Ridge Volcanism and Tectonism

Scott Veirs

Note: This chapter follows
0 mantle plumes, convection, hot spots
1 plate tectonics
and precedes
3 crustal lithologies and structure

  1. An ideal accreting plate margin
    1. Plates move apart with near constant velocities of a few cm/yr
    2. As they diverge, hot mantle rock flows upward to fill the gap
    3. Because the fresh oceanic lithosphere is hot and relatively buoyant, it forms a ridge on the seafloor
    4. Isostacy explains the elevation of the volcanic ridge relative to the abyssal plains
      1. The lithosphere cools as it moves away from the ridge, becoming denser and subsiding into the underlying asthenosphere
      2. Seafloor elevation (and conductive heat flux) is proportional to (t)^-1/2
      3. Show some depth vs age data
      4. This might be a good place to make convective/conductive heat flow vs age
        1. Lister's observations
        2. pre-1977 predictions of hydrothermal cooling of crust
  2. Sea floor topography is more complicated
    1. The real ridge system is segmented
      1. Volcanic ridges form at the boundaries of plates that make up a near-spherical planet
      2. Along a ridge there can be variations the magma supply
        1. Iceland
        2. near fracture zones
      3. The general pattern is low at the segment ends and higher in the middle
    2. Real ridges don't have smooth flanks
      1. In addition to getting cooler as it ages, the oceanic crust becomes brittle
      2. The spreading rate relates to the magma supply rate
      3. Different balances between magma supply dynamics and spreading rate create different topographies
        1. Fast spreading rate (EPR)
        2. Slow spreading rate (MAR)
        3. Intermediate spreading rate (our beloved JdFR)
    3. All ridges have an axial valley due to the extensional state of horizontal stress
      1. bounded by normal faults (typical dips and throes?)
      2. containing horsts and grabens
  3. The Endeavour segment as a case study?
    1. Bathymetry
    2. Segmentation and off-axis ridges
    3. Observations of faults (SSS, axial visual, off-axis transects)
    4. Seismicity
    5. Evidence of recent volcanism
    6. Periodic supply of magma? (frequency=1/200ky?)

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