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Lecture

Lecture7a.pdf

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Department
Archaeology
Course
ARH306Y1
Professor
All Professors
Semester
N/A

Description
Lecture  7     • Rocks  are  the  only  record  we  have  of  earth  in  the  remote  past   • Moon  rocks  are  very  valuable   o 3  sources  (US  Apollo  missions;  Soviet  Luna  missions;  meteorites)   o Many  given  as  gifts  to  other  nations   o Many  missing   o Mostly  basalts  and  anorthocite   o Believed  that  moon  originate  from  Earth  by  huge  impact   • The  rock  cycle  (Fig.  1.11)   • 3  main  groups  of  rocks   o Sedimentary  –  result  of  weathering  and  erosion   § Clay,  sand,  boulders   § Deposited  in  sedimentary  basins   § Lake  Ontario,  Hudson  Bay   § Often  buried  to  several  kilometers   § Lithification   § Can  get  caught  in  active  plate  tectonic  settings   o Metamorphic  –  changed  rock  due  to  heat  and  pressure   § Large  variety   § Difficult  to  identify  parent  rocks   § Transition  from  low  grade  to  high  grade   o Igneous  –  originate  from  magma   § Intrusive  –  never  surface  unless  weathered  and  exposed   • Underground,  minerals  have  time  to  grow   • Coarse  grained,  cool  slowly   § Extrusive  –  erupted,  rise  to  the  surface   • Fine  grained   • Cool  quickly   • Because  of  plate  tectonics,  all  these  rocks  can  be  uplifted,  eroded,  producing   sediment   o Whole  cycle  starts  again   • Very  oftern  there  are  individual  mineals  in  sedimentary  rock   o Quartz  –  very  resistant  mineral   o Constituent  grains  may  be  much  older  than  rest  of  rock   o Polycyclic  grains   o Acastanice  –  about  4  billion  years  old   § Tectonic  cycle  was  operating  since  then   § If  we  look  at  the  grains,  we  get  older  ages  (4.2,  4.3)   • Mantle  convection  is  driven  by  radioactive  decay   o Unstable  isotopes  in  mantle  that  release  heat  as  they  break  down   o Uranium  decay   o In  turn  produces  the  Earth’s  magnetic  field   o Protective  shield   o Without
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