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Divergent Plate Boundaries

Summer 2008 . Divergent Plate Boundaries. EARTH’S CRUST and INTERIOR. Fast-spreading ridges (e.g. East Pacific Rise) are broad; they lack the axial rift and have a few seamount volcanoes on their flanks

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Divergent Plate Boundaries

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  1. Summer 2008 Divergent Plate Boundaries EARTH’S CRUST and INTERIOR

  2. 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. Headlines:

  3. Continents breaking up Davidson et al. (2002)

  4. Narrow zones of shallow earthquakes

  5. Global Ocean Ridge System

  6. Sites of formation of new crust • oceans spread from here • youngest and thinnest part of the lithosphere • closest approach of mantle to Earth’s surface • Earth’s greatest mountain chain • longest 3000m peak system • longest valley (no river!) • yet in the thinnest and most mafic lithosphere • held up by heat • Earth’s 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 THE GLOBAL OCEAN RIDGES

  7. 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 • Moderate50 – 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<20 mm/yr e.g. SW Indian and Arctic oceans SPREADING RATES:

  8. DETAILS of CENTRAL RIFT Earthquakes above melting zone Melting below seismogenic zone

  9. NEXT TIME • Continental Divergent Boundaries • e.g. East African Rift

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