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EESA06H3 (560)
Nick Eyles (492)
Lecture

Lecture4b

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Department
Environmental Science
Course
EESA06H3
Professor
Nick Eyles
Semester
Summer

Description
Tectonic  forces  acting  to  move  plates  (Fig.2.12)   • Earthquake  epicenters  track  the  descending  slab;  magma  starts  to  melt  and   then  rises  as  volcanoes   o Process  of  frictional  generation  of  earthquakes  goes  on  for  700  km   o Referred  to  as  slab  pull;  weight  pulling  down  the  rest  of  the  oceanic   crust  behind  it   • Ridge  push  –  effects  of  the  MOR  itself  pushing  the  ocean  floor  away;  sea  floor   spreading   • Rock  making  up  MORs  is  basalt  –  relatively  dense  and  low  in  silica   o Low  viscosity  and  flow  readily   • Melting  slabs  with  sediments  on  them  produces  magma  rich  in  silica   o Andesitic  rock  results;  high  viscosity   o Gases  (water  vapor,  carbon  dioxide,  sulfur)  not  able  to  escape  from   magma  easily  and  thus  volcano  is  explosive   o Results  when  ocean  crust  is  pushed  very  deep  in  subduction;  chemical   changes     MOR  spreading  Fig.  10.19   • Gabbro  is  coarser  equivalent  of  basalt   • Mantle  plume  feeds  hot  rock  to  ridge  system   o This  is  above  magma  chamber  –  where  rock  is  actually  molten   • Huge  volumes  of  water  also  circulating  through  the  system   o Moves  metal  in  solution;  leach  the  surrounding  rocks  in  low   concentrations   o When  it  reaches  the  top  of  the  ridge,  material  is  precipitated  (comes   out  of  solution)   o Builds  up  mineral  deposits   o Massive  sulfides   o Economic  importance:  much  of  Canadian  shield  and  Newfoundland  is   made  up  of  ancient  ocean  floors  where  this  process  occurred  and   deposited  minerals   • Suggestion  that  life  originated  at  MORs     How  oceans  widen:  continuous  intrusion  of  dikes  at  MORs   • Peridotite  –  dominates  the  mantle  plume   o Rich  in  iron  and  magnesium   o What  the  mantle  is  made  out  of   o Has  a  lot  of  olivene  or  peridote  –  soft,  green  mineral   o Is  plastic  and  hot   o In  areas  of  low  pressure  (near  earth’s  surface)  it  begins  to  melt   o Forced  up  into  magma  chamber;  cooling  around  perimeter  forms   gabbro   o Mineral  crystals  forming  in  chamber  and  falling  down  like  snow  to  the   bottom  resulting  in  layered  gabbro   o Gabbro  is  coarse  
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