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EESA06H3 (568)
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Lecture

Lecture4c

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Department
Environmental Science
Course
EESA06H3
Professor
Nick Eyles
Semester
Summer

Description
• Why  does  a  MOR  come  up  on  land?   • In  Iceland,  the  MOR  is  underlain  by  a  hot  spot   o Large  mantle  plume   • Distribution  of  hot  spots  –  Fig.  2.39   • Another  important  hot  spot  is  under  Hawaii   • Their  positions  do  not  change;  fixed   • Overlying  plate,  through  which  the  plume  is  burning,  however,  does  change   • Hot  spot  tracks  –  plate  moves  over  hot  spot  creating  a  trail  of  dead  volcanoes   o Appendix  coming  off  Hawaii  is  an  island  chain  (hot  spot  track)     Reykjanes  ridge:  where  the  mid-­‐Atlantic  ridge  comes  onshore  in  Iceland   • Iceland  is  being  torn  in  two;  right  astride  the  North  American  plate  and  the   Eurasian  plate   • Upper  arm  immediately  below  Iceland  is  probably  a  transform   • Shield  volcanoes  occur  along  this  ridge   o Low  relief  mounded  volcanoes  characterized  by  basaltic  eruptions   • Iceland  is  expanding  –  oldest  rocks  occur  on  the  outer  edges  of  the  island   o Older  than  3  million  years   • There  are  ice  sheets  on  top  of  the  mid-­‐ocean  ridge   • Fissures  or  cracks  is  where  crust  is  being  stretched  or  broken;  run  for   kilometers   • Fissure  eruptions  –  basaltic  magma  emerges;  spectacular  but  not  dangerous   because  of  low  viscosity   o Some  reach  400-­‐500  m  in  height   • Spatter  or  cinder  cones  –  magma  is  blown  up  into  the  air,  cools,  and  falls   down  in  black  cindery  masses  forming  a  cone   o May  be  torn  in  two  along  MOR   • Drilling  down  into  the  MOR  to  tap  into  hot  water  circulating  under  ridge   • 1000  AD  –  famous  lava  flow   o People  were  living  there   o Now  green  with  moss   • Different  types  of  volcanoes  –  Fig.  7.15   o Shield  volcano   o Composite  volcano   o Pyroclastic  volcano   • Magmas  that  contain  lots  of  silica  are  acidic  and  create  more  violent,  pointed   volcanoes   o Found  at  active  plate  margins;  subduction  (Fig.  6.7)   o Associated  rocks  are  granite,  rhyolite,  diorite,  andesite     Quiet  Volcanism   • Basaltic  lavas  flow  easily   • Low  viscosity   • Two  types  of  lava   o Pahoehoe  –  ropey  (hot  lava)   § 1200  degrees  cenegrade   §
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