Transcript Lab 25
Lab 25 ONLINE LESSON If viewing this lesson in Powerpoint Use down or up arrows to navigate If viewing this lesson in Flash Use the control bar to navigate Do take notes as we peruse through this lesson… Radiometric Dating A question that is often asked… How old is the earth? And the answer is… Pretty Old Radiometric dating is the process of measuring the age of rocks By measuring the age of rocks, geologists can determine the age of a number of geologic events... To include the age of the earth!!! Radiometric dating is based on the concept of radioactivity…the disintegration (falling apart) of atoms What is an atom? An atom is the smallest particle in the universe with a distinct set of chemical characteristics….. An atom is a structure…it is a package of particles Everything in the universe is comprised of atoms…bicycles Cellphones Dumbasses Everything An atom is comprised of three basic components ATOM P PROTON (positively charged) N NEUTRON (neutral) e ELECTRON (negatively charged) The electron orbits the nucleus which contains the eprotons and neutrons NUCLEUS RULE: The number of protons determines the chemical identity of an element For instance… -all Hydrogen contains 1 proton… -all Helium contains 2 protons… -all Uranium contains 92 protons… Element = a substance that cannot be broken down into other substances by chemical means… For instance… Hydrogen is an element Helium is an element Uranium is an element Carbon-12 is a species of carbon… CARBON 12C It contains 6 protons and 6 neutrons…for a total of 12 particles in the nucleus CARBON 12C Carbon-14 is another species of carbon… CARBON 14C Carbon-14 contains 6 protons but 8 neutrons…for a total of 14 particles… CARBON 14C The neutron count may vary but the proton count remains the same…6 protons… CARBON 14C SO: The number of protons determines the chemical identity of an element Decay types are part of Radioactivity …there are three types to be concerned with… DECAY TYPES P ALPHA N BETA ELECTRON CAPTURE P N e N P P N - DECAY TYPES ALPHA P N P N 2 protons and 2 neutrons are thrown out of the nucleus…this reduces the number of protons and thus changes the element… DECAY TYPES A neutron turns into a proton BETA e N P This will change the element DECAY TYPES A proton turns into a neutron This will change the element ELECTRON CAPTURE P N - With all 3 decay types…elements will change their chemical identity because of the change in the number of protons… A common decay series is 238 206 U/ Pb URANIUM = PARENT LEAD = DAUGHTER A decay series is the progression of the disintegration of elements…a start to finish process… This beginning to end process is the essence of radiometric dating… 238U 92 For instance… Uranium 238 is a parent element and it decays over time to to Lead 206… 238U 92 The decay process starts out with 92 protons and 146 neutrons for a total of 238 particles in the nucleus… 238U 92 And includes a series of steps before uranium turns into lead… 238U 92 2 protons and 2 neutrons are ejected from the nucleus and changes the element… uranium to thorium 234Th A neutron is converted to a proton 90 and changes thorium to paladium… 234Pa And so on and so on… 91 234U 92 230Th 90 226Ra 88 222Rn 86 218Po 84 214Pb 82 214Bi 84 214Po 84 210Pb 82 210Po 84 206Pb 82 Through a series of steps…uranium is changed to lead… So how does radiometric dating allow geologists to date rocks? A rock is found in the field…an igneous rock… It is formed as a result of melted material from the interior of the planet… That eventually cooled… The minerals inside the rock solidified with some radioactive material inside the crystals… Radioactive elements started at 100% parent material (dark blue dots)… Then over time…the parent material decayed to daughter material (light blue dots) one atom at a time… Over time a reduction of parent material will occur as well as an increase of daughter material Principally, this means that we can make a ratio between parent material and daughter material… This ratio can be placed into an equation… T= ln(n0/nt)(100000000) 0.0154 “n0” Leftover “nt” Original amount of parent amount of material (daughter parent amount subtracted material from parent material) ln(n0/nt)(100000000) T= 0.0154 “T” Age of a rock The bean counting exercise involves counting brown beans which represents parent material nt The amount of parent material nt is determined by counting the total number of beans…brown and white… The white beans represent daughter material… n0 is calculated by number of brown beans Values are then placed into this equation T= ln(n0/nt)(100000000) 0.0154 And the age of a rock can be calculated… T= ln(n0/nt)(100000000) 0.0154 This is referred to as absolute time… T= ln(n0/nt)(100000000) 0.0154 Absolute time is used in concert with a Relative Time Scale… Periods such as Cretaceous and Jurassic are relative time periods… Meaning one period is older or younger than another period… Cretaceous rocks are younger than Jurassic rocks… Ordovician rocks are older than Triassic rocks… A Relative Time scale is older at the bottom And younger at the top… The Relative Time scale was developed to show the relationship of sedimentary rocks… Superimposed relative to each other… The Relative Time scale was developed with no absolute time values… However, with radiometric dating…real absolute time values can be assigned to each relative time period… Geologic Time Scale… Lab 25 ONLINE LESSON END