EARTHS CRUST

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Divergent Plate Boundaries
Summer 2008

• EARTHS CRUST
• and
• INTERIOR

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• Headlines

• Fast-spreading ridges (e.g. East Pacific Rise)
  are broad they lack the axial rift and have a
  few seamount volcanoes on their flanks
• Slow-spreading ridges (e.g. Mid Atlantic
  Ridge) are narrow and have pronounced axial rifts
  with numerous seamount volcanoes.
• Ridge and rise topography is maintained by heat
  flow faster spreading carries hot elevated
  crust farther from axis it is a balance of one
  cooling rate (same in both cases) and varied
  spreading rates.

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Continents breaking up
Davidson et al. (2002)
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Narrow zones of shallow earthquakes
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Global Ocean Ridge System
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• THE GLOBAL OCEAN RIDGES

• Sites of formation of new crust
• oceans spread from here
• youngest and thinnest part of the lithosphere
• closest approach of mantle to Earths surface
• Earths greatest mountain chain
• longest 3000m peak system
• longest valley (no river!)
• yet in the thinnest and most mafic lithosphere
• held up by heat
• Earths greatest surface heat flow
• huge volumes of pillow lavas
• rows of smokers (black- white- and
  yellow-smokers)
• rich life without photosynthesis
• biological light sources geothermal life
  support

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• SPREADING RATES

• FAST 100 200 mm/yr e.g. East Pacific
  Rise
• Broader, gentler flanks axial rift narrow and
  shallow or absent
• Deeper, wider, negative gravity anomaly
• Less segmented by transform offsets smoother
  topography
• Voluminous fissure eruptions along axis
• Sparse central-vent seamounts on flanks active
  up to 80km
• Moderate 50 100 mm/yr
• SLOW 20 50 mm/yr e.g. Mid Atlantic
  Ridge
• Steeper, narrower flanks deep axial rift valley
  (10km wide, 3km deep)
• Shallower, narrower, negative gravity anomaly
• More segmented by transform offsets rougher
  topography
• Numerous small (50-600m) seamount volcanoes along
  rift valley
• Super Slow lt20 mm/yr e.g. SW Indian and
  Arctic oceans

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DETAILS of CENTRAL RIFT
Earthquakes above melting zone
Melting below seismogenic zone
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NEXT TIME

• Continental Divergent Boundaries
• e.g. East African Rift