Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38NSCI 314LIFE IN THE COSMOS5 – FOSSILS AND RADIOACTIVE DATING ANDGEOLOGY AND THE ORIGIN OF THE EARTH Dr. Karen KolehmainenDepartment of Physics CSUSBCourse website:http://physics.csusb.edu/~karen/TYPES OF ROCKS IGNEOUS: MADE FROM MOLTEN ROCK (FROM A VOLCANO) THAT COOLED AND SOLIDIFIED.SEDIMENTARY: MADE FROM SEDIMENTS THAT ARE DEPOSITED AND COMPRESSED GRADUALLY, USUALLY AT THE BOTTOM OF A BODY OF WATER. DEEPER LAYERS ARE OLDER, TOP LAYERS ARE YOUNGER.METAMORPHIC: MADE FROM ROCK THAT HAS BEEN HEATED (BUT NOT ENOUGH TO MELT IT) AND/OR COMPRESSED ENOUGH TO CHANGE ITS STRUCTURE.NOTE THAT ONE TYPE OF ROCK CAN CHANGE INTO ANOTHER TYPE VIA MELTING, EROSION, COMPRESSION, HEATING, ETC.ROCKS AND MINERALSROCK TYPES (IGNEOUS, SEDIMENTARY, AND METAMORPHIC) ARE BASED ON HOW THE ROCK IS FORMED, NOT ITS CHEMICAL COMPOSITION. THE TERM MINERAL IS USED TO SPECIFY CHEMICAL COMPOSITION.IN A PARTICULAR ROCK (OF ANY OF THE THREE TYPES), THERE MAY BE CRYSTALS OR GRAINS OF VARIOUS DIFFERENT MINERALS PRESENT.FOSSILS RELICS OF ANCIENT ORGANISMS THAT HAVE BEEN PRESERVED IN SEDIMENTARY ROCKS, INCLUDING:–HARD COMPONENTS (BONES, TEETH, SHELLS, ETC.) OF ORGANISMS (THE SOFT PARTS DECAY MORE EASILY)–MINERALS THAT HAVE REPLACED DECAYED OR DECAYING ORGANIC MATERIAL, RETAINING THE SHAPE OF THE ORGANISM (FOR EXAMPLE, PETRIFIED WOOD)–CORPSES PARTIALLY PRESERVED BY BEING SEALED IN AN ENVIRONMENT THAT LIMITS DECAY (FOR EXAMPLE, ICE, AMBER, MATERIAL AT THE BOTTOM OF SOME SWAMPS)–PRESERVED INDICATION OF ORGANISM’S BEHAVIOR (FOR EXAMPLE, DINOSAUR FOOTPRINTS)MORE ON FOSSILSONLY A TINY FRACTION OF ORGANISMS BECOME FOSSILIZED WHEN THEY DIE. MOST ORGANISMS DECAY COMPLETELY INSTEAD OF FOSSILIZING.FOSSILS OCCUR ONLY IN SEDIMENTARY (NOT IGNEOUS OR METAMORPHIC) ROCKS.SINCE SEDIMENTARY ROCKS FORM LAYERS OF DIFFERENT AGES, DEEPER FOSSILS ARE OLDER.THIS FACT IS USED TO ESTABLISH RELATIVE AGES (WHICH IS OLDER) BUT CANNOT GIVE ABSOLUTE AGES (HOW MANY YEARS AGO).THE FOSSIL RECORD IN ANY ONE LOCATION IS INCOMPLETE BECAUSE SEDIMENTS WERE DEPOSITED THERE ONLY FOR SOME INTERVAL OF TIME.THE FOSSIL RECORD CAN BE PIECED TOGETHER BECAUSE ROCKS IN VARIOUS LOCATIONS OVERLAP IN THEIR PERIODS OF DEPOSITION.HOW IS THE ABSOLUTE AGE OF A ROCK OR FOSSIL DETERMINED?RADIOACTIVE DATINGTO UNDERSTAND HOW, WE MUST FIRST UNDERSTAND THE NATURE OF RADIOACTIVITY.TYPES OF RADIOACTIVITYALPHA DECAY–A 4He NUCLEUS (2 PROTONS + 2 NEUTRONS) IS EJECTED FROM A LARGE NUCLEUS.BETA DECAY–A NEUTRON CHANGES INTO A PROTON, CAUSING AN ELECTRON TO BE CREATED AND EJECTED FROM THE NUCLEUS, OR…–A PROTON CHANGES INTO A NEUTRON, ACCOMPANIED BY EITHER CREATION AND EJECTION OF A POSITRON (ANTIPARTICLE OF THE ELECTRON) OR THE CAPTURE OF A ELECTRON BY THE NUCLEUS GAMMA DECAY–PROTONS AND NEUTRONS REARRANGE THEMSELVES INSIDE THE NUCLEUS, CAUSING A HIGH ENERGY PHOTON (GAMMA RAY) TO BE CREATED AND EJECTED FROM THE NUCLEUS.NOTE: IN BOTH ALPHA AND BETA (BUT NOT GAMMA) DECAY, THE DAUGHTER NUCLEUS IS A DIFFERENT ELEMENT THAN THE PARENT NUCLEUS (SINCE