The age of the Earth

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(1)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.

(2) Amount of Time Required for Some Geologic Processes and Events.

(3) Relative dating of geological strata: Builds primarily on Steno´s principles of superposition and cross-cutting relationships, as well as biostratigraphical divisions of strata.

(4) Stratigraphic nomenclature Era = 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.

(5) Lithologic (rock) correlation Sandstone. 10 km correla. Sandstone tion. Shale correla. Limestone. tion. Shale. Limestone.

(6) Age correlation Sandstone. Extinction of trilobite species. Sandstone. Shale. Limestone. Origin of urchin species. Shale. Limestone.

(7) Biostratigraphy - The study of the spatial and temporal distribution of fossil organisms, often interpolated with paleoenvironmental information, as a means of dating rock strata.

(8) Biostratigraphic units • Index fossils (aka “guide fossils”) are fossils that are particularly useful in biostratigraphic correlation. Characteristics of index fossils are: – – – – –. Abundant Easily identified Geographically widespread Occurs in a variety of rock types Short stratigraphic range (rapid rates of evolution).

(9) 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 •.

(10) Stratigraphic correlations.

(11) Worldwide Correlation.

(12) International Geological Correlation Chart based on lithostratigraphical and biostratigraphical correlations. http://www.palaeos.com/Timescale/timescale.html.

(13) What is Absolute/Numerical Dating? The age of an Earth material or event in years Relative age 3rd 2nd 1st youngest. Absolute 12 kya 95 mya 1.7 bya 8 kya.

(14) Attempts to estimate the age of the Earth • In the pre-scientific world view the issue of the age of the Earth was a theological question. In 1640, Bishop Ussher produced his famous calculation that the Earth was created in 4004 BC.. The Earth was created on the 23rd of October 4004 BC. • There was no single estimate of the Earth's age in the mid 1800's and no good way to arrive at one. There were various attempts to estimate the Earth'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 such efforts. The first is that the geological history was still being reconstructed. The second is that the rates of the physical processes in question are variable and knowledge of them was incomplete..

(15) Estimating Earth's Age Bishop Usher – Biblical Interpretation Having established the first day of creation as Sunday 23 October 4004 BC (refinements by others further pinpointed this to 9 in the morning, London time, or midnight in the Garden of 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 Ark touched down on Mt Ararat on 5 May 2348 BC `on a Wednesday'..

(16) History of ideas on the age of the Earth.

(17) 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.

(18) 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 Kelvin estimated the age of the Earth to be 98 million years. Later in 1897 he revised his estimate downwards to 20-40 million years..

(19) The Radioactive Clock Kelvin did not know about radioactivity and heating of the Earth'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 new vistas 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..

(20) Radioactive Revolution around 1900 AD • Radioactive decay – spontaneous transformation of an element to another isotope of the same element or another element..

(21) Radioactive Decay of Rubidium to Strontium and Uranium to Thorium.

(22) Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons, i.e.: 235U, 238U;. 87Sr, 86Sr; 14C, 12C.

(23) Rate of radioactive decay We 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..

(24) The fundamental assumption: The half-lives of radioactive isotopes are the same as they were billions of years ago..

(25) Half-life The 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.

(26) Half-Life Example: 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;.

(27) Time change of isotope populations. • As the amount of the parent nuclei decreases exponentially…. … the amount of the daughter nuclei increases accordingly..

(28) Some Radioactive Isotopes Commonly Used in Absolute Dating Parent Isotope. Stable Daughter Product. Currently Accepted Half-Life Values. Uranium-238. Lead-206. 4.5 billion years. Uranium-235. Lead-207. 704 million years. Thorium-232. Lead-208. 14.0 billion years. Rubidium-87. Strontium-87. 48.8 billion years. Potassium-40. Argon-40. 1.25 billion years. Samarium-147 Neodymium-143 Carbon-14. Nitrogen-14. Hydrogen-3. Helium-3. 106 billion years 5.730 years 12.3. What rock types are most likely to be useful for radiometric dating?.

(29) Which rocks can be dated? Only certain types of rocks, chiefly igneous rocks, can be dated directly by radiometric methods; but these rocks do not ordinarily contain fossils... Most sedimentary rocks such as sandstone, limestone, and shale are related to the radiometric time scale by bracketing them within time zones that are determined by dating igneous rocks.

(30) Uranium (U) - Thorium (Th) - Lead (Pb) 238U decays to 206Pb 235U decays to 207Pb 232Th decays to 208Pb. (4.5 billion) (713 million) (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.

(31) Dating rocks – theory and reality The mathematical expression that relates radioactive decay 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..

(32) 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 reversed itself repeatedly over the course of Earth’s history.

(33) Magnetic “striping” of oceanic crust.

(34) The Magnetic Time Scale. Magnetic field reversals: • 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..

(35) Dating techniques that can be used on this geologic column: • absolute dating • paleomagnetism • superposition • fossil correlation.

(36) 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 from the Isua sequence in Greenland are probably of similar age.

(37) Dating oldest rocks on Earth. This example comes from some of the oldest rocks on Greenland. A suite of different samples from a group of granites and granitic gneisses (sheared and recrystallised granite) were analysed..

(38) The ancient supracrustal rocks of Isua and Akilia, West Greenland, date between 3,800 and 3,700 Ma. These metamorphosed rocks were originally deposited as sediments or volcanic flows in shallow water..

(39) The oldest material found on Earth In Western Australia, single zircon crystals found in younger sedimentary rocks 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 Moon were returned to Earth by the Apollo and Luna missions. The oldest dated moon rocks have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbor..

(40) The Solar system formed ~4.6 BY ago.

(41) History of thought as to the age of Earth.

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(43) Constructing geological time scales.

(44) 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.

(45) References, further reading, useful web pages Stanley, Earth System History, chapter 6 Lewis, C. 2000: The Dating Game – One Man´s Search for the Age of the Earth. Cambridge University Press http://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_introd uce.html http://www.palaeos.com/Timescale/timescale.html http://pubs.usgs.gov/gip/geotime/ http://www.earth.uni.edu/~groves/24.

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