Rocks and Time 2 - Started October 13th, 2011

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Unformatted text preview: Started October 13th, 2011 !""#$"%&'&"()* •  !"#$%&'()+,-./01*2%(#3&,*45(6*/(*789:;<8=:/'>* –  *++'),--"./'%/$-)01)23-$)/3'%(* •  ?@A*4:58*****!*;*B"-C6(* •  D=<*4:48****B#*;*@&E(#"* •  D=7*::48*****@C#*;*F$C-%* –  4+5)!678)0&"/.)1+5&)-$5#%/$)9:);3&#)3/#)</+=)1+5&)>30) -%;?+/)/5'0%&@* •  !"#$%&')A+B%&3.%CD+&'3$* –  /GG*G&%($,&)*/".*G/3*'/(&,-/G*$H*(#*/".*-"%G$.-"C*(6&*IJK* G&%($,&>* •  L/,(*!8*M&N"-O#")*P4:*Q$&)O#"R*4:*'/,S)T* •  L/,(*U8*V#%/G*!,&/*PW*Q$&)O#")R*4:*'/,S)T* •  L/,(*?8*K6#,(*!")E&,*PX*Q$&)O#")R*=4*'/,S)T* –  73'E>%)!"#$%&')F5%-?+/-)E+-$%#)+/),B%/5%@* !""#$"%&'&"()* •  Y/%Z#%S)*[$-\*%G#)&)*/(*589:H'*#"*+,-./01* 2%(#3&,*45(6* •  @#*V/3)*E&&S*#]*2%(#3&,*4:(6* •  @#*V/3*2^%&*A#$,)*E&&S*#]*2%(#3&,*4:(6* •  ?#"(&"(*2^%&*A#$,)* –  _&."&)./01*2%(#3&,*4=(6*],#'*=;989:*-"*UKU*95:U* `"a-,*K%*4I:9* D+>#"/.)3/#)G35>?/.() H%+>+.";)7$&5;$5&%-) Envir Sc 1G03: Earth & the Environment 1. Relative geologic time Rocks and Time: 2.  Absolute geologic time Relative and Absolute Dating 4>  Z&a-&E*#]*Z&G/Oa&* I&#G#C-%*D-'&* •  L,-"%-HG&)* •  b"%#"]#,'-O&)* =>  !3)#G$(&*M/O"C* •  Z/.-#/%Oa&*.&%/0* •  J)#(#H-%*./O"C* •  Z/.-#%/,3#"*./O"C** What sequence of events is recorded here? +-C*4<>=* Info for this in Rocks & Time 1 •  Relative geologic time –  •  Absolute geologic time –  +-C*4<>4=* 2. Absolute Dating Also known as Atoms –  nucleus composed of protons and neutrons –  atomic number = # of protons –  elements have multiple i sotopes (same number of protons, different number of neutrons) ISOTOPIC DATING e.g. 238U 235U - - 92 protons - 146 neutrons - (146 + 92 =238) - - 92 protons - 143 neutrons (143+92 = 235) Thus 238U and 235U different masses have - isotopes are identified and measured with a mass spectrometer What is radioactive decay? •  Some isotopes have unstable nuclei –  protons/neutrons leave nucleus –  energy is produced (measured with geiger counter) When protons are Lost •  – atom becomes a different element •  (daughter) 238U decays to 206Pb 40K decays to 40Ar How can we use this to date rocks? Isotopic dating •  Determining the age of a rock through its radioactive elements •  Radioactive elements decay through time - proportional decay rate (half-life) - produce 'unique' daughter products 206Pb only produced by decay of 238U 40Ar produced by decay of 40K Fig. 19.23 •  Half life –  amount of time required to reduce the amount of radioactive isotope by half 238U: half-life is 4.5 billion years *Need to know this one* (approximate age of earth) helps date age of earth and is common •  •  40K decays to 40Ar half-life of 40K is 1.25 billion years If rock begins with 1 mg 40 K how much is left after 1.25 by? 0.5 g after 2.5? 0.25g D o not memorize! Table 19.3 BUT, memorize this one To date a rock! •  Measure amount of 40K and 40Ar (daughter) in the rock •  Calculate amount of 40K in rock when it formed •  Using known half-life, calculate age What assumptions are made? •  System has remained closed - no daughter product escaped •  No daughter products present initially •  Enough parent and daughter atoms to measure good to know these.. first 2 or more i mportant to know Date these rocks! •  Half-life of 40K is 1.25 billion years •  Rock 1 –  ratio of 40K to 40Ar is 1:1 •  Rock 2 –  Ratio of 40K to 40Ar is 1:3 question like this on final... helps to draw circle and split i t up. rock 1 circle has 50:50 of the two rock 2 circle has 25:75 of the two What are the best isotopes to use for dating purposes? •  •  those commonly found in rock-forming minerals ex. 40K occurs in micas, feldspars, and hornblendes? those with a unique daughter product What are the best rocks to date? - Igneous rocks - lava flows - Metamorphic rocks Sedimentary rocks difficult ~ WHY? stopped October 13th, 2011 started October 18th, 2011 Radiocarbon dating • 14C is a radioactive isotope of carbon •  •  Half-life of 5,730 years Living matter incorporates 14C i nto tissues Fig. 19.23 •  •  On dying, 14C begins to decay Calculate age from ratio of 14C to other forms of carbon in a sample •  What materials is 14C most useful for dating? •  - archaeological materials - <40,000 years old Fig. 19.23 Geological time scale •  - Geological events through time - Eras, periods, epochs dont memorize whole table, just these 4 +-C>*4<>=W* Eras: Precambrian: few fossils Paleozoic: old life Mesozoic: middle life Cenozoic: new life* an extinction will determine a change in era (big changes) D6-"C)*(#*.#c* •  K($.0*]#,*(6&*'-.(&,'c* ...
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This note was uploaded on 11/28/2011 for the course ENV SCI 1G03 taught by Professor Padden during the Fall '11 term at McMaster University.

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