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Relative versus Absolute Age of Rocks
Jarðsaga 1 – Þróun Lífs og Lands –
Ólafur Ingólfsson
• Relative dating• Estimating the age of the Earth• Radiometric (absolute) dating
• The age of the Earth
Amount of Time Required for
Some Geologic Processes and
Events
Relativedating of geological
strata:Builds primarilyon Steno´s principles of superposition andcross-cuttingrelationships, as well as biostrati-graphical divisions of strata
Stratigraphic nomenclatureEra = Öld
• Paleozoic Era = Fornlífsöld
• Mezozoic Era = Miðlífsöld
• Cenozoic Era = Nýlífsöld
Period = Tímabil• Devonian Period = Devon tímabilið
• Tertiary Period = Tertíer tímabilið
Epoch = Tími• Paleocene Epoch = Paleocentími• Pleistocene Epoch = Ísöld, Ísaldartími
Lithologic (rock) correlation
correlation
correlation
10 kmSandstoneSandstone
ShaleShale
LimestoneLimestone
Age correlation
Extinction oftrilobite species
Origin ofurchin species
SandstoneSandstone
ShaleShale
LimestoneLimestone
Biostratigraphy- The study of the spatial and temporal distribution of fossil organisms, often interpolated with paleo-
environmental information, as a means of dating rock strata
Biostratigraphic units• Index fossils (aka “guide fossils”) are fossils
that are particularly useful in biostratigraphiccorrelation. Characteristics of index fossils are:– Abundant– Easily identified– Geographically widespread– Occurs in a variety of rock types– Short stratigraphic range (rapid rates of evolution)
Principles of Stratigraphy• Index fossils: Organisms evolve, thrive and then become extinct. If they can be recognized in the rock, these fossils can be used to correlate rock in one area with rocks in a distant area
Stratigraphic correlations
WorldwideCorrelation
International Geological Correlation Chart based onlithostratigraphical and biostratigraphical correlations
http://www.palaeos.com/Timescale/timescale.html
What is Absolute/Numerical Dating?
The age of an Earth material or event in years
AbsoluteRelative age
3rd 12 kya
1st2nd
1.7 bya95 mya
8 kyayoungest
Attempts to estimate the age of the Earth• In the pre-scientific world view the issue of the ageof the Earth was a theological question. In 1640, Bishop Ussher produced his famous calculation that the Earth was created in 4004 BC.
• There was no single estimate of the Earth's agein the mid 1800's and no good way to arrive at one. There were various attempts to estimate theEarth's age, working back from sedimentation rates and other geophysical phenomena. The attempts produced estimates from about 100 million years up to several billion years.
• There were two major problems with suchefforts. The first is that the geological historywas still being reconstructed. The second is that the rates of the physical processes in question are variable and knowledge of them was incomplete.
The Earthwas createdon the 23rd of October
4004 BC
Estimating Earth's Age Bishop Usher – Biblical Interpretation
Having established the first day of creation asSunday 23 October 4004 BC (refinements byothers further pinpointed this to 9 in themorning, London time, or midnight in the Gardenof Eden), Ussher calculated the dates of other biblical events, concluding, for example, that Adam and Eve were driven from Paradise on Monday 10 November 4004 BC, and that the Arktouched down on Mt Ararat on 5 May 2348 BC `on a Wednesday'.
History of ideas on the age of the Earth
Estimating Earth's Age -Early (failed) Attempt
Sedimentation rates - 3 my – 500 my
Halley/Joly - Ocean Salinity – 100 my
Lord Kelvin – Rate of Cooling – 100-20 my
Lord Kelvin´s attempts to date the Earth
Kelvin attempted to calculate the amount of time the Earth has been a solid body by estimating cooling rates by conduction. In 1862 Kelvinestimated the age of the Earth to be 98 million years. Later in 1897
he revised his estimate downwards to 20-40 million years.
The Radioactive ClockKelvin did not know aboutradioactivity and heating of theEarth's crust by radioactive decay; for this reason his estimates were completely wrong.
The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, opened newvistas in science. In 1905, the British physicist Lord Rutherford made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly.
Radioactive Revolutionaround 1900 AD
• Radioactive decay – spontaneous transformation of an element to another isotope of the same element or another element.
Radioactive Decay of Rubidium to Strontium and
Uranium to Thorium
IsotopesDifferent forms of the sameelement containing the samenumber of protons, but varyingnumbers of neutrons, i.e.i.e.:235235U, U, 238238U;U; 8787Sr, Sr, 8686Sr;Sr; 1414C, C, 1212CC
Rate of radioactive decayWe must know three things to calculate an age:
1. The current parent/daughter isotope ratio.
2.The original parent/daughter isotope ratio.
3.The rate of decay, or the half-life
• The rate of decay is an intrinsic property of the isotope, independent of temperature, pressure and chemistry.
• We know on average what percentage will decay in a given time. This is the decay rate.
The fundamental assumption:
The half-lives of radioactive isotopes are the same as they
were billions of years ago.
