere that moves relative to each other by gliding over the asthenosphere 2. As the plates move they slip past one another along immense fractures that form the boundaries between adjacent plates 3. The slippage is not smooth and continuous, but occurs as rapid jerks as one plate suddenly slips, causing an earthquake.
EARTHQUAKES, FAULTS, AND TECTONIC PLATESA. Fault 1. A fracture in rock along which movement has occurred in the past 2. Earthquakes start on old faults that have moved many times in the past and will move again in the future. 3. If new stresses develop in a region of the crust, new faults form and earthquakes can occur in places that were previously earthquake free. 4. fault creep a. Slow movement of plate past one another at a continuous snail-like pace b. The movement occurs without violent and destructive earthquakes c. seismic gap: fuzzy area on graph- little or no seismic activity
p.266bInactive fault- pink rocks on left against white ro