THE NUMBER OF PROTONS HAS CHANGED).MORE ON RADIOACTIVITYCERTAIN ISOTOPES OF CERTAIN ELEMENTS ARE RADIOACTIVE (WILL UNDERGO RADIOACTIVE DECAY), WHEREAS OTHERS (STABLE ISOTOPES) ARE NOT.RADIOACTIVE DECAY IS A RANDOM PROCESS (LIKE THE FLIP OF A COIN). WE CAN’T PREDICT EXACTLY WHEN A RADIOACTIVE NUCLEUS WILL DECAY, ONLY ITS PROBABILITY OF DOING SO WITHIN A CERTAIN INTERVAL OF TIME.FOR A SAMPLE OF MATERIAL CONTAINING A LARGE NUMBER OF RADIOACTIVE NUCLEI, WE CAN PREDICT APPROXIMATELY HOW MANY OF THEM WILL DECAY WITHIN A CERTAIN PERIOD OF TIME. THIS IS SIMILAR TO FLIPPING A COIN. WE CAN”T PREDICT WHETHER A SINGLE FLIP WILL GIVE HEADS OR TAILS. HOWEVER, IF WE FLIP A COIN A LARGE NUMBER OF TIMES, WE KNOW WE WILL GET CLOSE TO A 50-50 MIXTURE OF HEADS AND TAILS.HALF-LIFETHE HALF-LIFE IS DEFINED AS THE INTERVAL OF TIME DURING WHICH A NUCLEUS HAS A 50% CHANCE OF DECAYING.AFTER ONE HALF-LIFE HAS ELAPSED, APPROXIMATELY HALF OF THE NUCLEI IN A SAMPLE WILL HAVE DECAYED.EACH RADIOACTIVE ISOTOPE HAS A DIFFERENT HALF-LIFE, WITH VALUES RANGE FROM A FRACTION OF A SECOND TO BILLIONS OF YEARS.THE PROBABILITY OF DECAY DOESN’T CHANGE WITH TIME. IN OTHER WORDS, A NUCLEUS DOESN’T “REMEMBER” HOW LONG IT HAS ALREADY EXISTED WITHOUT DECAYING. THEREFORE ANY NUCLEUS THAT HAS ALREADY EXISTED FOR ONE HALF-LIFE WITHOUT DECAYING HAS A 50% CHANCE OF DECAYING DURING THE NEXT HALF-LIFE.HALF-LIFE EXAMPLE: CONSIDER A MATERIAL WITH A HALF-LIFE OF 1 YEAR. LET'S SAY THAT INITIALLY, 1000 NUCLEI ARE PRESENT IN A PARTICULAR SAMPLE.AFTER 1 YEAR HAS ELAPSED, APPROXIMATELY 500 (OR 1/2) HAVE DECAYED, AND 500 (1/2) HAVE NOT YET DECAYED. PARENT AND DAUGHTER ARE EQUALLY ABUNDANT.AFTER 2 YEARS, APPROXIMATELY 750 (3/4) HAVE DECAYED, AND 250 (I/4) HAVE NOT YET DECAYED. DAUGHTER IS 3 TIMES AS ABUNDANT AS PARENT.AFTER 3 YEARS, APPROXIMATELY 875 (7/8) HAVE DECAYED, AND 125 (1/8) HAVE NOT YET DECAYED. DAUGHTER IS 7 TIMES AS ABUNDANT AS PARENT.DECAYS CONTINUE IN THIS MANNER.RADIOACTIVE DATINGCOMPARE ABUNDANCES OF THE PARENT AND DAUGHTER ISOTOPES TO DETERMINE HOW MUCH TIME HAS ELAPSED. (SINCE WHEN?)RADIOACTIVE DATING OF A ROCK MEASURES THE TIME SINCE THE ROCK SOLIDIFIED.FOR AN IGNEOUS ROCK, THIS GIVES THE AGE OF THE ROCK.RADIOACTIVE DATINGTHE TECHNIQUE ISN’T DIRECTLY USEFUL FOR A SEDIMENTARY ROCK (THE ONLY TYPE IN WHICH FOSSILS ARE FOUND). IF YOU DO RADIOACTIVE DATING OF A SEDIMENTARY ROCK, IT WILL TELL YOU THE ELAPSED TIME SINCE THE SMALL GRAINS INSIDE THE ROCK SOLIDIFIED, NOT THE TIME SINCE THE SEDIMENTS WERE DEPOSITED OR COMPRESSED TO FORM THE SEDIMENTARY ROCK. THUS IT WON'T TELL YOU THE AGE OF THE SEDIMENTARY ROCK OR ANY FOSSILS IN IT.HOWEVER, IF A SEDIMENTARY ROCK LAYER IS “SANDWICHED” BETWEEN TWO IGNEOUS ROCK LAYERS, THE TECHNIQUE WILL PUT CONSTRAINTS ON THE AGE IN THE IN-BETWEEN LAYER. FOR EXAMPLE, CONSIDER A SEDIMENTARY ROCK LAYER SANDWICHED BETWEEN TWO LAYERS OF IGNEOUS ROCK. THE LAYER OF IGNEOUS ROCK ON TOP IS 100 MILLION YEARS OLD, AND THE LAYER OF IGNEOUS ROCK UNDERNEATH IS 110 MILLION YEARS OLD. THEN THE SEDIMENTARY ROCK (AND ANY FOSSILS IN