Half-lifeThe fixed period of time during which half the parent atoms present in a closed system decay to form daughter atoms - the time for one half of the radioactive atoms to decay
Half-LifeExample: a particular isotope has a half life of 1 year. If
we initially had 64 grams of the substance, then after:
• 1 year we have 32 g of the parent, 32 g of the daughter;
• 2 years we have 16 g of the parent, 48 g of the daughter;
• 3 years we have 8 g of the parent, 56 g of the daughter;
• 4 years we have 4 g of the parent, 60 g of the daughter;
• 5 years we have 2 g of the parent, 62 g of the daughter;
• 6 years we have 1 g of the parent, 63 g of the daughter;
Time change of isotope populations
• As the amount of the parent nuclei decreases exponentially…
… the amount of the daughter nuclei increases accordingly.
Some Radioactive Isotopes Commonly Used in Absolute Dating
12.3Helium-3Hydrogen-35.730 yearsNitrogen-14Carbon-14
106 billion years Neodymium-143 Samarium-147 1.25 billion years Argon-40 Potassium-40 48.8 billion years Strontium-87 Rubidium-87 14.0 billion years Lead-208 Thorium-232 704 million years Lead-207 Uranium-235 4.5 billion years Lead-206 Uranium-238
Currently Accepted Half-Life Values
Stable DaughterProduct
Parent Isotope
What rock types are most likely to be useful forradiometric dating?
Which rocks can be dated?Only certain typesof rocks, chieflyigneous rocks, canbe dated directlyby radiometric methods; but these rocks do not ordinarily contain fossils...
Most sedimentary rocks such as sandstone, limestone, and shale arerelated to the radiometric time scale by bracketing them within time zones that are determined by dating igneous rocks
Uranium (U) - Thorium (Th) - Lead (Pb)
238U decays to 206Pb (4.5 billion)235U decays to 207Pb (713 million)232Th decays to 208Pb (14.1 billion)
• Rocks containing Uranium provide 3 possible techniques.
• Because all 3 occur together, it allows a method to cross-check dates.
• Uses zircons, uraninite and uranium ores
Dating rocks – theory and realityThe mathematical expression that relates radioactivedecay to geologic time is called the age equation and is:
Dating rocks by radioactive decay is simple in theory, but the laboratory procedures are complex.
Magnetic stratigraphy• Motion of magnetic
material (iron) in the liquid outer core is responsible for Earth’s magnetic field
• The magnetic field is polarized—north and south poles
• Polarity has reverseditself repeatedly over the course of Earth’s history
Magnetic “striping” of oceanic crust
The Magnetic Time Scale
• Polarity: The N-S directionality of the Earth’s magnetic field• normal polarity• reverse polarity -- every 1/2 million years or so the Earth
reverses its polarity in what is called a magnetic reversal.
Magnetic field reversals:
Dating techniquesthat can be usedon this geologiccolumn:
• absolute dating
• paleomagnetism
• superposition
• fossil correlation
The Earth’s Oldest Crustal Rocks•The Acasta gneiss in northern Canada was formed 4.0 billion years ago and is the most ancient body of rock discovered thus far.
Rocks fromthe Isuasequence inGreenland are probably of similar age
Dating oldestrocks on Earth
This example comes from some of the oldest rocks onGreenland. A suite of different samples from a group of granites and granitic gneisses (sheared and recrystallised granite) were analysed.
The ancient supracrustal rocksof Isua and Akilia, WestGreenland, date between3,800 and 3,700 Ma. These metamorphosed rocks were originally deposited as sediments or volcanic flows in shallow water.
The oldest material found on EarthIn Western Australia, single zirconcrystals found in younger sedimentaryrocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be found on Earth so far.
About 380 kg of rocks from the Moonwere returned to Earth by the Apolloand Luna missions. The oldest datedmoon rocks have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbor.
The Solar system formed ~4.6 BY ago
History of thought as to the age of Earth
Constructing geological time scales
The Geologic time scale
• Divisions in the worldwide stratigraphic column based on variations in preserved fossils
• Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages
Skoðið þessa vefsíðu: http://www.palaeos.com/Timescale/timescale.html
References, further reading, useful web pagesStanley, Earth System History, chapter 6
Lewis, C. 2000: The Dating Game – One Man´s Search for the Age of the Earth. Cambridge University Presshttp://www.ucmp.berkeley.edu/help/timeform.html
http://earth.leeds.ac.uk/dynamicearth/dating/index.htm
http://www.enchantedlearning.com/subjects/Geologictime.html
http://www.palaeos.com/Timescale/default.htm
http://www.geo.ilstu.edu/geology/POG/MrShields/LectureNotes/24
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/clkroc.html
http://www2.nature.nps.gov/geology/usgsnps/gtime/ageofearth.html#age
http://gpc.edu/~pgore/geology/geo102/age.htm
http://www.whfreeman.com/presssiever/content/instructor/content/ppl/1
http://www.gustavus.edu/oncampus/academics/geology/nobel_display/nobel_introduce.html
http://www.palaeos.com/Timescale/timescale.html
http://pubs.usgs.gov/gip/geotime/
http://www.earth.uni.edu/~groves/24
