Dating of the Seafloor

Methods to date the seafloor:

  1. Fossils--these give the age of the sediment layer enclosing them.  The lowest fossils, just above the pillow basalts, will give the age of the crust.  This requires drilling to the basalts.
  2. Depth--due to thermal subsidence, the depth will give a rough age for seafloor younger than 70-100 Ma.  At that age the crust has finished cooling and has reached abyssal plain depth.  Depths deeper than the abyssal plain only occur at trenches, where they do not correlate with age.  Since this method works with just an echo sounder, it is the easiest.
  3. Magnetic anomalies--identification of the anomaly allows getting the sea floor age from the magnetic time scale. This requires towing a magnetometer.
  4. Radiometric dating--this gives the age of igneous and metamorphic rocks, for the last time they were heated enough to reset the radiometric clocks.  This requires a sample of the sea floor basalt (hard access, even if not deeply buried by sediments), which are frequently hydrothermally altered (hard analysis).

Radiometric systems.  While most people usually think of radiocarbon dating (carbon 14), its short half life means it is only good for the upper part of the Quaternary, or the last 30,000 to 50,000 years because by then 98% of the radioactive carbon is gone.  Geologically that is nothing.  The isotope pairs used for older crystalline(igneous and metamorphic) rocks include:


Last revision 10/3/2016